Created by: CK-12/Adapted by Christine Miller

Rebecca’s family tree, as illustrated in the pedigree above (Figure 5.18.1), shows a high incidence of cancer among close relatives. But are genes the cause of cancer in this family? Only genetic testing, which is the sequencing of specific genes in an individual, can reveal whether a cancer-causing gene is being inherited in this family.

Fortunately for Rebecca, the results of her genetic testing show that she does not have the mutations in the BRCA1 and BRCA2 genes that most commonly increase a person’s risk of getting cancer. This doesn't mean, however, that she doesn’t have other mutations in these genes that could increase her risk of getting cancer. There are many other mutations in BRCA genes whose effect on cancer risk is still not known — and there may be many more yet to be discovered. Figure 5.18.2 from the National Cancer Institute illustrates many of the different types of known mutations in the BRCA1 gene. It is important to continue to study the variations in genes such as BRCA in different people to better assess their possible contribution to the development of disease. As you now know from this chapter, many mutations are harmless, while others can cause significant health effects, depending on the specific mutation and the gene involved.

Mutations in BRCA genes are particularly likely to cause cancer because these genes encode for tumor-suppressor proteins that normally repair damaged DNA and control cell division. If these genes are mutated in a way that causes the proteins to not function properly, other mutations can accumulate and cell division can run out of control, which can cause cancer.

BRCA1 and BRCA2 are on chromosomes 17 and 13, respectively, which are autosomes. As Rebecca’s genetic counselor mentioned, mutations in these genes have a dominant inheritance pattern. Now that you know the pattern of inheritance of autosomal dominant genes, if Rebecca’s grandmother did have one copy of a mutated BRCA gene, what are the chances that Rebecca’s mother also has this mutation? Because it is dominant, only one copy of the gene is needed to increase the risk of cancer, and because it is on autosomes instead of sex chromosomes, the sex of the parent or offspring does not matter in the inheritance pattern. In this situation, Rebecca’s grandmother’s eggs would have had a 50 per cent chance of having a BRCA gene mutation (Mendel’s law of segregation). Therefore, Rebecca’s mother would have had a 50 per cent chance of inheriting this gene. Even though Rebecca does not have the most common BRCA mutations that increase the risk of cancer, it does not mean that her mother does not, because there would also only be a 50 per cent chance that she would pass it on to Rebecca. Rebecca’s mother, therefore, should consider getting tested for mutations in the BRCA genes, as well. Ideally, the individuals with cancer in a family should be tested first when a genetic cause is suspected, so that if there is a specific mutation being inherited, it can be identified, and the other family members can be tested for that same mutation.

Mutations in both BRCA1 and BRCA2 are often found in Ashkenazi Jewish families. However, these genes are not linked in the chromosomal sense, because they are on different chromosomes and are therefore inherited independently, in accordance with Mendel’s law of independent assortment. Why would certain gene mutations be prevalent in particular ethnic groups? If people within an ethnic group tend to produce offspring with each other, their genes will remain prevalent within the group. These may be genes for harmless variations such as skin, hair, or eye colour, or harmful variations such as the mutations in the BRCA genes. Other genetically based diseases and disorders are sometimes more commonly found in particular ethnic groups, such as cystic fibrosis in people of European descent, and sickle cell anemia in people of African descent. You will learn more about the prevalence of certain genes and traits in particular ethnic groups and populations in the chapter on Human Variation.

As you learned in this chapter, genetics is not the sole determinant of phenotype. The environment can also influence many traits, including adult height and skin colour. The environment plays a major role in the development of cancer, too. Ninety to 95 per cent of all cancers do not have an identified genetic cause, and are often caused by mutagens in the environment, such as UV radiation from the sun or toxic chemicals in cigarette smoke. But for families like Rebecca’s, knowing their family health history and genetic makeup may help them better prevent or treat diseases that are caused by their genetic inheritance. If a person knows they have a gene that can increase their risk of cancer, they can make lifestyle changes and have early and more frequent cancer screenings. They may even choose to have preventative surgeries that can help reduce their risk of getting cancer and increase their odds of long-term survival if cancer does occur. The next time you go to the doctor and they ask whether any members of your family have had cancer, you will have a deeper understanding why this information is so important to your health.

Chapter 5 Attributions and References

Unit 5.18 Image Attributions

• Figure 5.18.1 Rebeccas Pedigree Cancer by CK-12 Foundation is used under a  CC BY 3.0 (https://creativecommons.org/licenses/by/3.0/) license. ©CK-12 Foundation Licensed under  • Terms of Use • Attribution

• Figure 5.18.2 Mutations_on_BRCA1 by National Cancer Institute (NCI) on Wikimedia Commons is in the public domain (https://en.wikipedia.org/wiki/Public_domain).

Reference

Brainard, J/ CK-12 Foundation. (2016). Figure 1 Pedigree for Rebecca's family, as described in the beginning of this chapter, [digital image]. In CK-12 College Human Biology (Section 5.17) [online Flexbook]. CK12.org. https://www.ck12.org/book/ck-12-college-human-biology/section/5.17/

Created by: CK-12/Adapted by Christine Miller

The simple cut-away model of an animal cell (Figure 4.4.1) shows that a cell resembles a plastic bag full of Jell-O. Its basic structure is a plasma membrane filled with cytoplasm. Like Jell-O containing mixed fruit (Figure 4.4.2), the cytoplasm of the cell also contains various structures, including a nucleus and other organelles. Your body is composed of trillions of cells, but all of them perform the same basic life functions. They all obtain and use energy, respond to the environment, and reproduce. How do your cells carry out these basic functions and keep themselves — and you — alive? To answer these questions, you need to know more about the structures that make up cells, starting with the plasma membrane.

The plasma membrane is a structure that forms a barrier between the cytoplasm inside the cell and the environment outside the cell. Without the plasma membrane, there would be no cell. Although it is very thin and flexible, the plasma membrane protects and supports the cell by controlling everything that enters and leaves it. It allows only certain substances to pass through, while keeping others in or out. To understand how the plasma membrane controls what passes into or out of the cell, you need to know its basic structure.

The plasma membrane is composed mainly of phospholipids, which consist of fatty acids and alcohol. The phospholipids in the plasma membrane are arranged in two layers, called a phospholipid bilayer. As shown in the simplified diagram in Figure 4.4.3, each individual  phospholipid molecule has a phosphate group head (in red) and two fatty acid tails (in yellow). The head “loves” water (hydrophilic) and the tails “hate” water (hydrophobic). The water-hating tails are on the interior of the membrane, whereas the water-loving heads point outward, toward either the cytoplasm (intracellular) or the fluid that surrounds the cell (extracellular).

Hydrophobic molecules can easily pass through the plasma membrane if they are small enough, because they are water-hating like the interior of the membrane. Hydrophilic molecules, on the other hand, cannot pass through the plasma membrane — at least not without help — because they are water-loving like the exterior of the membrane.

The plasma membrane also contains other molecules, primarily other lipids and proteins. The yellow molecules in the diagram here, for example, are the lipid cholesterol. Molecules of the steroid lipid cholesterol help the plasma membrane keep its shape. Proteins in the plasma membrane (shown blue in Figure 4.4.4) include: transport proteins that assist other substances in crossing the cell membrane, receptors that allow the cell to respond to chemical signals in its environment, and cell-identity markers that indicate what type of cell it is and whether it belongs in the body.

The plasma membrane may have extensions, such as whip-like flagella (singular flagellum) or brush-like cilia (singular cilium), shown below (Figure 4.4.5), that give it other functions. In single-celled organisms, these membrane extensions may help the organisms move. In multicellular organisms, the extensions have different functions. For example, the cilia on human lung cells sweep foreign particles and mucus toward the mouth and nose, while the flagellum on a human sperm cell allows it to swim.

If you smoke or use e-cigarettes (vaping) and need another reason to quit, here's a good one. We usually think of lung cancer as the major disease caused by smoking. But smoking and vaping can have devastating effects on the body's ability to protect itself from repeated, serious respiratory infections, such as bronchitis and pneumonia.

Cilia are microscopic, hair-like projects on cells that line the respiratory, reproductive, and digestive systems. Cilia in the respiratory system line most of your airways, where they have the job of trapping and removing dust, germs, and other foreign particles before they can make you sick. Cilia secrete mucus that traps particles, and they move in a continuous wave-like motion that sweeps the mucus and particles upward toward the throat, where they can be expelled from the body. When you are sick and cough up phlegm, that's what you are doing.

Smoking prevents cilia from performing these important functions. Chemicals in tobacco smoke paralyze the cilia so they can't sweep mucus out of the airways. Those chemicals also inhibit the cilia from producing mucus. Fortunately, these effects start to wear off soon after the most recent exposure to tobacco smoke. If you stop smoking, your cilia will return to normal. Even if prolonged smoking has destroyed cilia, they will regrow and resume functioning in a matter of months after you stop smoking.

Attributions

Figure 4.4.1

Animal Cell Unannotated, by Kelvin Song on Wikimedia Commons is used under a CC0 1.0 (https://creativecommons.org/publicdomain/zero/1.0/deed.en) public domain dedication license.

Figure 4.4.2

Jello mold at the mexican bakery photo by Aimée Knight on Flickr is used under a CC BY 2.0 (https://creativecommons.org/licenses/by/2.0/) license.

Figure 4.4.3

Phospholipid_Bilayer by OpenStax on Wikimedia Commons is used under a CC BY 4.0 (https://creativecommons.org/licenses/by/4.0) license.

Figure 4.4.4

Lipid bilayer by OpenStax on Wikimedia Commons is used under a CC BY 4.0 (https://creativecommons.org/licenses/by/4.0) license.

Figure 4.4.5

Spermatozoa-human-3140x by No specific author on Wikimedia Commons is released into the public domain (https://en.wikipedia.org/wiki/Public_domain).

Figure 4.4.6

Cilia/ Bronchiolar epithelium 3 - SEM by Charles Daghlian on Wikimedia Commons is released into the public domain (https://en.wikipedia.org/wiki/Public_domain).

Figure 4.4.7

Adverse effects of vaping (raster) by Mikael Häggström on Wikimedia Commons is released into the public domain (https://en.wikipedia.org/wiki/Public_domain).

References

Amoeba Sisters. (2018, February 27). Inside the cell membrane. YouTube. https://www.youtube.com/watch?v=qBCVVszQQNs&feature=youtu.be

Betts, J.G., Young, K.A., Wise, J.A., Johnson, E., Poe, B., Kruse, D.H., Korol, O., Johnson, J.E.. Womble, M., DeSaix. P. (2013, April 25). Figure 3.3 Phospolipid Bilayer [digital image]. In Anatomy and Physiology. OpenStax. https://openstax.org/books/anatomy-and-physiology/pages/3-1-the-cell-membrane

Betts, J.G., Young, K.A., Wise, J.A., Johnson, E., Poe, B., Kruse, D.H., Korol, O., Johnson, J.E.. Womble, M., DeSaix. P. (2013, April 25). Figure 3.4 Cell Membrane [digital image]. In Anatomy and Physiology. OpenStax. https://openstax.org/books/anatomy-and-physiology/pages/3-1-the-cell-membrane

Ghosh, A., Coakley, R. C., Mascenik, T., Rowell, T. R., Davis, E. S., Rogers, K., Webster, M. J., Dang, H., Herring, L. E., Sassano, M. F., Livraghi-Butrico, A., Van Buren, S. K., Graves, L. M., Herman, M. A., Randell, S. H., Alexis, N. E., & Tarran, R. (n.d.). Chronic E-Cigarette Exposure Alters the Human Bronchial Epithelial Proteome. American Journal of Respiratory and Critical /Care Medicine, 198(1), 67–76. https://doi-org.ezproxy.tru.ca/10.1164/rccm.201710-2033OC

TED-Ed. (2013, February 26). Insights into cell membranes via dish detergent - Ethan Perlstein. YouTube. https://www.youtube.com/watch?v=yAXnYcUjn5k&feature=youtu.be

 

Created by CK-12 Foundation/Adapted by Christine Miller

Does the word muscle make you think of the well-developed muscles of a weightlifter, like the woman in Figure 12.2.1? Her name is Natalia Zabolotnaya, and she’s a Russian Olympian. The muscles that are used to lift weights are easy to feel and see, but they aren’t the only muscles in the human body. Many muscles are deep within the body, where they form the walls of internal organs and other structures. You can flex your biceps at will, but you can’t control internal muscles like these. It’s a good thing that these internal muscles work without any conscious effort on your part, because movement of these muscles is essential for survival. Muscles are the organs of the muscular system.

The muscular system consists of all the muscles of the body. The largest percentage of muscles in the muscular system consists of skeletal muscles, which are attached to bones and enable voluntary body movements (shown in Figure 12.2.2). There are almost 650 skeletal muscles in the human body, many of them shown in Figure 12.2.2. Besides skeletal muscles, the muscular system also includes cardiac muscle, which makes up the walls of the heart, and smooth muscles, which control movement in other internal organs and structures.

Muscles are organs composed mainly of muscle cells, which are also called muscle fibres (mainly in skeletal and cardiac muscle) or myocytes (mainly in smooth muscle). Muscle cells are long, thin cells that are specialized for the function of contracting. They contain protein filaments that slide over one another using energy in ATP. The sliding filaments increase the tension in — or shorten the length of — muscle cells, causing a contraction. Muscle contractions are responsible for virtually all the movements of the body, both inside and out.

Skeletal muscles are attached to bones of the skeleton. When these muscles contract, they move the body. They allow us to use our limbs in a variety of ways, from walking to turning cartwheels. Skeletal muscles also maintain posture and help us to keep balance.

Smooth muscles in the walls of blood vessels contract to cause vasoconstriction, which may help conserve body heat. Relaxation of these muscles causes vasodilation, which may help the body lose heat. In the organs of the digestive system, smooth muscles squeeze food through the gastrointestinal tract by contracting in sequence to form a wave of muscle contractions called peristalsis. Think of squirting toothpaste through a tube by applying pressure in sequence from the bottom of the tube to the top, and you have a good idea of how food is moved by muscles through the digestive system. Peristalsis of smooth muscles also moves urine through the urinary tract.

Cardiac muscle tissue is found only in the walls of the heart. When cardiac muscle contracts, it makes the heart beat. The pumping action of the beating heart keeps blood flowing through the cardiovascular system.

Muscles can grow larger, or hypertrophy. This generally occurs through increased use, although hormonal or other influences can also play a role. The increase in testosterone that occurs in males during puberty, for example, causes a significant increase in muscle size. Physical exercise that involves weight bearing or resistance training can increase the size of skeletal muscles in virtually everyone. Exercises (such as running) that increase the heart rate may also increase the size and strength of cardiac muscle. The size of muscle, in turn, is the main determinant of muscle strength, which may be measured by the amount of force a muscle can exert.

Muscles can also grow smaller, or atrophy, which can occur through lack of physical activity or from starvation. People who are immobilized for any length of time — for example, because of a broken bone or surgery — lose muscle mass relatively quickly. People in concentration or famine camps may be so malnourished that they lose much of their muscle mass, becoming almost literally just “skin and bones.” Astronauts on the International Space Station may also lose significant muscle mass because of weightlessness in space (see Figure 12.2.3).

Many diseases, including cancer and AIDS, are often associated with muscle atrophy. Atrophy of muscles also happens with age. As people grow older, there is a gradual decrease in the ability to maintain skeletal muscle mass, known as sarcopenia. The exact cause of sarcopenia is not known, but one possible cause is a decrease in sensitivity to growth factors that are needed to maintain muscle mass. Because muscle size determines strength, muscle atrophy causes a corresponding decline in muscle strength.

In both hypertrophy and atrophy, the number of muscle fibres does not change. What changes is the size of the muscle fibres. When muscles hypertrophy, the individual fibres become wider. When muscles atrophy, the fibres become narrower.

Muscles cannot contract on their own. Skeletal muscles need stimulation from motor neurons in order to contract. The point where a motor neuron attaches to a muscle is called a neuromuscular junction. Let’s say you decide to raise your hand in class. Your brain sends electrical messages through motor neurons to your arm and shoulder. The motor neurons, in turn, stimulate muscle fibres in your arm and shoulder to contract, causing your arm to rise.

Involuntary contractions of smooth and cardiac muscles are also controlled by electrical impulses, but in the case of these muscles, the impulses come from the autonomic nervous system (smooth muscle) or specialized cells in the heart (cardiac muscle). Hormones and some other factors also influence involuntary contractions of cardiac and smooth muscles. For example, the fight-or-flight hormone adrenaline increases the rate at which cardiac muscle contracts, thereby speeding up the heartbeat.

Muscles cannot move the body on their own. They need the skeletal system to act upon. The two systems together are often referred to as the musculoskeletal system. Skeletal muscles are attached to the skeleton by tough connective tissues called tendons. Many skeletal muscles are attached to the ends of bones that meet at a joint. The muscles span the joint and connect the bones. When the muscles contract, they pull on the bones, causing them to move. The skeletal system provides a system of levers that allow body movement. The muscular system provides the force that moves the levers.

Involuntary, striated muscle found only in the walls of the heart; also called myocardium.

Created by CK-12 Foundation/Adapted by Christine Miller

Imagine the man in Figure 12.3.1 turns his eyes in your direction. This is a very small movement, considering the conspicuously large and strong external eye muscles that control eyeball movements. These muscles have been called the strongest muscles in the human body relative to the work they do. However, the external eye muscles actually do a surprising amount of work. Eye movements occur almost constantly during waking hours, especially when we are scanning faces or reading. Eye muscles are also exercised nightly during the phase of sleep called rapid eye movement sleep. External eye muscles can move the eyes because they are made mainly of muscle tissue.

Muscle tissue is a soft tissue that makes up most of the tissues in the muscles of the human muscular system. Other tissues in muscles are connective tissues, such as tendons that attach skeletal muscles to bones and sheaths of connective tissues that cover or line muscle tissues. Only muscle tissue per se, has cells with the ability to contract.

There are three major types of muscle tissues in the human body: skeletal, smooth, and cardiac muscle tissues. Figure 12.3.2 shows how the three types of muscle tissues appear under magnification. When you read about each type below, you will learn why the three types appear as they do.

Skeletal muscle is muscle tissue that is attached to bones by tendons, which are bundles of collagen fibres. Whether you are moving your eyes or running a marathon, you are using skeletal muscles. Contractions of skeletal muscles are voluntary, or under conscious control of the central nervous system via the somatic nervous system. Skeletal muscle tissue is the most common type of muscle tissue in the human body. By weight, an average adult male is about 42% skeletal muscles, and the average adult female is about 36% skeletal muscles. Some of the major skeletal muscles in the human body are labeled in Figure 12.3.3 below.

Skeletal Muscle Pairs

To move bones in opposite directions, skeletal muscles often consist of muscle pairs that work in opposition to one another, also called antagonistic muscle pairs.  For example, when the biceps muscle (on the front of the upper arm) contracts, it can cause the elbow joint to flex or bend the arm, as shown in Figure 12.3.4. When the triceps muscle (on the back of the upper arm) contracts, it can cause the elbow to extend or straighten the arm. The biceps and triceps muscles, also shown in Figure 12.3.4, are an example of a muscle pair where the muscles work in opposition to each other.

Skeletal Muscle Structure

Each skeletal muscle consists of hundreds — or even thousands — of skeletal muscle fibres, which are long, string-like cells. As shown in Figure 12.3.5 below, skeletal muscle fibres are individually wrapped in connective tissue called endomysium. The skeletal muscle fibres are bundled together in units called muscle fascicles, which are surrounded by sheaths of connective tissue called perimysium. Each fascicle contains between ten and 100 (or even more!) skeletal muscle fibres. Fascicles, in turn, are bundled together to form individual skeletal muscles, which are wrapped in connective tissue called epimysium. The connective tissues in skeletal muscles have a variety of functions. They support and protect muscle fibres, allowing them to withstand the forces of contraction by distributing the forces applied to the muscle. They also provide pathways for nerves and blood vessels to reach the muscles. In addition, the epimysium anchors the muscles to tendons.

The same bundles-within-bundles structure is replicated within each muscle fibre. As shown in Figure 12.3.6, a muscle fibre consists of a bundle of myofibrils, which are themselves bundles of protein filaments. These protein filaments consist of thin filaments of the protein actin, which are anchored to structures called Z discs, and thick filaments of the protein myosin. The filaments are arranged together within a myofibril in repeating units called sarcomeres, which run from one Z disc to the next. The sarcomere is the basic functional unit of skeletal and cardiac muscles. It contracts as actin and myosin filaments slide over one another. Skeletal muscle tissue is said to be striated, because it appears striped. It has this appearance because of the regular, alternating A (dark) and I (light) bands of filaments arranged in sarcomeres inside the muscle fibres. Other components of a skeletal muscle fibre include multiple nuclei and mitochondria.

Slow- and Fast-Twitch Skeletal Muscle Fibres

Skeletal muscle fibres can be divided into two types, called slow-twitch (or type I) muscle fibres and fast-twitch (or type II) muscle fibres.

• Slow-twitch muscle fibres are dense with capillaries and rich in mitochondria and myoglobin, which is a protein that stores oxygen until needed for muscle activity. Relative to fast-twitch fibres, slow-twitch fibres can carry more oxygen and sustain aerobic (oxygen-using) activity. Slow-twitch fibres can contract for long periods of time, but not with very much force. They are relied upon primarily in endurance events, such as distance running or cycling.
• Fast-twitch muscle fibres contain fewer capillaries and mitochondria and less myoglobin. This type of muscle fibre can contract rapidly and powerfully, but it fatigues very quickly. Fast-twitch fibres can sustain only short, anaerobic (non-oxygen-using) bursts of activity. Relative to slow-twitch fibres, fast-twitch fibres contribute more to muscle strength and have a greater potential for increasing in mass. They are relied upon primarily in short, strenuous events, such as sprinting or weightlifting.

Proportions of fibre types vary considerably from muscle to muscle and from person to person. Individuals may be genetically predisposed to have a larger percentage of one type of muscle fibre than the other. Generally, an individual who has more slow-twitch fibres is better suited for activities requiring endurance, whereas an individual who has more fast-twitch fibres is better suited for activities requiring short bursts of power.

Smooth muscle is muscle tissue in the walls of internal organs and other internal structures such as blood vessels. When smooth muscles contract, they help the organs and vessels carry out their functions. When smooth muscles in the stomach wall contract, for example, they squeeze the food inside the stomach, helping to mix and churn the food and break it into smaller pieces. This is an important part of digestion. Contractions of smooth muscles are involuntary, so they are not under conscious control. Instead, they are controlled by the autonomic nervous system, hormones, neurotransmitters, and other physiological factors.

Structure of Smooth Muscle

The cells that make up smooth muscle are generally called myocytes. Unlike the muscle fibres of striated muscle tissue, the myocytes of smooth muscle tissue do not have their filaments arranged in sarcomeres. Therefore, smooth tissue is not striated. However, the myocytes of smooth muscle do contain myofibrils, which in turn contain bundles of myosin and actin filaments. The filaments cause contractions when they slide over each other, as shown in Figure 12.3.7.

Unlike striated muscle, smooth muscle can sustain very long-term contractions. Smooth muscle can also stretch and still maintain its contractile function, which striated muscle cannot. The elasticity of smooth muscle is enhanced by an extracellular matrix secreted by myocytes. The matrix consists of elastin, collagen, and other stretchy fibres. The ability to stretch and still contract is an important attribute of smooth muscle in organs such as the stomach and uterus (see Figures 12.3.8 and 12.3.9), both of which must stretch considerably as they perform their normal functions.

The following list indicates where many smooth muscles are found, along with some of their specific functions.

• Walls of organs of the gastrointestinal tract (such as the esophagus, stomach, and intestines), moving food through the tract by peristalsis
• Walls of air passages of the respiratory tract (such as the bronchi), controlling the diameter of the passages and the volume of air that can pass through them
• Walls of organs of the male and female reproductive tracts; in the uterus, for example, pushing a baby out of the uterus and into the birth canal
• Walls of structures of the urinary system, including the urinary bladder, allowing the bladder to expand so it can hold more urine, and then contract as urine is released
• Walls of blood vessels, controlling the diameter of the vessels and thereby affecting blood flow and blood pressure
• Walls of lymphatic vessels, squeezing the fluid called lymph through the vessels
• Iris of the eyes, controlling the size of the pupils and thereby the amount of light entering the eyes
• Arrector pili in the skin, raising hairs in hair follicles in the dermis

Cardiac muscle is found only in the wall of the heart. It is also called myocardium. As shown in Figure 12.3.10, myocardium is enclosed within connective tissues, including the endocardium on the inside of the heart and pericardium on the outside of the heart. When cardiac muscle contracts, the heart beats and pumps blood. Contractions of cardiac muscle are involuntary, like those of smooth muscles. They are controlled by electrical impulses from specialized cardiac muscle cells in an area of the heart muscle called the sinoatrial node.

Like skeletal muscle, cardiac muscle is striated because its filaments are arranged in sarcomeres inside the muscle fibres. However, in cardiac muscle, the myofibrils are branched at irregular angles rather than arranged in parallel rows (as they are in skeletal muscle). This explains why cardiac and skeletal muscle tissues look different from one another.

The cells of cardiac muscle tissue are arranged in interconnected networks. This arrangement allows rapid transmission of electrical impulses, which stimulate virtually simultaneous contractions of the cells. This enables the cells to coordinate contractions of the heart muscle.

The heart is the muscle that performs the greatest amount of physical work in the course of a lifetime. Although the power output of the heart is much less than the maximum power output of some other muscles in the human body, the heart does its work continuously over an entire lifetime without rest. Cardiac muscle contains a great many mitochondria, which produce ATP for energy and help the heart resist fatigue.

Cardiomyopathy is a disease in which the muscles of the heart are no longer able to effectively pump blood to the body — extreme forms of this disease can lead to heart failure.  There are four main types of cardiomyopathy (also illustrated in Figure 12.3.11):

• Dilated (congestive) cardiomyopathy: the left ventricle (the chamber itself) of the heart becomes enlarged and can't pump blood our to the body.  This is normally related to coronary artery disease and/or heart attack
• Hypertrophic cardiomyopathy: abnormal thickening of the muscular walls of the left ventricle make the chamber less able to work properly.  This condition is more common in patients with a family history of the disease.
• Restrictive cardiomyopathy: the myocardium becomes abnormally rigid and inelastic and is unable to expand in between heartbeats to refill with blood.  Restrictive cardiomyopathy typically affects older people.
• Arrhythmogenic right ventricular cardiomyopathy: the right ventricular muscle is replaced by adipose or scar tissue, reducing elasticity and interfering with normal heartbeat and rhythm.  This disease is often caused by genetic mutations.

Cardiomyopathy is typically diagnosed with a physical exam supplemented by medical and family history, an angiogram, blood tests, chest x-rays and electrocardiograms.  In some cases your doctor would also requisition a CT scan and/or genetic testing.

When treating cardiomyopathy, the goal is to reduce symptoms that affect everyday life.  Certain medications can help regularize and slow heart rate, decrease chances of blood clots and cause vasodilation in the coronary arteries.  If medication is not sufficient to manage symptoms, a pacemaker or even a heart transplant may be the best option.  Lifestyle can also help manage the symptoms of cardiomyopathy — people living with this disease are encouraged to avoid drug and alcohol use, control high blood pressure, eat a healthy diet, get ample rest and exercise, as well as reduce stress levels.

 

Attributions

Figure 12.3.1

Look by ali-yahya-155huuQwGvA [photo] by Ali Yahya on Unsplash is used under the Unsplash License (https://unsplash.com/license).

Figure 12.3.2

Skeletal_Smooth_Cardiac by OpenStax College on Wikimedia Commons is used under a CC BY 3.0 (https://creativecommons.org/licenses/by/3.0) license.

Figure 12.3.3

Anterior_and_Posterior_Views_of_Muscles by OpenStax on Wikimedia Commons is used under a CC BY 4.0 (https://creativecommons.org/licenses/by/4.0) license.

Figure 12.3.4

Antagonistic Muscle Pair by Laura Guerin at CK-12 Foundation on Wikimedia Commons is used under a CC BY-NC 3.0 (https://creativecommons.org/licenses/by-nc/3.0/) license. 

Figure 12.3.5

Muscle_Fibes_(large) by OpenStax on Wikimedia Commons is used under a CC BY 4.0 (https://creativecommons.org/licenses/by/4.0) license.

Figure 12.3.6

Muscle_Fibers_(small) by OpenStax on Wikimedia Commons is used under a CC BY 4.0 (https://creativecommons.org/licenses/by/4.0) license.

Figure 12.3.7

Smooth_Muscle_Contraction by OpenStax on Wikimedia Commons is used under a CC BY 4.0 (https://creativecommons.org/licenses/by/4.0) license.

Figure 12.3.8

Blausen_0747_Pregnancy by BruceBlaus on Wikimedia Commons is used under a CC BY 3.0 (https://creativecommons.org/licenses/by/3.0) license.

Figure 12.3.9

Size_of_Uterus_Throughout_Pregnancy-02 by OpenStax College on Wikimedia Commons is used under a CC BY 3.0 (https://creativecommons.org/licenses/by/3.0) license.

Figure 12.3.10

1024px-Blausen_0470_HeartWall by BruceBlaus on Wikimedia Commons is used under a CC BY 3.0 (https://creativecommons.org/licenses/by/3.0) license.

Figure 12.3.11

Tipet_e_kardiomiopative by Npatchett at English Wikipedia on Wikimedia Commons is used under a CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0) license. (Work derived from Blausen 0165 Cardiomyopathy Dilated by BruceBlaus)

References

Betts, J. G., Young, K.A., Wise, J.A., Johnson, E., Poe, B., Kruse, D.H., Korol, O., Johnson, J.E., Womble, M., DeSaix, P. (2013, June 19). Figure 4.18 Muscle tissue [digital image].  In Anatomy and Physiology (Section 4.4). OpenStax. https://openstax.org/books/anatomy-and-physiology/pages/4-4-muscle-tissue-and-motion

Betts, J. G., Young, K.A., Wise, J.A., Johnson, E., Poe, B., Kruse, D.H., Korol, O., Johnson, J.E., Womble, M., DeSaix, P. (2013, June 19). Figure 28.18 Size of uterus throughout pregnancy [digital image].  In Anatomy and Physiology (Section 28.4). OpenStax. https://openstax.org/books/anatomy-and-physiology/pages/28-4-maternal-changes-during-pregnancy-labor-and-birth

Betts, J. G., Young, K.A., Wise, J.A., Johnson, E., Poe, B., Kruse, D.H., Korol, O., Johnson, J.E., Womble, M., DeSaix, P. (2016, May 18). Figure 10.3 The three connective tissue layers [digital image].  In Anatomy and Physiology (Section 10.2). OpenStax. https://openstax.org/books/anatomy-and-physiology/pages/10-2-skeletal-muscle

Betts, J. G., Young, K.A., Wise, J.A., Johnson, E., Poe, B., Kruse, D.H., Korol, O., Johnson, J.E., Womble, M., DeSaix, P. (2016, May 18). Figure 10.4 Muscle fiber [digital image].  In Anatomy and Physiology (Section 10.2). OpenStax. https://openstax.org/books/anatomy-and-physiology/pages/10-2-skeletal-muscle

Betts, J. G., Young, K.A., Wise, J.A., Johnson, E., Poe, B., Kruse, D.H., Korol, O., Johnson, J.E., Womble, M., DeSaix, P. (2016, May 18). Figure 10.24 Muscle contraction [digital image].  In Anatomy and Physiology (Section 10.8). OpenStax. https://openstax.org/books/anatomy-and-physiology/pages/10-8-smooth-muscle

Betts, J. G., Young, K.A., Wise, J.A., Johnson, E., Poe, B., Kruse, D.H., Korol, O., Johnson, J.E., Womble, M., DeSaix, P. (2016, May 18). Figure 11.5 Overview of the muscular system [digital image].  In Anatomy and Physiology (Section 11.2). OpenStax. https://openstax.org/books/anatomy-and-physiology/pages/11-2-naming-skeletal-muscles

Blausen.com staff. (2014). Medical gallery of Blausen Medical 2014. WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.010. ISSN 2002-4436.

Brainard, J/ CK-12 Foundation. (2012). Figure 5 Triceps and biceps muscles in the upper arm are opposing muscles. [digital image]. In CK-12 Biology (Section 21.3) [online Flexbook]. CK12.org. https://www.ck12.org/book/ck-12-biology/section/21.3/ (Last modified August 11, 2017.)

khanacademymedicine. (2012, October 19). Three types of muscle | Circulatory system physiology | NCLEX-RN | Khan Academy. YouTube.

TED-Ed. (2017, February 14).  What happens during a heart attack? - Krishna Sudhir. YouTube. https://www.youtube.com/watch?v=3_PYnWVoUzM&feature=youtu.be

Created by: CK-12/Adapted by Christine Miller

Figure 6.7.1 Men from Maasai Mara, Kenya.

Built for Heat

These tall, slender men live near the equator in Kenya, East Africa — one of the hottest regions of the world. These and the other people of his tribal group, called the Maasai, are among the tallest, most linear people on the planet. Their body build is thought to be an adaptation to their climate, which is hot year-round.

Climate refers to the average weather conditions in a region over a long period of time. One of the main determinants of climate is temperature. Both hot and cold temperatures are serious environmental stresses on the human body.

In the cold, there is risk of hypothermia, which is a dangerous decrease in core body temperature. The normal temperature of the human body is 37 degrees C (98.6 degrees F). Hypothermia sets in when body temperature drops to 34.4 degrees C (94 degrees F). If body temperature falls below 29.4 degrees C (85 degrees F), it starts to cool very rapidly because the body’s temperature regulation mechanism starts to fail.

The opposite problem occurs in the heat, where the risk is hyperthermia, which is a dangerous increase in core body temperature. If human body temperature rises above about 40.6 degrees C (105 degrees F), hyperthermia may become life threatening. If a temperature this high persists more than a few days, it generally damages the brain and other internal organs, leading to death.

Humans are the most widespread species on the planet, and they have lived in extreme climates for tens of thousands of years. As a result, many human populations have had to cope with extreme temperatures for hundreds of generations, which has forced them to develop genetic adaptations to these climate extremes.

The size and proportions of the human body may play an important role in how well an individual is able to handle hot or cold temperatures. In general, people with a tall, slender build, like the Maasai man pictured in Figure 6.7.1, are well adapted to heat, whereas people with a short, stocky build (like the Indigenous North American Inuit pictured in Figure 6.7.2) are well adapted to cold. These relationships between body build and climate were first noticed in other animal species in the 1800s by biologists Carl Bergmann and Joel Allen. These scientists formulated what are now known as Bergmann’s and Allen’s rules.

Figure 6.7.2 Indigenous North American Inuit.

Bergmann’s Rule

Bergmann's rule states that within a broadly distributed taxonomic group, populations or species of larger size are found in colder environments, whereas populations or species of smaller size are found in warmer environments. Bergmann’s rule has been shown to generally apply to widespread species of mammals and birds, although there are also many exceptions to the rule.

What explains Bergmann’s rule? Larger animals have a lower surface area to volume ratio than smaller animals, which is illustrated in Table 6.7.1 for a simple shape, a cube. From the table, you can see how the surface area to volume ratio of a cube decreases dramatically as the size of the cube increases. Because heat is lost through the surface of the body, an animal with a smaller surface area to volume ratio radiates less body heat per unit of mass. The larger body mass also allows the animal to generate more heat. A larger animal has more cells, so it can produce more body heat as a byproduct of cellular metabolism. Both of these factors allow a larger animal to stay warmer in a cold climate.

Table 6.7.1

Relationship of Surface Area to Volume in Cubes of Different Sizes

Relationship of Surface Area to Volume in Cubes of Different Sizes
Side of Cube (cm)	Surface Area of Cube (cm2)	Volume of Cube (cm3)	Surface Area:Volume Ratio
2	24	8	3:1
4	96	64	3:2
6	216	216	3:3
12	864	1728	3:6
20	2400	8000	3:10

Warmer climates impose the opposite problem: body heat generated by metabolism needs to be dissipated quickly rather than stored within the body. Smaller animals have a higher surface area to volume ratio that maximizes heat loss through the surface of the body and helps cool the body. With less mass and fewer cells, smaller animals also generate less heat due to cellular metabolism.

Anthropologists have found that many human populations tend to follow Bergmann’s rule. For example, a study of 100 human populations in the 1950s found a strong negative correlation between mean body mass and average yearly temperature. In other words, higher body mass was generally found in colder places, and lower body mass was generally found in hotter places.

There are also exceptions to the rule, in part because we use cultural responses to temper environmental stresses so we do not need to change genetically or physiologically in order to cope. Humans, for example, use clothing and heated buildings to stay warm in cold climates, which tends to counter the effects of natural selection changing human body shape in cold climates.

Allen’s Rule

Allen’s rule is a corollary of Bergmann’s rule. It states that animals living in hotter climates generally have longer extremities (such as limbs, tails, snouts, and ears) than closely related animals living in colder climates. The explanation for Allen’s rule is similar to the rationale behind Bergmann’s rule. Longer extremities maximize an animal’s surface area, allowing greater heat loss through the surface of the body. Therefore, having long extremities is adaptive in hot climates where the main challenge is dissipating body heat.

Anthropologists have noted that, in populations that have lived in tropical regions for long periods of time, the limbs of people tend to be longer in proportion to overall body height. The Maasai man pictured in Figure 6.7.1 is a clear example. His exceptionally long limbs — like those of other members of his population — are optimally proportioned for the hot climate in Kenya. The shorter-limbed body proportions of the Inuit people (Figure 6.7.2) suit them well for their cold climate. Marked differences in limb length have also been observed in related populations that have lived for long periods of time at different altitudes. High altitudes have colder climates than lower altitudes and — consistent with Allen's rule —people tend to have shorter limbs at higher altitudes.

Humans exhibit several other responses to high temperatures that are generally considered either short-term physiological responses or examples of longer-term acclimatization.

Sweating and Humidity

Because humans are basically tropical animals, we generally have an easier time dealing with excessive heat than excessive cold. Evaporation of sweat is the main way we cool the body. The dancer in Figure 6.7.4 is sweating copiously while working out in a hot environment. Why does sweating cool us? When sweat evaporates from the skin, it requires heat. The heat comes from the surface of the body, resulting in evaporative cooling.

The heat index (HI) is a number that combines air temperature and relative humidity to indicate how hot the air feels due to the humidity. The heat index is also called "apparent temperature.” Figure 6.7.5 shows the heat index at different combinations of air temperature and relative humidity. As you can see, when the humidity is very high, even a 90-degree F (32 degrees C) temperature can be very dangerous.

 

If humidity is low, evaporation of sweat can be an effective way to keep the body from overheating. However, the loss of water and salts in sweat can also be dangerous. In very hot conditions, an adult may lose up to four litres of sweat per hour and up to 14 litres per day. Such water losses may cause severe dehydration if the water is not replaced by drinking much more than usual. The loss of salts may also upset the normal salt balance in the body, which can be dangerous. Becoming acclimatized to heat by gradually increasing the exposure time to high temperatures — particularly while exercising or doing physical work — can reduce the risk of these effects.

It may take up to 14 days to attain maximum heat acclimatization. As the body becomes acclimatized, sweat output increases, and sweating begins sooner. The salt content of the sweat also declines, as does the output of urine. These and other physiological changes help the body lose heat through the evaporation of sweat, while maintaining the proper balance of salts and fluids in the body. There may also be increased blood flow to the body surface through the widening of blood vessels near the skin. This is called vasodilation. This brings more heat from the body core to the skin, and from there it may be radiated out into the environment.

Becoming acclimatized to heat allows one to safely perform more exercise or work in the heat. It also helps prevent heat-related illnesses by reducing strain on the body. Heat-related illnesses — from least to most serious — include heat cramps, heat exhaustion, and heat stroke.

• Heat cramps are muscle spasms caused by loss of water and salts. They often follow prolonged sweating brought on by over-exertion in hot weather.
• Heat exhaustion is a condition in which over-heating of the body causes dizziness, headache, profuse sweating, rapid heartbeat. and other symptoms. Without prompt treatment, heat exhaustion can lead to heat stroke.
• Heat stroke is potentially life threatening and a medical emergency. Heat stroke results from prolonged exposure to high temperatures, usually in combination with dehydration. It leads to failure of the body's temperature control system and is diagnosed when the core body temperature exceeds 105 degrees F (40 degrees C). Symptoms may include nausea, seizures, confusion, disorientation, and coma.

Acclimatization to heat, like other types of acclimatization, is a reversible process. Just as quickly as heat acclimatization occurs, the physiological changes fade away in the absence of heat exposure. The body returns to its baseline state within a week or two of no longer exercising or working at high temperatures.

Besides genetic difference in body build, there are two major ways the human body can respond to the cold. One way is by producing more body heat, and the other way is by conserving more body heat. An immediate response to cooling of the body is shivering. This is an involuntary and simultaneous contraction of many tiny muscles in the body. These muscle contractions generate a small amount of heat. Another early response to cold temperature is a narrowing of blood vessels near the skin. This is called vasoconstriction. This helps to shunt blood away from the body surface so more heat is held at the body core. The skin cools down and radiates less heat into the environment.

Hunting Response

At temperatures below freezing, vasoconstriction can be dangerous if it lasts too long. The extremities become too cold because of lack of blood flow, and cold injury (such as frostbite) may occur. Frostbite is tissue destruction that occurs when tissue freezes. You can see a mild-to-moderate case of frostbite of the fingers in Figure 6.7.7. If frostbite is severe, it may lead to gangrene and amputation of the affected extremities.

The body counters the possibility of cold injury with a reaction called the hunting response. This is a process of alternating vasoconstriction and vasodilation in extremities exposed to cold. About five to ten minutes after the start of cold exposure, the blood vessels in the extremities suddenly dilate, which increases blood flow and subsequently the temperature of the extremities. This is soon followed by another phase of vasoconstriction, and then the process repeats.

The hunting response occurs in most people, but several factors may influence the strength of the response. People who live or work regularly in cold environments show an increased hunting response. Through acclimatization, however, tropical residents can develop an increased response, which is indistinguishable from that of arctic residents. Genetic factors may play a role in the hunting response, but this is uncertain because it is difficult to differentiate between adaptation and acclimatization.

Persistent Vasoconstriction

Where temperatures rarely fall below freezing but are repeatedly very chilly, the hunting response may not occur. Instead, vasoconstriction may persist to keep heat within the body at the expense of cooling the skin. As long as the temperature stays above freezing, cold injury (such as frostbite) will not occur. This type of response has been shown to occur in indigenous desert dwellers in southern Africa and Australia, where the temperature is hot during the day and very cold at night. People in these populations also tend to deposit fat around the organs in their chest and abdomen. The fat serves as insulation, protecting vital structures from the cold.

High-Fat Diet

Besides shivering, another way to increase body heat is to raise the basal metabolic rate. The basal metabolic rate (BMR) is the amount of energy that a person needs to keep the body functioning at rest. The higher the BMR, the more heat the body generates, even without exercise or physical labor. The BMR can be increased by consuming large quantities of high-calorie fatty foods. People living in very cold subarctic regions, including the Inuit, traditionally ate whale and seal blubber and other high-fat foods, which helped them maintain a high BMR and stay warm.

Figure 6.7.8  Whale and seal blubber (mainly on abundant ring seals) is an important part of the traditional Inuit diet. 

Too many news stories report young children being seriously injured or dying from heat stroke in hot vehicles. On average, 38 children die in hot vehicles each year from heat-related deaths after being trapped inside. Most often, this happens by accident, when a parent or caregiver unknowingly leaves a sleeping child in a car. In other cases, children get into cars on their own, and then cannot get out again.

A child’s thermoregulatory system is not as efficient as that of an adult, and a child’s body temperature may increase as much as five times faster. This makes children prime candidates for heat stroke. A motor vehicle is also easily heated by direct sun. The windows of the vehicle allow solar radiation to pass through and heat up objects inside. A dark-coloured dashboard or seat may quickly reach a temperature of more than 180 degrees F (82 degrees C)! These hot surfaces can just as quickly heat the adjacent air, rapidly increasing the temperature of the air trapped inside the vehicle.

Here are several simple tips that parents and caregivers can follow to prevent heat stroke tragedies:

• Never leave children alone in or around cars — not even for a minute.
• Always open the back door and check the back seat before leaving your vehicle to be sure no child has been left behind.
• Put something you will need, such as your cell phone or handbag, in the back seat so you will have to open the back door to retrieve it whenever you park the car.
• Keep a large stuffed animal in the child's car seat, and when the child is placed in the car seat, put the stuffed animal in the front passenger seat as a visual reminder that the child is in the back.
• Make sure you have a strict policy in place with everyone involved in the care of your child that you should always be called whenever your child does not show up at daycare or school as scheduled.
• Keep vehicles locked at all times, even in driveways and garages. Ask home visitors, child care providers, and neighbors to do the same.
• Keep car keys and remote vehicle openers out of reach of children.
• If a child is missing, immediately check the inside passenger compartments and trunks of all vehicles in the area. Check vehicles even if they are locked, because a child may lock a vehicle after entering and not be able to unlock it again to get out.
• If you see a child alone in a vehicle, call 911 immediately. If the child seems hot or sick, get them out of the vehicle as quickly as possible.
• Pay for gas at the pump and use drive-throughs at the bank, pharmacy, or wherever else they are available.

Attributions

Figure 6.7.1

• Maasai warrior by Ninaras on Wikimedia Commons is used under a CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/deed.en) license.
• Smiling man from Maasai Mara, Kenya by Sneha on Unsplash is used under the Unsplash License (https://unsplash.com/license). 

Figure 6.7.2

• Inuit-Kleidung women by Ansgar Walk on Wikimedia Commons is used under a CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0/deed.lv) license.
• Kulusuk, Tunumiit Inuit couple by Arian Zwegers on Wikimedia Commons is used under a CC BY 2.0 (https://creativecommons.org/licenses/by/2.0/deed.en) license.
• Inuit girls by Susan van Gelder on Flickr is used under a CC BY-NC-SA 2.0 (https://creativecommons.org/licenses/by-nc-sa/2.0/) license.

Figure 6.7.3

Bergmann’s_rule_-_Canis_lupus by Dhaval Vargiya at Yellowstone National Park on Wikimedia Commons is in the public domain (https://en.wikipedia.org/wiki/Public_domain).

Figure 6.7.4

Sweating [photo] by Avi Richards on Unsplash is used under the Unsplash License (https://unsplash.com/license). 

Figure 6.7.5

Heat_Index by U.S. National Oceanic and Atmospheric Administration (NOAA) on Wikimedia Commons is in the public domain (https://en.wikipedia.org/wiki/Public_domain).

Figure 6.7.6

Thirsty [photo] by Dylan Alcock on Unsplash is used under the Unsplash License (https://unsplash.com/license). 

Figure 6.7.7

Frostbitten_hands by Winky from Oxford, UK on Wikimedia Commons is used under a CC BY 2.0 (https://creativecommons.org/licenses/by/2.0/deed.en) license.

Figure 6.7.8

• Ringed seal preparation by Ansgar Walk on Wikimedia Commons is used under a CC BY-SA 2.5 (https://creativecommons.org/licenses/by-sa/2.5/deed.en) license.
• Butchering a narwhal by Spencer & Carole on Flickr is used under a CC BY-NC-SA 2.0 (https://creativecommons.org/licenses/by-nc-sa/2.0/) license.
• Inuit children playing while the family is on seal hunt, by GRID-Arendal on Flickr is used under a CC BY-NC-SA 2.0 (https://creativecommons.org/licenses/by-nc-sa/2.0/) license.

References

It's Okay To Be Smart. (2017, January 9). How an igloo keeps you warm. YouTube. https://www.youtube.com/watch?v=1L7EI0vKVuU&feature=youtu.be

National Geographic. (2014, April 7). Hailstones' Inupiaq traditions | Life below zero. YouTube. https://www.youtube.com/watch?v=_Ifq73REJiM&feature=youtu.be

TED-Ed. (2014, July 21). What happens when you get heat stroke? - Douglas J. Casa. YouTube. https://www.youtube.com/watch?v=PpHM4DfPZQU&feature=youtu.be

TED-Ed. (2018, May 15). Why do we sweat? - John Murnan. YouTube. https://www.youtube.com/watch?v=fctH_1NuqCQ&feature=youtu.be

Wim Hof. (2011, June 19). Wim Hof Method. YouTube. https://www.youtube.com/watch?v=j3sY67aGFXY&feature=youtu.be

Created by: CK-12/Adapted by Christine Miller

Got Lactase?

Do you remember the American “got milk?” slogan from the 1990s?  It was used in advertisements for milk in which celebrities were pictured wearing milk “mustaches.” While the purpose of the “got milk?” ads was to sell more milk, there is no denying that drinking milk can actually be good for one’s health. Milk is naturally high in protein and minerals. It can also be low in fat or even fat-free if treated to remove the lipids that naturally occur in milk. However, before you reach for a tall, cold glass of milk,  you might want to ask yourself another question: “got lactase?”

Do you drink milk? Or do you avoid drinking milk and consuming milk products because they cause you discomfort? If the latter is the case, then you may have trouble digesting milk.

Milk, Lactose, and Lactase

Milk naturally contains more than just proteins and lipids — it also contains carbohydrates. Specifically, milk contains the sugar lactose, which is a disaccharide (two-sugar) compound that consists of one molecule each of galactose and glucose, as shown in the structural formula below (Figure 6.8.2). Lactose makes up between two and eight per cent of milk by weight. The exact amount varies both within and between species.

Lactose Intolerance

Lactose intolerance is the inability of older children and adults to digest lactose in milk. People who are lactose intolerant may be able to drink small quantities of milk without any problems, but if they try to consume larger amounts, they are likely to suffer adverse effects. For example, they may have abdominal bloating and cramping, flatulence (gas), diarrhea, nausea, and vomiting. The symptoms may occur from 30 minutes to two hours after milk is consumed, and they're generally worse when the quantity of milk consumed is greater. The symptoms result from the small intestine's inability to digest and absorb lactose, so the lactose is passed on to the large intestine, where normal intestinal bacteria start breaking it down through the process of fermentation. This process releases gas and causes the other symptoms of lactose intolerance.

Lactose intolerance is actually the original and normal condition of the human species, as well as all other mammalian species. Early humans were hunter-gatherers that subsisted on wild plant and animal foods. The animal foods may have included meat and eggs, but did not include milk because animals had not been domesticated. Therefore, beyond the weaning period, milk was not available for people to drink in early human populations. It makes good biological sense to stop synthesizing an enzyme that the body does not need. After a young child is weaned, it is a waste of materials and energy to keep producing lactase when milk is no longer likely to be consumed.

Overall, an estimated 60 per cent of the world’s adult human population is thought to be lactose intolerant today. You can see the geographic distribution of modern human lactose intolerance on the map in Figure 6.8.4. Lactose intolerance (dark blue) approaches 100 per cent in populations throughout southern South America, southern Africa, and East and Southeast Asia.

Lactose intolerance is not considered a medical problem, because its symptoms can be avoided by avoiding milk or milk products. Dietary control of lactose intolerance may be a matter of trial and error, however, because different people may be able to consume different quantities of milk or milk products before symptoms occur. If you are lactose intolerant, be aware that low-fat and fat-free milk may contain somewhat more lactose than full-fat milk because the former often have added milk solids that are relatively high in lactose.

Lactase Persistence

Lactase persistence is the opposite of lactose intolerance. People who are lactase persistent continue to produce the enzyme lactase beyond infancy and generally throughout life. As a consequence they are able to digest lactose and drink milk at older ages without adverse effects. The map in Figure 6.8.4 can also be read to show where lactase persistence occurs today. Populations with a low percentage of lactose intolerance (including most North Americans and Western and Northern Europeans) have high percentages of lactase-persistent people.

Lactase persistence is a uniquely human trait that is not found in any other mammalian species. Why did lactase persistence evolve in humans? When some human populations began domesticating and keeping herds of animals, animal milk became a potential source of food. Animals such as cows, sheep, goats, camels, and even reindeer (see Figure 6.8.5) can be kept for their milk. These animal milks also contain lactose, so natural selection would be strong for any individuals who kept producing lactase beyond infancy and could make use of this nutritious food. Eventually, the trait of lactase persistence would increase in frequency and come to be the predominant trait in dairying populations.

It is likely that lactase persistence occurs as a result of both genes and environment. Some people inherit genes that help them keep producing lactase after infancy. Geneticists think that several different mutations for lactase persistence arose independently in different populations within the last ten thousand years. Part of lactase persistence may be due to continued exposure to milk in the childhood and adulthood diet. In other words, a person may be genetically predisposed to synthesize lactase at older ages because of a mutation, but they may need the continued stimulation of milk drinking to keep producing lactase.

Besides variation in lactase persistence, human populations may vary in how efficiently they use calories in the foods they consume. People in some populations seem able to get by on quantities of food that would be inadequate for others, so they tend to gain weight easily. What explains these differences in people?

Thrifty Gene Hypothesis

In 1962, human geneticist James Neel proposed the thrifty gene hypothesis. According to this hypothesis, so-called “thrifty genes” evolved in some human populations because they allowed people to get by on fewer calories and store the rest as body fat when food was plentiful. According to Neel’s hypothesis, thrifty genes would have increased in frequency through natural selection, because they would help people survive during times of famine. People with the genes would be fatter and able to rely on their stored body fat for calories when food was scarce.

Such thrifty genes would have been advantageous in early human populations of hunter-gatherers if food scarcity was a recurrent stress. However, in modern times, when most people have access to enough food year-round, thrifty genes would no longer be advantageous. In fact, under conditions of plentiful food, having thrifty genes would predispose people to gain weight and develop obesity. They would also tend to develop the chronic diseases associated with obesity, particularly type II diabetes. Diabetes mellitus is a disease that occurs when there are problems with the pancreatic hormone insulin, which normally helps cells take up glucose from the blood and controls blood glucose levels. In type II diabetes, body cells become relatively resistant to insulin, leading to high blood glucose. This causes symptoms like excessive thirst and urination. Without treatment, diabetes can lead to serious consequences, such as blindness and kidney failure.

Neel proposed his thrifty gene hypothesis not on the basis of genetic evidence for thrifty genes, but as a possible answer to the mystery of why genes that seem to promote diabetes have not been naturally selected out of some populations. The mystery arose from observations that certain populations — such as South Pacific Islanders, sub-Saharan Africans, and southwestern Native Americans — developed high levels of obesity and diabetes after they abandoned traditional diets and adopted Western diets.

Assessing the Thrifty Gene Hypothesis

One of the assumptions underlying the thrifty gene hypothesis is that human populations that recently developed high rates of obesity and diabetes after Western contact had a long history of recurrent famine. Anthropological evidence contradicts this assumption for at least some of the populations in question. South Pacific Islanders, for example, have long lived in a “land of plenty,” with lush tropical forests year-round on islands surrounded by warm waters full of fish. Another assumption underlying the thrifty gene hypothesis is that hunter-gatherer people became significantly fatter during periods of plenty. Again, there is little or no evidence that hunter-gatherers traditionally deposited large fat stores when food was readily available.

Some geneticists have searched directly for so-called thrifty genes. Studies have revealed many genes with small effects associated with obesity or diabetes. However, these genes can explain only a few percentage points of the total population variation in obesity or diabetes.

The Drifty Gene and Other Hypotheses

Given the lack of evidence for the thrifty gene hypothesis, several researchers have suggested alternative hypotheses to explain population variation in obesity and diabetes. One hypothesis posits that susceptibility to obesity and diabetes may be a side effect of heat adaptation. According to this idea, some populations evolved lower metabolic rates as an adaptation to heat stress, because lower metabolic rates reduced the amount of heat that the body produced. The lower metabolic rates also predisposed people to gain excess weight and develop obesity and diabetes.

A thrifty phenotype hypothesis has also been proposed. This hypothesis suggests that individuals who have inadequate nutrition during fetal development might develop an insulin-resistant phenotype. The insulin-resistant phenotype would supposedly prepare these individuals for a life of famine, based on the environment within the womb. In a famine-free environment, however, the thrifty phenotype would lead to the development of diabetes.

The most recent alternative to the thrifty gene hypothesis is the drifty gene hypothesis, which was proposed by biologist John Speakman. He argues that genes protecting humans from obesity were under strong natural selection pressure for a very long period of time while human ancestors were subject to the risk of predation. According to this view, being able to outrun predators would have been an important factor selecting against fatness. When the risk of predation was lessened — perhaps as early as two million years ago — genes keeping fatness in check would no longer be selected for. Without selective pressure for these genes, their frequencies could change randomly due to genetic drift. In some populations, by chance, frequencies of the genes could decrease to relatively low levels, whereas in other populations the frequencies could be much higher.

Myth: Lactose intolerance is an allergy to milk.

Reality: Lactose intolerance is not an allergy because it is not an immune system response. It is a sensitivity to milk caused by lactase deficiency so the sugar in milk cannot be digested. Milk allergy does exist, but it is a different condition that occurs in only about four per cent of people. It results when milk proteins (not milk sugar) trigger an immune reaction. How can you determine whether you have lactose intolerance or milk allergy? If you can drink lactose-free milk without symptoms, it is likely that you are lactose intolerant and not allergic to milk. However, if lactose-free milk also produces symptoms, it is likely that you have milk allergy. Note that it is possible to have both conditions.

Myth: If you are lactose intolerant, you will never be able to drink milk or consume other dairy products without suffering adverse physical symptoms.

Reality: Lactose intolerance does not mean that consuming milk and other dairy products is out of the question. Besides lactose-free milk, which is widely available, many dairy products have relatively low levels of lactose, so you may be able to consume at least small amounts of them without discomfort. You may be able to consume milk in the form of yogurt without any problems because the bacteria in yogurt produce lactase that breaks down the lactose. Greek yogurt may be your best bet, because it is lower in lactose to begin with. Aged cheeses also tend to have relatively low levels of lactose, because of the cheese-making process. Finally, by gradually adding milk or milk products to your diet, you may be able to increase your tolerance to lactose.

 

Attributions

Figure 6.8.1

IMG_4325 Milk Mustache licking 3 by Cedar Summit Farm on Flickr is used under a CC BY SA 2.0 (https://creativecommons.org/licenses/by-sa/2.0/) license.

Figure 6.8.2

Lactose Haworth by NEUROtiker on Wikimedia Commons is in the public domain (https://en.wikipedia.org/wiki/Public_domain).

Figure 6.8.3

Lactase by Boghog2 on Wikimedia Commons is released into the public domain (https://en.wikipedia.org/wiki/Public_domain).

Figure 6.8.4

Lactose Intolerance by Rainer Z ... on Wikimedia Commons is released into the public domain (https://en.wikipedia.org/wiki/Public_domain).

Figure 6.8.5

Reindeer_herding by Mats Andersson on Wikimedia Commons is used under a CC BY 2.0 (https://creativecommons.org/licenses/by/2.0/deed.en) license.

Figure 6.8.6

Milk Photo [photo] by Eiliv-Sonas Aceron on Unsplash is used under the Unsplash License (https://unsplash.com/license).

References

Great Big Story. (2018, November 29). The last nomadic reindeer herders in the world. YouTube. https://www.youtube.com/watch?v=4O8k9qe8fjI&feature=youtu.be

Super Scienced. (2016, February 26). Why are people lactose intolerant? YouTube. https://www.youtube.com/watch?v=G1NGzycaQV0&feature=youtu.be

National Geographic. (2018, November 27). Experience a traditional whale hunt in northern Alaska | Short film showcase. YouTube. https://www.youtube.com/watch?v=XIYag5MWhPU&feature=youtu.be

TED. (2013, June 25). Peter Attia: What if we're wrong about diabetes? YouTube. https://www.youtube.com/watch?v=UMhLBPPtlrY&feature=youtu.be

Wikipedia contributors. (2019, December 15). James V. Neel. In Wikipedia. https://en.wikipedia.org/w/index.php?title=James_V._Neel&oldid=930860629

Wikipedia contributors. (2020, June 9). John Speakman. In Wikipedia. https://en.wikipedia.org/w/index.php?title=John_Speakman&oldid=961610417

 

A complex organic chemical that provides energy to drive many processes in living cells, e.g. muscle contraction, nerve impulse propagation, and chemical synthesis. Found in all forms of life, ATP is often referred to as the "molecular unit of currency" of intracellular energy transfer.

Created by CK-12 Foundation/Adapted by Christine Miller

Figure 7.2.1 Complex machines.

A Fantastic Machine

These robots were created for research or to do complex tasks, but they look like they might be fun to play with too! They are all complex machines. Think about some other, more familiar machines, such as power drills, washing machines, and lawn mowers. Each machine consists of many parts, and each part does a specific job, yet all the parts work together to perform certain functions. Many people have compared the human body to a machine, albeit an extremely complex one. Like real machines, the human body also consists of many parts that work together to perform certain functions. In this case, these parts and functions keep the organism alive. The human body may be the most fantastic machine on Earth, as you will discover when you learn more about it in this concept.

Imagine a machine that has all of the following attributes:

• It can generate a “wind” of 166 km/hr (100 mi/hr).
• It can relay messages faster than 400 km/hr (249 mi/hr).
• It contains a pump that moves about a million barrels of fluid over its lifetime.
• It has a control center that contains billions of individual components.
• It can repair itself, if necessary.
• It may not wear out for up to a century or more.

This machine has all of these abilities, and yet it consists mainly of water. What is it? It is the human body.

The human body is a complicated, highly organized structure that consists of trillions of parts that function together to achieve all the functions needed to maintain life. The biology of the human body incorporates:

• The body’s structure, the study of which is called anatomy.
• The body’s functioning, the study of which is called physiology.

The organization of the human body can be seen as a hierarchy of increasing size and complexity, starting at the level of atoms and molecules, and ending at the level of the entire organism, which is an individual living thing. You can see the intervening levels of organization in Figure 7.2.2. Read about the levels in the sections that follow.

 

Cells

The basic units of structure and function of the human body — as in all living things — are cells. By the time the average person reaches adulthood, their body has an amazing 37 trillion of them! Each cell carries out basic life processes that allow the body to survive. In addition, most human cells are specialized in structure and function to carry out other specific roles. In fact, the human body may consist of as many as 200 different types of cells, each of which has a special job to do. Just a few of these different human cell types are pictured in Figure 7.2.3. These cells have obvious differences in structure that reflect their different functions. For example, nerve cells have long projections sticking out from the body of the cell. These projections help them carry electrical messages to other cells.

Tissues

The next level of organization in the human body is tissues. A tissue is a group of connected cells that have a similar function. There are four basic types of human tissues: epithelial, muscle, nervous, and connective tissues. These four tissue types (shown in Figure 7.2.4) make up all the organs of the human body.

Organs are the next level of organization of the human body. An organ is a structure that consists of two or more types of tissues that work together to do the same job. Examples of human organs include the heart, brain, lungs, skin, and kidneys. Human organs are organized into organ systems, which are shown in Figure 7.2.5. An organ system is a group of organs that work together to carry out a complex overall function. Each organ of the system does part of the larger job.

Figure 7.2.5 The Human Organ Systems. Some of the system names shown in this illustration differ from the terminology used in this book, but the systems are the same.

 

All of the organs and organ systems of the human body normally work together like a well-oiled machine, because they are closely regulated by the nervous and endocrine systems. The nervous system controls virtually all body activities, and the endocrine system secretes hormones that help to regulate these activities. Functioning together, the organ systems supply body cells with all the substances they need and eliminate their wastes. They also keep temperature, pH, and other conditions at just the right levels to support life.

 

Attributions

Figure 7.2.1

• White and brown human robot illustration by Franck V. on Unsplash is used under the Unsplash License (https://unsplash.com/license).
• Mighty Mouse, a Robotic Vehicle Range (RVR) by Science in HD on Unsplash is used under the Unsplash License (https://unsplash.com/license).
• Lauron4c 2009 FZI Karlsruhe from the FZI Research Center for Information Technology - Department IDS (Germany) on Wikimedia Commons is released for free use.
• NASA Mars Rover (artist's concept) by NASA/JPL/Cornell University, Maas Digital LLC (#PIA04413)  on Wikimedia Commons is in the public domain (https://en.wikipedia.org/wiki/Public_domain).

Figure 7.2.2

101_Levels_of_Org_in_Body by OpenStax  on Wikimedia Commons is used under a CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/deed.en) licence.

Figure 7.2.3

Feature_Stem_Cell_new by OpenStax College on Wikimedia Commons is used under a CC BY 3.0 (https://creativecommons.org/licenses/by/3.0/deed.en) license.

Figure 7.2.4

Four types of tissues by CK-12 Foundation/ Zachary Wilson is used under a CC BY-NC 3.0 (https://creativecommons.org/licenses/by-nc/3.0/) license.

Figure 7.2.5

Organ Systems 1 by Connexions/ OpenStax on Wikimedia Commons is used under a CC BY 3.0 (https://creativecommons.org/licenses/by/3.0/deed.en) license.

 

References

Betts, J. G., Young, K.A., Wise, J.A., Johnson, E., Poe, B., Kruse, D.H., Korol, O., Johnson, J.E., Womble, M., DeSaix, P. (2013, April 25). Figure 1.3 Levels of Structural Organization of the Human Body [digital image]. In Anatomy and Physiology (Section 1.2). OpenStax. https://openstax.org/books/anatomy-and-physiology/pages/1-2-structural-organization-of-the-human-body

Betts, J. G., Young, K.A., Wise, J.A., Johnson, E., Poe, B., Kruse, D.H., Korol, O., Johnson, J.E., Womble, M., DeSaix, P. (2013, April 25). Figure 1.4 Organ Systems of the Human Body [digital image]. In Anatomy and Physiology (Section 1.2). OpenStax. https://openstax.org/books/anatomy-and-physiology/pages/1-2-structural-organization-of-the-human-body 

Betts, J. G., Young, K.A., Wise, J.A., Johnson, E., Poe, B., Kruse, D.H., Korol, O., Johnson, J.E., Womble, M., DeSaix, P. (2013, April 25). Figure 3.36 Stem Cells [digital image]. In Anatomy and Physiology (Section 3.6). OpenStax. https://openstax.org/books/anatomy-and-physiology/pages/3-6-cellular-differentiation

Brainard, J/ CK-12 Foundation. (2016). Figure 4 The human body contains these four types of tissues [digital image]. In CK-12 College Human Biology (Section 9.12) [online Flexbook]. CK12.org. https://www.ck12.org/book/ck-12-college-human-biology/section/9.2/

Fw: Thinking. (2014, May 14). Computers that think like humans. YouTube. https://www.youtube.com/watch?v=I43hq13MnYM&feature=youtu.be

TED. (2015, Febuary 23). Rob Knight: How our microbes make us who we are. YouTube. https://www.youtube.com/watch?v=i-icXZ2tMRM&feature=youtu.be

 

Groups of connected cells form tissues. The cells in a tissue may all be the same type, or they may be of multiple types. In either case, the cells in the tissue work together to carry out a specific function, and they are always specialized to be able to carry out that function better than any other type of tissue.  There are four main types of human tissues: connective, epithelial, muscle, and nervous tissues. We use tissues to build organs and organ systems.  The 200 types of cells that the body can produce based on our single set of DNA can create all the types of tissue in the body.

Epithelial Tissue

Epithelial tissue is made up of cells that line inner and outer body surfaces, such as the skin and the inner surface of the digestive tract. Epithelial tissue that lines inner body surfaces and body openings is called mucous membrane. This type of epithelial tissue produces mucus, a slimy substance that coats mucous membranes and traps pathogens, particles, and debris. Epithelial tissue protects the body and its internal organs, secretes substances (such as hormones) in addition to mucus, and absorbs substances (such as nutrients).

The key identifying feature of epithelial tissue is that it contains a free surface and a basement membrane.  The free surface is not attached to any other cells and is either open to the outside of the body, or is open to the inside of a hollow organ or body tube.  The basement membrane anchors the epithelial tissue to underlying cells.

Epithelial tissue is identified and named by shape and layering.  Epithelial cells exist in three main shapes: squamous, cuboidal, and columnar.  These specifically shaped cells can, depending on function, be layered several different ways: simple, stratified, pseudostratified, and transitional.

Epithelial tissue forms coverings and linings and is responsible for a range of functions including diffusion, absorption, secretion and protection.  The shape of an epithelial cell can maximize its ability to perform a certain function.  The thinner an epithelial cell is, the easier it is for substances to move through it to carry out diffusion and/or absorption.  The larger an epithelial cell is, the more room it has in its cytoplasm to be able to make products for secretion, and the more protection it can provide for underlying tissues. Their are three main shapes of epithelial cells: squamous (which is shaped like a pancake- flat and oval), cuboidal (cube shaped), and columnar (tall and rectangular).

Figure 7.4.2 The shape of epithelial tissues is important.  

Epithelial tissue will also organize into different layerings depending on their function.  For example, multiple layers of cells provide excellent protection, but would no longer be efficient for diffusion, whereas a single layer would work very well for diffusion, but no longer be as protective; a special type of layering called transitional is needed for organs that stretch, like your bladder.  Your tissues exhibit the layering that makes them most efficient for the function they are supposed to perform. There are four main layerings found in epithelial tissue: simple (one layer of cells), stratified (many layers of cells), pseudostratified (appears stratified, but upon closer inspection is actually simple), and transitional (can stretch, going from many layers to fewer layers).

Figure 7.4.3 The layerings found in epithelial tissues is important.  

See Table 7.4.1 for a summary of the different layering types and shapes epithelial cells can form and their related functions and locations.

Table 7.4.1

Summary of Epithelial Tissue Cells

So far, we have identified epithelial tissue based on shape and layering.  The representative diagrams we have seen so far are helpful for visualizing the tissue structures, but it is important to look at real examples of these cells.  Since cells are too tiny to see with the naked eye, we rely on microscopes to help us study them.  Histology is the study of the microscopic anatomy and cells and tissues.  See Table 7.4.2 to see some examples of slides of epithelial tissues prepared for the purpose of histology.

Table 7.4.2

Epithelial Tissues and Histological Samples

 

Epithelial Tissue Type	Tissue Diagram	Histological Sample
Stratified squamous (from skin)
Simple cuboidal (from kidney tubules)
Pseudostratified ciliated columnar (from trachea)

Connective Tissue

Bone and blood are examples of connective tissue. Connective tissue is very diverse. In general, it forms a framework and support structure for body tissues and organs. It's made up of living cells separated by non-living material, called extracellular matrix, which can be solid or liquid. The extracellular matrix of bone, for example, is a rigid mineral framework. The extracellular matrix of blood is liquid plasma.

The key identifying feature of connective tissue is that is is composed of a scattering of cells in a non-cellular matrix. There are three main categories of connective tissue, based on the nature of the matrix. They  look very different from one another, which is a reflection of their different functions:

1. Fibrous connective tissue: is characterized by a matrix which is flexible and is made of protein fibres including collagen, elastin and possibly reticular fibres.  These tissues are found making up tendons, ligaments, and body membranes.
2. Supportive connective tissue: is characterized by a solid matrix and is what is used to make bone and cartilage.  These tissues are used for support and protection.
3. Fluid connective tissue: is characterized by a fluid matrix and includes both blood and lymph.

Fibrous Connective Tissue

Fibrous connective tissue contains cells called fibroblasts.  These cells produce fibres of collagen, elastin, or reticular fibre which makes up the matrix of this type of connective tissue.  Based on how tightly packed these fibres are and how they are oriented changes the properties, and therefore the function of the fibrous connective tissue.

 

• Loose fibrous connective tissue:  composed of a loose and disorganized weave of collagen and elastin fibres, creating a tissue that is thin and flexible, yet still tough.  This tissue, which is also sometimes referred to as "areolar tissue", is found in membranes and surrounding blood vessels and most body organs.  As you can see from the diagram in Figure 7.4.4, loose fibrous connective tissue fulfills the definition of connectives tissue since it is a scattering of cells (fibroblasts) in a non-cellular matrix (a mesh of collagen and elastin fibres).  There are two types of specialized loose fibrous connective tissue: reticular and adipose.  Adipose tissue stores fat and reticular tissue forms the spleen and lymph nodes.
  
• Dense Fibrous Connective Tissue: composed of a dense mat of parallel collagen fibres and a scattering of fibroblasts, creating a tissue that is very strong.  Dense fibrous connective tissue forms tendons and ligaments, which connect bones to muscles and/or bones to neighbouring bones.
  

Supportive Connective Tissue

Supportive connective tissue exhibits the defining feature of connective tissue in that it is a scattering of cells in a non-cellular matrix; what sets it apart from other connective tissues is its solid matrix.  In this tissue group, the matrix is solid- either bone or cartilage.  While fibrous connective tissue contained cells called fibroblasts which produced fibres, supportive connective tissue contains cells that either create bone (osteocytes) or cells that create cartilage (chondrocytes).

Cartilage

Chondrocytes produce the cartilage matrix in which they reside.  Cartilage is made up of protein fibres and chondrocytes in lacunae.  This is tissue is strong yet flexible and is used many places in the body for protection and support.  Cartilage is one of the few tissues that is not vascular (doesn't have a direct blood supply) meaning it relies on diffusion to obtain nutrients and gases; this is the cause of slow healing rates in injuries involving cartilage.  There are three main types of cartilage:

• Hyaline cartilage: a smooth, strong and flexible tissue.  Found at the ends of ribs and long bones, in the nose, and comprising the entire fetal skeleton.
• Fibrocartilage: a very strong tissue containing thick bundles of collagen.  Found in joints that need cushioning from high impact (knees, jaw).
• Elastic cartilage: contains elastic fibres in addition to collagen,  giving support with the benefit of elasticity.  Found in earlobes and the epiglottis.
  

  

Bone

Osteocytes produce the bone matrix in which they reside.  Since bone is very solid, these cells reside in small spaces called lacunae.  This bone tissue is composed of collagen fibres embedded in calcium phosphate giving it strength without brittleness.  There are two types of bone: compact and spongy.

• Compact bone: has a dense matrix organized into cylindrical units called osteons.  Each osteon contains a central canal (sometimes called a Harversian Canal) which allows for space for blood vessels and nerves, as well as concentric rings of bone matrix and osteocytes in lacunae, as per the diagram here.  Compact bone is found in long bones and forms a shell around spongy bone.

• Spongy bone: a very porous type of bone which most often contains bone marrow.  It is found at the end of long bones, and makes up the majority of the ribs, shoulder blades and flat bones of the cranium.

Fluid Connective Tissue

Fluid connective tissue has a matrix that is fluid; unlike the other two categories of connective tissue, the cells that reside in the matrix do not actually produce the matrix. Fibroblasts make the fibrous matrix, chondrocytes make the cartilaginous matrix, osteocytes make the bony matrix, yet blood cells do not make the fluid matrix of either lymph or plasma.  This tissue still fits the definition of connective tissue in that it is still a scattering of cells in a non-cellular matrix.

There are two types of fluid connective tissue:

• Blood: blood contains three types of cells suspended in plasma, and is contained in the cardiovascular system.
  • Eryththrocytes, more commonly called red blood cells, are present in high numbers (roughly 5 million cells per mL) and are responsible for delivering oxygen from to the lungs to all the other areas of the body. These cells are relatively small in size with a diameter of around 7 micrometres and live no longer than 120 days.
  • Leukocytes, often referred to as white blood cells, are present in lower numbers (approximately 5 thousand cells per mL) are responsible for various immune functions.  They are typically larger than erythrocytes, but can live much longer, particularly white blood cells responsible for long term immunity.  The number of leukocytes in your blood can go up or down based on whether or not you are fighting an infection.
  • Thrombocytes, also known as platelets, are very small cells responsible for blood clotting.  Thrombocytes are not actually true cells, they are fragments of a much larger cell called a megakaryocyte.
• Lymph: contains a liquid matrix and white blood cells and is contained in the lymphatic system, which ultimately drains into the cardiovascular system.

Figure 7.4.11 A stained lymphocyte surrounded by red blood cells viewed using a light microscope. 

Muscular Tissue

Muscular tissue is made up of cells  that have the unique ability to contract- which is the defining feature of muscular tissue.  There are three major types of muscle tissue, as pictured in Figure 7.4.12 skeletal, smooth, and cardiac muscle tissues.

Skeletal Muscle

Skeletal muscles are voluntary muscles, meaning that you exercise conscious control over them.  Skeletal muscles are attached to bones by tendons, a type of connective tissue. When these muscles shorten to pull on the bones to which they are attached, they enable the body to move. When you are exercising, reading a book, or making dinner, you are using skeletal muscles to move your body to carry out these tasks.

Under the microscope, skeletal muscles are striated (or striped) in appearance, because of their internal structure which contains alternating protein fibres of actin and myosin.  Skeletal muscle is described as multinucleated, meaning one "cell" has many nuclei.  This is because in utero, individual cells destined to become skeletal muscle fused, forming muscle fibres in a process known as myogenesis.  You will learn more about skeletal muscle and how it contracts in the Muscular System.

Smooth Muscle

Smooth muscles are nonstriated muscles- they still contain the muscle fibres actin and myosin, but not in the same alternating arrangement seen in skeletal muscle.   Smooth muscle is found in the tubes of the body - in the walls of blood vessels and in the reproductive, gastrointestinal, and respiratory tracts. Smooth muscles are not under voluntary control meaning that they operate unconsciously, via the autonomic nervous system.  Smooth muscles move substances through a wave of contraction which cascades down the length of a tube, a process termed peristalsis. 

Watch the YouTube video "What is Peristalsis" by Mister Science to see peristalsis in action.

https://www.youtube.com/watch?v=kVjeNZA5pi4

What is Peristalsis, Mister Science, 2018.

 

 

Cardiac Muscle

Cardiac muscles work involuntarily, meaning they are regulated by the autonomic nervous system.  This is probably a good thing, since you wouldn't want to have to consciously concentrate on keeping your heart beating all the time! Cardiac muscle, which is found only in the heart, is mononucleated and striated (due to alternating bands of myosin and actin). Their contractions cause the heart to pump blood. In order to make sure entire sections of the heart contract in unison, cardiac muscle tissue contains special cell junctions called intercalated discs, which conduct the electrical signals used to "tell" the chambers of the heart when to contract.

Nervous Tissue

Nervous tissue is made up of neurons and a group of cells called neuroglia (also known as glial cells).  Nervous tissue makes up the central nervous system (mainly the brain and spinal cord) and peripheral nervous system (the network of nerves that runs throughout the rest of the body).  The defining feature of nervous tissue is that it is specialized to be able to generate and conduct nerve impulses.  This function is carried out by neurons, and the purpose of neuroglia is to support neurons.

A neuron has several parts to its structure:

• Dendrites which collect incoming nerve impulses
• A cell body, or soma, which contains the majority of the neuron's organelles, including the nucleus
• An axon, which carries nerve impulses away from the soma, to the next neuron in the chain
• A myelin sheath, which encases the axon and increases that rate at which nerve impulses can be conducted
• Axon terminals, which maintain physical contact with the dendrites of neighbouring neurons

 

Attributions

Figure 7.4.1

• Construction man kneeling in front of wall by Charles Deluvio on Unsplash is used under the Unsplash License (https://unsplash.com/license).
• Beige wooden frame by Charles Deluvio on Unsplash is used under the Unsplash License (https://unsplash.com/license).
• Tambour on green by Pierre Châtel-Innocention Unsplash is used under the Unsplash License (https://unsplash.com/license).
• Tags: Construction Studs Plumbing Wiring by JWahl on Pixabay is used under the Pixabay License (https://pixabay.com/es/service/license/).

Figure 7.4.2 and Figure 7.4.3

• Simple columnar epithelium tissue by Kamil Danak on Wikimedia Commons is used under a CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0/deed.en) license. 
• Simple cuboidal epithelium by Kamil Danak on Wikimedia Commons is used under a CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0/deed.en) license.
• Simple squamous epithelium by Kamil Danak on Wikimedia Commons is used under a CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0/deed.en) license. 

Figure 7.4.4

Loose fibrous connective tissue by CNX OpenStax. Biology. on Wikimedial Commons is used under a CC BY 4.0. (https://creativecommons.org/licenses/by/4.0) license.

Figure 7.4.5

Connective Tissue: Loose Aerolar by Berkshire Community College Bioscience Image Library on Flickr is used under a CC0 1.0 Universal public domain dedication (https://creativecommons.org/publicdomain/zero/1.0/) license.

Figure 7.4.6

Dense Fibrous Connective Tissue by by CNX OpenStax. Biology. on Wikimedial Commons is used under a CC BY 4.0. (https://creativecommons.org/licenses/by/4.0) license.

Figure 7.4.7

Dense_connective_tissue-400x by J Jana on Wikimedia Commons is used under a  CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/deed.en) license.

Figure 7.4.8

Types_of_Cartilage-new by OpenStax College on Wikipedia Commons is used under a CC BY 3.0 (https://creativecommons.org/licenses/by/3.0) license. 

Figure 7.4.9

Compact_bone_histology_2014 by Athikhun.suw on Wikimedia Commons is used under a  CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/deed.en) license.

Figure 7.4.10

Bone_normal_and_degraded_micro_structure by Gtirouflet on Wikimedia Commons is used under a CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0/deed.en) license.

Figure 7.4.11

Lymphocyte2 by NicolasGrandjean on Wikimedia Commons is used under a CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0/deed.en) license.  [No machine-readable author provided. NicolasGrandjean is assumed, based on copyright claims.]

Figure 7.4.12

Skeletal_muscle_横纹肌1 by 乌拉跨氪 on Wikimedia Commons is used under a CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/deed.en) license.

Figure 7.4.13

Smooth_Muscle_new by OpenStax on Wikimedia Commons is used under a CC BY 4.0  (https://creativecommons.org/licenses/by/4.0/deed.en) license.

Figure 7.4.14

Peristalsis by OpenStax on Wikimedia Commons is used under a CC BY 3.0 (https://creativecommons.org/licenses/by/3.0/deed.en) license.

Figure 7.4.15

400x Cardiac Muscle by Jessy731 on Flickr is used and adapted by Christine Miller under a CC BY-NC 2.0 (https://creativecommons.org/licenses/by-nc/2.0/) license.

Figure 7.4.16

Neuron.svg by User:Dhp1080 on Wikimedia Commons is used under a CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0/deed.en) license.

Figure 7.4.17

400x Nervous Tissue  by Jessy731 on Flickr is used under a CC BY-NC 2.0 (https://creativecommons.org/licenses/by-nc/2.0/) license.

Table 7.4.1

Summary of Epithelial Tissue Cells, by OpenStax College on Wikipedia Commons is used under a CC BY 3.0 (https://creativecommons.org/licenses/by/3.0) license. 

Table 7.4.2

• Epithelial_Tissues_Stratified_Squamous_Epithelium_(40230842160) by
  Berkshire Community College Bioscience Image Library on Wikimedia Commons is used under a  CC0 1.0 Universal Public Domain Dedication (https://creativecommons.org/publicdomain/zero/1.0/) license.
• Simple cuboidal epithelial tissue histology by Berkshire Community College on Flickr is used under a  CC0 1.0 Universal Public Domain Dedication (https://creativecommons.org/publicdomain/zero/1.0/) license.
• Pseudostratified_Epithelium by OpenStax College on Wikimedia Commons is used under a CC BY 3.0 (https://creativecommons.org/licenses/by/3.0) license.

References

Betts, J. G., Young, K.A., Wise, J.A., Johnson, E., Poe, B., Kruse, D.H., Korol, O., Johnson, J.E., Womble, M., DeSaix, P. (2013, April 25). Figure 4.8 Summary of epithelial tissue cells [digital image].  In Anatomy and Physiology (Section 4.2). OpenStax. https://openstax.org/books/anatomy-and-physiology/pages/4-2-epithelial-tissue

Betts, J. G., Young, K.A., Wise, J.A., Johnson, E., Poe, B., Kruse, D.H., Korol, O., Johnson, J.E., Womble, M., DeSaix, P. (2013, April 25). Figure 4.16 Types of cartilage [digital image].  In Anatomy and Physiology (Section 4.3). OpenStax. https://openstax.org/books/anatomy-and-physiology/pages/4-3-connective-tissue-supports-and-protects

Betts, J. G., Young, K.A., Wise, J.A., Johnson, E., Poe, B., Kruse, D.H., Korol, O., Johnson, J.E., Womble, M., DeSaix, P. (2013, April 25). Figure 10.23 Smooth muscle [digital micrograph].  In Anatomy and Physiology (Section 10.8). OpenStax. https://openstax.org/books/anatomy-and-physiology/pages/10-8-smooth-muscle (Micrograph provided by the Regents of University of Michigan Medical School © 2012)

Betts, J. G., Young, K.A., Wise, J.A., Johnson, E., Poe, B., Kruse, D.H., Korol, O., Johnson, J.E., Womble, M., DeSaix, P. (2013, April 25). Figure 22.5 Pseudostratified ciliated columnar epithelium [digital micrograph].  In Anatomy and Physiology (Section 22.1). OpenStax. https://openstax.org/books/anatomy-and-physiology/pages/22-1-organs-and-structures-of-the-respiratory-system

Betts, J. G., Young, K.A., Wise, J.A., Johnson, E., Poe, B., Kruse, D.H., Korol, O., Johnson, J.E., Womble, M., DeSaix, P. (2013, April 25). Figure 23.5 Peristalsis [diagram]. In Anatomy and Physiology (Section 23.2). OpenStax. https://openstax.org/books/anatomy-and-physiology/pages/23-2-digestive-system-processes-and-regulation

Mister Science. (2018). What is peristalsis? YouTube. https://www.youtube.com/watch?v=kVjeNZA5pi4

MoomooMath and Science. (2017, May 18). Types of human body tissue. YouTube. https://www.youtube.com/watch?v=O0ZvbPak4ck&feature=youtu.be

Open Stax. (2016, May 27). Figure 6 Loose connective tissue [digital image]. In OpenStax Biology (Section 33.2). OpenStax CNX. https://cnx.org/contents/GFy_h8cu@10.53:-LfhWRES@4/Animal-Primary-Tissues

Open Stax. (2016, May 27). Figure 7 Fibrous connective tissue from the tendon [digital image]. In OpenStax Biology (Section 33.2). OpenStax CNX. https://cnx.org/contents/GFy_h8cu@10.53:-LfhWRES@4/Animal-Primary-Tissues

TED-Ed. (2019, October 17). How to 3D print human tissue - Taneka Jones. YouTube. https://www.youtube.com/watch?v=uHbn7wLN_3k&feature=youtu.be

TED-Ed. (2020, January 27). How bones make blood - Melody Smith. YouTube. https://www.youtube.com/watch?v=1Qfmkd6C8u8&feature=youtu.be

 

 

Created by CK-12 Foundation/Adapted by Christine Miller

The woman in Figure 18.12.1 is holding a home pregnancy test. The two pink lines in the middle are the type of result that Alicia and Victor are desperately hoping to see themselves one day — a positive pregnancy test. In the beginning of the chapter you learned that Alicia and Victor have been actively trying to get pregnant for a year, which, as you now know, is the time frame necessary for infertility to be diagnosed.

Alicia and Victor tried having sexual intercourse on day 14 of her menstrual cycle to optimize their chances of having his sperm meet her ovum. Why might this not be successful, even if they do not have fertility problems? Although the average menstrual cycle is 28 days, with ovulation occurring around day 14, many women vary widely from these averages (either consistently or variably) from month to month. Recall, for example, that menstrual cycles may vary from 21 to 45 days in length, and a woman’s cycle is considered to be regular if it varies within as many as eight days from shortest to longest cycle. This variability means that ovulation often does not occur on or around day 14, particularly if the woman has significantly shorter, longer, or irregular cycles — like Alicia does. Therefore, by aiming for day 14 without knowing when Alicia is actually ovulating, they may not be successful in helping Victor’s sperm encounter Alicia’s egg.

Lack of ovulation entirely can also cause variability in menstrual cycle length. As you have learned, the regulation of the menstrual cycle depends on an interplay of hormones from the pituitary gland and ovaries, including FSH and LH from the pituitary and estrogen and progesterone from the ovary — specifically from the follicle which surrounds the maturing egg and becomes the corpus luteum after ovulation. Shifts in these hormones and processes can affect ovulation and menstrual cycle length. This is why Alicia was concerned about her long and irregular menstrual cycles. If they are a sign that she is not ovulating, that could be the reason why she is having trouble getting pregnant.

In order to get a better idea of whether Alicia is ovulating, Dr. Bashir recommended that she take her basal body temperature (BBT) each morning before getting out of bed, and track it throughout her menstrual cycle. As you have learned, BBT typically rises slightly and stays high after ovulation. While tracking BBT is not a particularly effective form of contraception, since the temperature rise occurs only after ovulation, it can be a good way to see whether a woman is ovulating at all. Although not every woman will see a clear shift in BBT after ovulation, it is a relatively easy way to start assessing a woman’s fertility and is used as part of a more comprehensive fertility assessment by some physicians.

Dr. Bashir also recommended that Alicia use a home ovulation predictor kit. This is another relatively cheap and easy way to assess ovulation. Most ovulation predictor kits work by detecting the hormone LH in urine using test strips, like the ones shown in Figure 18.12.2. Why can this predict ovulation? Think about what you have learned about how ovulation is triggered. Rising levels of estrogen from the maturing follicle in the ovary causes a surge in the level of LH secreted from the pituitary gland, which triggers ovulation. This surge in LH can be detected by the home kit, which compares the level of LH in a woman’s urine to that of a control on the strip. After the LH surge is detected, ovulation will typically occur within one to two days.

By tracking her BBT and using the ovulation predictor kit, Alicia has learned that she is most likely ovulating, but not in every cycle, and sometimes she ovulates much later than day 14. Because frequent anovulatory cycles can be a sign of an underlying hormonal disorder, such as polycystic ovary syndrome (PCOS) or problems with the pituitary or other glands that regulate the reproductive system, Dr. Bashir orders blood tests for Alicia and sets up an appointment for a physical exam.

However, because Alicia is sometimes ovulating, the problem may not lie solely with her. Recall that infertility occurs in similar proportions in men and women, and can be due to problems in both partners. This is why it is generally recommended that both partners get assessed for fertility issues when they are having trouble getting pregnant after a year of trying.

Therefore, Victor proceeds with the semen analysis that Dr. Bashir recommended. In this process, the man provides a semen sample by ejaculating into a cup or special condom, and the semen is then examined under a microscope. The semen is then checked for sperm number, shape, and motility. Sperm with an abnormal shape or trouble moving will likely have trouble reaching and fertilizing an egg. A low number of sperm will also reduce the chances of conception. In this way, semen analysis can provide insight into the possible underlying causes of infertility. For instance, a low sperm count could indicate problems in sperm production or a blockage in the male reproductive tract that is preventing sperm from being emitted from the penis. Further testing would have to be done to distinguish between these two possible causes.

Victor had been worried that past injuries to his testes may have affected his fertility. You may remember the testes are where sperm are produced, and because they are external to the body, they are vulnerable to injury. In addition to physical damage to the testes and other parts of the male reproductive tract, a testicular injury could potentially cause the creation of antibodies against a man’s own sperm. As you have learned, Sertoli cells lining the seminiferous tubules are tightly packed so that the developing sperm are normally well-separated from the body’s immune system. However, in the case of an injury, this barrier can be breached, which can cause the creation of antisperm antibodies. These antibodies can hamper fertility by killing the sperm, or otherwise interfering with their ability to move or fertilize an egg. When infertility is due to such antibodies, it is called “immune infertility.”

Victor’s semen analysis shows that he has normal numbers of healthy sperm. Dr. Bashir recommends that while they investigate whether Alicia has an underlying medical issue, she continue to track her BBT and use ovulation predictor kits to try to pinpoint when she is ovulating. She recommends that once Alicia sees an LH surge, the couple try to have intercourse within three days to maximize their chances of conception. If Alicia is found to have a medical problem that is inhibiting ovulation, depending on what it is, they may either address the problem directly, or she can take medication that stimulates ovulation, such as clomiphene citrate (often sold under the brand name Clomid). This medication works by increasing the amount of FSH secreted by the pituitary.

Fortunately, tracking ovulation at home and timing intercourse appropriately was all Alicia and Victor needed to do to finally get pregnant! After their experience, they, like you, now have a much deeper understanding of the intricacies of the reproductive system and the complex biology that is involved in the making of a new human organism.

References

Created by: CK-12/Adapted by Christine Miller

The colourful image in Figure 4.3.1 shows a bacterial cell (in green) attacking human red blood cells. The bacterium causes a disease called relapsing fever. The bacterial and human cells look very different in size and shape. Although all living cells have certain things in common — such as a plasma membrane and cytoplasm — different types of cells, even within the same organism, may have their own unique structures and functions. Cells with different functions generally have different shapes that suit them for their particular job. Cells vary not only in shape, but also in size, as this example shows. In most organisms, however, even the largest cells are no bigger than the period at the end of this sentence. Why are cells so small?

Most organisms, even very large ones, have microscopic cells. Why don't cells get bigger instead of remaining tiny and multiplying? Why aren't you one giant cell rolling around school? What limits cell size?

Once you know how a cell functions, the answers to these questions are clear. To carry out life processes, a cell must be able to quickly pass substances in and out of the cell. For example, it must be able to pass nutrients and oxygen into the cell and waste products out of the cell. Anything that enters or leaves a cell must cross its outer surface. The size of a cell is limited by its need to pass substances across that outer surface.

Look at the three cubes in Figure 4.3.2. A larger cube has less surface area relative to its volume than a smaller cube. This relationship also applies to cells — a larger cell has less surface area relative to its volume than a smaller cell. A cell with a larger volume also needs more nutrients and oxygen, and produces more waste. Because all of these substances must pass through the surface of the cell, a cell with a large volume will not have enough surface area to allow it to meet its needs. The larger the cell is, the smaller its ratio of surface area to volume, and the more difficult it will be for the cell to get rid of its waste and take in necessary substances. This is what limits the size of the cell.

 

 

 

Cells with different functions often have varying shapes. The cells pictured below (Figure 4.3.3) are just a few examples of the many different shapes that human cells may have. Each type of cell  has characteristics that help it do its job. The job of the nerve cell, for example, is to carry messages to other cells. The nerve cell has many long extensions that reach out in all directions, allowing it to pass messages to many other cells at once. Do you see the tail of each tiny sperm cell? Its tail helps a sperm cell "swim" through fluids in the female reproductive tract in order to reach an egg cell. The white blood cell has the job of destroying bacteria and other pathogens. It is a large cell that can engulf foreign invaders.

The nucleus is a basic cell structure present in many — but not all — living cells. The nucleus of a cell is a structure in the cytoplasm that is surrounded by a membrane (the nuclear membrane) and contains DNA. Based on whether or not they have a nucleus, there are two basic types of cells: prokaryotic cells and eukaryotic cells.

Prokaryotic Cells

Prokaryotic cells are cells without a nucleus. The DNA in prokaryotic cells is in the cytoplasm, rather than enclosed within a nuclear membrane.  In addition, these cells are typically smaller than eukaryotic cells and contain fewer organelles.  Prokaryotic cells are found in single-celled organisms, such as the bacterium represented by the model in Figure 4.3.3. Organisms with prokaryotic cells are called prokaryotes. They were the first type of organisms to evolve, and they are still the most common organisms today.

Eukaryotic Cells

Eukaryotic cells are cells that contain a nucleus. A typical eukaryotic cell is represented by the model in Figure 4.3.4. Eukaryotic cells are usually larger than prokaryotic cells. They are found in some single-celled and all multicellular organisms. Organisms with eukaryotic cells are called eukaryotes, and they range from fungi to humans.

Besides a nucleus, eukaryotic cells also contain other organelles. An organelle is a structure within the cytoplasm that performs a specific job in the cell. Organelles called mitochondria, for example, provide energy to the cell, and organelles called vesicles store substances in the cell. Organelles allow eukaryotic cells to carry out more functions than prokaryotic cells can.

Interestingly, scientists think that mitochondria were once free-living prokaryotes that infected (or were engulfed by) larger cells. The two organisms developed a symbiotic relationship that was beneficial to both of them, resulting in the smaller prokaryote becoming an organelle within the larger cell. This is called endosymbiotic theory, and it is supported by a lot of evidence, including the fact that mitochondria have their own DNA separate from the DNA in the nucleus of the eukaryotic cell. Endosymbiotic theory will be described in more detail in later sections, and it's also discussed in the video below.

Figure 4.3.4

Model of a prokaryotic cell: bacterium by Mariana Ruiz Villarreal [LadyofHats] on Wikimedia Commons is released into the public domain (https://en.wikipedia.org/wiki/Public_domain).

Figure 4.3.5

Animal Cell adapted by Christine Miller is used under a CC0 1.0 (https://creativecommons.org/publicdomain/zero/1.0/deed.en) public domain dedication license. (Original image, Animal Cell Unannotated, is by Kelvin Song on Wikimedia Commons.)

References

Amoeba Sisters. (2017, May 3). Endosymbiotic theory. YouTube. https://www.youtube.com/watch?v=FGnS-Xk0ZqU&feature=youtu.be

Amoeba Sisters. (2018, July 30). Prokaryotic vs. eukaryotic cells (updated). YouTube. https://www.youtube.com/watch?v=Pxujitlv8wc&feature=youtu.be

Brinkmann, V. (November 2005). Neutrophil engulfing Bacillus anthracis. PLoS Pathogens 1 (3): Cover page [digital image]. DOI:10.1371. https://journals.plos.org/plospathogens/issue?id=10.1371/issue.ppat.v01.i03

Lee, W.C.A., Huang, H., Feng, G., Sanes, J.R., Brown, E.N., et al. (2005, December 27) Figure 6f, slightly altered (plus scalebar, minus letter "f".) [digital image]. Dynamic Remodeling of Dendritic Arbors in GABAergic Interneurons of Adult Visual Cortex. PLoS Biology, 4(2), e29. doi:10.1371/journal.pbio.0040029. https://journals.plos.org/plosbiology/article?id=10.1371/journal.pbio.0040029

TED-Ed. (2015, February 17). How we think complex cells evolved - Adam Jacobson. https://www.youtube.com/watch?v=9i7kAt97XYU&feature=youtu.be

 

 

A nervous system cell that provides support for neurons and helps them transmit nerve impulses.

Created by CK-12 Foundation/Adapted by Christine Miller

Aiko, 22, and Larissa, 23, met through mutual friends and hit it off right away. They began dating and just four months later, they are now madly in love. They spend as much time as they can with each other, and have decided to move in together when Larissa’s roommate moves out. They are even discussing getting married one day.

Inspired by his passion for Larissa, Aiko is considering getting her name tattooed on his arm. As you probably know, tattoos are designs on the skin created by injecting pigments into the skin with a needle. Aiko looks up different tattoo styles online, and starts to envision what he would want in a tattoo.

One day at a street festival, Aiko sees a sign that says “Henna Tattoos.” Henna tattoos are not technically tattoos — they are temporary designs that artists can create on the skin using a paste made out of the leaves of the henna plant. The henna stains the skin a reddish-brown colour, and once the paste is scraped off, the design typically remains on the skin for a few weeks. The use of henna to create designs on the skin is called mehndi. It is traditionally used by people in and from regions such as India, Pakistan, the Middle East, and Africa to celebrate special occasions, particularly weddings. Mendhi is often done on the palms of the hands and soles of the feet, where the designs usually come out darker than on other areas of the skin. You can see some examples of henna art in the images below.

Aiko asks the mehndi artist to inscribe Larissa’s name on his arm, so that he can see whether he likes it without making the permanent commitment of a real tattoo. Two days later, Aiko visits his parents. They are not familiar with mehndi, and they have a moment of panic when they think he got a real tattoo. Aiko reassures them that it is temporary, but tells them that he is thinking about getting a real tattoo.

His parents are concerned. His father points out that he has not known Larissa long — what if they break up and he regrets the tattoo? His mother additionally worries about whether tattoos are safe. Aiko says that he doesn’t think he will regret the decision, but if he does, he can cover it up with another tattoo or get it removed with laser treatments. He also tells them that he would go to an artist and shop that are reputable, and take appropriate safety precautions. His parents warn him that getting a tattoo removed may not be as simple as he thinks, and that he should think very carefully before making such a permanent decision.

Humans have long decorated and adorned their skin with tattoos, makeup, and piercings. They also colour, cut, straighten, curl, and remove their hair; and paint, grow, and cut their nails. The skin, hair, and nails make up the integumentary system. As you read this chapter, you will learn about the important biological functions that these organs carry out, beyond being a convenient canvas for personal expression. At the end of the chapter you will find out if Aiko got his tattoo. You will also learn more about how tattoos, mehndi, and laser tattoo removal work, as well as the important considerations to protect your health if you are thinking about getting a tattoo.

 

Attributions

Figure 10.1.1

Arm tattoo by telly telly on Flickr is used under a CC BY 2.0  (https://creativecommons.org/licenses/by/2.0/) license.

Figure 10.1.2

• Henna for hair by Andrey "A.I." Sitnik ( www.sitnik.ru ) on Wikimedia Commons is released into the public domain (https://en.wikipedia.org/wiki/Public_domain).
• Henna on foot in Morocco by Bjørn Christian Tørrissen on Wikimedia Commons is used under a CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0/deed.en) license.
• Mehndi (front) by AKS.9955 on Wikimedia Commons is used under a CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/deed.en) license.
• Tags: Hand Jewelry Ornaments. . .Henna by BenBernardBags on Pixabay is used under the Pixabay License (https://pixabay.com/ja/service/license/).

Created by CK-12 Foundation/Adapted by Christine Miller

Giving yourself an injection can be difficult, but for someone with diabetes, it may be a matter of life or death. The person in the photo has diabetes and is injecting themselves with insulin, the hormone that helps control the level of glucose in the blood. Insulin is produced by the pancreas.

The pancreas is a large gland located in the upper left abdomen behind the stomach, as shown in Figure 9.7.2. The pancreas is about 15 cm (6 in) long, and it has a flat, oblong shape. Structurally, the pancreas is divided into a head, body, and tail. Functionally, the pancreas serves as both an endocrine gland and an exocrine gland.

• As an endocrine gland, the pancreas is part of the endocrine system. As such, it releases hormones (such as insulin) directly into the bloodstream for transport to cells throughout the body.
• As an exocrine gland, the pancreas is part of the digestive system. As such, it releases digestive enzymes into ducts that carry the enzymes to the gastrointestinal tract, where they assist with digestion. In this concept, the focus is on the pancreas as an endocrine gland. You can read about the pancreas as an exocrine gland in Chapter 15 Digestive System.

The tissues within the pancreas that have an endocrine role exist as clusters of cells called pancreatic islets. They are also called the islets of Langerhans. You can see pancreatic tissue, including islets, in Figure 9.7.3. There are approximately three million pancreatic islets, and they are crisscrossed by a dense network of capillaries. The capillaries are lined by layers of islet cells that have direct contact with the blood vessels, into which they secrete their endocrine hormones.

The pancreatic islets consist of four main types of cells, each of which secretes a different endocrine hormone. All of the hormones produced by the pancreatic islets, however, play crucial roles in glucose metabolism and the regulation of blood glucose levels, among other functions.

1. Islet cells called alpha (α) cells secrete the hormone glucagon. The function of glucagon is to increase the level of glucose in the blood. It does this by stimulating the liver to convert stored glycogen into glucose, which is released into the bloodstream.
2. Islets cells called beta (β) cells secrete the hormone insulin. The function of insulin is to decrease the level of glucose in the blood. It does this by promoting the absorption of glucose from the blood into fat, liver, and skeletal muscle cells. In these tissues, the absorbed glucose is converted into glycogen, fats (triglycerides), or both.
3. Islet cells called delta (δ) cells secrete the hormone somatostatin. This hormone is also called growth hormone inhibiting hormone, because it inhibits the anterior lobe of the pituitary gland from producing growth hormone. Somatostatin also inhibits the secretion of pancreatic endocrine hormones and pancreatic exocrine enzymes.
4. Islet cells called gamma (γ) cells secrete the hormone pancreatic polypeptide. The function of pancreatic polypeptide is to help regulate the secretion of both endocrine and exocrine substances by the pancreas.

There are a variety of disorders that affect the pancreas. They include pancreatitis, pancreatic cancer, and diabetes mellitus.

Pancreatitis

Pancreatitis is inflammation of the pancreas. It has a variety of possible causes, including gallstones, chronic alcohol use, infections (such as measles or mumps), and certain medications. Pancreatitis occurs when digestive enzymes produced by the pancreas damage the gland’s tissues, which causes problems with fat digestion. The disorder is usually associated with intense pain in the central abdomen, and the pain may radiate to the back. Yellowing of the skin and whites of the eyes (see Figure 9.7.4), which is called jaundice, is a common sign of pancreatitis. People with pancreatitis may also have pale stools and dark urine. Treatment of pancreatitis includes administering drugs to manage pain, and addressing the underlying cause of the disease, for example, by removing gallstones.

Pancreatic Cancer

There are several different types of pancreatic cancer that may affect either the endocrine or the exocrine tissues of the gland. Cancers affecting the endocrine tissues are all relatively rare. However, their incidence has been rising sharply. It is unclear to what extent this reflects increased detection, especially through medical imaging techniques. Unfortunately, pancreatic cancer is usually diagnosed at a relatively late stage when it is too late for surgery, which is the only way to cure the disorder. In 2020 it is estimated that 6,000 Canadians will be newly diagnosed with pancreatic cancer, and that during this same year, 5,300 will die of pancreatic cancer.

While it is rare before the age of 40, pancreatic cancer occurs most often after the age of 60. Factors that increase the risk of developing pancreatic cancer include smoking, obesity, diabetes, and a family history of the disease. About one in four cases of pancreatic cancer are attributable to smoking. Certain rare genetic conditions are also risk factors for pancreatic cancer.

Diabetes Mellitus

By far the most common type of pancreatic disorder is diabetes mellitus, more commonly called simply diabetes. There are many different types of diabetes, but diabetes mellitus is the most common. It occurs in two major types, type 1 diabetes and type 2 diabetes. The two types have different causes and may also have different treatments, but they generally produce the same initial symptoms, which include excessive urination and thirst. These symptoms occur because the kidneys excrete more urine in an attempt to rid the blood of excess glucose. Loss of water in urine stimulates greater thirst. Other signs and symptoms of diabetes are listed in Figure 9.7.5.

When diabetes is not well controlled, it is likely to have several serious long-term consequences. Most of these consequences are due to damage to small blood vessels caused by high glucose levels in the blood. Damage to blood vessels, in turn, may lead to increased risk of coronary artery disease and stroke. Damage to blood vessels in the retina of the eye can result in gradual vision loss and blindness. Damage to blood vessels in the kidneys can lead to chronic kidney disease, sometimes requiring dialysis or kidney transplant. Long-term consequences of diabetes may also include damage to the nerves of the body, known as diabetic neuropathy. In fact, this is the most common complication of diabetes. Symptoms of diabetic neuropathy may include numbness, tingling, and pain in the extremities.

Type 1 Diabetes

Type 1 diabetes is a chronic autoimmune disorder in which the immune system attacks the insulin-secreting beta cells of the pancreas. As a result, people with type 1 diabetes lack the insulin needed to keep blood glucose levels within the normal range. Type 1 diabetes may develop in people of any age, but is most often diagnosed before adulthood. For type 1 diabetics, insulin injections are critical for survival.

Type 2 Diabetes

Type 2 diabetes is the single most common form of diabetes. The cause of high blood glucose in this form of diabetes usually includes a combination of insulin resistance and impaired insulin secretion. Both genetic and environmental factors play roles in the development of type 2 diabetes. Type 2 diabetes can be managed with changes in diet and physical activity, which may increase insulin sensitivity and help reduce blood glucose levels to normal ranges. Medications may also be used as part of the treatment, as may insulin injections.

Some patients with type 1 diabetes have been given pancreatic islet cells transplants from other human donors. If the transplanted cells are not rejected by the recipient’s immune system, they can cure the patient of diabetes. However, because of a shortage of appropriate human donors, only about one thousand such surgeries have been performed over the past ten years.

In June of 2016, a research team led by Dr. David K.C. Cooper at the Thomas E. Starzl Transplantation Institute in Pittsburgh, Pennsylvania, reported on their work developing pig islet cells for transplant into human diabetes patients. The researchers genetically engineered the pig islet cells to be protected from the human immune response. As a result, patients receiving the transplanted cells would require only minimal suppression of their immune system after the surgery. The pig islet cells would also be less likely to transmit pathogenic agents, because the animals could be raised in a controlled environment.

The researchers have successfully transplanted the pig islet cells into monkey models of type 1 diabetes. As of June 2016, the scientists were looking for funding to undertake clinical trials in humans with type 1 diabetes. Dr. Cooper predicted then that if the human trials go as well as expected, the pig islet cells could be available for curing patients in as little as two years.

Attributions

Figure 9.7.1

Insulin_Application by Mr Hyde at Czech Wikipedia (Original text: moje foto) on Wikimedia Commons is released into the public domain (https://en.wikipedia.org/wiki/Public_domain).

Figure 9.7.2

Blausen_0699_PancreasAnatomy2 by BruceBlaus on Wikimedia Commons is used under a  CC BY 3.0 (https://creativecommons.org/licenses/by/3.0) license.

Figure 9.7.3

Exocrine_and_Endocrine_Pancreas by OpenStax College is used under a CC BY 3.0 (https://creativecommons.org/licenses/by/3.0/deed.en) license.

Figure 9.7.4

Jaundice_eye_new by Info-farmer on Wikimedia Commons is in the public domain (https://en.wikipedia.org/wiki/Public_domain). (Original image, File:Jaundice eye.jpg, is from Centers for Disease Control and Prevention's Public Health Image Library (PHIL), with identification number #2860)

Figure 9.7.5

Main_symptoms_of_diabetes.svg by Mikael Häggström on Wikimedia Commons is released into the public domain (https://en.wikipedia.org/wiki/Public_domain).

 

References

Betts, J. G., Young, K.A., Wise, J.A., Johnson, E., Poe, B., Kruse, D.H., Korol, O., Johnson, J.E., Womble, M., DeSaix, P. (2013, July 19). Figure 23.26 Exocrine and endocrine pancreas [digital image].  In Anatomy and Physiology (Section 23.6). OpenStax. https://openstax.org/books/anatomy-and-physiology/pages/23-6-accessory-organs-in-digestion-the-liver-pancreas-and-gallbladder

Blausen.com Staff. (2014). Medical gallery of Blausen Medical 2014. WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.010. ISSN 2002-4436.

Children's Health. (2008, June 13). Type 2 diabetes in children. YouTube. https://www.youtube.com/watch?v=qlzLSbAGMqA&feature=youtu.be

Hering, B. J., Cozzi, E., Spizzo, T., Cowan, P. J., Rayat, G. R., Cooper, D. K. C., Denner, J. (2016, March 4). First update of the International Xenotransplantation Association consensus statement on conditions for undertaking clinical trials of porcine islet products in type 1 diabetes—Executive summary. Xenotransplantation 2016, 23: 3– 13. https://doi.org/10.1111/xen.12231

TED-Ed. (2015, February 19). What does the pancreas do? - Emma Bryce. YouTube. https://www.youtube.com/watch?v=8dgoeYPoE-0&feature=youtu.be

TEDx Talks. (2015, May 4). Reversing Type 2 diabetes starts with ignoring the guidelines | Sarah Hallberg | TEDxPurdueU. YouTube. https://www.youtube.com/watch?v=da1vvigy5tQ&feature=youtu.be

Created by CK-12 Foundation/Adapted by Christine Miller

Painting nails with coloured polish for aesthetic reasons is nothing new. In fact, there is evidence of this practice dating back to at least 3000 BCE. Today, painting and otherwise decorating the nails is big business, with annual revenues in the billions of dollars in North America alone! With all the attention (and money) given to nails as decorative objects, it’s easy to forget that they also have important biological functions.

Nails are accessory organs of the skin. They are made of sheets of dead keratinocytes and are found on the far (or distal) ends of the fingers and toes. The keratin in nails makes them hard, but flexible. Nails serve a number of purposes, including protecting the digits, enhancing sensations, and acting as tools.

A nail has three main parts: the root, plate, and free margin. Other structures around or under the nail include the nail bed, cuticle, and nail fold. These structures are shown in Figure 10.6.2.

• The nail root is the portion of the nail found under the surface of the skin at the near (or proximal) end of the nail. It is where the nail begins.
• The nail plate (or body) is the portion of the nail that is external to the skin. It is the visible part of the nail.
• The free margin is the portion of the nail that protrudes beyond the distal end of the finger or toe. This is the part that is cut or filed to keep the nail trimmed.
• The nail bed is the area of skin under the nail plate. It is pink in colour, due to the presence of capillaries in the dermis.
• The cuticle is a layer of dead epithelial cells that overlaps and covers the edge of the nail plate. It helps to seal the edges of the nail to prevent infection of the underlying tissues.
• The nail fold is a groove in the skin in which the side edges of the nail plate are embedded.

Nails grow from a deep layer of living epidermal tissue, known as the nail matrix, at the proximal end of the nail (see the bottom of the diagram in Figure 10.6.2). The nail matrix surrounds the nail root. It contains stem cells that divide to form keratinocytes, which are cells that produce keratin and make up the nail.

Formation of the Nail Root and Nail Plate

The keratinocytes produced by the nail matrix accumulate to form tough, hard, translucent sheets of dead cells filled with keratin. The sheets make up the nail root, which slowly grows out of the skin and becomes the nail plate when it reaches the skin surface. As the nail grows longer, the cells of the nail root and nail plate are pushed toward the distal end of the finger or toe by new cells being formed in the nail matrix. The upper epidermal cells of the nail bed also move along with the nail plate as it grows toward the tip of the digit.

The proximal end of the nail plate near the root has a whitish crescent shape called the lunula. This is where a small amount of the nail matrix is visible through the nail plate. The lunula is most pronounced in the nails of the thumbs, and may not be visible in the nails of the little fingers.

Rate of Nail Growth

Nails grow at an average rate of 3 mm a month. Fingernails, however, grow up to four times as fast as toenails. If a fingernail is lost, it takes between three and six months to regrow completely, whereas a toenail takes between 12 and 18 months to regrow. The actual rate of growth of an individual’s nails depends on many factors, including age, sex, season, diet, exercise level, and genes. If protected from breaking, nails can sometimes grow to be very long. The Chinese doctor in the photo below (Figure 10.6.3) has very long nails on two fingers of his left hand. This picture was taken in 1920 in China, where having long nails was a sign of aristocracy since it implied that one was wealthy enough to not have to do physical labour.

Both fingernails and toenails protect the soft tissues of the fingers and toes from injury. Fingernails also serve to enhance sensation and precise movements of the fingertips through the counter-pressure exerted on the pulp of the fingers by the nails. In addition, fingernails can function as several different types of tools. For example, they enable a fine precision grip like tweezers, and can also be used for cutting and scraping.

Healthcare providers, particularly EMTs, often examine the fingernail beds as a quick and easy indicator of oxygen saturation of the blood, or the amount of blood reaching the extremities. If the nail beds are bluish or purple, it is generally a sign of low oxygen saturation. To see if blood flow to the extremities is adequate, a blanch test may be done. In this test, a fingernail is briefly depressed to turn the nail bed white by forcing the blood out of its capillaries. When the pressure is released, the pink colour of the nail bed should return within a second or two if there is normal blood flow. If the return to a pink colour is delayed, then it can be an indicator of low blood volume, due to dehydration or shock.

How the visible portion of the nails appears can be used as an indicator of recent health status. In fact, nails have been used as diagnostic tools for hundreds — if not thousands — of years. Nail abnormalities, such as deep grooves, brittleness, discolouration, or unusually thin or thick nails, may indicate various illnesses, nutrient deficiencies, drug reactions, or other health problems.

Nails — especially toenails — are common sites of fungal infections (shown in Figure 10.6.4), causing nails to become thickened and yellowish in colour. Toenails are more often infected than fingernails because they are often confined in shoes, which creates a dark, warm, moist environment where fungi can thrive. Toes also tend to have less blood flow than fingers, making it harder for the immune system to detect and stop infections in toenails.

Although nails are harder and tougher than skin, they are also more permeable. Harmful substances may be absorbed through the nails and cause health problems. Some of the substances that can pass through the nails include the herbicide Paraquat, fungicidal agents such as miconazole (e.g., Monistat), and sodium hypochlorite, which is an ingredient in common household bleach. Care should be taken to protect the nails from such substances when handling or immersing the hands in them by wearing latex or rubber gloves.

Do you get regular manicures or pedicures from a nail technician? If so, there is a chance that you are putting your health at risk. Nail tools that are not properly disinfected between clients may transmit infections from one person to another. Cutting the cuticles with scissors may create breaks in the skin that let infective agents enter the body. Products such as acrylics, adhesives, and UV gels that are applied to the nails may be harmful, especially if they penetrate the nails and enter the skin.

Use the Internet to find several reliable sources that address the health risks of professional manicures or pedicures. Try to find answers to the following questions:

1. What training and certification are required for professional nail technicians?
2. What licenses and inspections are required for nail salons?
3. What hygienic practices should be followed in nail salons to reduce the risk of infections being transmitted to clients?
4. Which professional nail products are potentially harmful to the human body and which are safer?
5. How likely is it to have an adverse health consequence when you get a professional manicure or pedicure?
6. What steps can you take to ensure that a professional manicure or pedicure is safe?

Attributions

Figure 10.6.1

Nails by allison-christine-vPrqHSLdF28 [photo] by allison christine on Unsplash is used under the Unsplash License (https://unsplash.com/license).

Figure 10.6.2

Blausen_0406_FingerNailAnatomy by BruceBlaus  on Wikimedia Commons is used under a CC BY 3.0 (https://creativecommons.org/licenses/by/3.0) license.

Figure 10.6.3

Chinese_doctor_with_long_finger_nails_(an_aristocrat),_ca.1920_(CHS-249) by Pierce, C.C. (Charles C.), 1861-1946 from the USC Digital Library on Wikimedia Commons is in the public domain (https://en.wikipedia.org/wiki/public_domain).

Figure 10.6.4

Toenail fungus Nagelpilz-3 by Pepsyrock on Wikimedia Commons is released into the public domain (https://en.wikipedia.org/wiki/public_domain).

Figure 10.6.5

OLYMPUS DIGITAL CAMERA by Stoive at the English language Wikipedia, on Wikimedia Commons is used under a CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0/) license.

References

Blausen.com staff. (2014). Medical gallery of Blausen Medical 2014. WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.010. ISSN 2002-4436.

Guiness World Records. (2014, December 8). Longest fingernails - Guinness World Records 60th Anniversary. YouTube. https://www.youtube.com/watch?v=G35kPhbUZdg

SciShow. (2015, September 14). 5 things your nails can say about your health. YouTube. https://www.youtube.com/watch?v=aTSVHwzkYI4

TED-Ed. (2019, October 29). Claws vs. nails - Matthew Borths. YouTube. https://www.youtube.com/watch?v=7w2gCBL1MCg

Created by CK-12 Foundation/Adapted by Christine Miller

Summer sun may feel good on your body, but its invisible UV rays wreak havoc on your skin. Exposing the skin to UV light causes photo-aging: premature wrinkling, brown discolourations, and other unattractive signs of sun exposure. Even worse, UV light increases your risk of skin cancer.

Skin cancer is a disease in which skin cells grow out of control. It is caused mainly by excessive exposure to UV light, which damages DNA. Therefore, skin cancer most often develops on areas of the skin that are frequently exposed to UV light. However, it can also occur on areas that are rarely exposed to UV light. Skin cancer affects people of all skin colours, including those with dark skin. It also affects more people altogether than all other cancers combined. One in five Canadians develops skin cancer in his or her lifetime.

Skin cancer begins in the outer layer of skin, the epidermis. There are three common types of skin cancer: basal cell carcinoma, squamous cell carcinoma, and melanoma.

Basal Cell Carcinoma

Basal cell carcinoma occurs in basal cells of the epidermis. Basal cells are stem cells in the stratum basale layer that divide to form all the keratinocytes of the epidermis. Basal cell carcinoma is the most common form of skin cancer and 1 in 8 Canadians will develop basal cell carcinoma during their lifetime.  A basal cell carcinoma may appear as a pearly or waxy sore, like the one shown in Figure 10.7.2. Basal cell carcinomas rarely spread (or undergo metastasis), so they can generally be cured with a biopsy, in which the lesion is cut out of the skin and analyzed in a medical lab.

Squamous cell carcinoma occurs in squamous cells of the epidermis. Squamous cells are flattened, keratin-filled cells in upper layers of the epidermis. Squamous cell carcinoma is the second most common form of skin cancer. More than two million cases occur in the United States each year. A squamous cell carcinoma may appear as a firm, red nodule, or as a flat lesion with a scaly or crusty surface, like the one pictured in Figure 10.7.3. Squamous cell carcinomas are generally localized and unlikely to metastasize, so they are usually curable surgically.

 

Melanoma occurs in melanocytes of the epidermis. Melanocytes are the melanin-producing cells in the stratum basale of the epidermis. Melanoma is the rarest type of skin cancer, accounting for less than one per cent of all skin cancer cases. Melanoma, however, is the most deadly type of skin cancer. It causes the vast majority of skin cancer deaths, because melanoma is malignant. If not treated, it will metastasize and spread to other parts of the body. If melanoma is detected early and while it is still localized in the skin, most patients survive for at least five years. If melanoma is discovered only after it has already metastasized to distant organs, there is only a 17% of patients surviving for five years. You can see an example of a melanoma in Figure 10.7.4.

Melanoma can develop anywhere on the body. It may develop in otherwise normal skin, or an existing mole may become cancerous. Signs of melanoma may include a:

• Mole that changes in size, feel, or colour.
• Mole that bleeds.
• Large brown spot on the skin sprinkled with darker specks.
• Small lesion with an irregular border and parts that appear red, white, blue, or blue-black.
• Dark lesion on the palms, soles, fingertips, toes, or mucous membranes.

Exposure to UV radiation causes about 90 per cent of all skin cancer cases. The connection between skin cancer and UV light is so strong that the World Health Organization has classified UV radiation (whether from tanning beds or the sun) as a Group 1 carcinogen (cancer-causing agent). Group 1 carcinogens are those carcinogens that are known with virtual certainty to cause cancer. In addition to UV light, Group 1 carcinogens include tobacco and plutonium. In terms of numbers of cancers caused, UV radiation is far worse than tobacco. More people develop skin cancer because of UV light exposure than develop lung cancer because of smoking. The increase in cancer risk due to UV light is especially great if you have ever had blistering sunburns as a child or teen.

Besides UV light exposure, other risk factors for skin cancer include:

• Having light coloured skin.
• Having a lot of moles.
• Being diagnosed with precancerous skin lesions.
• Having a family history of skin cancer.
• Having a personal history of skin cancer.
• Having a weakened immune system.
• Being exposed to other forms of radiation or to certain toxic substances such as arsenic.

As with most types of cancer, skin cancer is easiest to treat and most likely to be cured the earlier it is detected. The skin is one of the few organs that you can monitor for cancer yourself, as long as you know what to look for. A brown spot on the skin is likely to be a harmless mole, but it could be a sign of skin cancer. As shown in Figure 10.7.5 below, unlike moles, skin cancers may be asymmetrical, have irregular borders, may be very dark in colour, and may have a relatively great diameter. These characteristics can be remembered with the acronym ABCD.

With the help of mirrors, you should check all of your skin regularly. Look for new skin growths or changes in any existing moles, freckles, bumps, or birthmarks. Report anything suspicious or different to your doctor.

If you have risk factors for skin cancer, it’s a good idea to have an annual skin check by a dermatologist. This helps ensure that cancerous or precancerous lesions will be detected before they grow too large and become difficult to cure, or in the case of melanoma, before they metastasize.

 

Attributions

Figure 10.7.1

Stolen_Moment_in_the_Sun by Angie Garrett on Wikimedia Commons is used under a CC BY 2.0 (https://creativecommons.org/licenses/by/2.0) license.

Figure 10.7.2

Basal_cell_carcinoma,_ulcerated by Kelly Nelson (Photographer) from National Cancer Institute (part of the National Institutes of Health) with the ID 9237 on Wikimedia Commons was released into the public domain (https://en.wikipedia.org/wiki/Public_domain).

Figure 10.7.3

Squamous_cell_carcinoma_(1) by Kelly Nelson (Photographer) from National Cancer Institute (part of the National Institutes of Health) with the ID 9248 on Wikimedia Commons was released into the public domain (https://en.wikipedia.org/wiki/Public_domain).

Figure 10.7.4

Melanoma by Unknown author (Photographer) from National Cancer Institute (part of the National Institutes of Health) with the AV-8500-3850/ ID 9186 on Wikimedia Commons was released into the public domain (https://en.wikipedia.org/wiki/Public_domain).

Figure 10.7.5

ABCDs of skin cancer by CK-12 Foundation is used under a CC BY-NC 3.0 (https://creativecommons.org/licenses/by-nc/3.0/) license. (Original images courtesy of NCI: ID numbers 2362; 2363; 2364; and 2184)

References

Brainard, J/ CK-12 Foundation. (2016). Figure 5 ABCDs of skin cancer[digital image]. In CK-12 College Human Biology (Section 12.7) [online Flexbook]. CK12.org. https://www.ck12.org/book/ck-12-college-human-biology/section/12.7/

Public Health Agency of Canada. (2019, December 9). Non melanoma skin cancer. Canada.ca. https://www.canada.ca/en/public-health/services/chronic-diseases/cancer/non-melanoma-skin-cancer.html

Robert Miller. (2014, July 22). Cancer of the vulva. YouTube. https://www.youtube.com/watch?v=ID-O-Ion3EQ

TED-Ed. (2012, December 5). How do cancer cells behave differently from healthy ones? - George Zaidan. YouTube. https://www.youtube.com/watch?v=BmFEoCFDi-w

TEDx Talks. (2014, March 28). The skin 'beauty' and the sun 'beast': Charareh Pourzand at TEDxBathUniveristy. YouTube. https://www.youtube.com/watch?v=60e-t4zglBk

Created by CK-12 Foundation/Adapted by Christine Miller

Giving yourself an injection can be difficult, but for someone with diabetes, it may be a matter of life or death. The person in the photo has diabetes and is injecting themselves with insulin, the hormone that helps control the level of glucose in the blood. Insulin is produced by the pancreas.

The pancreas is a large gland located in the upper left abdomen behind the stomach, as shown in Figure 9.7.2. The pancreas is about 15 cm (6 in) long, and it has a flat, oblong shape. Structurally, the pancreas is divided into a head, body, and tail. Functionally, the pancreas serves as both an endocrine gland and an exocrine gland.

• As an endocrine gland, the pancreas is part of the endocrine system. As such, it releases hormones (such as insulin) directly into the bloodstream for transport to cells throughout the body.
• As an exocrine gland, the pancreas is part of the digestive system. As such, it releases digestive enzymes into ducts that carry the enzymes to the gastrointestinal tract, where they assist with digestion. In this concept, the focus is on the pancreas as an endocrine gland. You can read about the pancreas as an exocrine gland in Chapter 15 Digestive System.

The tissues within the pancreas that have an endocrine role exist as clusters of cells called pancreatic islets. They are also called the islets of Langerhans. You can see pancreatic tissue, including islets, in Figure 9.7.3. There are approximately three million pancreatic islets, and they are crisscrossed by a dense network of capillaries. The capillaries are lined by layers of islet cells that have direct contact with the blood vessels, into which they secrete their endocrine hormones.

The pancreatic islets consist of four main types of cells, each of which secretes a different endocrine hormone. All of the hormones produced by the pancreatic islets, however, play crucial roles in glucose metabolism and the regulation of blood glucose levels, among other functions.

1. Islet cells called alpha (α) cells secrete the hormone glucagon. The function of glucagon is to increase the level of glucose in the blood. It does this by stimulating the liver to convert stored glycogen into glucose, which is released into the bloodstream.
2. Islets cells called beta (β) cells secrete the hormone insulin. The function of insulin is to decrease the level of glucose in the blood. It does this by promoting the absorption of glucose from the blood into fat, liver, and skeletal muscle cells. In these tissues, the absorbed glucose is converted into glycogen, fats (triglycerides), or both.
3. Islet cells called delta (δ) cells secrete the hormone somatostatin. This hormone is also called growth hormone inhibiting hormone, because it inhibits the anterior lobe of the pituitary gland from producing growth hormone. Somatostatin also inhibits the secretion of pancreatic endocrine hormones and pancreatic exocrine enzymes.
4. Islet cells called gamma (γ) cells secrete the hormone pancreatic polypeptide. The function of pancreatic polypeptide is to help regulate the secretion of both endocrine and exocrine substances by the pancreas.

There are a variety of disorders that affect the pancreas. They include pancreatitis, pancreatic cancer, and diabetes mellitus.

Pancreatitis

Pancreatitis is inflammation of the pancreas. It has a variety of possible causes, including gallstones, chronic alcohol use, infections (such as measles or mumps), and certain medications. Pancreatitis occurs when digestive enzymes produced by the pancreas damage the gland’s tissues, which causes problems with fat digestion. The disorder is usually associated with intense pain in the central abdomen, and the pain may radiate to the back. Yellowing of the skin and whites of the eyes (see Figure 9.7.4), which is called jaundice, is a common sign of pancreatitis. People with pancreatitis may also have pale stools and dark urine. Treatment of pancreatitis includes administering drugs to manage pain, and addressing the underlying cause of the disease, for example, by removing gallstones.

Pancreatic Cancer

There are several different types of pancreatic cancer that may affect either the endocrine or the exocrine tissues of the gland. Cancers affecting the endocrine tissues are all relatively rare. However, their incidence has been rising sharply. It is unclear to what extent this reflects increased detection, especially through medical imaging techniques. Unfortunately, pancreatic cancer is usually diagnosed at a relatively late stage when it is too late for surgery, which is the only way to cure the disorder. In 2020 it is estimated that 6,000 Canadians will be newly diagnosed with pancreatic cancer, and that during this same year, 5,300 will die of pancreatic cancer.

While it is rare before the age of 40, pancreatic cancer occurs most often after the age of 60. Factors that increase the risk of developing pancreatic cancer include smoking, obesity, diabetes, and a family history of the disease. About one in four cases of pancreatic cancer are attributable to smoking. Certain rare genetic conditions are also risk factors for pancreatic cancer.

Diabetes Mellitus

By far the most common type of pancreatic disorder is diabetes mellitus, more commonly called simply diabetes. There are many different types of diabetes, but diabetes mellitus is the most common. It occurs in two major types, type 1 diabetes and type 2 diabetes. The two types have different causes and may also have different treatments, but they generally produce the same initial symptoms, which include excessive urination and thirst. These symptoms occur because the kidneys excrete more urine in an attempt to rid the blood of excess glucose. Loss of water in urine stimulates greater thirst. Other signs and symptoms of diabetes are listed in Figure 9.7.5.

When diabetes is not well controlled, it is likely to have several serious long-term consequences. Most of these consequences are due to damage to small blood vessels caused by high glucose levels in the blood. Damage to blood vessels, in turn, may lead to increased risk of coronary artery disease and stroke. Damage to blood vessels in the retina of the eye can result in gradual vision loss and blindness. Damage to blood vessels in the kidneys can lead to chronic kidney disease, sometimes requiring dialysis or kidney transplant. Long-term consequences of diabetes may also include damage to the nerves of the body, known as diabetic neuropathy. In fact, this is the most common complication of diabetes. Symptoms of diabetic neuropathy may include numbness, tingling, and pain in the extremities.

Type 1 Diabetes

Type 1 diabetes is a chronic autoimmune disorder in which the immune system attacks the insulin-secreting beta cells of the pancreas. As a result, people with type 1 diabetes lack the insulin needed to keep blood glucose levels within the normal range. Type 1 diabetes may develop in people of any age, but is most often diagnosed before adulthood. For type 1 diabetics, insulin injections are critical for survival.

Type 2 Diabetes

Type 2 diabetes is the single most common form of diabetes. The cause of high blood glucose in this form of diabetes usually includes a combination of insulin resistance and impaired insulin secretion. Both genetic and environmental factors play roles in the development of type 2 diabetes. Type 2 diabetes can be managed with changes in diet and physical activity, which may increase insulin sensitivity and help reduce blood glucose levels to normal ranges. Medications may also be used as part of the treatment, as may insulin injections.

Some patients with type 1 diabetes have been given pancreatic islet cells transplants from other human donors. If the transplanted cells are not rejected by the recipient’s immune system, they can cure the patient of diabetes. However, because of a shortage of appropriate human donors, only about one thousand such surgeries have been performed over the past ten years.

In June of 2016, a research team led by Dr. David K.C. Cooper at the Thomas E. Starzl Transplantation Institute in Pittsburgh, Pennsylvania, reported on their work developing pig islet cells for transplant into human diabetes patients. The researchers genetically engineered the pig islet cells to be protected from the human immune response. As a result, patients receiving the transplanted cells would require only minimal suppression of their immune system after the surgery. The pig islet cells would also be less likely to transmit pathogenic agents, because the animals could be raised in a controlled environment.

The researchers have successfully transplanted the pig islet cells into monkey models of type 1 diabetes. As of June 2016, the scientists were looking for funding to undertake clinical trials in humans with type 1 diabetes. Dr. Cooper predicted then that if the human trials go as well as expected, the pig islet cells could be available for curing patients in as little as two years.

Attributions

Figure 9.7.1

Insulin_Application by Mr Hyde at Czech Wikipedia (Original text: moje foto) on Wikimedia Commons is released into the public domain (https://en.wikipedia.org/wiki/Public_domain).

Figure 9.7.2

Blausen_0699_PancreasAnatomy2 by BruceBlaus on Wikimedia Commons is used under a  CC BY 3.0 (https://creativecommons.org/licenses/by/3.0) license.

Figure 9.7.3

Exocrine_and_Endocrine_Pancreas by OpenStax College is used under a CC BY 3.0 (https://creativecommons.org/licenses/by/3.0/deed.en) license.

Figure 9.7.4

Jaundice_eye_new by Info-farmer on Wikimedia Commons is in the public domain (https://en.wikipedia.org/wiki/Public_domain). (Original image, File:Jaundice eye.jpg, is from Centers for Disease Control and Prevention's Public Health Image Library (PHIL), with identification number #2860)

Figure 9.7.5

Main_symptoms_of_diabetes.svg by Mikael Häggström on Wikimedia Commons is released into the public domain (https://en.wikipedia.org/wiki/Public_domain).

 

References

Betts, J. G., Young, K.A., Wise, J.A., Johnson, E., Poe, B., Kruse, D.H., Korol, O., Johnson, J.E., Womble, M., DeSaix, P. (2013, July 19). Figure 23.26 Exocrine and endocrine pancreas [digital image].  In Anatomy and Physiology (Section 23.6). OpenStax. https://openstax.org/books/anatomy-and-physiology/pages/23-6-accessory-organs-in-digestion-the-liver-pancreas-and-gallbladder

Blausen.com Staff. (2014). Medical gallery of Blausen Medical 2014. WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.010. ISSN 2002-4436.

Children's Health. (2008, June 13). Type 2 diabetes in children. YouTube. https://www.youtube.com/watch?v=qlzLSbAGMqA&feature=youtu.be

Hering, B. J., Cozzi, E., Spizzo, T., Cowan, P. J., Rayat, G. R., Cooper, D. K. C., Denner, J. (2016, March 4). First update of the International Xenotransplantation Association consensus statement on conditions for undertaking clinical trials of porcine islet products in type 1 diabetes—Executive summary. Xenotransplantation 2016, 23: 3– 13. https://doi.org/10.1111/xen.12231

TED-Ed. (2015, February 19). What does the pancreas do? - Emma Bryce. YouTube. https://www.youtube.com/watch?v=8dgoeYPoE-0&feature=youtu.be

TEDx Talks. (2015, May 4). Reversing Type 2 diabetes starts with ignoring the guidelines | Sarah Hallberg | TEDxPurdueU. YouTube. https://www.youtube.com/watch?v=da1vvigy5tQ&feature=youtu.be

Created by CK-12/Adapted by Christine Miller

As you read in the beginning of this chapter, new parents Samantha and Aki left their pediatrician’s office still unsure whether or not to vaccinate baby James. Dr. Rodriguez gave them a list of reputable sources where they could look up information about the safety of vaccines, including the Centers for Disease Control and Prevention (CDC). Samantha and Aki read that the consensus within the scientific community is that there is no link between vaccines and autism. They find a long list of studies published in peer-reviewed scientific journals that disprove any link. Additionally, some of the studies are “meta-analyses” that analyzed the findings from many individual studies. The new parents are reassured by the fact that many different researchers, using a large number of subjects in numerous well-controlled and well-reviewed studies, all came to the same conclusion.

Samantha also went back to the web page that originally scared her about the safety of vaccines. She found that the author was not a medical doctor or scientific researcher, but rather a self-proclaimed “child wellness expert.” He sold books and advertising on his site, some of which were related to claims of vaccine injury. She realized that he was both an unqualified and potentially biased source of information.

Samantha also realized that some of his arguments were based on correlations between autism and vaccines, but, as the saying goes, “correlation does not imply causation.” For instance, the recent rise in autism rates may have occurred during the same time period as an increase in the number of vaccines given in childhood, but Samantha could think of many other environmental and social factors that have also changed during this time period. There are just too many variables to come to the conclusion that vaccines, or anything else, are the cause of the rise in autism rates based on that type of argument alone. Also, she learned that the age of onset of autism symptoms happens to typically be around the time that the MMR vaccine is first given, so the apparent association in the timing may just be a coincidence.

Finally, Samantha came across news about  a measles outbreak in Vancouver, British Columbia in the winter of 2019. Measles wasn’t just a disease of the past! She learned that measles and whooping cough, which had previously been rare thanks to widespread vaccinations, are now on the rise, and that people choosing not to vaccinate their children seems to be one of the contributing factors. She realized that it is important to vaccinate her baby against these diseases, not only to protect him from their potentially deadly effects, but also to protect others in the population.

In their reading, Samantha and Aki learn that scientists do not yet know the causes of autism, but they feels reassured by the abundance of data that disproves any link with vaccines. Both parents think that the potential benefits of protecting their baby’s health against deadly diseases outweighs any unsubstantiated claims about vaccines. They will be making an appointment to get baby James his shots soon.

Attribution

Figure 1.8.1

[Photo of person sitting in front of personal computer] by Avel Chuklanov on Unsplash is used under the Unsplash License (https://unsplash.com/license).

Created by CK-12 Foundation/Adapted by Christine Miller

If you had a skin rash like the one shown in Figure 15.7.1, you probably wouldn’t assume that it was caused by a digestive system disease. However, that’s exactly why the individual in the picture has a rash. He has a gastrointestinal (GI) tract disorder called Crohn’s disease. This disease is one of a group of GI tract disorders that are known collectively as inflammatory bowel disease. Unlike other inflammatory bowel diseases, signs and symptoms of Crohn’s disease may not be confined to the GI tract.

Inflammatory bowel disease (IBD) is a collection of inflammatory conditions primarily affecting the intestines. The two principal inflammatory bowel diseases are Crohn’s disease and ulcerative colitis. Unlike Crohn’s disease — which may affect any part of the GI tract and the joints, as well as the skin — ulcerative colitis mainly affects just the colon and rectum. Both diseases occur when the body’s own immune systemno post attacks the digestive system. Both diseases typically first appear in the late teens or early twenties, and occur equally in males and females.  Approximately 270,000 Canadians are currently living with IBD, 7,000 of which are children.  The annual cost of caring for these Canadians is estimated at $1.28 billion.  The number of cases of IBD has been steadily increasing and it is expected that by 2030 the number of Canadians suffering from IBD will grow to 400,000.

Crohn’s Disease

Crohn’s disease is a type of inflammatory bowel disease that may affect any part of the GI tract from the mouth to the anus, among other body tissues. The most commonly affected region is the ileum, which is the final part of the small intestine. Signs and symptoms of Crohn’s disease typically include abdominal pain, diarrhea (with or without blood), fever, and weight loss. Malnutrition because of faulty absorption of nutrients may also occur. Potential complications of Crohn’s disease include obstructions and abscesses of the bowel. People with Crohn’s disease are also at slightly greater risk than the general population of developing bowel cancer. Although there is a slight reduction in life expectancy in people with Crohn’s disease, if the disease is well-managed, affected people can live full and productive lives.  Approximately 135,000 Canadians are living with Crohn's disease.

Crohn’s disease is caused by a combination of genetic and environmental factors that lead to impairment of the generalized immune response (called innate immunity). The chronic inflammation of Crohn’s disease is thought to be the result of the immune system “trying” to compensate for the impairment. Dozens of genes are likely to be involved, only a few of which have been identified. Because of the genetic component, close relatives such as siblings of people with Crohn’s disease are many times more likely to develop the disease than people in the general population. Environmental factors that appear to increase the risk of the disease include smoking tobacco and eating a diet high in animal proteins. Crohn’s disease is typically diagnosed on the basis of a colonoscopy, which provides a direct visual examination of the inside of the colon and the ileum of the small intestine.

People with Crohn’s disease typically experience recurring periods of flare-ups followed by remission. There are no medications or surgical procedures that can cure Crohn’s disease, although medications such as anti-inflammatory or immune-suppressing drugs may alleviate symptoms during flare-ups and help maintain remission. Lifestyle changes, such as dietary modifications and smoking cessation, may also help control symptoms and reduce the likelihood of flare-ups. Surgery may be needed to resolve bowel obstructions, abscesses, or other complications of the disease.

Ulcerative Colitis

Ulcerative colitis is an inflammatory bowel disease that causes inflammation and ulcers (sores) in the colon and rectum. Unlike Crohn’s disease, other parts of the GI tract are rarely affected in ulcerative colitis. The primary symptoms of the disease are lower abdominal pain and bloody diarrhea. Weight loss, fever, and anemia may also be present. Symptoms typically occur intermittently with periods of no symptoms between flare-ups. People with ulcerative colitis have a considerably increased risk of colon cancer and should be screened for colon cancer more frequently than the general population. Ulcerative colitis, however, seems to primarily reduce the quality of life, and not the lifespan.

The exact cause of ulcerative colitis is not known. Theories about its cause involve immune system dysfunction, genetics, changes in normal gut bacteria, and lifestyle factors, such as a diet high in animal protein and the consumption of alcoholic beverages. Genetic involvement is suspected in part because ulcerative colitis tens to “run” in families. It is likely that multiple genes are involved. Diagnosis is typically made on the basis of colonoscopy and tissue biopsies.

Lifestyle changes, such as reducing the consumption of animal protein and alcohol, may improve symptoms of ulcerative colitis. A number of medications are also available to treat symptoms and help prolong remission. These include anti-inflammatory drugs and drugs that suppress the immune system. In cases of severe disease, removal of the colon and rectum may be required and can cure the disease.

Diverticulitis is a digestive disease in which tiny pouches in the wall of the large intestine become infected and inflamed. Symptoms typically include lower abdominal pain of sudden onset. There may also be fever, nausea, diarrhea or constipation, and blood in the stool. Having large intestine pouches called diverticula (see Figure 15.7.2) that are not inflamed is called diverticulosis. Diverticulosis is thought to be caused by a combination of genetic and environmental factors, and is more common in people who are obese. Infection and inflammation of the pouches (diverticulitis) occurs in about 10–25% of people with diverticulosis, and is more common at older ages. The infection is generally caused by bacteria.

Diverticulitis can usually be diagnosed with a CT scan and can be monitored with a colonoscopy (as seen in Figure 15.7.3). Mild diverticulitis may be treated with oral antibiotics and a short-term liquid diet. For severe cases, intravenous antibiotics, hospitalization, and complete bowel rest (no nourishment via the mouth) may be recommended. Complications such as abscess formation or perforation of the colon require surgery.

A peptic ulcer is a sore in the lining of the stomach or the duodenum (first part of the small intestine). If the ulcer occurs in the stomach, it is called a gastric ulcer. If it occurs in the duodenum, it is called a duodenal ulcer. The most common symptoms of peptic ulcers are upper abdominal pain that often occurs in the night and improves with eating. Other symptoms may include belching, vomiting, weight loss, and poor appetite. Many people with peptic ulcers, particularly older people, have no symptoms. Peptic ulcers are relatively common, with about ten per cent of people developing a peptic ulcer at some point in their life.

The most common cause of peptic ulcers is infection with the bacterium Helicobacter pylori, which may be transmitted by food, contaminated water, or human saliva (for example, by kissing or sharing eating utensils). Surprisingly, the bacterial cause of peptic ulcers was not discovered until the 1980s. The scientists who made the discovery are Australians Robin Warren and Barry J. Marshall. Although the two scientists eventually won a Nobel Prize for their discovery, their hypothesis was poorly received at first. To demonstrate the validity of their discovery, Marshall used himself in an experiment. He drank a culture of bacteria from a peptic ulcer patient and developed symptoms of peptic ulcer in a matter of days. His symptoms resolved on their own within a couple of weeks, but, at his wife's urging, he took antibiotics to kill any remaining bacteria. Marshall’s self-experiment was published in the Australian Medical Journal, and is among the most cited articles ever published in the journal.  Figure 15.7.4 shows how H. pylori cause peptic ulcers.

Another relatively common cause of peptic ulcers is chronic use of non-steroidal anti-inflammatory drugs (NSAIDs), such as aspirin or ibuprofen. Additional contributing factors may include tobacco smoking and stress, although these factors have not been demonstrated conclusively to cause peptic ulcers independent of H. pylori infection. Contrary to popular belief, diet does not appear to play a role in either causing or preventing peptic ulcers. Eating spicy foods and drinking coffee and alcohol were once thought to cause peptic ulcers. These lifestyle choices are no longer thought to have much (if any) of an effect on the development of peptic ulcers.

Peptic ulcers are typically diagnosed on the basis of symptoms or the presence of H. pylori in the GI tract. However, endoscopy (shown in Figure 15.7.5), which allows direct visualization of the stomach and duodenum with a camera, may be required for a definitive diagnosis. Peptic ulcers are usually treated with antibiotics to kill H. pylori, along with medications to temporarily decrease stomach acid and aid in healing. Unfortunately, H. pylori has developed resistance to commonly used antibiotics, so treatment is not always effective. If a peptic ulcer has penetrated so deep into the tissues that it causes a perforation of the wall of the stomach or duodenum, then emergency surgery is needed to repair the damage.

 

Gastroenteritis, also known as infectious diarrhea or stomach flu, is an acute and usually self-limiting infection of the GI tract by pathogens. Symptoms typically include some combination of diarrhea, vomiting, and abdominal pain. Fever, lack of energy, and dehydration may also occur. The illness generally lasts less than two weeks, even without treatment, but in young children it is potentially deadly. Gastroenteritis is very common, especially in poorer nations. Worldwide, up to five billion cases occur each year, resulting in about 1.4 million deaths.

Commonly called “stomach flu,” gastroenteritis is unrelated to the influenza virus, although viruses are the most common cause of the disease (see Figure 15.7.6). In children, rotavirus is most often the cause which is why the British Columbia immunization schedule now includes a rotovirus vaccine. Norovirus is more likely to be the cause of gastroenteritis in adults. Besides viruses, other potential causes of gastroenteritis include fungi, bacteria (most often E. coli or Campylobacter jejuni), and protozoa(including Giardia lamblia, more commonly called Beaver Fever, described below). Transmission of pathogens may occur due to eating improperly prepared foods or foods left to stand at room temperature, drinking contaminated water, or having close contact with an infected individual.

Gastroenteritis is less common in adults than children, partly because adults have acquired immunity after repeated exposure to the most common infectious agents. Adults also tend to have better hygiene than children. If children have frequent repeated incidents of gastroenteritis, they may suffer from malnutrition, stunted growth, and developmental delays. Many cases of gastroenteritis in children can be avoided by giving them a rotavirus vaccine. Frequent and thorough handwashing can cut down on infections caused by other pathogens.

Treatment of gastroenteritis generally involves increasing fluid intake to replace fluids lost in vomiting or diarrhea. Oral rehydration solution, which is a combination of water, salts, and sugar, is often recommended. In severe cases, intravenous fluids may be needed. Antibiotics are not usually prescribed, because they are ineffective against viruses that cause most cases of gastroenteritis.

Giardiasis, popularly known as beaver fever, is a type of gastroenteritis caused by a GI tract parasite, the single-celled protozoan Giardia lamblia (pictured in Figure 15.7.7). In addition to human beings, the parasite inhabits the digestive tract of a wide variety of domestic and wild animals, including cows, rodents, and sheep, as well as beavers (hence its popular name). Giardiasis is one of the most common parasitic infections in people the world over, with hundreds of millions of people infected worldwide each year.

Transmission of G. lamblia is via a fecal-oral route (as in, you got feces in your food). Those at greatest risk include travelers to countries where giardiasis is common, people who work in child-care settings, backpackers and campers who drink untreated water from lakes or rivers, and people who have close contact with infected people or animals in other settings. In Canada, Giardia is the most commonly identified intestinal parasite and approximately 3,000 Canadians will contract the parasite annually.

Symptoms of giardiasis can vary widely. About one-third third of people with the infection have no symptoms, whereas others have severe diarrhea with poor absorption of nutrients. Problems with absorption occur because the parasites inhibit intestinal digestive enzyme production, cause detrimental changes in microvilli lining the small intestine, and kill off small intestinal epithelial cells. The illness can result in weakness, loss of appetite, stomach cramps, vomiting, and excessive gas. Without treatment, symptoms may continue for several weeks. Treatment with anti-parasitic medications may be needed if symptoms persist longer or are particularly severe.

Attributions

Figure 15.7.1

BADAS_Crohn by Dayavathi Ashok and Patrick Kiely/ Journal of medical case reports on Wikimedia Commons is used under a CC BY 2.0 (https://creativecommons.org/licenses/by/2.0) license.

Figure 15.7.2

512px-Ds00070_an01934_im00887_divert_s_gif.webp by Lfreeman04  on Wikimedia Commons is used under a CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0) license.

Figure 15.7.3

Colon_diverticulum by melvil on Wikimedia Commons is used under a CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0) license.

Figure 15.7.4

H_pylori_ulcer_diagram by Y_tambe on Wikimedia Commons is used under a CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0/) license.

Figure 15.7.5

1024px-Endoscopy_training by Yuya Tamai on Wikimedia Commons is used under a CC BY 2.0 (https://creativecommons.org/licenses/by/2.0) license.

Figure 15.7.6

Gastroenteritis_viruses by Dr. Graham Beards [en:User:Graham Beards] at en.wikipedia on Wikimedia Commons is used under a CC BY 3.0 (https://creativecommons.org/licenses/by/3.0) license.

Figure 15.7.7

Giardia_lamblia_SEM_8698_lores by Janice Haney Carr from CDC/ Public Health Image Library (PHIL) ID# 8698 on Wikimedia Commons is in the public domain (https://en.wikipedia.org/wiki/public_domain).

References

Ashok, D., & Kiely, P. (2007). Bowel associated dermatosis - arthritis syndrome: a case report. Journal of medical case reports, 1, 81. https://doi.org/10.1186/1752-1947-1-81

Marshall, B. J., Armstrong, J. A., McGechie, D. B., & Glancy, R. J. (1985). Attempt to fulfil Koch's postulates for pyloric Campylobacter. The Medical Journal of Australia, 142(8), 436–439.

Marshall, B. J., McGechie, D. B., Rogers, P. A., & Glancy, R. J. (1985). Pyloric campylobacter infection and gastroduodenal disease. The Medical Journal of Australia, 142(8), 439–444.

TED-Ed. (2017, September 28). The surprising cause of stomach ulcers - Rusha Modi. YouTube. https://www.youtube.com/watch?v=V_U6czbDHLE&feature=youtu.be

TED-Ed. (2018, January 4). Who's at risk for colon cancer? - Amit H. Sachdev and Frank G. Gress. YouTube. https://www.youtube.com/watch?v=H5zin8jKeT0&feature=youtu.be

division of the peripheral nervous system that controls involuntary activities

A hormone is a signaling molecule produced by glands in multicellular organisms that target distant organs to regulate physiology and behavior.

Created by CK-12 Foundation/Adapted by Christine Miller

As you learned in the beginning of this chapter, botulinum toxin — one form of which is sold under the brand name Botox — does much more than smooth out wrinkles. It can be used to treat a number of disorders involving excessive muscle contraction, including cervical dystonia. You also learned that cervical dystonia, which Edward suffers from, causes abnormal, involuntary muscle contractions of the neck. This results in jerky movements of the head and neck, and/or a sustained abnormal tilt to the head. It is often painful and can significantly interfere with a person’s life.

How could a toxin actually help treat a muscular disorder? The botulinum toxin is produced by the soil bacterium, Clostridium botulinum, and it is the cause of the potentially deadly disease called botulism. Botulism is often a foodborne illness, commonly caused by foods that are improperly canned. Other forms of botulism are caused by wound infections, or occur when infants consume spores of the bacteria from soil or honey.

Botulism can be life-threatening, because it paralyzes muscles throughout the body, including those involved in breathing. When a very small amount of botulinum toxin is injected carefully into specific muscles by a trained medical professional, however, it can be useful in inhibiting unwanted muscle contractions.

For cosmetic purposes, botulinum toxin injected into the facial muscles relaxes them to reduce the appearance of wrinkles. When used to treat cervical dystonia, it is injected into the muscles of the neck to inhibit excessive muscle contractions. For many patients, this helps relieve the abnormal positioning, movements, and pain associated with the disorder. The effect is temporary, so the injections must be repeated every three to four months to keep the symptoms under control.

How does botulinum toxin inhibit muscle contraction? First, recall how skeletal muscle contraction works. A motor neuron instructs skeletal muscle fibres to contract at a synapse between them called the neuromuscular junction. A nerve impulse called an action potential travels down to the axon terminal of the motor neuron, where it causes the release of the neurotransmitter acetylcholine (ACh) from synaptic vesicles. The ACh travels across the synaptic cleft and binds to ACh receptors on the muscle fibre, signaling the muscle fibre to contract. According to the sliding filament theory, the contraction of the muscle fibre occurs due to the sliding of myosin and actin filaments across each other. This causes the Z discs of the sacromeres to move closer together, shortening the sacromeres and causing the muscle fibre to contract.

If you wanted to inhibit muscle contraction, at what points could you theoretically interfere with this process? Inhibiting the action potential in the motor neuron, the release of ACh, the activity of ACh receptors, or the sliding filament process in the muscle fibre would all theoretically impair this process and inhibit muscle contraction. For example, in the disease myasthenia gravis, the function of the ACh receptors is impaired, causing a lack of sufficient muscle contraction. As you have learned, this results in muscle weakness that can eventually become life-threatening. Botulinum toxin works by inhibiting the release of ACh from the motor neurons, thereby removing the signal instructing the muscles to contract.

Fortunately, Edward’s excessive muscle contractions and associated pain improved significantly thanks to botulinum toxin injections. Although cervical dystonia cannot currently be cured, botulinum toxin injections have improved the quality of life for many patients with this and other disorders involving excessive involuntary muscle contractions.

As you have learned in this chapter, our muscular system allows us to do things like make voluntary movements, digest our food, and pump blood through our bodies. Whether they are in your arm, heart, stomach, or blood vessels, muscle tissue works by contracting. But as you have seen here, too much contraction can be a very bad thing. Fortunately, scientists and physicians have found a way to put a potentially deadly toxin — and wrinkle-reducing treatment — to excellent use as a medical treatment for some muscular system disorders.

Attributions

Figure 12.7.1

Botox, he whispered by Michael Reuter on Flickr is used under a CC BY 2.0 (https://creativecommons.org/licenses/by/2.0/) license.

Figure 12.7.2

botulism by jason wilson on Flickr is used under a CC BY 2.0 (https://creativecommons.org/licenses/by/2.0/) license.

Reference

Pearson Scott Foresman. (2020, April 14). File:Biceps (PSF).jpg [digital image]. Wikimedia Commons. https://commons.wikimedia.org/w/index.php?title=File:Biceps_(PSF).jpg&oldid=411251538. [Public Domain (https://en.wikipedia.org/wiki/Public_domain)]

 

A double-membrane-bound organelle found in most eukaryotic organisms. Mitochondria convert oxygen and nutrients into adenosine triphosphate (ATP). ATP is the chemical energy "currency" of the cell that powers the cell's metabolic activities.

Created by CK-12 Foundation/Adapted by Christine Miller

Why do bodybuilders have such prominent veins? Bulging muscles push surface veins closer to the skin. Combine that with a virtual lack of subcutaneous fat, and you have bulging veins, as well as bulging muscles. Veins are one of three major types of blood vessels in the cardiovascular system.

Blood vessels are the part of the cardiovascular system that transports blood throughout the human body. There are three major types of blood vessels. Besides veins, they include arteries and capillaries.

Arteries

Arteries are defined as blood vessels that carry blood away from the heart. Blood flows through arteries largely because it is under pressure from the pumping action of the heart. It should be noted that coronary arteries, which supply heart muscle cells with blood, travel toward the heart, but not as part of the blood flow that travels through the chambers of the heart. Most arteries, including coronary arteries, carry oxygenated blood, but there are a few exceptions, most notably the pulmonary artery. This artery carries deoxygenated blood from the heart to the lungs, where it picks up oxygen and releases carbon dioxide. In virtually all other arteries, the hemoglobin in red blood cells is highly saturated with oxygen (95–100 per cent). These arteries distribute oxygenated blood to tissues throughout the body.

The largest artery in the body is the aorta, which is connected to the heart and extends down into the abdomen (see Figure 14.4.2). The aorta has high-pressure, oxygenated blood pumped directly into it from the left ventricle of the heart. The aorta has many branches, and the branches subdivide repeatedly, with the subdivisions growing smaller and smaller in diameter. The smallest arteries are called arterioles.

Veins

Veins are defined as blood vessels that carry blood toward the heart. Blood traveling through veins is not under pressure from the beating heart. It gets help moving along by the squeezing action of skeletal muscles, for example, when you walk or breathe. It is also prevented from flowing backward by valves in the larger veins, as illustrated in Figure 14.4.3. and as seen in the ultrasonography image in Figure 14.4.4.  Veins are called capacitance blood vessels, because the majority of the body’s total volume of blood (about 60 per cent) is contained within veins.

 

Most veins carry deoxygenated blood, but there are a few exceptions, including the four pulmonary veins. These veins carry oxygenated blood from the lungs to the heart, which then pumps the blood to the rest of the body. In virtually all other veins, hemoglobin is relatively unsaturated with oxygen (about 75 per cent).

The two largest veins in the body are the superior vena cava — which carries blood from the upper body directly to the right atrium of the heart — and the inferior vena cava, which carries blood from the lower body directly to the right atrium (shown in Figure 14.4.5).  Like arteries, veins form a complex, branching system of larger and smaller vessels. The smallest veins are called venules. They receive blood from capillaries and transport it to larger veins. Each venule receives blood from multiple capillaries.  See the major veins of the human body in Figure 14.4.6.

Capillaries

Capillaries are the smallest blood vessels in the cardiovascular system. They are so small that only one red blood cell at a time can squeeze through a capillary, and then only if the red blood cell deforms. Capillaries connect arterioles and venules, as shown in Figure 14.4.7. Capillaries generally form a branching network of vessels, called a capillary bed, that provides a large surface area for the exchange of substances between the blood and surrounding tissues.

Structure of Blood Vessels

All blood vessels are basically hollow tubes with an internal space, called a lumen, through which blood flows. The lumen of an artery is shown in cross section in the photomicrograph (Figure 14.4.8). The width of blood vessels varies, but they all have a lumen. The walls of blood vessels differ depending on the type of vessel. In general, arteries and veins are more similar to one another than to capillaries in the structure of their walls.

Walls of Arteries and Veins

The walls of both arteries and veins have three layers: the tunica intima, tunica media, and tunica adventitia. You can see the three layers for an artery in the Figure 14.4.9.

1. The tunica intima is the inner layer of arteries and veins. It is also the thinnest layer, consisting of a single layer of endothelial cells surrounded by a thin layer of connective tissues. It reduces friction between the blood and the inside of the blood vessel walls.
2. The tunica media is the middle layer of arteries and veins. In arteries, this is the thickest layer. It consists mainly of elastic fibres and connective tissues. In arteries, this is the thickest layer, because it also contains smooth muscle tissues, which control the diameter of the vessels- as such, the width of the tunic media can be helpful in distinguishing arteries from veins.
3. The tunica externa (also called tunica adventitia) is the outer layer of arteries and veins. It consists of connective tissue, and also contains nerves. In veins, this is the thickest layer. In general, the tunica externa protects and strengthens vessels, and attaches them to surrounding structures.

Capillary Walls

The walls of capillaries consist of little more than a single layer of epithelial cells. Being just one cell thick, the walls are well-suited for the exchange of substances between the blood inside them and the cells of surrounding tissues. Substances including water, oxygen, glucose, and other nutrients, as well as waste products (such as carbon dioxide), can pass quickly and easily through the extremely thin walls of capillaries.  See figure 14.4.9 for a comparison of the structure of arteries, veins and capillaries.

The blood in arteries is normally under pressure because of the beating of the heart. The pressure is highest when the heart contracts and pumps out blood, and lowest when the heart relaxes and refills with blood. (You can feel this variation in pressure in your wrist or neck when you count your pulse.) Blood pressure is a measure of the force that blood exerts on the walls of arteries. It is generally measured in millimetres of mercury (mm Hg), and expressed as a double number — a higher number for systolic pressure when the ventricles contract, and a lower number for diastolic pressure when the ventricles relax. Normal blood pressure is generally defined as less than 120 mm Hg (systolic)/80 mm Hg (diastolic) when measured in the arm at the level of the heart. It decreases as blood flows farther away from the heart and into smaller arteries.

As arteries grow smaller, there is increasing resistance to blood flow through them, because of the blood's friction against the arterial walls. This resistance restricts blood flow so that less blood reaches smaller, downstream vessels, thus reducing blood pressure before the blood flows into the tiniest vessels, the capillaries. Without this reduction in blood pressure, capillaries would not be able to withstand the pressure of the blood without bursting. By the time blood flows through the veins, it is under very little pressure. The pressure of blood against the walls of veins is always about the same — normally no more than 10 mm Hg.

Smooth muscles in the walls of arteries can contract or relax to cause vasoconstriction (narrowing of the lumen of blood vessels) or vasodilation (widening of the lumen of blood vessels). This allows the arteries — especially the arterioles — to contract or relax as needed to help regulate blood pressure. In this regard, the arterioles act like an adjustable nozzle on a garden hose. When they narrow, the increased friction with the arterial walls causes less blood to flow downstream from the narrowing, resulting in a drop in blood pressure. These actions are controlled by the autonomic nervous system in response to pressure-sensitive sensory receptors in the walls of larger arteries.

Arteries can also dilate or constrict to help regulate body temperature, by allowing more or less blood to flow from the warm body core to the body’s surface. In addition, vasoconstriction and vasodilation play roles in the fight-or-flight response, under control of the sympathetic nervous system. Vasodilation allows more blood to flow to skeletal muscles, and vasoconstriction reduces blood flow to digestive organs.

Varicose veins are veins that have become enlarged and twisted, because their valves have become ineffective (see Figure 14.4.11). As a result, blood pools in the veins and stretches them out. Varicose veins occur most frequently in the superficial veins of the legs, but they may also occur in other parts of the body. They are most common in older adults, females, and people who have a family history of the condition. Obesity and pregnancy also increase the risk of developing varicose veins. A job that requires standing for long periods of time, chronic constipation, and long-term alcohol consumption are additional risk factors.

Varicose veins usually are not serious. For many people, they are only a cosmetic issue. In severe cases, however, varicose veins may cause pain and other problems. The affected leg(s) may feel heavy and achy, especially after long periods of standing. Ankles may become swollen by the end of the day. Minor injuries may bleed more than normal. The skin over the varicosity may become red, dry, and itchy. In very severe cases, skin ulcers may develop.

If you are concerned about varicose veins, call them to the attention of your doctor, who can determine the best course of action for your case. There are many potential treatments for varicose veins. Some of the treatments have potential adverse side effects, and with many of the treatments, varicose veins may return. The best treatment for a given patient depends in part on the severity of the condition.

• If varicose veins are not serious, conservative treatment options may be recommended. These include avoiding standing or sitting for long periods, frequently elevating the legs, and wearing graduated compression stockings.
• For more serious cases, less conservative, but non-surgical options may be advised. These include sclerotherapy, in which medicine is injected into the veins to make them shrink. Another non-surgical approach is endovenous thermal ablation. In this type of treatment, laser light, radio-frequency energy, or steam is used to heat the walls of the veins, causing them to shrink and collapse.
• For the most serious cases, surgery may be the best option. The most invasive surgery is vein stripping, in which all or part of the main trunk of a vein is tied off and removed from the leg while the patient is under general anesthesia. In a less invasive surgery, called ambulatory phlebectomy, short segments of a vein are removed through tiny incisions under local anesthesia.

Attributions

Figure 14.4.1

bodybuilding_PNG24 from pngimg.com is used under a CC BY-NC 4.0 (https://creativecommons.org/licenses/by-nc/4.0/) license.

Figure 14.4.2

Arterial_System_en.svg by Mariana Ruiz Villarreal [LadyofHats] on Wikimedia Commons is in the public domain (https://en.wikipedia.org/wiki/Public_domain).

Figure 14.4.3

Skeletal_Muscle_Vein_Pump by OpenStax College on Wikimedia Commons is used under a CC BY 3.0 (https://creativecommons.org/licenses/by/3.0) license.

Figure 14.4.4

Venous_valve_00013 by Nevit Dilmen on Wikimedia Commons is used under a CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0) license.

Figure 14.4.5

Superior and Inferior Vena Cava by ArtFavor (acquired from OCAL) from Freestockphotos.biz, is used under a CC0 1.0 Universal public domain dedication license (https://creativecommons.org/publicdomain/zero/1.0/). Work adapted by Christine Miller.

Figure 14.4.6

Venous_system_en.svg by Mariana Ruiz Villarreal [LadyofHats] on Wikimedia Commons is in the public domain (https://en.wikipedia.org/wiki/Public_domain).

Figure 14.4.7

1024px-2105_Capillary_Bed by OpenStax College on Wikimedia Commons is used under a CC BY 3.0 (https://creativecommons.org/licenses/by/3.0) license.

Figure 14.4.8

Artery by Lord of Konrad on Wikimedia Commons is used under a CC0 1.0 Universal public domain dedication license (https://creativecommons.org/publicdomain/zero/1.0/).

Figure 14.4.9

Blausen_0055_ArteryWallStructure by BruceBlaus on Wikimedia Commons is used under a CC BY 3.0 (https://creativecommons.org/licenses/by/3.0) license. 

Figure 14.4.10

Artery Vein Capillary Comparison by Christinelmiller on Wikimedia Commons is used under a CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0) license.

Figure 14.4.11

Varicose-veins by Jackerhack at English Wikipedia on Wikimedia Commons is used under a CC BY-SA 2.5 (https://creativecommons.org/licenses/by-sa/2.5) license.

Figure 14.4.12

Varicose_veins-en.svg by Jmarchn on Wikimedia Commons is used under a CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0) license. [Work modified from Varicose veins.jpg on Wikimedia Commons from National Heart Lung and Blood Institute (NIH)]

References

Betts, J. G., Young, K.A., Wise, J.A., Johnson, E., Poe, B., Kruse, D.H., Korol, O., Johnson, J.E., Womble, M., DeSaix, P. (2013, June 19). Figure 20.6 Capillary bed [digital image].  In Anatomy and Physiology (Section 20.1). OpenStax. https://openstax.org/books/anatomy-and-physiology/pages/20-1-structure-and-function-of-blood-vessels

Betts, J. G., Young, K.A., Wise, J.A., Johnson, E., Poe, B., Kruse, D.H., Korol, O., Johnson, J.E., Womble, M., DeSaix, P. (2013, June 19). Figure 20.15 Skeletal muscle pump [digital image].  In Anatomy and Physiology (Section 20.2). OpenStax. https://openstax.org/books/anatomy-and-physiology/pages/20-2-blood-flow-blood-pressure-and-resistance

Blausen.com Staff. (2014). Medical gallery of Blausen Medical 2014. WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.010. ISSN 2002-4436.

Cleveland Clinic. (2019, December 30). What are varicose veins? YouTube. https://www.youtube.com/watch?v=9Wf8bLXVwFI&feature=youtu.be

MooMooMath and Science. (2018, April 5). Arteries vs veins ( Circulatory System ). YouTube. https://www.youtube.com/watch?v=hnjMdXSyA5o&feature=youtu.be

TED-Ed. (2015, July 23). How blood pressure works - Wilfred Manzano. YouTube. https://www.youtube.com/watch?v=Ab9OZsDECZw&feature=youtu.be

 

Created by CK-12 Foundation/Adapted by Christine Miller

From Bram Stoker’s famous novel about Count Dracula, to films such as Van Helsing and the Twilight Saga, fantasies featuring vampires (like the one in Figure 14.5.1) have been popular for decades. Vampires, in fact, are found in centuries-old myths from many cultures. In such myths, vampires are generally described as creatures that drink blood — preferably of the human variety — for sustenance. Dracula, for example, is based on Eastern European folklore about a human who attains immortality (and eternal damnation) by drinking the blood of others.

The average adult body contains between 4.7 and 5.7 litres of blood. More than half of that amount is fluid. Most of the rest of that amount consists of blood cells. The relative amounts of the various components in blood are illustrated in Figure 14.5.2. The components are also described in detail below.

Blood performs many important functions in the body. Major functions of blood include:

• Supplying tissues with oxygen, which is needed by all cells for aerobic cellular respiration.
• Supplying cells with nutrients, including glucose, amino acids, and fatty acids.
• Removing metabolic wastes from cells, including carbon dioxide, urea, and lactic acid.
• Helping to defend the body from pathogens and other foreign substances.
• Forming clots to seal broken blood vessels and stop bleeding.
• Transporting hormones and other messenger molecules.
• Regulating the pH of the body, which must be kept within a narrow range (7.35 to 7.45).
• Helping regulate body temperature (through vasoconstriction and vasodilation).

Plasma is the liquid component of human blood. It makes up about 55% of blood by volume. It is about 92% water, and contains many dissolved substances. Most of these substances are proteins, but plasma also contains trace amounts of glucose, mineral ions, hormones, carbon dioxide, and other substances. In addition, plasma contains blood cells. When the cells are removed from plasma, as in Figure 14.5.2 above, the remaining liquid is clear but yellow in colour.

The cells in blood include erythrocytes, leukocytes, and thrombocytes. These different types of blood cells are shown in the photomicrograph (Figure 14.5.3) and described in the sections that follow.

Erythrocytes

The most numerous cells in blood are red blood cells, also called erythrocytes. One microlitre of blood contains between 4.2 and 6.1 million red blood cells, and red blood cells make up about 25% of all the cells in the human body. The cytoplasm of a mature erythrocyte is almost completely filled with hemoglobin, the iron-containing protein that binds with oxygen and gives the cell its red colour. In order to provide maximum space for hemoglobin, mature erythrocytes lack a cell nucleus and most organelles. They are little more than sacks of hemoglobin.

Erythrocytes also carry proteins called antigens that determine blood type. Blood type is a genetic characteristic. The best known human blood type systems are the ABO and Rhesus systems.

• In the ABO system, there are two common antigens, called antigen A and antigen B. There are four ABO blood types, A (only A antigen), B (only B antigen), AB (both A and B antigens), and O (neither A nor B antigen). The ABO antigens are illustrated in Figure 14.5.4.
• In the Rhesus system, there is just one common antigen. A person may either have the antigen (Rh+) or lack the antigen (Rh-).

Blood type is important for medical reasons. A person who needs a blood transfusion must receive blood of a compatible type. Blood that is compatible lacks antigens that the patient's own blood also lacks. For example, for a person with type A blood (no B antigen), compatible types include any type of blood that lacks the B antigen. This would include type A blood or type O blood, but not type AB or type B blood. If incompatible blood is transfused, it may cause a potentially life-threatening reaction in the patient’s blood.

Leukocytes

Leukocytes (also called white blood cells) are cells in blood that defend the body against invading microorganisms and other threats. There are far fewer leukocytes than red blood cells in blood. There are normally only about 1,000 to 11,000 white blood cells per microlitre of blood. Unlike erythrocytes, leukocytes have a nucleus. White blood cells are part of the body’s immune systemno post. They destroy and remove old or abnormal cells and cellular debris, as well as attack pathogens and foreign substances. There are five main types of white blood cells, which are described in Table 14.5.1: neutrophils, eosinophils, basophils, lymphocytes, and monocytes. The five types differ in their specific immune functions.

Table 14.5.1: Major Types of White Blood Cells

Type of Leukocyte	Per cent of All Leukocytes	Main Function(s)
Neutrophil	62%	Phagocytize (engulf and destroy) bacteria and fungi in blood.
Eosinophil	2%	Attack and kill large parasites; carry out allergic responses.
Basophil	less than 1%	Release histamines in inflammatory responses.
Lymphocyte	30%	Attack and destroy virus-infected and tumor cells; create lasting immunity to specific pathogens.
Monocyte	5%	Phagocytize pathogens and debris in tissues.

Thrombocytes

Thrombocytes, also called platelets, are actually cell fragments. Like erythrocytes, they lack a nucleus and are more numerous than white blood cells. There are about 150 thousand to 400 thousand thrombocytes per microlitre of blood.

The main function of thrombocytes is blood clotting, or coagulation. This is the process by which blood changes from a liquid to a gel, forming a plug in a damaged blood vessel. If blood clotting is successful, it results in hemostasis, which is the cessation of blood loss from the damaged vessel. A blood clot consists of both platelets and proteins, especially the protein fibrin. You can see a scanning electron microscope photomicrograph of a blood clot in Figure 14.5.5.

 

Coagulation begins almost instantly after an injury occurs to the endothelium of a blood vessel. Thrombocytes become activated and change their shape from spherical to star-shaped, as shown in Figure 14.5.6. This helps them aggregate with one another (stick together) at the site of injury to start forming a plug in the vessel wall. Activated thrombocytes also release substances into the blood that activate additional thrombocytes and start a sequence of reactions leading to fibrin formation. Strands of fibrin crisscross the platelet plug and strengthen it, much as rebar strengthens concrete.

Formation and Degradation of Blood Cells

Blood is considered a connective tissue, because blood cells form inside bones. All three types of blood cells are made in red marrow within the medullary cavity of bones in a process called hematopoiesis. Formation of blood cells occurs by the proliferation of stem cells in the marrow. These stem cells are self-renewing — when they divide, some of the daughter cells remain stem cells, so the pool of stem cells is not used up. Other daughter cells follow various pathways to differentiate into the variety of blood cell types. Once the cells have differentiated, they cannot divide to form copies of themselves.

Eventually, blood cells die and must be replaced through the formation of new blood cells from proliferating stem cells. After blood cells die, the dead cells are phagocytized (engulfed and destroyed) by white blood cells, and removed from the circulation. This process most often takes place in the spleen and liver.

Many human disorders primarily affect the blood. They include cancers, genetic disorders, poisoning by toxins, infections, and nutritional deficiencies.

• Leukemia is a group of cancers of the blood-forming tissues in the bone marrow. It is the most common type of cancer in children, although most cases occur in adults. Leukemia is generally characterized by large numbers of abnormal leukocytes. Symptoms may include excessive bleeding and bruising, fatigue, fever, and an increased risk of infections. Leukemia is thought to be caused by a combination of genetic and environmental factors.
• Hemophilia refers to any of several genetic disorders that cause dysfunction in the blood clotting process. People with hemophilia are prone to potentially uncontrollable bleeding, even with otherwise inconsequential injuries. They also commonly suffer bleeding into the spaces between joints, which can cause crippling.
• Carbon monoxide poisoning occurs when inhaled carbon monoxide (in fumes from a faulty home furnace or car exhaust, for example) binds irreversibly to the hemoglobin in erythrocytes. As a result, oxygen cannot bind to the red blood cells for transport throughout the body, and this can quickly lead to suffocation. Carbon monoxide is extremely dangerous, because it is colourless and odorless, so it cannot be detected in the air by human senses.
• HIV is a virus that infects certain types of leukocytes and interferes with the body’s ability to defend itself from pathogens and other causes of illness. HIV infection may eventually lead to AIDS (acquired immunodeficiency syndrome). AIDS is characterized by rare infections and cancers that people with a healthy immune systemno post almost never acquire.
• Anemia is a disorder in which the blood has an inadequate volume of erythrocytes, reducing the amount of oxygen that the blood can carry, and potentially causing weakness and fatigue. These and other signs and symptoms of anemia are shown in Figure 14.5.8. Anemia has many possible causes, including excessive bleeding, inherited disorders (such as sickle cell hemoglobin), or nutritional deficiencies (iron, folate, or B12). Severe anemia may require transfusions of donated blood.

Donating blood saves lives. In fact, with each blood donation, as many as three lives may be saved. According to Government Canada, up to 52% of Canadians have reported that they or a family member have needed blood or blood products at some point in their lifetime. Many donors agree that the feeling that comes from knowing you have saved lives is well worth the short amount of time it takes to make a blood donation. Nonetheless, only a minority of potential donors actually donate blood. There are many myths about blood donation that may help explain the small percentage of donors. Knowing the facts may reaffirm your decision to donate if you are already a donor — and if you aren’t a donor already, getting the facts may help you decide to become one.

Myth	Reality
"Your blood might become contaminated with an infection during the donation."	There is no risk of contamination because only single-use, disposable catheters, tubing, and other equipment are used to collect blood for a donation.
"You are too old (or too young) to donate blood."	There is no upper age limit on donating blood, as long as you are healthy. The minimum age is 16 years.
"You can’t donate blood if you have high blood pressure."	As long as your blood pressure is below 180/100 at the time of donation, you can give blood. Even if you take blood pressure medication to keep your blood pressure below this level, you can donate.
"You can’t give blood if you have high cholesterol."	Having high cholesterol does not affect your ability to donate blood. Taking cholesterol-lowering medication also does not disqualify you.
"You can’t donate blood if you have had a flu shot."	Having a flu shot has no effect on your ability to donate blood. You can even donate on the same day that you receive a flu shot.
"You can’t donate blood if you take medication."	As long as you are healthy, in most cases, taking medication does not preclude you from donating blood.
"Your blood isn’t needed if it’s a common blood type."	All types of blood are in constant demand.

Attributions

Figure 14.5.1

vampire_PNG32 from pngimg.com is used under a CC BY-NC 4.0 (https://creativecommons.org/licenses/by-nc/4.0/) license.

Figure 14.5.2

Blood-centrifugation-scheme by KnuteKnudsen at English Wikipedia on Wikimedia Commons is used under a CC BY 3.0 (https://creativecommons.org/licenses/by/3.0) license.

Figure 14.5.3

SEM_blood_cells by Bruce Wetzel and Harry Schaefer (Photographers)/ NCI AV-8202-3656 on Wikimedia Commons is in the public domain (https://en.wikipedia.org/wiki/en:Public_domain).

Figure 14.5.4

ABO_blood_type.svg by InvictaHOG on Wikimedia Commons is in the public domain (https://en.wikipedia.org/wiki/en:Public_domain).

Figure 14.5.5

Blood_clot_in_scanning_electron_microscopy by Janice Carr from CDC/ Public Health Image LIbrary (PHIL) ID #7308 on Wikimedia Commons is in the public domain (https://en.wikipedia.org/wiki/en:Public_domain).

Figure 14.5.6

Blausen_0740_Platelets by BruceBlaus on Wikimedia Commons is used under a CC BY 3.0 (https://creativecommons.org/licenses/by/3.0) license. 

Figure 14.5.7

Platelet_Party_900x by Awkward Yeti (used with permission of the author) © All Rights Reserved

Figure 14.5.8

Symptoms_of_anemia.svg by Mikael Häggström on Wikimedia Commons is in the public domain (https://en.wikipedia.org/wiki/en:public_domain).

References

Blausen.com Staff. (2014). Medical gallery of Blausen Medical 2014. WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.010. ISSN 2002-4436.

Blood, organ and tissue donation. (2020, April 28). Government of Canada. https://www.canada.ca/en/public-health/services/healthy-living/blood-organ-tissue-donation.html#a3

Canadian Blood Services. (n.d.). There is an immediate need for blood as demand is rising. https://www.blood.ca

Science Plus. (2016, March 2). Can synthetic blood help the world's blood shortage? https://www.youtube.com/watch?v=hgp8LtwFSBA&feature=youtu.be

TED. (2014, November 20). Joe Landolina: This gel can make you stop bleeding instantly. YouTube. https://www.youtube.com/watch?v=e-5wqwp64MM&feature=youtu.be

TED-Ed. (2020, January 27). How bones make blood - Melody Smith. YouTube. https://www.youtube.com/watch?v=1Qfmkd6C8u8&feature=youtu.be

Created by CK-12 Foundation/Adapted by Christine Miller

If you’re an ice cream lover, then just the sight of this yummy ice cream cone may make your mouth water. The “water” in your mouth is actually saliva, a fluid released by glands that are part of the digestive system. Saliva contains digestive enzymes, among other substances important for digestion. When your mouth waters at the sight of a tasty treat, it’s a sign that your digestive system is preparing to digest food.

The digestive system consists of organs that break down food, absorb its nutrients, and expel any remaining waste. Organs of the digestive system are shown in Figure 15.2.2. Most of these organs make up the gastrointestinal (GI) tract, through which food actually passes. The rest of the organs of the digestive system are called accessory organs. These organs secrete enzymes and other substances into the GI tract, but food does not actually pass through them.

The digestive system has three main functions relating to food: digestion of food, absorption of nutrients from food, and elimination of solid food waste. Digestion is the process of breaking down food into components the body can absorb. It consists of two types of processes: mechanical digestion and chemical digestion. Mechanical digestion is the physical breakdown of chunks of food into smaller pieces, and it takes place mainly in the mouth and stomach. Chemical digestion is the chemical breakdown of large, complex food molecules into smaller, simpler nutrient molecules that can be absorbed by body fluids (blood or lymph). This type of digestion begins in the mouth and continues in the stomach, but occurs mainly in the small intestine.

After food is digested, the resulting nutrients are absorbed. Absorption is the process in which substances pass into the bloodstream or lymph system to circulate throughout the body. Absorption of nutrients occurs mainly in the small intestine. Any remaining matter from food that is not digested and absorbed passes out of the body through the anus in the process of elimination.

The gastrointestinal (GI) tract is basically a long, continuous tube that connects the mouth with the anus. If it were fully extended, it would be about nine metres long in adults. It includes the mouth, pharynx, esophagus, stomach, and small and large intestines. Food enters the mouth, and then passes through the other organs of the GI tract, where it is digested and/or absorbed. Finally, any remaining food waste leaves the body through the anus at the end of the large intestine. It takes up to 50 hours for food or food waste to make the complete trip through the GI tract.

Tissues of the GI Tract

The walls of the organs of the GI tract consist of four different tissue layers, which are illustrated in Figure 15.2.3: mucosa, submucosa, muscularis externa, and serosa.

1. The mucosa is the innermost layer surrounding the lumen (open space within the organs of the GI tract). This layer consists mainly of epithelium with the capacity to secrete and absorb substances. The epithelium can secret digestive enzymes and mucus, and it can absorb nutrients and water.
2. The submucosa layer consists of connective tissue that contains blood and lymph vessels, as well as nerves. The vessels are needed to absorb and carry away nutrients after food is digested, and nerves help control the muscles of the GI tract organs.
3. The muscularis externa layer contains two types of smooth muscle: longitudinal muscle and circular muscle. Longitudinal muscle runs the length of the GI tract organs, and circular muscle encircles the organs. Both types of muscles contract to keep food moving through the tract by the process of peristalsis, which is described below.
4. The serosa layer is the outermost layer of the walls of GI tract organs. This is a thin layer that consists of connective tissue and separates the organs from surrounding cavities and tissues.

Peristalisis in the GI Tract

The muscles in the walls of GI tract organs enable peristalsis, which is illustrated in Figure 15.2.5. Peristalsis is a continuous sequence of involuntary muscle contraction and relaxation that moves rapidly along an organ like a wave, similar to the way a wave moves through a spring toy. Peristalsis in organs of the GI tract propels food through the tract.

Watch the video "What is peristalsis?" by Mister Science to see peristalsis in action:

https://youtu.be/kVjeNZA5pi4

What is peristalsis?, Mister Science, 2018.

Immune Function of the GI Tract

The GI tract plays an important role in protecting the body from pathogens. The surface area of the GI tract is estimated to be about 32 square metres (105 square feet), or about half the area of a badminton court. This is more than three times the area of the exposed skin of the body, and it provides a lot of area for pathogens to invade the tissues of the body. The innermost mucosal layer of the walls of the GI tract provides a barrier to pathogens so they are less likely to enter the blood or lymph circulations. The mucus produced by the mucosal layer, for example, contains antibodies that mark many pathogenic microorganisms for destruction. Enzymes in some of the secretions of the GI tract also destroy pathogens. In addition, stomach acids have a very low pH that is fatal for many microorganisms that enter the stomach.

Divisions of the GI Tract

The GI tract is often divided into an upper GI tract and a lower GI tract. For medical purposes, the upper GI tract is typically considered to include all the organs from the mouth through the first part of the small intestine, called the duodenum. For our instructional purposes, it makes more sense to include the mouth through the stomach in the upper GI tract, and all of the small intestine — as well as the large intestine — in the lower GI tract.

Upper GI Tract

The mouth is the first digestive organ that food enters. The sight, smell, or taste of food stimulates the release of digestive enzymes and other secretions by salivary glands inside the mouth. The major salivary gland enzyme is amylase. It begins the chemical digestion of carbohydrates by breaking down starches into sugar. The mouth also begins the mechanical digestion of food. When you chew, your teeth break, crush, and grind food into increasingly smaller pieces. Your tongue helps mix the food with saliva and also helps you swallow.

A lump of swallowed food is called a bolus. The bolus passes from the mouth into the pharynx, and from the pharynx into the esophagus. The esophagus is a long, narrow tube that carries food from the pharynx to the stomach. It has no other digestive functions. Peristalsis starts at the top of the esophagus when food is swallowed and continues down the esophagus in a single wave, pushing the bolus of food ahead of it.

From the esophagus, food passes into the stomach, where both mechanical and chemical digestion continue. The muscular walls of the stomach churn and mix the food, thus completing mechanical digestion, as well as mixing the food with digestive fluids secreted by the stomach. One of these fluids is hydrochloric acid (HCl). In addition to killing pathogens in food, it gives the stomach the low pH needed by digestive enzymes that work in the stomach. One of these enzymes is pepsin, which chemically digests proteins. The stomach stores the partially digested food until the small intestine is ready to receive it. Food that enters the small intestine from the stomach is in the form of a thick slurry (semi-liquid) called chyme.

Lower GI Tract

The small intestine is a narrow, but very long tubular organ. It may be almost seven metres long in adults. It is the site of most chemical digestion and virtually all absorption of nutrients. Many digestive enzymes are active in the small intestine, some of which are produced by the small intestine itself, and some of which are produced by the pancreas, an accessory organ of the digestive system. Much of the inner lining of the small intestine is covered by tiny finger-like projections called villi, each of which is covered by even tinier projections called microvilli. These projections, shown in the drawing below (Figure 15.2.6), greatly increase the surface area through which nutrients can be absorbed from the small intestine.

From the small intestine, any remaining nutrients and food waste pass into the large intestine. The large intestine is another tubular organ, but it is wider and shorter than the small intestine. It connects the small intestine and the anus. Waste that enters the large intestine is in a liquid state. As it passes through the large intestine, excess water is absorbed from it. The remaining solid waste — called feces — is eventually eliminated from the body through the anus.

Accessory organs of the digestive system are not part of the GI tract, so they are not sites where digestion or absorption take place. Instead, these organs secrete or store substances needed for the chemical digestion of food. The accessory organs include the liver, gallbladder, and pancreas. They are shown in Figure 15.2.7 and described in the text that follows.

• The liver is an organ with multitude of functions. Its main digestive function is producing and secreting a fluid called bile, which reaches the small intestine through a duct. Bile breaks down large globules of lipids into smaller ones that are easier for enzymes to chemically digest. Bile is also needed to reduce the acidity of food entering the small intestine from the highly acidic stomach, because enzymes in the small intestine require a less acidic environment in order to work.
• The gallbladder is a small sac below the liver that stores some of the bile from the liver. The gallbladder also concentrates the bile by removing some of the water from it. It then secretes the concentrated bile into the small intestine as needed for fat digestion following a meal.
• The pancreas secretes many digestive enzymes, and releases them into the small intestine for the chemical digestion of carbohydrates, proteins, and lipids. The pancreas also helps lessen the acidity of the small intestine by secreting bicarbonate, a basic substance that neutralizes acid.

 

Attributions

Figure 15.2.1

Ice Cream [photo] by Mark Cruz on Unsplash is used under the Unsplash License (https://unsplash.com/license).

Figure 15.2.2

Blausen_0316_DigestiveSystem by BruceBlaus on Wikimedia Commons is used under a CC BY 3.0 (https://creativecommons.org/licenses/by/3.0) license.

Figure 15.2.3

Intestinal_layers by Boumphreyfr on Wikimedia Commons is used under a CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0) license.

Figure 15.2.4

512px-Normal_gastric_mucosa_intermed_mag by Nephron on Wikimedia Commons is used under a CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0) license.

Figure 15.2.5

Peristalsis pushes food through the GI tract by CK-12 Foundation is used under a CC BY NC 3.0 (https://creativecommons.org/licenses/by-nc/3.0/) license.

Figure 15.2.6

Villi_&_microvilli_of_small_intestine.svg by BallenaBlanca on Wikimedia Commons is used under a CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0) license.

Figure 15.2.7

Blausen_0428_Gallbladder-Liver-Pancreas_Location by BruceBlaus  on Wikimedia Commons is used under a CC BY 3.0 (https://creativecommons.org/licenses/by/3.0) license.

References

Blausen.com Staff. (2014). Medical gallery of Blausen Medical 2014. WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.010. ISSN 2002-4436.

Brainard, J/ CK-12 Foundation. (2016). Figure 4 Peristalsis pushes food through the GI tract. [digital image]. In CK-12 College Human Biology (Section 17.2) [online Flexbook]. CK12.org. https://www.ck12.org/book/ck-12-college-human-biology/section/17.2/

Mister Science. (2018). What is peristalsis? YouTube. https://www.youtube.com/channel/UCxTlkZfjArUobBAeVwzJjYg/videos

TED-Ed. (2017, November 13). How does your body know you're full? - Hilary Coller. YouTube. https://www.youtube.com/watch?v=YVfyYrEmzgM&feature=youtu.be

TED-Ed. (2017, December 14). How your digestive system works - Emma Bryce. YouTube. https://www.youtube.com/watch?v=Og5xAdC8EUI&feature=youtu.be

 

 

Created by CK-12 Foundation/Adapted by Christine Miller

Feel the Burn

The person in Figure 10.3.1 is no doubt feeling the burn — sunburn, that is. Sunburn occurs when the outer layer of the skin is damaged by UV light from the sun or tanning lamps. Some people deliberately allow UV light to burn their skin, because after the redness subsides, they are left with a tan. A tan may look healthy, but it is actually a sign of skin damage. People who experience one or more serious sunburns are significantly more likely to develop skin cancer. Natural pigment molecules in the skin help protect it from UV light damage. These pigment molecules are found in the layer of the skin called the epidermis.

The epidermis is the outer of the two main layers of the skin. The inner layer is the dermis. It averages about 0.10 mm thick, and is much thinner than the dermis. The epidermis is thinnest on the eyelids (0.05 mm) and thickest on the palms of the hands and soles of the feet (1.50 mm). The epidermis covers almost the entire body surface. It is continuous with — but structurally distinct from — the mucous membranes that line the mouth, anus, urethra, and vagina.

There are no blood vessels and very few nerve cells in the epidermis. Without blood to bring epidermal cells oxygen and nutrients, the cells must absorb oxygen directly from the air and obtain nutrients via diffusion of fluids from the dermis below. However, as thin as it is, the epidermis still has a complex structure. It has a variety of cell types and multiple layers.

Cells of the Epidermis

There are several different types of cells in the epidermis. All of the cells are necessary for the important functions of the epidermis.

• The epidermis consists mainly of stacks of keratin-producing epithelial cells called keratinocytes. These cells make up at least 90 per cent of the epidermis. Near the top of the epidermis, these cells are also called squamous cells.
• Another eight per cent of epidermal cells are melanocytes. These cells produce the pigment melanin that protects the dermis from UV light.
• About one per cent of epidermal cells are Langerhans cells. These are immune system cells that detect and fight pathogens entering the skin.
• Less than one per cent of epidermal cells are Merkel cells, which respond to light touch and connect to nerve endings in the dermis.

Layers of the Epidermis

The epidermis in most parts of the body consists of four distinct layers. A fifth layer occurs in the palms of the hands and soles of the feet, where the epidermis is thicker than in the rest of the body. The layers of the epidermis are shown in Figure 10.3.2, and described in the following text.

Stratum Basale

The stratum basale is the innermost (or deepest) layer of the epidermis. It is separated from the dermis by a membrane called the basement membrane. The stratum basale contains stem cells — called basal cells — which divide to form all the keratinocytes of the epidermis. When keratinocytes first form, they are cube-shaped and contain almost no keratin. As more keratinocytes are produced, previously formed cells are pushed up through the stratum basale. Melanocytes and Merkel cells are also found in the stratum basale. The Merkel cells are especially numerous in touch-sensitive areas, such as the fingertips and lips.

Stratum Spinosum

Just above the stratum basale is the stratum spinosum. This is the thickest of the four epidermal layers. The keratinocytes in this layer have begun to accumulate keratin, and they have become tougher and flatter. Spiny cellular projections form between the keratinocytes and hold them together. In addition to keratinocytes, the stratum spinosum contains the immunologically active Langerhans cells.

Stratum Granulosum

The next layer above the stratum spinosum is the stratum granulosum. In this layer, keratinocytes have become nearly filled with keratin, giving their cytoplasm a granular appearance. Lipids are released by keratinocytes in this layer to form a lipid barrier in the epidermis. Cells in this layer have also started to die, because they are becoming too far removed from blood vessels in the dermis to receive nutrients. Each dying cell digests its own nucleus and organelles, leaving behind only a tough, keratin-filled shell.

Stratum Lucidum

Only on the palms of the hands and soles of the feet, the next layer above the stratum granulosum is the stratum lucidum. This is a layer consisting of stacks of translucent, dead keratinocytes that provide extra protection to the underlying layers.

Stratum Corneum

The uppermost layer of the epidermis everywhere on the body is the stratum corneum. This layer is made of flat, hard, tightly packed dead keratinocytes that form a waterproof keratin barrier to protect the underlying layers of the epidermis. Dead cells from this layer are constantly shed from the surface of the body. The shed cells are continually replaced by cells moving up from lower layers of the epidermis. It takes a period of about 48 days for newly formed keratinocytes in the stratum basale to make their way to the top of the stratum corneum to replace shed cells.

The epidermis has several crucial functions in the body. These functions include protection, water retention, and vitamin D synthesis.

Protective Functions

The epidermis provides protection to underlying tissues from physical damage, pathogens, and UV light.

Protection from Physical Damage

Most of the physical protection of the epidermis is provided by its tough outer layer, the stratum corneum. Because of this layer, minor scrapes and scratches generally do not cause significant damage to the skin or underlying tissues. Sharp objects and rough surfaces have difficulty penetrating or removing the tough, dead, keratin-filled cells of the stratum corneum. If cells in this layer are pierced or scraped off, they are quickly replaced by new cells moving up to the surface from lower skin layers.

Protection from Pathogens

When pathogens such as viruses and bacteria try to enter the body, it is virtually impossible for them to enter through intact epidermal layers. Generally, pathogens can enter the skin only if the epidermis has been breached, for example by a cut, puncture, or scrape (like the one pictured in Figure 10.3.3). That’s why it is important to clean and cover even a minor wound in the epidermis. This helps ensure that pathogens do not use the wound to enter the body. Protection from pathogens is also provided by conditions at or near the skin surface. These include relatively high acidity (pH of about 5.0), low amounts of water, the presence of antimicrobial substances produced by epidermal cells, and competition with non-pathogenic microorganisms that normally live on the epidermis.

 

Protection from UV Light

UV light that penetrates the epidermis can damage epidermal cells. In particular, it can cause mutations in DNA that lead to the development of skin cancer, in which epidermal cells grow out of control. UV light can also destroy vitamin B9 (in forms such as folate or folic acid), which is needed for good health and successful reproduction. In a person with light skin, just an hour of exposure to intense sunlight can reduce the body’s vitamin B9 level by 50 per cent.

Melanocytes in the stratum basale of the epidermis contain small organelles called melanosomes, which produce, store, and transport the dark brown pigment melanin. As melanosomes become full of melanin, they move into thin extensions of the melanocytes. From there, the melanosomes are transferred to keratinocytes in the epidermis, where they absorb UV light that strikes the skin. This prevents the light from penetrating deeper into the skin, where it can cause damage. The more melanin there is in the skin, the more UV light can be absorbed.

Water Retention

Skin's ability to hold water and not lose it to the surrounding environment is due mainly to the stratum corneum. Lipids arranged in an organized way among the cells of the stratum corneum form a barrier to water loss from the epidermis. This is critical for maintaining healthy skin and preserving proper water balance in the body.

Although the skin is impermeable to water, it is not impermeable to all substances. Instead, the skin is selectively permeable, allowing certain fat-soluble substances to pass through the epidermis. The selective permeability of the epidermis is both a benefit and a risk.

• Selective permeability allows certain medications to enter the bloodstream through the capillaries in the dermis. This is the basis of medications that are delivered using topical ointments, or patches (see Figure 10.3.4) that are applied to the skin. These include steroid hormones, such as estrogen (for hormone replacement therapy), scopolamine (for motion sickness), nitroglycerin (for heart problems), and nicotine (for people trying to quit smoking).
• Selective permeability of the epidermis also allows certain harmful substances to enter the body through the skin. Examples include the heavy metal lead, as well as many pesticides.

Vitamin D Synthesis

Vitamin D is a nutrient that is needed in the human body for the absorption of calcium from food. Molecules of a lipid compound named 7-dehydrocholesterol are precursors of vitamin D. These molecules are present in the stratum basale and stratum spinosum layers of the epidermis. When UV light strikes the molecules, it changes them to vitamin D3. In the kidneys, vitamin D3 is converted to calcitriol, which is the form of vitamin D that is active in the body.

Melanin in the epidermis is the main substance that determines the colour of human skin. It explains most of the variation in skin colour in people around the world. Two other substances also contribute to skin colour, however, especially in light-skinned people: carotene and hemoglobin.

• The pigment carotene is present in the epidermis and gives skin a yellowish tint, especially in skin with low levels of melanin.
• Hemoglobin is a red pigment found in red blood cells. It is visible through skin as a pinkish tint, mainly in skin with low levels of melanin. The pink colour is most visible when capillaries in the underlying dermis dilate, allowing greater blood flow near the surface.

Hear what Bill Nye has to say about the subject of skin colour in the video here.

The surface of the human skin normally provides a home to countless numbers of bacteria. Just one square inch of skin normally has an average of about 50 million bacteria. These generally harmless bacteria represent roughly one thousand bacterial species (including the one in Figure 10.3.5) from 19 different bacterial phyla. Typical variations in the moistness and oiliness of the skin produce a variety of rich and diverse habitats for these microorganisms. For example, the skin in the armpits is warm and moist and often hairy, whereas the skin on the forearms is smooth and dry. These two areas of the human body are as diverse to microorganisms as rainforests and deserts are to larger organisms. The density of bacterial populations on the skin depends largely on the region of the skin and its ecological characteristics. For example, oily surfaces, such as the face, may contain over 500 million bacteria per square inch. Despite the huge number of individual microorganisms living on the skin, their total volume is only about the size of a pea.

In general, the normal microorganisms living on the skin keep one another in check, and thereby play an important role in keeping the skin healthy. If the balance of microorganisms is disturbed, however, there may be an overgrowth of certain species, and this may result in an infection. For example, when a patient is prescribed antibiotics, it may kill off normal bacteria and allow an overgrowth of single-celled yeast. Even if skin is disinfected, no amount of cleaning can remove all of the microorganisms it contains. Disinfected areas are also quickly recolonized by bacteria residing in deeper areas (such as hair follicles) and in adjacent areas of the skin.

Because of the negative health effects of excessive UV light exposure, it is important to know the facts about protecting the skin from UV light.

Myth	Reality
"Sunblock and sunscreen are just different names for the same type of product. They both work the same way and are equally effective."	Sunscreens and sunblocks are different types of products that protect the skin from UV light in different ways. They are not equally effective. Sunblocks are opaque, so they do not let light pass through. They prevent most of the rays of UV light from penetrating to the skin surface. Sunblocks are generally stronger and more effective than sunscreens. Sunblocks also do not need to be reapplied as often as sunscreens. Sunscreens, in contrast, are transparent once they are applied the skin. Although they can prevent most UV light from penetrating the skin when first applied, the active ingredients in sunscreens tend to break down when exposed to UV light. Sunscreens, therefore, must be reapplied often to remain effective.
"The skin needs to be protected from UV light only on sunny days. When the sky is cloudy, UV light cannot penetrate to the ground and harm the skin."	Even on cloudy days, a significant amount of UV radiation penetrates the atmosphere to strike Earth’s surface. Therefore, using sunscreens or sunblocks to protect exposed skin is important even when there are clouds in the sky.
"People who have dark skin, such as African Americans, do not need to worry about skin damage from UV light."	No matter what colour skin you have, your skin can be damaged by too much exposure to UV light. Therefore, even dark-skinned people should use sunscreens or sunblocks to protect exposed skin from UV light.
"Sunscreens with an SPF (sun protection factor) of 15 are adequate to fully protect the skin from UV light."	Most dermatologists recommend using sunscreens with an SPF of at least 35 for adequate protection from UV light. They also recommend applying sunscreens at least 20 minutes before sun exposure and reapplying sunscreens often, especially if you are sweating or spending time in the water.
"Using tanning beds is safer than tanning outside in natural sunlight."	The light in tanning beds is UV light, and it can do the same damage to the skin as the natural UV light in sunlight. This is evidenced by the fact that people who regularly use tanning beds have significantly higher rates of skin cancer than people who do not. It is also the reason that the use of tanning beds is prohibited in many places in people who are under the age of 18, just as youth are prohibited from using harmful substances, such as tobacco and alcohol.

 

Attributions

Figure 10.3.1

Sunburn by QuinnHK at English Wikipedia on Wikimedia Commons is released into the public domain (https://en.wikipedia.org/wiki/Public_domain).

Figure 10.3.2

Blausen_0353_Epidermis by BruceBlaus on Wikimedia Commons is used under a CC BY 3.0 (https://creativecommons.org/licenses/by/3.0) license.

Figure 10.3.3

Isaac's scraped knee close-up by Alpha on Flickr is used under a CC BY-NC-SA 2.0 (https://creativecommons.org/licenses/by-nc-sa/2.0/) license.

Figure 10.3.4

Nicoderm by RegBarc on Wikimedia Commons is used under a CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0/) license. (No machine-readable author provided for original.)

Figure 10.3.5

Staphylococcus aureus bacteria, MRSA by Microbe World on Flickr is used under a CC BY-NC-SA 2.0 (https://creativecommons.org/licenses/by-nc-sa/2.0/) license.

References

Blausen.com staff. (2014). Medical gallery of Blausen Medical 2014. WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.010. ISSN 2002-4436.

Jeff Bone 'n' Pookie. (2020, July 19). Bill Nye the science guy explains we have different skin color. Youtube. https://www.youtube.com/watch?v=zOkj5jgC4sM&feature=youtu.be

SciShow. (2014, July 15). Why do we blush? YouTube. https://www.youtube.com/watch?v=9AcQXnOscQ8

TED. (2015, July 17). Jonathan Eisen: Meet your microbes. YouTube. https://www.youtube.com/watch?v=27lMmdmy-b8

TED-Ed. (2016, February 16). The science of skin color - Angela Koine Flynn. YouTube. https://youtu.be/_r4c2NT4naQ

Created by CK-12 Foundation/Adapted by Christine Miller

Pictured in Figure 10.2.1, is Maud Stevens Wagner, a tattoo artist from 1907. Tattoos are not just a late 20th and early 21st century trend. They have been popular in many eras and cultures. Tattoos literally illustrate the biggest organ of the human body: the skin. The skin is very thin, but it covers a large area — about 2 m² in adults. The skin is the major organ in the integumentary system.

In addition to the skin, the integumentary system includes the hair and nails, which are organs that grow out of the skin. Because the organs of the integumentary system are mostly external to the body, you may think of them as little more than accessories, like clothing or jewelry, but they serve vital physiological functions. They provide a protective covering for the body, sense the environment, and help the body maintain homeostasis.

The skin is remarkable not only because it is the body’s largest organ: the average square inch of skin has 20 blood vessels, 650 sweat glands, and more than 1,000 nerve endings. Incredibly, it also has 60,000 pigment-producing cells. All of these structures are packed into a stack of cells that is just 2 mm thick. Although the skin is thin, it consists of two distinct layers: the epidermis and dermis, as shown in the diagram (Figure 10.2.2).

Outer Layer of Skin

The outer layer of skin is the epidermis. This layer is thinner than the inner layer (the dermis). The epidermis consists mainly of epithelial cells, called keratinocytes, which produce the tough, fibrous protein keratin. The innermost cells of the epidermis are stem cells that divide continuously to form new cells. The newly formed cells move up through the epidermis toward the skin surface, while producing more and more keratin. The cells become filled with keratin and die by the time they reach the surface, where they form a protective, waterproof layer. As the dead cells are shed from the surface of the skin, they are replaced by other cells that move up from below. The epidermis also contains melanocytes, the cells that produce the brown pigment melanin, which gives skin most of its colour. Although the epidermis contains some sensory receptor cells — called Merkel cells — it contains no nerves, blood vessels, or other structures.

Inner Layer of Skin

The dermis is the inner, thicker layer of skin. It consists mainly of tough connective tissue, and is attached to the epidermis by collagen fibres. The dermis contains many structures (as shown in Figure 10.2.2), including blood vessels, sweat glands, and hair follicles, which are structures where hairs originate. In addition, the dermis contains many sensory receptors, nerves, and oil glands.

Functions of the Skin

The skin has multiple roles in the body. Many of these roles are related to homeostasis. The skin’s main functions are preventing water loss from the body and serving as a barrier to the entry of microorganisms. Another function of the skin is synthesizing vitamin D, which occurs when the skin is exposed to ultraviolet (UV) light. Melanin in the epidermis blocks some of the UV light and protects the dermis from its damaging effects.

Another important function of the skin is helping to regulate body temperature. When the body is too warm, for example, the skin lowers body temperature by producing sweat, which cools the body when it evaporates. The skin also increases the amount of blood flowing near the body surface through vasodilation (widening of blood vessels), bringing heat from the body core to radiate out into the environment. The sweaty hair and flushed skin of the young man pictured in Figure 10.2.3 reflect these skin responses to overheating.

Hair is a fibre found only in mammals. It consists mainly of keratin-producing keratinocytes. Each hair grows out of a follicle in the dermis. By the time the hair reaches the surface, it consists mainly of dead cells filled with keratin. Hair serves several homeostatic functions. Head hair is important in preventing heat loss from the head and protecting its skin from UV radiation. Hairs in the nose trap dust particles and microorganisms in the air, and prevent them from reaching the lungs. Hair all over the body provides sensory input when objects brush against it, or when it sways in moving air. Eyelashes and eyebrows (see Figure 10.2.4) protect the eyes from water, dirt, and other irritants.

Fingernails and toenails consist of dead keratinocytes filled with keratin. The keratin makes them hard but flexible, which is important for the functions they serve. Nails prevent injury by forming protective plates over the ends of the fingers and toes. They also enhance sensation by acting as a counterforce to the sensitive fingertips when objects are handled. In addition, the fingernails can be used as tools.

The skin and other parts of the integumentary system work with other organ systems to maintain homeostasis.

• The skin works with the immune system to defend the body from pathogens by serving as a physical barrier to microorganisms.
• Vitamin D is needed by the digestive system to absorb calcium from food. By synthesizing vitamin D, the skin works with the digestive system to ensure that calcium can be absorbed.
• To control body temperature, the skin works with the cardiovascular system to either lose body heat, or to conserve it through vasodilation or vasoconstriction.
• To detect certain sensations from the outside world, the nervous system depends on nerve receptors in the skin.

National Geographic. (2008). Scarification | National Geographic. YouTube. https://www.youtube.com/watch?v=Lfhot7tQcWs&t=1s

TED-Ed. (2018, March 12). The science of skin - Emma Bryce. YouTube. https://www.youtube.com/watch?v=OxPlCkTKhzY&feature=youtu.be

TED-Ed. (2013, August 6). Why do we have to wear sunscreen? - Kevin P. Boyd. YouTube. https://www.youtube.com/watch?v=ZSJITdsTze0&feature=youtu.be

 

Created by CK-12 Foundation/Adapted by Christine Miller

 

Head Stand

Did you ever wonder what would happen if you tried to swallow food while standing on your head like this person in Figure 15.4.1? Many people think that food travels down the gullet from the mouth by the force of gravity. If that were the case, then food you swallowed would stay in your throat while you were standing on your head. In reality, your position doesn’t have much to do with your ability to swallow. Food will travel from your mouth to your stomach whether you are standing upright or upside down. That’s because the tube the food travels through — the esophagus — moves the food along via muscular contractions known as peristalsis. The esophagus is one of several organs that make up the upper gastrointestinal tract.

Besides the esophagus, organs of the upper gastrointestinal (GI) tract include the mouth, pharynx, and stomach. These hollow organs are all connected to form a tube through which food passes during digestion. The only role in digestion played by the pharynx and esophagus is to move food through the GI tract. The mouth and stomach, in contrast, are organs where digestion — or the breakdown of food — also occurs. In both of these organs, food is broken into smaller pieces (mechanical digestion), as well as broken down chemically (chemical digestion). It should be noted that the first part of the small intestine (duodenum) is considered in some contexts to be part of the upper GI tract, but that practice is not followed here.

The mouth is the first organ of the GI tract. Most of the oral cavity is lined with mucous membrane. This tissue produces mucus, which helps moisten, soften, and lubricate food. Underlying the mucous membrane is a thin layer of smooth muscle to which the mucous membrane is only loosely connected. This gives the mucous membrane considerable ability to stretch as you eat food. The roof of the mouth, called the palate, separates the oral cavity from the nasal cavity. The front part is hard, consisting of mucous membrane covering a plate of bone. The back part of the palate is softer and more pliable, consisting of mucous membrane over muscle and connective tissue. The hard surface of the front of the palate allows for pressure needed in chewing and mixing food. The soft, pliable surface of the back of the palate can move to accommodate the passage of food while swallowing. Muscles at either side of the soft palate contract to create the swallowing action.

Several specific structures in the mouth are specialized for digestion. These include salivary glands, tongue, and teeth.

Salivary Glands

The mouth contains three pairs of major salivary glands, shown in Figure 15.4.2. These three pairs are all exocrine glands that secrete saliva into the mouth through ducts.

1. The largest of the three major pairs of salivary glands are the parotid glands, which are located on either side of the mouth in front of the ears.
2. The next largest pair is the submandibular glands, located beneath the lower jaw.
3. The third pair is the sublingual glands, located underneath the tongue.

In addition to these three pairs of major salivary glands, there are also hundreds of minor salivary glands in the oral mucosa lining the mouth and on the tongue. Along with the major glands, most of the minor glands secrete the digestive enzyme amylase, which begins the chemical digestion of starch and glycogen (polysaccharides). However, the minor salivary glands on the tongue secrete the fat-digesting enzyme lipase, which in the mouth is called lingual lipase (to distinguish it from pancreatic lipase secreted by the pancreas).

Saliva secreted by the salivary glands mainly helps digestion, but it also plays other roles. It helps maintain dental health by cleaning the teeth, and it contains antibodies that help protect against infection. By keeping the mouth lubricated, saliva also allows the mouth movements needed for speech.

Tongue

The tongue is a fleshy, muscular organ that is attached to the floor of the mouth by a band of ligaments that gives it great mobility. This is necessary so the tongue can manipulate food for chewing and swallowing. Movements of the tongue are also necessary for speaking. The upper surface of the tongue is covered with tiny projections called papillae, which contain taste buds. The latter are collections of chemoreceptor cells (shown in Figure 15.4.3). These sensory cells sense chemicals in food and send the information to the brain via cranial nerves, thus enabling the sense of taste.

There are five basic tastes detected by the chemoreceptor cells in taste buds: saltiness, sourness, bitterness, sweetness, and umami (often described as a meaty taste). Contrary to popular belief, taste buds for the five basic tastes are not located on different parts of the tongue. Why does taste matter? The taste of food helps to stimulate the secretion of saliva from the salivary glands. It also helps us to eat foods that are good for us, instead of rotten or toxic foods. The detection of saltiness, for example, enables the control of salt intake and salt balance in the body. The detection of sourness may help us avoid spoiled foods, which often taste sour due to fermentation by bacteria. The detection of bitterness warns of poisons, because many plants defend themselves with toxins that taste bitter. The detection of sweetness guides us to foods that supply quick energy. The detection of umami may signal protein-rich foods.

Teeth

The teeth are complex structures made of a bone-like material called dentin and covered with enamel, which is the hardest tissue in the body. Adults normally have a total of 32 teeth, with 16 in each jaw. The right and left sides of each jaw are mirror images in terms of the numbers and types of teeth they contain. Teeth have different shapes to suit them for different aspects of mastication (chewing). The different types of teeth are illustrated in Figure 15.4.4.

• Incisors are the sharp, blade-like teeth at the front of the mouth. They are used for cutting or biting off pieces of food. In adults, there are normally four incisors in each jaw, or eight in total.
• Canines are the pointed teeth on either side of the incisors. They are used for tearing foods that are tough or stringy. Adults normally have two canines in each jaw, or four altogether.
• Premolars and molars are cuboid teeth with cusps and grooves that are located on the sides and toward the back of the jaws. Premolars are closer to the front of the mouth. Molars are larger and have more cusps than premolars, but both are used for crushing and grinding food. Adults normally have two premolars and three molars on each side of each jaw, for a total of eight premolars and twelve molars.

The tube-like pharynx (see Figure 15.4.5 below) plays a dual role as an organ of both respiration and digestion. As part of the respiratory system, it conducts air between the nasal cavity and larynx. As part of the digestive system, it allows swallowed food to pass from the oral cavity to the esophagus. Anything swallowed has priority over inhaled air when passing through the pharynx. During swallowing, the backward motion of the tongue causes a flap of elastic cartilage — called the epiglottis — to close over the opening to the larynx. This prevents food or drink from entering the larynx.

Esophagus

The esophagus (shown in Figure 15.4.6) is a muscular tube through which food is pushed from the pharynx to the stomach. The esophagus passes through an opening in the diaphragm (the large breathing muscle that separates the abdomen from the thorax) before reaching the stomach. In adults, the esophagus averages about 25 cm (about 9.8 inches) in length, depending on a person’s height. The inner lining of the esophagus consists of mucous membrane, which provides a smooth, slippery surface for the passage of food. The cells of this membrane are constantly being replaced as they are worn away from the frequent passage of food over them.

When food is not being swallowed, the esophagus is closed at both ends by upper and lower esophageal sphincters. Sphincters are rings of muscle that can contract to close off openings between structures. The upper esophageal sphincter is triggered to relax and open by the act of swallowing, allowing a bolus of food to enter the esophagus from the pharynx. Then, the esophageal sphincter closes again to prevent food from moving back into the pharynx from the esophagus.

Once in the esophagus, the food bolus travels down to the stomach, pushed along by the rhythmic contraction and relaxation of muscles (peristalsis). The lower esophageal sphincter is located at the junction between the esophagus and the stomach. This sphincter opens when the bolus reaches it, allowing the food to enter the stomach. The sphincter normally remains closed at other times to prevent the contents of the stomach from entering the esophagus. Failure of this sphincter to remain completely closed can lead to heartburn. If it happens chronically, it can lead to gastroesophageal reflux disease (GERD), in which the mucous membrane of the esophagus may become damaged by the highly acidic contents of the stomach.

See the video below to see how the parts of the upper GI tract work together to carry out swallowing:

https://youtu.be/pNcV6yAfq-g

Swallowing, uploaded by Alejandra Cork, 2012.

Stomach

The stomach is a J-shaped organ (shown in Figure 15.4.7) that is joined to the esophagus at its upper end, and to the first part of the small intestine (duodenum) at its lower end. When the stomach is empty of food, it normally has a volume of about 75 millilitres, but it can expand to hold up to about a litre of food. Waves of muscle contractions (peristalsis) passing through the muscular walls of the stomach cause the food inside to be mixed and churned. The wall of the stomach has an extra layer of muscle tissue not found in other organs of the GI tract that helps it squeeze and mix the food. These movements of the stomach wall contribute greatly to mechanical digestion by breaking the food into much smaller pieces. The churning also helps mix the food with stomach secretions that aid in its chemical digestion.

Secretions of the stomach include gastric acid, which consists mainly of hydrochloric acid (HCl). This makes the stomach contents highly acidic, which is necessary so that the enzyme pepsin — also secreted by the stomach — can begin the digestion of protein. Mucus is secreted by the lining of stomach to provide a slimy protective coating against the otherwise damaging effects of gastric acid. The fat-digesting enzyme lipase is secreted in small amounts in the stomach, but very little fat digestion occurs there.

By the time food has been in the stomach for about an hour, it has become the thick, semi-liquid chyme. When the small intestine is ready to receive chyme, a sphincter between the stomach and duodenum — called the pyloric sphincter — opens to allow the chyme to enter the small intestine for further digestion and absorption.

The ongoing epidemic of obesity in the wealthier nations of the world, including Canada, has led to the development of several different bariatric surgeries that modify the stomach to help obese patients reduce their food intake and lose weight. Go online to learn more about bariatric surgery. Find sources you judge to be reliable that answer the following questions:

1. Who qualifies for bariatric surgery?
2. Describe the bariatric surgeries commonly called stomach stapling, lap band, and gastric sleeve. How does each type of surgery modify the stomach? In terms of weight loss, how effective is each type?
3. What are the major potential risks of bariatric surgery?
4. Besides weight loss, what other benefits have been shown to result from bariatric surgery?

Attributions

Figure 15.4.1

Handstand, Pender Island, B.C. [photo] by Jasper Garratt on Unsplash is used under the Unsplash License (https://unsplash.com/license).

Figure 15.4.2

Blausen_0780_SalivaryGlands by BruceBlaus on Wikimedia Commons is used under a CC BY 3.0 (https://creativecommons.org/licenses/by/3.0) license.

Figure 15.4.3

1402_The_Tongue by OpenStax on Wikimedia Commons is used under a CC BY 4.0 (https://creativecommons.org/licenses/by/4.0) license.

Figure 15.4.4

1024px-3D_Medical_Animation_Still_Showing_Types_of_Teeth by http://www.scientificanimations.com on Wikimedia Commons is used under a CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0) license.

Figure 15.4.5

Illu01_head_neck by Arcadian from NCI/ SEER Training Modules on Wikimedia Common is in the public domain (https://en.wikipedia.org/wiki/public_domain).

Figure 15.4.6

ZenkerSchraeg by Bernd Brägelmann Braegel on Wikimedia Commons is used under a CC BY 3.0  (https://creativecommons.org/licenses/by/3.0) license.  (Courtesy of Dr. Martin Steinhoff. It is not known whether there is a possibly necessary approval from the patient.)

Figure 15.4.7

Anatomy stomach – white by www.medicalgraphics.de from MedicalGraphics is used under a  CC BY-ND 4.0 (https://creativecommons.org/licenses/by-nd/4.0/) license.

References

Alejandra Cork. (2012). Swallowing. YouTube. https://www.youtube.com/watch?v=pNcV6yAfq-g&t=4s

Betts, J. G., Young, K.A., Wise, J.A., Johnson, E., Poe, B., Kruse, D.H., Korol, O., Johnson, J.E., Womble, M., DeSaix, P. (2016, May 27). Figure 14.3 The tongue [digital image]. In Anatomy and Physiology (Section 14.1). OpenStax. https://openstax.org/books/anatomy-and-physiology/pages/14-1-sensory-perception

Blausen.com Staff. (2014). Medical gallery of Blausen Medical 2014. WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.010. ISSN 2002-4436.

Mayo Clinic. (2014, August 26). Gastric bypass surgery: One patient’s journey - Mayo Clinic. https://www.youtube.com/watch?v=twJBEypJDfU&feature=youtu.be

Mayo Clinic Staff. (n.d.). Gastroesophageal reflux disease (GERD) [online article]. MayoClinic.org. https://www.mayoclinic.org/diseases-conditions/gerd/symptoms-causes/syc-20361940

Tech Insider. (2018, March 20). Here's what happens in your body when you swallow gum | The human body. YouTube. https://www.youtube.com/watch?v=u_1sVri3b2w&feature=youtu.be

TED-Ed. (2020, April 9). How does alcohol make you drunk? - Judy Grisel. YouTube. https://www.youtube.com/watch?v=gCrmFbgT37I&feature=youtu.be

TED-Ed. (2016, October 17). What causes cavities? - Mel Rosenberg. YouTube. https://www.youtube.com/watch?v=zGoBFU1q4g0&feature=youtu.be

 

Created by CK-12 Foundation/Adapted by Christine Miller

If you had a skin rash like the one shown in Figure 15.7.1, you probably wouldn’t assume that it was caused by a digestive system disease. However, that’s exactly why the individual in the picture has a rash. He has a gastrointestinal (GI) tract disorder called Crohn’s disease. This disease is one of a group of GI tract disorders that are known collectively as inflammatory bowel disease. Unlike other inflammatory bowel diseases, signs and symptoms of Crohn’s disease may not be confined to the GI tract.

Inflammatory bowel disease (IBD) is a collection of inflammatory conditions primarily affecting the intestines. The two principal inflammatory bowel diseases are Crohn’s disease and ulcerative colitis. Unlike Crohn’s disease — which may affect any part of the GI tract and the joints, as well as the skin — ulcerative colitis mainly affects just the colon and rectum. Both diseases occur when the body’s own immune systemno post attacks the digestive system. Both diseases typically first appear in the late teens or early twenties, and occur equally in males and females.  Approximately 270,000 Canadians are currently living with IBD, 7,000 of which are children.  The annual cost of caring for these Canadians is estimated at $1.28 billion.  The number of cases of IBD has been steadily increasing and it is expected that by 2030 the number of Canadians suffering from IBD will grow to 400,000.

Crohn’s Disease

Crohn’s disease is a type of inflammatory bowel disease that may affect any part of the GI tract from the mouth to the anus, among other body tissues. The most commonly affected region is the ileum, which is the final part of the small intestine. Signs and symptoms of Crohn’s disease typically include abdominal pain, diarrhea (with or without blood), fever, and weight loss. Malnutrition because of faulty absorption of nutrients may also occur. Potential complications of Crohn’s disease include obstructions and abscesses of the bowel. People with Crohn’s disease are also at slightly greater risk than the general population of developing bowel cancer. Although there is a slight reduction in life expectancy in people with Crohn’s disease, if the disease is well-managed, affected people can live full and productive lives.  Approximately 135,000 Canadians are living with Crohn's disease.

Crohn’s disease is caused by a combination of genetic and environmental factors that lead to impairment of the generalized immune response (called innate immunity). The chronic inflammation of Crohn’s disease is thought to be the result of the immune system “trying” to compensate for the impairment. Dozens of genes are likely to be involved, only a few of which have been identified. Because of the genetic component, close relatives such as siblings of people with Crohn’s disease are many times more likely to develop the disease than people in the general population. Environmental factors that appear to increase the risk of the disease include smoking tobacco and eating a diet high in animal proteins. Crohn’s disease is typically diagnosed on the basis of a colonoscopy, which provides a direct visual examination of the inside of the colon and the ileum of the small intestine.

People with Crohn’s disease typically experience recurring periods of flare-ups followed by remission. There are no medications or surgical procedures that can cure Crohn’s disease, although medications such as anti-inflammatory or immune-suppressing drugs may alleviate symptoms during flare-ups and help maintain remission. Lifestyle changes, such as dietary modifications and smoking cessation, may also help control symptoms and reduce the likelihood of flare-ups. Surgery may be needed to resolve bowel obstructions, abscesses, or other complications of the disease.

Ulcerative Colitis

Ulcerative colitis is an inflammatory bowel disease that causes inflammation and ulcers (sores) in the colon and rectum. Unlike Crohn’s disease, other parts of the GI tract are rarely affected in ulcerative colitis. The primary symptoms of the disease are lower abdominal pain and bloody diarrhea. Weight loss, fever, and anemia may also be present. Symptoms typically occur intermittently with periods of no symptoms between flare-ups. People with ulcerative colitis have a considerably increased risk of colon cancer and should be screened for colon cancer more frequently than the general population. Ulcerative colitis, however, seems to primarily reduce the quality of life, and not the lifespan.

The exact cause of ulcerative colitis is not known. Theories about its cause involve immune system dysfunction, genetics, changes in normal gut bacteria, and lifestyle factors, such as a diet high in animal protein and the consumption of alcoholic beverages. Genetic involvement is suspected in part because ulcerative colitis tens to “run” in families. It is likely that multiple genes are involved. Diagnosis is typically made on the basis of colonoscopy and tissue biopsies.

Lifestyle changes, such as reducing the consumption of animal protein and alcohol, may improve symptoms of ulcerative colitis. A number of medications are also available to treat symptoms and help prolong remission. These include anti-inflammatory drugs and drugs that suppress the immune system. In cases of severe disease, removal of the colon and rectum may be required and can cure the disease.

Diverticulitis is a digestive disease in which tiny pouches in the wall of the large intestine become infected and inflamed. Symptoms typically include lower abdominal pain of sudden onset. There may also be fever, nausea, diarrhea or constipation, and blood in the stool. Having large intestine pouches called diverticula (see Figure 15.7.2) that are not inflamed is called diverticulosis. Diverticulosis is thought to be caused by a combination of genetic and environmental factors, and is more common in people who are obese. Infection and inflammation of the pouches (diverticulitis) occurs in about 10–25% of people with diverticulosis, and is more common at older ages. The infection is generally caused by bacteria.

Diverticulitis can usually be diagnosed with a CT scan and can be monitored with a colonoscopy (as seen in Figure 15.7.3). Mild diverticulitis may be treated with oral antibiotics and a short-term liquid diet. For severe cases, intravenous antibiotics, hospitalization, and complete bowel rest (no nourishment via the mouth) may be recommended. Complications such as abscess formation or perforation of the colon require surgery.

A peptic ulcer is a sore in the lining of the stomach or the duodenum (first part of the small intestine). If the ulcer occurs in the stomach, it is called a gastric ulcer. If it occurs in the duodenum, it is called a duodenal ulcer. The most common symptoms of peptic ulcers are upper abdominal pain that often occurs in the night and improves with eating. Other symptoms may include belching, vomiting, weight loss, and poor appetite. Many people with peptic ulcers, particularly older people, have no symptoms. Peptic ulcers are relatively common, with about ten per cent of people developing a peptic ulcer at some point in their life.

The most common cause of peptic ulcers is infection with the bacterium Helicobacter pylori, which may be transmitted by food, contaminated water, or human saliva (for example, by kissing or sharing eating utensils). Surprisingly, the bacterial cause of peptic ulcers was not discovered until the 1980s. The scientists who made the discovery are Australians Robin Warren and Barry J. Marshall. Although the two scientists eventually won a Nobel Prize for their discovery, their hypothesis was poorly received at first. To demonstrate the validity of their discovery, Marshall used himself in an experiment. He drank a culture of bacteria from a peptic ulcer patient and developed symptoms of peptic ulcer in a matter of days. His symptoms resolved on their own within a couple of weeks, but, at his wife's urging, he took antibiotics to kill any remaining bacteria. Marshall’s self-experiment was published in the Australian Medical Journal, and is among the most cited articles ever published in the journal.  Figure 15.7.4 shows how H. pylori cause peptic ulcers.

Another relatively common cause of peptic ulcers is chronic use of non-steroidal anti-inflammatory drugs (NSAIDs), such as aspirin or ibuprofen. Additional contributing factors may include tobacco smoking and stress, although these factors have not been demonstrated conclusively to cause peptic ulcers independent of H. pylori infection. Contrary to popular belief, diet does not appear to play a role in either causing or preventing peptic ulcers. Eating spicy foods and drinking coffee and alcohol were once thought to cause peptic ulcers. These lifestyle choices are no longer thought to have much (if any) of an effect on the development of peptic ulcers.

Peptic ulcers are typically diagnosed on the basis of symptoms or the presence of H. pylori in the GI tract. However, endoscopy (shown in Figure 15.7.5), which allows direct visualization of the stomach and duodenum with a camera, may be required for a definitive diagnosis. Peptic ulcers are usually treated with antibiotics to kill H. pylori, along with medications to temporarily decrease stomach acid and aid in healing. Unfortunately, H. pylori has developed resistance to commonly used antibiotics, so treatment is not always effective. If a peptic ulcer has penetrated so deep into the tissues that it causes a perforation of the wall of the stomach or duodenum, then emergency surgery is needed to repair the damage.

 

Gastroenteritis, also known as infectious diarrhea or stomach flu, is an acute and usually self-limiting infection of the GI tract by pathogens. Symptoms typically include some combination of diarrhea, vomiting, and abdominal pain. Fever, lack of energy, and dehydration may also occur. The illness generally lasts less than two weeks, even without treatment, but in young children it is potentially deadly. Gastroenteritis is very common, especially in poorer nations. Worldwide, up to five billion cases occur each year, resulting in about 1.4 million deaths.

Commonly called “stomach flu,” gastroenteritis is unrelated to the influenza virus, although viruses are the most common cause of the disease (see Figure 15.7.6). In children, rotavirus is most often the cause which is why the British Columbia immunization schedule now includes a rotovirus vaccine. Norovirus is more likely to be the cause of gastroenteritis in adults. Besides viruses, other potential causes of gastroenteritis include fungi, bacteria (most often E. coli or Campylobacter jejuni), and protozoa(including Giardia lamblia, more commonly called Beaver Fever, described below). Transmission of pathogens may occur due to eating improperly prepared foods or foods left to stand at room temperature, drinking contaminated water, or having close contact with an infected individual.

Gastroenteritis is less common in adults than children, partly because adults have acquired immunity after repeated exposure to the most common infectious agents. Adults also tend to have better hygiene than children. If children have frequent repeated incidents of gastroenteritis, they may suffer from malnutrition, stunted growth, and developmental delays. Many cases of gastroenteritis in children can be avoided by giving them a rotavirus vaccine. Frequent and thorough handwashing can cut down on infections caused by other pathogens.

Treatment of gastroenteritis generally involves increasing fluid intake to replace fluids lost in vomiting or diarrhea. Oral rehydration solution, which is a combination of water, salts, and sugar, is often recommended. In severe cases, intravenous fluids may be needed. Antibiotics are not usually prescribed, because they are ineffective against viruses that cause most cases of gastroenteritis.

Giardiasis, popularly known as beaver fever, is a type of gastroenteritis caused by a GI tract parasite, the single-celled protozoan Giardia lamblia (pictured in Figure 15.7.7). In addition to human beings, the parasite inhabits the digestive tract of a wide variety of domestic and wild animals, including cows, rodents, and sheep, as well as beavers (hence its popular name). Giardiasis is one of the most common parasitic infections in people the world over, with hundreds of millions of people infected worldwide each year.

Transmission of G. lamblia is via a fecal-oral route (as in, you got feces in your food). Those at greatest risk include travelers to countries where giardiasis is common, people who work in child-care settings, backpackers and campers who drink untreated water from lakes or rivers, and people who have close contact with infected people or animals in other settings. In Canada, Giardia is the most commonly identified intestinal parasite and approximately 3,000 Canadians will contract the parasite annually.

Symptoms of giardiasis can vary widely. About one-third third of people with the infection have no symptoms, whereas others have severe diarrhea with poor absorption of nutrients. Problems with absorption occur because the parasites inhibit intestinal digestive enzyme production, cause detrimental changes in microvilli lining the small intestine, and kill off small intestinal epithelial cells. The illness can result in weakness, loss of appetite, stomach cramps, vomiting, and excessive gas. Without treatment, symptoms may continue for several weeks. Treatment with anti-parasitic medications may be needed if symptoms persist longer or are particularly severe.

Attributions

Figure 15.7.1

BADAS_Crohn by Dayavathi Ashok and Patrick Kiely/ Journal of medical case reports on Wikimedia Commons is used under a CC BY 2.0 (https://creativecommons.org/licenses/by/2.0) license.

Figure 15.7.2

512px-Ds00070_an01934_im00887_divert_s_gif.webp by Lfreeman04  on Wikimedia Commons is used under a CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0) license.

Figure 15.7.3

Colon_diverticulum by melvil on Wikimedia Commons is used under a CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0) license.

Figure 15.7.4

H_pylori_ulcer_diagram by Y_tambe on Wikimedia Commons is used under a CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0/) license.

Figure 15.7.5

1024px-Endoscopy_training by Yuya Tamai on Wikimedia Commons is used under a CC BY 2.0 (https://creativecommons.org/licenses/by/2.0) license.

Figure 15.7.6

Gastroenteritis_viruses by Dr. Graham Beards [en:User:Graham Beards] at en.wikipedia on Wikimedia Commons is used under a CC BY 3.0 (https://creativecommons.org/licenses/by/3.0) license.

Figure 15.7.7

Giardia_lamblia_SEM_8698_lores by Janice Haney Carr from CDC/ Public Health Image Library (PHIL) ID# 8698 on Wikimedia Commons is in the public domain (https://en.wikipedia.org/wiki/public_domain).

References

Ashok, D., & Kiely, P. (2007). Bowel associated dermatosis - arthritis syndrome: a case report. Journal of medical case reports, 1, 81. https://doi.org/10.1186/1752-1947-1-81

Marshall, B. J., Armstrong, J. A., McGechie, D. B., & Glancy, R. J. (1985). Attempt to fulfil Koch's postulates for pyloric Campylobacter. The Medical Journal of Australia, 142(8), 436–439.

Marshall, B. J., McGechie, D. B., Rogers, P. A., & Glancy, R. J. (1985). Pyloric campylobacter infection and gastroduodenal disease. The Medical Journal of Australia, 142(8), 439–444.

TED-Ed. (2017, September 28). The surprising cause of stomach ulcers - Rusha Modi. YouTube. https://www.youtube.com/watch?v=V_U6czbDHLE&feature=youtu.be

TED-Ed. (2018, January 4). Who's at risk for colon cancer? - Amit H. Sachdev and Frank G. Gress. YouTube. https://www.youtube.com/watch?v=H5zin8jKeT0&feature=youtu.be

Glucose (also called dextrose) is a simple sugar with the molecular formula C6H12O6. Glucose is the most abundant monosaccharide, a subcategory of carbohydrates. Glucose is mainly made by plants and most algae during photosynthesis from water and carbon dioxide, using energy from sunlight.

Created by: CK-12/Adapted by Christine Miller

People tend to carry similar traits to their biological parents, as illustrated by the family tree. Beyond just appearance, you can also inherit traits from your parents that you can’t see.

Rebecca becomes very aware of this fact when she visits her new doctor for a physical exam. Her doctor asks several questions about her family medical history, including whether Rebecca has or had relatives with cancer. Rebecca tells her that her grandmother, aunt, and uncle — who have all passed away — had cancer. They all had breast cancer, including her uncle, and her aunt also had ovarian cancer. Her doctor asks how old they were when they were diagnosed with cancer. Rebecca is not sure exactly, but she knows that her grandmother was fairly young at the time, probably in her forties.

Rebecca’s doctor explains that while the vast majority of cancers are not due to inherited factors, a cluster of cancers within a family may indicate that there are mutations in certain genes that increase the risk of getting certain types of cancer, particularly breast and ovarian cancer. Some signs that cancers may be due to these genetic factors are present in Rebecca’s family, such as cancer with an early age of onset (e.g., breast cancer before age 50), breast cancer in men, and breast cancer and ovarian cancer within the same person or family.

Based on her family medical history, Rebecca’s doctor recommends that she see a genetic counselor, because these professionals can help determine whether the high incidence of cancers in her family could be due to inherited mutations in their genes. If so, they can test Rebecca to find out whether she has the particular variations of these genes that would increase her risk of getting cancer.

When Rebecca sees the genetic counselor, he asks how her grandmother, aunt, and uncle with cancer are related to her. She says that these relatives are all on her mother’s side — they are her mother’s mother and siblings. The genetic counselor records this information in the form of a specific type of family tree, called a pedigree, indicating which relatives had which type of cancer, and how they are related to each other and to Rebecca.

He also asks her ethnicity. Rebecca says that her family on both sides are Ashkenazi Jews (Jews whose ancestors came from central and eastern Europe). “But what does that have to do with anything?” she asks. The counselor tells Rebecca that mutations in two tumor-suppressor genes called BRCA1 and BRCA2, located on chromosome 17 and 13, respectively, are particularly prevalent in people of Ashkenazi Jewish descent and greatly increase the risk of getting cancer. About one in 40 Ashkenazi Jewish people have one of these mutations, compared to about one in 800 in the general population. Her ethnicity, along with the types of cancer, age of onset, and the specific relationships between her family members who had cancer, indicate to the counselor that she is a good candidate for genetic testing for the presence of these mutations.

Rebecca says that her 72-year-old mother never had cancer, nor had many other relatives on that side of the family. How could the cancers be genetic? The genetic counselor explains that the mutations in the BRCA1 and BRCA2 genes, while dominant, are not inherited by everyone in a family. Also, even people with mutations in these genes do not necessarily get cancer — the mutations simply increase their risk of getting cancer. For instance, 55 to 65 per cent of women with a harmful mutation in the BRCA1 gene will get breast cancer before age 70, compared to 12 per cent of women in the general population who will get breast cancer sometime over the course of their lives.

Rebecca is not sure she wants to know whether she has a higher risk of cancer. The genetic counselor understands her apprehension, but explains that if she knows that she has harmful mutations in either of these genes, her doctor will screen her for cancer more often and at earlier ages. Therefore, any cancers she may develop are likely to be caught earlier when they are often much more treatable. Rebecca decides to go through with the testing, which involves taking a blood sample, and nervously waits for her results.

Attributions

Figure 5.1.1

Family Tree [all individual face images] from Clker.com used and adapted by Christine Miller under a CC0 1.0 public domain dedication license (https://creativecommons.org/publicdomain/zero/1.0/).

Figure 5.1.2

Rebecca by Kyle Broad on Unsplash is used under the Unsplash License (https://unsplash.com/license).

References

Wikipedia contributors. (2020, June 27). Ashkenazi Jews. In Wikipedia.  https://en.wikipedia.org/w/index.php?title=Ashkenazi_Jews&oldid=964691647

Wikipedia contributors. (2020, June 22). BRCA1. In Wikipedia. https://en.wikipedia.org/w/index.php?title=BRCA1&oldid=963868423

Wikipedia contributors. (2020, May 25). BRCA2. In Wikipedia.  https://en.wikipedia.org/w/index.php?title=BRCA2&oldid=958722957

The process of producing cellular energy involving oxygen. Cells break down food in the mitochondria in a long, multi-step process that produces roughly 36 ATP. The first step in is glycolysis, the second is the Krebs cycle and the third is the electron transport system.

Respiration using electron acceptors other than molecular oxygen. Although oxygen is not the final electron acceptor, the process still uses a respiratory electron transport chain.

Created by CK-12 Foundation/Adapted by Christine Miller

Does the organism in Figure 17.2.1 look like a space alien? A scary creature from a nightmare? In fact, it’s a 1-cm long worm in the genus Schistosoma. It may invade and take up residence in the human body, causing a very serious illness known as schistosomiasis. The worm gains access to the human body while it is in a microscopic life stage. It enters through a hair follicle when the skin comes into contact with contaminated water. The worm then grows and matures inside the human organism, causing disease.

The Schistosoma worm has a parasitic relationship with humans. In this type of relationship, one organism, called the parasite, lives on or in another organism, called the host. The parasite always benefits from the relationship, and the host is always harmed. The human host of the Schistosoma worm is clearly harmed by the parasite when it invades the host’s tissues. The urinary tract or intestines may be infected, and signs and symptoms may include abdominal pain, diarrhea, bloody stool, or blood in the urine. Those who have been infected for a long time may experience liver damage, kidney failure, infertility, or bladder cancer. In children, Schistosoma infection may cause poor growth and difficulty learning.

Like the Schistosoma worm, many other organisms can make us sick if they manage to enter our body. Any such agent that can cause disease is called a pathogen. Most pathogens are microorganisms, although some — such as the Schistosoma worm — are much larger. In addition to worms, common types of pathogens of human hosts include bacteria, viruses, fungi, and single-celled organisms called protists. You can see examples of each of these types of pathogens in Table 17.1.1. Fortunately for us, our immune system is able to keep most potential pathogens out of the body, or quickly destroy them if they do manage to get in. When you read this chapter, you’ll learn how your immune system usually keeps you safe from harm — including from scary creatures like the Schistosoma worm!

Table 17.1.1: Types of Disease-Causing Pathogens

Type of Pathogen		Description	Disease Caused
Bacteria: Example shown: Escherichia coli		Single celled organisms without a nucleus	Strep throat, staph infections, tuberculosis, food poisoning, tetanus, pneumonia, syphillis
Viruses: Example shown: Herpes simplex		Non-living particles that reproduce by taking over living cells	Common cold, flu, genital herpes, cold sores, measles, AIDS, genital warts, chicken pox, small pox
Fungi: Example shown: Death cap mushroom		Simple organisms, including mushrooms and yeast, that grow as single cells or thread-like filaments	Ringworm, athletes foot, tineas, candidias, histoplasmomis, mushroom poisoning
Protozoa: Example shown: Giardia lamblia		Single celled organisms with a nucleus	Malaria, "traveller's diarrhea", giardiasis, typano somiasis ("sleeping sickness")

What is the Immune System?

The immune systemno post is a host defense system. It comprises many biological structures —ranging from individual leukocytes to entire organs — as well as many complex biological processes. The function of the immune system is to protect the host from pathogens and other causes of disease, such as tumor (cancer) cells. To function properly, the immune system must be able to detect a wide variety of pathogens. It also must be able to distinguish the cells of pathogens from the host’s own cells, and also to distinguish cancerous or damaged host cells from healthy cells. In humans and most other vertebrates, the immune system consists of layered defenses that have increasing specificity for particular pathogens or tumor cells. The layered defenses of the human immune system are usually classified into two subsystems, called the innate immune system and the adaptive immune system.

Innate Immune System

The innate immune system (sometimes referred to as "non-specific defense") provides very quick, but non-specific responses to pathogens. It responds the same way regardless of the type of pathogen that is attacking the host. It includes barriers — such as the skin and mucous membranes — that normally keep pathogens out of the body. It also includes general responses to pathogens that manage to breach these barriers, including chemicals and cells that attack the pathogens inside the human host. Certain leukocytes (white blood cells), for example, engulf and destroy pathogens they encounter in the process called phagocytosis, which is illustrated in Figure 17.2.2. Exposure to pathogens leads to an immediate maximal response from the innate immune system.

Adaptive Immune System

The adaptive immune system is activated if pathogens successfully enter the body and manage to evade the general defenses of the innate immune system. An adaptive response is specific to the particular type of pathogen that has invaded the body, or to cancerous cells. It takes longer to launch a specific attack, but once it is underway, its specificity makes it very effective. An adaptive response also usually leads to immunity. This is a state of resistance to a specific pathogen, due to the adaptive immune system's ability to “remember” the pathogen and immediately mount a strong attack tailored to that particular pathogen if it invades again in the future.

Both innate and adaptive immune responses depend on the immune system's ability to distinguish between self- and non-self molecules. Self molecules are those components of an organism’s body that can be distinguished from foreign substances by the immune system. Virtually all body cells have surface proteins that are part of a complex called major histocompatibility complex (MHC). These proteins are one way the immune system recognizes body cells as self. Non-self proteins, in contrast, are recognized as foreign, because they are different from self proteins.

Antigens and Antibodies

Many non-self molecules comprise a class of compounds called antigens. Antigens, which are usually proteins, bind to specific receptors on immune system cells and elicit an adaptive immune response. Some adaptive immune system cells (B cells) respond to foreign antigens by producing antibodies. An antibody is a molecule that precisely matches and binds to a specific antigen. This may target the antigen (and the pathogen displaying it) for destruction by other immune cells.

Antigens on the surface of pathogens are how the adaptive immune system recognizes specific pathogens. Antigen specificity allows for the generation of responses tailored to the specific pathogen. It is also how the adaptive immune system ”remembers” the same pathogen in the future.

Immune Surveillance

Another important role of the immune system is to identify and eliminate tumor cells. This is called immune surveillance. The transformed cells of tumors express antigens that are not found on normal body cells. The main response of the immune system to tumor cells is to destroy them. This is carried out primarily by aptly-named killer T cells of the adaptive immune system.

The lymphatic system is a human organ system that is a vital part of the adaptive immune system. It is also part of the cardiovascular system and plays a major role in the digestive system (see section 17.3 Lymphatic System). The major structures of the lymphatic system are shown in Figure 17.2.3 .

The lymphatic system consists of several lymphatic organs and a body-wide network of lymphatic vessels that transport the fluid called lymph. Lymph is essentially blood plasma that has leaked from capillaries into tissue spaces. It includes many leukocytes, especially lymphocytes, which are the major cells of the lymphatic system. Like other leukocytes, lymphocytes defend the body. There are several different types of lymphocytes that fight pathogens or cancer cells as part of the adaptive immune system.

Major lymphatic organs include the thymus and bone marrow. Their function is to form and/or mature lymphocytes. Other lymphatic organs include the spleen, tonsils, and lymph nodes, which are small clumps of lymphoid tissue clustered along lymphatic vessels. These other lymphatic organs harbor mature lymphocytes and filter lymph. They are sites where pathogens collect, and adaptive immune responses generally begin.

The brain and spinal cord are normally protected from pathogens in the blood by the selectively permeable blood-brain and blood-spinal cord barriers. These barriers are part of the neuroimmune system. The neuroimmune system has traditionally been considered distinct from the rest of the immune system, which is called the peripheral immune system — although that view may be changing. Unlike the peripheral system, in which leukocytes are the main cells, the main cells of the neuroimmune system are thought to be nervous system cells called neuroglia. These cells can recognize and respond to pathogens, debris, and other potential dangers. Types of neuroglia involved in neuroimmune responses include microglial cells and astrocytes.

• Microglial cells are among the most prominent types of neuroglia in the brain. One of their main functions is to phagocytize cellular debris that remains when neurons die. Microglial cells also “prune” obsolete synapses between neurons.
• Astrocytes are neuroglia that have a different immune function. They allow certain immune cells from the peripheral immune system to cross into the brain via the blood-brain barrier to target both pathogens and damaged nervous tissue.

“They’ll have to rewrite the textbooks!”

That sort of response to a scientific discovery is sure to attract media attention, and it did. It’s what Kevin Lee, a neuroscientist at the University of Virginia, said in 2016 when his colleagues told him they had discovered human anatomical structures that had never before been detected. The structures were tiny lymphatic vessels in the meningeal layers surrounding the brain.

How these lymphatic vessels could have gone unnoticed when all human body systems have been studied so completely is amazing in its own right. The suggested implications of the discovery are equally amazing:

• The presence of these lymphatic vessels means that the brain is directly connected to the peripheral immune system, presumably allowing a close association between the human brain and human pathogens. This suggests an entirely new avenue by which humans and their pathogens may have influenced each other’s evolution. The researchers speculate that our pathogens even may have influenced the evolution of our social behaviors.
• The researchers think there will also be many medical applications of their discovery. For example, the newly discovered lymphatic vessels may play a major role in neurological diseases that have an immune component, such as multiple sclerosis. The discovery might also affect how conditions such as autism spectrum disorders and schizophrenia are treated.

Attributions

Figure 17.1.1

Schistosome Parasite by Bruce Wetzel and Harry Schaefer (Photographers) from the National Cancer Institute, Visuals online is in the public domain (https://en.wikipedia.org/wiki/Public_domain).

Figure 17.1.2

Phagocytosis by Rlawson at en.wikibooks on Wikimedia Commons is used under a CC BY ­SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0/deed.en) license. (Transferred from en.wikibooks to Commons by User:Adrignola.)

Figure 17.1.3

2201_Anatomy_of_the_Lymphatic_System by OpenStax College on Wikimedia Commons is used under a CC BY 3.0 (https://creativecommons.org/licenses/by/3.0) license.

 

Table 17.1.1 

• EscherichiaColi NIAID [photo] by Rocky Mountain Laboratories,  NIH National Institute of Allergy and Infectious Diseases (NIAID) on Wikimedia Commons is in the public domain (https://en.wikipedia.org/wiki/Public_domain).
• Herpes simplex virus TEM B82-0474 lores by Dr. Erskine Palmer/ CDC Public Health Image Library (PHIL) on Wikimedia Commons is in the public domain (https://en.wikipedia.org/wiki/Public_domain).
• Red death cap mushroom by Rosendahl on Wikimedia Commons is in the public domain (https://en.wikipedia.org/wiki/Public_domain). (Transferred from Pixnio by Fæ.)
• Scanning electron micrograph (SEM) of Giardia lamblia by Janice Haney Carr/ CDC, Public Health Image Library (PHIL) Photo ID# 8698 is in the public domain (https://en.wikipedia.org/wiki/Public_domain).

References

Barney, J. (2016, March 21). They’ll have to rewrite the textbooks [online article]. Illimitable - Discovery. UVA Today/ University of Virginia. https://news.virginia.edu/illimitable/discovery/theyll-have-rewrite-textbooks

Betts, J. G., Young, K.A., Wise, J.A., Johnson, E., Poe, B., Kruse, D.H., Korol, O., Johnson, J.E., Womble, M., DeSaix, P. (2013, June 19). Figure 21.2 Anatomy of the lymphatic system [digital image].  In Anatomy and Physiology (Section 21.1). OpenStax. https://openstax.org/books/anatomy-and-physiology/pages/21-1-anatomy-of-the-lymphatic-and-immune-systems

Handwritten Tutorials. (2011, October 25). Overview of the immune system. YouTube. https://www.youtube.com/watch?v=Nw27_jMWw10&feature=youtu.be

ImmiflexImmuneSystem. (2013). Neutrophil phagocytosis - White blood cell eats staphylococcus aureus bacteria. YouTube. https://www.youtube.com/watch?v=Z_mXDvZQ6dU

Kurzgesagt – In a Nutshell. (2016, March 16). The antibiotic apocalypse explained. YouTube. https://www.youtube.com/watch?v=xZbcwi7SfZE&feature=youtu.be

Louveau, A., Smirnov, I., Keyes, T. J., Eccles, J. D., Rouhani, S. J., Peske, J. D., Derecki, N. C., Castle, D., Mandell, J. W., Lee, K. S., Harris, T. H., & Kipnis, J. (2015). Structural and functional features of central nervous system lymphatic vessels. Nature, 523(7560), 337–341. https://doi.org/10.1038/nature14432

Mayo Clinic Staff. (n.d.). Autism spectrum disorder [online article]. MayoClinic.org. https://www.mayoclinic.org/diseases-conditions/autism-spectrum-disorder/symptoms-causes/syc-20352928

Mayo Clinic Staff. (n.d.). Multiple sclerosis [online article]. MayoClinic.org. https://www.mayoclinic.org/diseases-conditions/multiple-sclerosis/symptoms-causes/syc-20350269

Mayo Clinic Staff. (n.d.). Schizophrenia [online article]. MayoClinic.org. https://www.mayoclinic.org/diseases-conditions/schizophrenia/symptoms-causes/syc-20354443

TED-Ed. (2016, April 19). The surprising reason you feel awful when you're sick - Marco A. Sotomayor. YouTube. https://www.youtube.com/watch?v=gVdY9KXF_Sg&feature=youtu.be

 

Created by CK-12 Foundation/Adapted by Christine Miller

The white patches on either side of the throat in Figure 17.3.1 are signs of tonsillitis. The tonsils are small structures in the throat that are very common sites of infection. The white spots on the tonsils pictured here are evidence of infection. The patches consist of large amounts of dead bacteria, cellular debris, and white blood cells — in a word: pus. Children with recurrent tonsillitis may have their tonsils removed surgically to eliminate this type of infection. The tonsils are organs of the lymphatic system.

The lymphatic system is a collection of organs involved in the production, maturation, and harboring of white blood cells called lymphocytes. It also includes a network of vessels that transport or filter the fluid known as lymph in which lymphocytes circulate. Figure 17.3.2 shows major lymphatic vessels and other structures that make up the lymphatic system. Besides the tonsils, organs of the lymphatic system include the thymus, the spleen, and hundreds of lymph nodes distributed along the lymphatic vessels.

The lymphatic vessels form a transportation network similar in many respects to the blood vessels of the cardiovascular system. However, unlike the cardiovascular system, the lymphatic system is not a closed system. Instead, lymphatic vessels carry lymph in a single direction — always toward the upper chest, where the lymph empties from lymphatic vessels into blood vessels.

The return of lymph to the bloodstream is one of the major functions of the lymphatic system. When blood travels through capillaries of the cardiovascular system, it is under pressure, which forces some of the components of blood (such as water, oxygen, and nutrients) through the walls of the capillaries and into the tissue spaces between cells, forming tissue fluid, also called interstitial fluid (see Figure 17.3.3). Interstitial fluid bathes and nourishes cells, and also absorbs their waste products. Much of the water from interstitial fluid is reabsorbed into the capillary blood by osmosis. Most of the remaining fluid is absorbed by tiny lymphatic vessels called lymph capillaries. Once interstitial fluid enters the lymphatic vessels, it is called lymph. Lymph is very similar in composition to blood plasma. Besides water, lymph may contain proteins, waste products, cellular debris, and pathogens. It also contains numerous white blood cells, especially the subset of white blood cells known as lymphocytes. In fact, lymphocytes are the main cellular components of lymph.

The lymph that enters lymph capillaries in tissues is transported through the lymphatic vessel network to two large lymphatic ducts in the upper chest. From there, the lymph flows into two major veins (called subclavian veins) of the cardiovascular system. Unlike blood, lymph is not pumped through its network of vessels. Instead, lymph moves through lymphatic vessels via a combination of contractions of the vessels themselves and the forces applied to the vessels externally by skeletal muscles, similarly to how blood moves through veins. Lymphatic vessels also contain numerous valves that keep lymph flowing in just one direction, thereby preventing backflow.

Lymphatic vessels called lacteals (see Figure 17.3.4) are present in the lining of the gastrointestinal tract, mainly in the small intestine. Each tiny villus in the lining of the small intestine has an internal bed of capillaries and lacteals. The capillaries absorb most nutrients from the digestion of food into the blood. The lacteals absorb mainly fatty acids from lipid digestion into the lymph, forming a fatty-acid-enriched fluid called chyle. Vessels of the lymphatic network then transport chyle from the small intestine to the main lymphatic ducts in the chest, from which it drains into the blood circulation. The nutrients in chyle then circulate in the blood to the liver, where they are processed along with the other nutrients that reach the liver directly via the bloodstream.

The primary immune function of the lymphatic system is to protect the body against pathogens and cancerous cells. This function of the lymphatic system is centred on the production, maturation, and circulation of lymphocytes. Lymphocytes are leukocytes that are involved in the adaptive immune system. They are responsible for the recognition of — and tailored defense against — specific pathogens or tumor cells. Lymphocytes may also create a lasting memory of pathogens, so they can be attacked quickly and strongly if they ever invade the body again. In this way, lymphocytes bring about long-lasting immunity to specific pathogens.

There are two major types of lymphocytes, called B cells and T cells. Both B cells and T cells are involved in the adaptive immune response, but they play different roles.

Production and Maturation of Lymphocytes

Like all other types of blood cells (including erythrocytes), both B cells and T cells are produced from stem cells in the red marrow inside bones. After lymphocytes first form, they must go through a complicated maturation process before they are ready to search for pathogens. In this maturation process, they “learn” to distinguish self from non-self. Only those lymphocytes that successfully complete this maturation process go on to actually fight infections by pathogens.

B cells mature in the bone marrow, which is why they are called B cells. After they mature and leave the bone marrow, they travel first to the circulatory system and then enter the lymphatic system to search for pathogens. T cells, on the other hand, mature in the thymus, which is why they are called T cells. The thymus is illustrated in Figure 17.3.5. It is a small lymphatic organ in the chest that consists of an outer cortex and inner medulla, all surrounded by a fibrous capsule. After maturing in the thymus, T cells enter the rest of the lymphatic system to join B cells in the hunt for pathogens. The bone marrow and thymus are called primary lymphoid organs because of their role in the production and/or maturation of lymphocytes.

Lymphocytes in Secondary Lymphoid Organs

The tonsils, spleen, and lymph nodes are referred to as secondary lymphoid organs. These organs do not produce or mature lymphocytes. Instead, they filter lymph and store lymphocytes. It is in these secondary lymphoid organs that pathogens (or their antigens) activate lymphocytes and initiate adaptive immune responses. Activation leads to cloning of pathogen-specific lymphocytes, which then circulate between the lymphatic system and the blood, searching for and destroying their specific pathogens by producing antibodies against them.

Tonsils

There are four pairs of human tonsils. Three of the four are shown in Figure 17.3.6. The fourth pair, called tubal tonsils, is located at the back of the nasopharynx. The palatine tonsils are the tonsils that are visible on either side of the throat. All four pairs of tonsils encircle a part of the anatomy where the respiratory and gastrointestinal tracts intersect, and where pathogens have ready access to the body. This ring of tonsils is called Waldeyer's ring.

Spleen

The spleen (Figure 17.3.7) is the largest of the secondary lymphoid organs, and is centrally located in the body. Besides harboring lymphocytes and filtering lymph, the spleen also filters blood. Most dead or aged erythrocytes are removed from the blood in the red pulp of the spleen. Lymph is filtered in the white pulp of the spleen. In the fetus, the spleen has the additional function of producing red blood cells. This function is taken over by bone marrow after birth.

Lymph Nodes

Each lymph node is a small, but organized collection of lymphoid tissue (see Figure 17.3.8) that contains many lymphocytes. Lymph nodes are located at intervals along the lymphatic vessels, and lymph passes through them on its way back to the blood.

There are at least 500 lymph nodes in the human body. Many of them are clustered at the base of the limbs and in the neck. Figure 17.3.9 shows the major lymph node concentrations, and includes the spleen and the region named Waldeyer’s ring, which consists of the tonsils.

When lymph nodes become enlarged and tender to the touch, they are obvious signs of immune system activity. Because it is easy to see and feel swollen lymph nodes, they are one way an individual can monitor his or her own health. To be useful in this way, it is important to know the myths and realities about swollen lymph nodes.

 

Myth	Reality
"You should see a doctor immediately whenever you have swollen lymph nodes."	Lymph nodes are constantly filtering lymph, so it is expected that they will change in size with varying amounts of debris or pathogens that may be present. A minor, unnoticed infection may cause swollen lymph nodes that may last for a few weeks. Generally, lymph nodes that return to their normal size within two or three weeks are not a cause for concern.
"Swollen lymph nodes mean you have a bacterial infection."	Although an infection is the most common cause of swollen lymph nodes, not all infections are caused by bacteria. Mononucleosis, for example, commonly causes swollen lymph nodes, and it is caused by viruses. There are also other causes of swollen lymph nodes besides infections, such as cancer and certain medications.
"A swollen lymph node means you have cancer."	Cancer is far less likely to be the cause of a swollen lymph node than is an infection. However, if a lymph node remains swollen longer than a few weeks — especially in the absence of an apparent infection — you should have your doctor check it.
"Cancer in a lymph node always originates somewhere else. There is no cancer of the lymph nodes."	Cancers do commonly spread from their site of origin to nearby lymph nodes and then to other organs, but cancer may also originate in the lymph nodes. This type of cancer is called lymphoma.

 

Attributions

Figure 17.3.1

512px-Tonsillitis by Michaelbladon at English Wikipedia on Wikimedia Commons is in the public domain (https://en.wikipedia.org/wiki/Public_domain).  (Transferred from en.wikipedia to Commons by Kauczuk)

Figure 17.3.2

Blausen_0623_LymphaticSystem_Female by BruceBlaus  on Wikimedia Commons is used under a CC BY 3.0 (https://creativecommons.org/licenses/by/3.0) license.

Figure 17.3.3

2201_Anatomy_of_the_Lymphatic_System (cropped) by OpenStax College on Wikimedia Commons is used under a CC BY 3.0 (https://creativecommons.org/licenses/by/3.0) license.

Figure 17.3.4

1000px-Intestinal_villus_simplified.svg by Snow93 on Wikimedia Commons is in the public domain (https://en.wikipedia.org/wiki/Public_domain).

Figure 17.3.5

2206_The_Location_Structure_and_Histology_of_the_Thymus by OpenStax College on Wikimedia Commons is used under a CC BY 3.0 (https://creativecommons.org/licenses/by/3.0) license.

Figure 17.3.6

Blausen_0861_Tonsils&Throat_Anatomy2 by BruceBlaus  on Wikimedia Commons is used under a CC BY 3.0 (https://creativecommons.org/licenses/by/3.0) license.

Figure 17.3.7

Figure_42_02_14 by CNX OpenStax on Wikimedia Commons is used under a CC BY 4.0 (https://creativecommons.org/licenses/by/4.0) license.

Figure 17.3.8

Illu_lymph_node_structure by NCI/ SEER Training on Wikimedia Commons is in the public domain (https://en.wikipedia.org/wiki/Public_domain). (Archives: https://web.archive.org/web/20070311015818/http://training.seer.cancer.gov/module_anatomy/unit8_2_lymph_compo1_nodes.html)

Figure 17.3.9

1000px-Lymph_node_regions.svg by Fred the Oyster (derivative work) on Wikimedia Commons is in the public domain (https://en.wikipedia.org/wiki/Public_domain). (Original by NCI/ SEER Training)

References

Betts, J. G., Young, K.A., Wise, J.A., Johnson, E., Poe, B., Kruse, D.H., Korol, O., Johnson, J.E., Womble, M., DeSaix, P. (2013, June 19). Figure 21.2 Anatomy of the lymphatic system [digital image].  In Anatomy and Physiology (Section 21.1). OpenStax. https://openstax.org/books/anatomy-and-physiology/pages/21-1-anatomy-of-the-lymphatic-and-immune-systems

Betts, J. G., Young, K.A., Wise, J.A., Johnson, E., Poe, B., Kruse, D.H., Korol, O., Johnson, J.E., Womble, M., DeSaix, P. (2013, June 19). Figure 21.7 Location, structure, and histology of the thymus [digital image].  In Anatomy and Physiology (Section 21.1). OpenStax. https://openstax.org/books/anatomy-and-physiology/pages/21-1-anatomy-of-the-lymphatic-and-immune-systems

Blausen.com Staff. (2014). Medical gallery of Blausen Medical 2014". WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.010. ISSN 2002-4436

Compton Care. (2016, March 7). What is lymphoedema or lymphedema? YouTube. https://www.youtube.com/watch?v=RMLPwOiYnII&feature=youtu.be

OpenStax. (2016, May 27) Figure 14. The spleen is similar to a lymph node but is much larger and filters blood instead of lymph [digital image]. In Open Stax, Biology (Section 42.2). OpenStax CNX.  https://cnx.org/contents/GFy_h8cu@10.8:etZobsU-@6/Adaptive-Immune-Response

Simple Nursing. (2015, June 28). Spleen physiology What does the spleen do in 2 minutes. YouTube. https://www.youtube.com/watch?v=ah74jT00jBA&feature=youtu.be

University Hospitals Bristol and Weston NHS FT. (2020, May 13). How to check your lymph nodes. YouTube. https://www.youtube.com/watch?v=L4KexZZAdyA&feature=youtu.be

 

Created by CK-12 Foundation/Adapted by Christine Miller

The photomicrograph in Figure 17.5.1 shows a group of killer T cells (green and red) surrounding a cancer cell (blue, centre). When a killer T cell makes contact with the cancer cell, it attaches to and spreads over the dangerous target. The killer T cell then uses special chemicals stored in vesicles (red) to deliver the killing blow. This event has thus been nicknamed “the kiss of death.” After the target cell is killed, the killer T cells move on to find the next victim. Killer T cells like these are important players in the adaptive immune system.

The adaptive immune system is a subsystem of the overall immune systemno post. It is composed of highly specialized cells and processes that eliminate specific pathogens and tumor cells. An adaptive immune response is set in motion by antigens that the immune system recognizes as foreign. Unlike an innate immune response, an adaptive immune response is highly specific to a particular pathogen (or its antigen). An important function of the adaptive immune system that is not shared by the innate immune system is the creation of immunological memory — or immunity — which occurs after the initial response to a specific pathogen. It allows for a faster, stronger response on subsequent encounters with the same pathogen, usually before the pathogen can even cause symptoms of illness.

Lymphocytes are the main cells of the adaptive immune system. They are leukocytes that arise and mature in organs of the lymphatic system, including the bone marrow and thymus. The human body normally has about 2 trillion lymphocytes, which constitute about 1/3 of all leukocytes. Most of the lymphocytes are normally sequestered within tissue fluid or organs of the lymphatic system, including the tonsils, spleen, and lymph nodes. Only about 2% of the lymphocytes are normally circulating in the blood. There are two main types of lymphocytes involved in adaptive immune responses, called T cells and B cells. T cells destroy infected cells or release chemicals that regulate immune responses. B cells secrete antibodies that bind with antigens[/pb_glossary] of [pb_glossary id="271"]pathogens so they can be removed by other immune cells or processes.

There are multiple types of T cells, or T lymphocytes. Major types are killer (or cytotoxic) T cells and helper T cells. Both types develop from immature T cells that become activated by exposure to an antigen.

T Cell Activation (or Cell-Mediated Immunity)

T cells must be activated to become either killer T cells or helper T cells. This requires presentation of a foreign antigen by antigen-presenting cells, as shown in Figure 17.5.2. Antigen-presenting cells may be dendritic cells, macrophages, or B cells. Activation occurs when T cells are presented with a foreign antigen coupled with an MHC self antigen. Helper T cells are more easily activated than killer T cells. Activation of killer T cells is strongly regulated and may require additional stimulation from helper T cells.

Killer T Cells

Activated killer T cells induce the death of cells that bear a specific non-self antigen because they are infected with pathogens or are cancerous. The antigen targets the cell for destruction by killer T cells, which travel through the bloodstream searching for target cells to kill. Killer T cells may use various mechanisms to kill target cells. One way is by releasing toxins in granules that enter and kill infected or cancerous cells (see Figure 17.5.3).

Helper T Cells

Activated helper T cells do not kill infected or cancerous cells. Instead, their role is to “manage” both innate and adaptive immune responses by directing other cells to perform these tasks. They control other cells by releasing cytokines, which are proteins that can influence the activity of many cell types, including killer T cells, B cells, and macrophages. Some cytokines released by helper T cells assist with the activation of killer T cells.

B cells, or B lymphocytes, are the major cells involved in the creation of antibodies that circulate in blood plasma and lymph. Antibodies are large, Y-shaped proteins used by the immune systemno post to identify and neutralize foreign invaders. Besides producing antibodies, B cells may also function as antigen-presenting cells, or secrete cytokines that help control other immune cells and responses.

B Cell Activation (or Antibody-Mediated Immunity)

Before B cells can actively function to defend the host, they must be activated. As shown in Figure 17.5.4, B cell activation begins when a B cell engulfs and digests an antigen. The antigen may be either free floating in the lymph, or it may be presented by an antigen-presenting cell, such as a dendritic cell or macrophage. In either case, the B cell then displays antigen fragments bound to its own MHC antigens. The MHC-antigen complex on the B cell attracts helper T cells. The helper T cells, in turn, secrete cytokines that help the B cell to multiply, and the daughter cells to mature into plasma cells.

Plasma Cells

Plasma cells are antibody-secreting cells that form from activated B cells. Each plasma cell is like a tiny antibody factory. It may secrete millions of copies of an antibody, each of which can bind to the specific antigen that activated the original B cell. The specificity of an antibody to a specific antigen is illustrated in Figure 17.5.5. When antibodies bind with antigens, it makes the cells bearing them easier targets for phagocytes to find and destroy. Antibody-antigen complexes may also trigger the complement system of the innate immune system, which destroys the cells in a cascade of protein enzymes. In addition, the complexes are likely to clump together (agglutinate). If this occurs, they are filtered out of the blood in the spleen or liver.

Once a pathogen has been cleared from the body, most activated T cells and B cells die within a few days. A few of the cells, however, survive and remain in the body as memory T cells or memory B cells. These memory cells are ready to activate an immediate response if they are exposed to the same antigen again in the future. This is the basis of immunity.

The earliest known reference to the concept of immunity relates to the bubonic plaque (see Figure 17.5.6). In 430 B.C., a Greek historian and general named Thucydides noted that people who had recovered from a previous bout of the plague could nurse people who were sick with the plague without contracting the illness a second time. We now know that this is true of many diseases, and that it occurs because of active immunity.

Active Immunity

Active immunity is the ability of the adaptive immune system to resist a specific pathogen because it has formed an immunological memory of the pathogen. Active immunity is adaptive, because it occurs during the lifetime of an individual as an adaptation to infection with a specific pathogen, and prepares the immune system for future challenges from that pathogen. Active immunity can come about naturally or artificially.

Naturally Acquired Active Immunity

Active immunity is acquired naturally when a pathogen invades the body and activates the adaptive immune system. When the initial infection is over, memory B cells and memory T cells remain, providing immunological memory of the pathogen. As long as the memory cells are alive, the immune system is ready to mount an immediate response if the same pathogen tries to infect the body again.

Artificially Acquired Active Immunity

Active immunity can also be acquired artificially through immunization. Immunization is the deliberate exposure of a person to a pathogen in order to provoke an adaptive immune response and the formation of memory cells specific to that pathogen. The pathogen is introduced in a vaccine — usually by injection, sometimes by nose or mouth (see Figure 17.5.7) — so immunization is also called vaccination.

Typically, only part of a pathogen, a weakened form of the pathogen, or a dead pathogen is used in a vaccine, which causes an adaptive immune response without making the immunized person sick. This is how you most likely became immune to diseases such as measles, mumps, and chicken pox. Immunizations may last for a lifetime, or they may require periodic booster shots to maintain immunity. While immunization generally has long-lasting effects, it usually takes several weeks to develop full immunity.

Immunization is the most effective method ever discovered of preventing infectious diseases. As many as 3 million deaths are prevented each year because of vaccinations. Widespread immunity from vaccinations is largely responsible for the worldwide eradication of smallpox, and the near elimination of several other infectious diseases from many populations, including polio and measles. Immunization is so successful because it exploits the natural specificity and inducibility of the adaptive immune system.

Passive Immunity

Passive immunity results when pathogen-specific antibodies or activated T cells are transferred to a person who has never been exposed to the pathogen. Passive immunity provides immediate protection from a pathogen, but the adaptive immune system does not develop immunological memory to protect the host from the same pathogen in the future. Unlike active immunity, passive immunity lasts only as long as the transferred antibodies or T cells survive in the blood — usually between a few days and a few months. However, like active immunity, passive immunity can be acquired both naturally and artificially.

Naturally Acquired Passive Immunity

Passive immunity is acquired naturally by a fetus through its mother’s blood. Antibodies are transported from mother to fetus across the placenta, so babies have high levels of antibodies at birth. Their antibodies have the same range of antigen specificities as their mother’s. Passive immunity may also be acquired by an infant through the mother’s breast milk. This gives young infants protection from common pathogens in their environment while their own immune system matures.

Artificially Acquired Passive Immunity

Older children and adults can acquire passive immunity artificially through the injection of antibodies or activated T cells, which may be done when there is a high risk of infection and insufficient time for the body to develop active immunity through vaccination. It may also be done to reduce symptoms of ongoing disease, or to compensate for immunodeficiency diseases.

Many pathogens have been around for a long time, living with human populations for generations. To persist, some have evolved mechanisms to evade the adaptive immune system of human hosts. One way they have done this is by rapidly changing their non-essential antigens. This is called antigenic variation. An example of a pathogen that takes this approach is human immunodeficiency virus (HIV). It mutates rapidly so the proteins on its viral envelope are constantly changing. By the time the adaptive immune system responds, the virus’s antigens have changed. Antigenic variation is the main reason that efforts to develop a vaccine against HIV have not yet been successful.

Another evasion approach that some pathogens may take is to mask pathogen antigens with host molecules so the host’s immune system cannot detect the antigens. HIV takes this approach, as well. The envelope that covers the virus is formed from the outermost membrane of the host cell.

If you think that immunizations are just for kids, think again. There are several vaccines recommended by HealthLinkBC for people over the age of 18. The tables below from HealthLinkBC show the vaccine schedules recommended for infants and children, school-aged children, and adults and senior. Additional vaccines may be recommended for certain adults based on specific travel plans, medical conditions or other indications. Are you up to date with your vaccines? You can check with your doctor to be sure.

 

 

Attributions

Figure 17.5.1

Killer_T_cells_surround_a_cancer_cell by Alex Ritter, Jennifer Lippincott Schwartz and Gillian Griffiths at the National Institutes of Health/ Visuals Online on Wikimedia Commons is in the public domain (https://en.wikipedia.org/wiki/Public_domain).

Figure 17.5.2

T_cell_activation.svg by Rehua (derivative work) on Wikimedia Commons is in the public domain (https://en.wikipedia.org/wiki/Public_domain). (Original image: T_cell_activation.png:  from The Immune System - NIH Publication No. 03–5423)

Figure 17.5.3

Cytotoxic T Cell function by CNX OpenStax on Wikimedia Commons is used under a CC BY 4.0 (https://creativecommons.org/licenses/by/4.0) license.

Figure 17.5.4

B_cell_activation.svg by Fred the Oyster on Wikimedia Commons is in the public domain (https://en.wikipedia.org/wiki/Public_domain). (Original from The Immune System - NIH Publication No. 03–5423)

Figure 17.5.5

Antibody.svg by Fvasconcellos  on Wikimedia Commons is in the public domain (https://en.wikipedia.org/wiki/Public_domain). (Original black and white image from the National Human Genome Research Institute's Talking Genetics Glossary)

Figure 17.5.7

immunizations by U.S. Air Force photo by Airman 1st Class Destinee Dougherty from Military Health System website, Health.mil, is in the public domain (https://en.wikipedia.org/wiki/Public_domain).

 

References

HealthLinkBC. (2018). B.C. immunization schedules.  Gov.BC.CA.  https://www.healthlinkbc.ca/tools-videos/bc-immunization-schedules

Mayo Clinic Staff. (n.d.). Measles [online article]. MayoClinic.org. https://www.mayoclinic.org/diseases-conditions/measles/symptoms-causes/syc-20374857

Mayo Clinic Staff. (n.d.). Mumps [online article]. MayoClinic.org. https://www.mayoclinic.org/diseases-conditions/mumps/symptoms-causes/syc-20375361

Mayo Clinic Staff. (n.d.). Polio [online article]. MayoClinic.org. https://www.mayoclinic.org/diseases-conditions/polio/symptoms-causes/syc-20376512

Mayo Clinic Staff. (n.d.). Smallpox [online article]. MayoClinic.org. https://www.mayoclinic.org/diseases-conditions/smallpox/symptoms-causes/syc-20353027

NIAID. (2016, August 11). An HIV vaccine: Mapping uncharted territory. YouTube. https://www.youtube.com/watch?v=X-rC78MKZvw&feature=youtu.be

OpenStax. (2016, March 23). Figure 4 Naïve CD4⁺ T cells engage MHC II molecules on antigen-presenting cells (APCs) and become activated. Clones of the activated helper T cell, in turn, activate B cells and CD8⁺ T cells, which become cytotoxic T cells. Cytotoxic T cells kill infected cells [digital image]. In OpenStax, Biology (Section 42.2). OpenStax CNX. https://cnx.org/contents/GFy_h8cu@10.53:etZobsU-@6/Adaptive-Immune-Response

Seeker. (2015, September 2). Why do we need a new flu shot every year? YouTube. https://www.youtube.com/watch?v=5THf6gTNqO8

TED-Ed. (2015, January 12). How do vaccines work? - Kelwalin Dhanasarnsombut. YouTube. https://www.youtube.com/watch?v=rb7TVW77ZCs&feature=youtu.be

TED-Ed. (2013, October 28). How we conquered the deadly smallpox virus - Simona Zompi. YouTube. https://www.youtube.com/watch?v=yqUFy-t4MlQ&feature=youtu.be

Created by CK-12 Foundation/Adapted by Christine Miller

These people are participating in a bike ride to raise funds for leukemia and lymphoma research (Figure 17.7.1). Leukemia and lymphoma are blood cancers. In 2020,  approximately 6,900 Canadians will be diagnosed with leukemia and 3,000 will die from this cancer. Lymphoma is the most common type of blood cancer. As a lymphoma patient, Hakeem, who you learned about in the beginning of this chapter, may eventually benefit from research funded by a bike ride like this one.

What type of blood cell is affected in lymphoma? As the name implies, lymphoma is a cancer that affects lymphocytes, which are a type of leukocyte. As you have learned in this chapter, there are different types of lymphocytes, including the B and T cells of the adaptive immune system. Different types of lymphoma affect different types of lymphocytes in different ways. It is important to correctly identify the type of lymphoma, so that patients can be treated appropriately.

You may recall that one of Hakeem’s symptoms was a swollen lymph node, and he was diagnosed with lymphoma after a biopsy of that lymph node. Swollen lymph nodes are a common symptom of lymphoma. As you have learned, lymph nodes are distributed throughout the body along lymphatic vessels, as part of the lymphatic system. The lymph nodes filter lymph and store lymphocytes. Therefore, they play an important role in fighting infections. Because of this, they will often swell in response to an infection. In Hakeem’s case, the swelling and other symptoms did not improve after several weeks and a course of antibiotics, which caused Dr. Hayes to suspect lymphoma instead. The biopsy showed that Hakeem did indeed have cancerous lymphocytes in his lymph nodes.

But which type of lymphocytes were affected? Lymphoma most commonly affects B or T lymphocytes. The two major types of lymphoma are called Hodgkin (HL) or non-Hodgkin lymphoma (NHL). NHL is more common than HL. In 2020, the Canadian Cancer Society estimates 10,400 Canadians will be diagnosed with non-Hodgkin lymphoma, whereas 1,000 will be diagnosed with Hodgkin lymphoma. While HL is one distinct type of lymphoma, NHL has about 60 different subtypes, depending on which specific cells are affected and how.

Hakeem was diagnosed with a type of NHL called diffuse large B-cell lymphoma (DLBCL) — the most common type of NHL. This type of lymphoma affects B cells and causes them to appear large under the microscope. In addition to Hakeem’s symptoms of fatigue, swollen lymph nodes, loss of appetite, and weight loss, common symptoms of this type of lymphoma include fever and night sweats. It is an aggressive and fast-growing type of lymphoma that is fatal if not treated. The good news is that with early detection and proper treatment, about 70% of patients with DLBCL can be cured.

How do physicians determine the specific type of lymphoma? Tissue obtained from a biopsy can be examined under a microscope to observe physical changes (such as abnormal cell size or shape) that are characteristic of a particular subtype of lymphoma. Additionally, tests can be performed on the tissue to determine which cell-surface antigens are present. Recall that antigens are molecules that bind to specific antibodies. Antibodies can be produced in the laboratory and labeled with compounds that can be identified by their colour under a microscope. When these antibodies are applied to a tissue sample, this colour will appear wherever the antigen is present, because it binds to the antibody. This technique was used in the photomicrograph in Figure 17.7.2 to identify the presence of a cell-surface antigen (shown as reddish-brown) in a sample of skin cells. This technique, called immunohistochemistry, is also commonly used to identify antigens in tissue samples from lymphoma patients.

Why would identifying cell-surface antigens be important in diagnosing and treating lymphoma? As you have learned, the immune system uses antigens present on the surface of cells or pathogens to distinguish between self and non-self, and to launch adaptive immune responses. Cells that become cancerous often change their cell-surface antigens. This is one way that the immune system can identify and destroy them. Also, different cell types in the body can sometimes be identified by the presence of specific cell-surface antigens. Knowing the types of cell-surface antigens present in a tissue sample can help physicians identify which cells are cancerous, and possibly the specific subtype of cancer. Knowing this information can be helpful in choosing more tailored and effective treatments.

One treatment for NHL is, in fact, the use of medications made from antibodies that bind to cell-surface antigens present on cells affected by the specific subtype of NHL. This is called immunotherapy. These drugs can directly bind to and kill the cancerous cells. For patients with DLBCL like Hakeem, immunotherapy is often used in conjunction with chemotherapy and radiation as a course of treatment. Although Hakeem has a difficult road ahead, he and his medical team are optimistic that — given the high success rate when DLBCL is caught and treated early — he may be cured. More research into how the immune system functions may lead to even better treatments for lymphoma — and other types of cancers — in the future.

Attributions

Figure 17.7.1

Cycling to Beat Blood Cancer by Blood Cancer UK (Formerly Bloodwise) on Flickr is used under a CC BY-NC-ND 2.0 (https://creativecommons.org/licenses/by-nc-nd/2.0/) license.

Figure 17.7.2

antigen stain by Ed Uthman from Houston, TX, USA on Wikimedia Commons is used under a CC BY 2.0 (https://creativecommons.org/licenses/by/2.0) license.

References

Hodgkin lymphoma statistics [online article]. (2020). Canadian Cancer Society. https://www.cancer.ca:443/en/cancer-information/cancer-type/hodgkin-lymphoma/statistics/?region=on

Non-Hodgkin lymphoma statistics [online article]. (2020). Canadian Cancer Society. https://www.cancer.ca:443/en/cancer-information/cancer-type/non-hodgkin-lymphoma/statistics/?region=on

TED-Ed. (2015, April 30). What is leukemia? - Danilo Allegra and Dania Puggioni. YouTube. https://www.youtube.com/watch?v=Z3B-AaqjyjE&feature=youtu.be

 

Created by CK-12 Foundation/Adapted by Christine Miller

A tiny sperm from dad breaks through the surface of a huge egg from mom. Voilà! In nine months, a new son or daughter will be born. Like most other multicellular organisms, human beings reproduce sexually. In human sexual reproduction, males produce sperm and females produce eggs, and a new offspring forms when a sperm unites with an egg. How do sperm and eggs form? And how do they arrive together at the right place and time so they can unite to form a new offspring? These are functions of the reproductive system.

The reproductive system is the human organ system responsible for the production and fertilization of gametes (sperm or eggs) and, in females, the carrying of a fetus. Both male and female reproductive systems have organs called gonads that produce gametes. A gamete is a haploid cell that combines with another haploid gamete during fertilization, forming a single diploid cell called a zygote. Besides producing gametes, the gonads also produce sex hormones. Sex hormones are endocrine hormones that control the development of sex organs before birth, sexual maturation at puberty, and reproduction once sexual maturation has occurred. Other reproductive system organs have various functions, such as maturing gametes, delivering gametes to the site of fertilization, and providing an environment for the development and growth of an offspring.

The reproductive system is the only human organ system that is significantly different between males and females. Embryonic structures that will develop into the reproductive system start out the same in males and females, but by birth, the reproductive systems have differentiated. How does this happen?

Sex Differentiation

Starting around the seventh week after conception in genetically male (XY) embryos, a gene called SRY on the Y chromosome (shown in Figure 18.2.2) initiates the production of multiple proteins. These proteins cause undifferentiated gonadal tissue to develop into male gonads (testes). The male gonads then secrete hormones — including the male sex hormone testosterone — that trigger other changes in the developing offspring (now called a fetus), causing it to develop a complete male reproductive system. Without a Y chromosome, an embryo will develop female gonads (ovaries) that will produce the female sex hormone estrogen. Estrogen, in turn, will lead to the formation of the other organs of a normal female reproductive system.

Homologous Structures

Undifferentiated embryonic tissues develop into different structures in male and female fetuses. Structures that arise from the same tissues in males and females are called homologous structures. The male testes and female ovaries, for example, are homologous structures that develop from the undifferentiated gonads of the embryo. Likewise, the male penis and female clitoris are homologous structures that develop from the same embryonic tissues.

Sex Hormones and Maturation

Male and female reproductive systems are different at birth, but they are immature and incapable of producing gametes or sex hormones. Maturation of the reproductive system occurs during puberty, when hormones from the hypothalamus and pituitary gland stimulate the testes or ovaries to start producing sex hormones again. The main sex hormones are testosterone in males and estrogen in females. Sex hormones, in turn, lead to the growth and maturation of the reproductive organs, rapid body growth, and the development of secondary sex characteristics. Secondary sex characteristics are traits that are different in mature males and females, but are not directly involved in reproduction. They include facial hair in males and breasts in females.

The main structures of the male reproductive system are external to the body and illustrated in Figure 18.2.3. The two testes (singular, testis) hang between the thighs in a sac of skin called the scrotum. The testes produce both sperm and testosterone. Resting atop each testis is a coiled structure called the epididymis (plural, epididymes). The function of the epididymes is to mature and store sperm. The penis is a tubular organ that contains the urethra and has the ability to stiffen during sexual arousal. Sperm passes out of the body through the urethra during a sexual climax (orgasm). This release of sperm is called ejaculation.

In addition to these organs, the male reproductive system consists of several ducts and glands that are internal to the body. The ducts, which include the vas deferens (also called the ductus deferens), transport sperm from the epididymis to the urethra. The glands, which include the prostate gland and seminal vesicles, produce fluids that become part of semen. Semen is the fluid that carries sperm through the urethra and out of the body. It contains substances that control pH and provide sperm with nutrients for energy.

The main structures of the female reproductive system are internal to the body and shown in the following figure. They include the paired ovaries, which are small, ovoid structures that produce ova and secrete estrogen. The two oviducts (sometimes called Fallopian tubes or uterine tubes) start near the ovaries and end at the uterus. Their function is to transport ova from the ovaries to the uterus. If an egg is fertilized, it usually occurs while it is traveling through an oviduct. The uterus is a pear-shaped muscular organ that functions to carry a fetus until birth. It can expand greatly to accommodate a growing fetus, and its muscular walls can contract forcefully during labour to push the baby out of the uterus and into the vagina. The vagina is a tubular tract connecting the uterus to the outside of the body. The vagina is where sperm are usually deposited during sexual intercourse and ejaculation. The vagina is also called the birth canal because a baby travels through the vagina to leave the body during birth.

The external structures of the female reproductive system are referred to collectively as the vulva. They include the clitoris, which is homologous to the male penis. They also include two pairs of labia (singular, labium), which surround and protect the openings of the urethra and vagina.

Attributions

Figure 18.2.1

Sperm-egg by Unknown author on Wikimedia Commons is in the public domain (https://en.wikipedia.org/wiki/public_domain).  

Figure 18.2.2

Y Chromosome by Christinelmiller on Wikimedia Commons is used under a CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0) license. 

Figure 18.2.3

3D_Medical_Animation_Vas_Deferens by https://www.scientificanimations.com on Wikimedia Commons is used under a CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0) license.

Figure 18.2.4

Blausen_0399_FemaleReproSystem_01 by BruceBlaus on Wikimedia Commons is used under a CC BY 3.0 (https://creativecommons.org/licenses/by/3.0) license.

References 

Blausen.com Staff. (2014). Medical gallery of Blausen Medical 2014. WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.010. ISSN 2002-4436.

Reactions. (2015, June 8). Hormones and gender transition. YouTube. https://www.youtube.com/watch?v=l5knvmy1Z3s&feature=youtu.be

TED-Ed. (2012, April 23). Sex determination: More complicated than you thought. YouTube. https://www.youtube.com/watch?v=kMWxuF9YW38&feature=youtu.be

TED-Ed. (2017, April 24). The evolution of animal genitalia - Menno Schilthuizen. YouTube. https://www.youtube.com/watch?v=vcPJkz-D5II&feature=youtu.be

 

Created by CK-12 Foundation/Adapted by Christine Miller

This false-colour image (Figure 18.4.1) shows real human sperm. The tiny gametes are obviously greatly magnified in the picture, because they are actually the smallest of all human cells. In fact, human sperm cells are small, even when compared with sperm cells of other animals. Mice sperm are about twice the length of human sperm! Human sperm may be small in size, but in a normal, healthy man, huge numbers of them are usually released during each ejaculation. There may be hundreds of millions of sperm cells in a single teaspoon of semen. Producing sperm is one of the major functions of the male reproductive system.

A mature sperm cell has several structures that help it reach and penetrate an egg. These are labeled in the drawing of a sperm shown in Figure 18.4.2.

• The head is the part of the sperm that contains the nucleus — and not much else. The nucleus, in turn, contains tightly coiled DNA that is the male parent’s contribution to the genetic makeup of a zygote (if one forms). Each sperm is a haploid cell, containing half the chromosomal complement of a normal, diploid body cell.
• The front of the head is an area called the acrosome. The acrosome contains enzymes that help the sperm penetrate an ovum (if it reaches one).
• The midpiece is the part of the sperm between the head and the flagellum. The midpiece is packed with mitochondria that produce the energy needed to move the flagellum.
• The flagellum (also called the tail) can rotate like a propeller, allowing the sperm to “swim” through the female reproductive tract to reach an ovum if one is present.

The process of producing sperm is known as spermatogenesis. Spermatogenesis normally starts when a male reaches puberty, and it usually continues uninterrupted until death, although a decrease in sperm production generally occurs at older ages. A young, healthy male may produce hundreds of millions of sperm a day! Only about half of these, however, are likely to become viable, mature sperm.

Where Sperm Are Produced

Spermatogenesis occurs in the seminiferous tubules in the testes. Spermatogenesis requires high concentrations of testosterone. Testosterone is secreted by Leydig cells, which are adjacent to the seminiferous tubules in the testes.

Sperm production in the seminiferous tubules is very sensitive to temperature. This may be the most important reason the testes are located outside the body in the scrotum. The temperature inside the scrotum is generally about 2 degrees Celsius cooler than core body temperature. This lower temperature is optimal for spermatogenesis. The scrotum regulates its internal temperature as needed by contractions of the smooth muscles lining the scrotum. When the temperature inside the scrotum becomes too low, the scrotal muscles contract. The contraction of the muscles pulls the scrotum higher against the body, where the temperature is warmer. The opposite occurs when the temperature inside the scrotum becomes too high.

Events of Spermatogenesis

Figure 18.4.3 summarizes the main cellular events that occur in the process of spermatogenesis. The process begins with a diploid stem cell called a spermatogonium (plural, spermatogonia), and involves several cell divisions. The entire process takes at least ten weeks to complete, including maturation in the epididymis.

1. A spermatogonium undergoes mitosis to produce two diploid cells called primary spermatocytes. One of the primary spermatocytes goes on to produce sperm. The other replenishes the reserve of spermatogonia.
2. The primary spermatocyte undergoes meiosis I to produce two haploid daughter cells called secondary spermatocytes.
3. The secondary spermatocytes rapidly undergo meiosis II to produce a total of four haploid daughter cells called spermatids.
4. The spermatids begin to form a tail, and their DNA becomes highly condensed. Unnecessary cytoplasm and organelles are removed from the cells, and they form a head, midpiece, and flagellum. The resulting cells are sperm (spermatozoa).

As shown in Figure 18.4.4, the events of spermatogenesis begin near the wall of the seminiferous tubule — where spermatogonia are located — and continue inward toward the lumen of the tubule. Sertoli cells extend from the wall of the seminiferous tubule inward toward the lumen, so they are in contact with developing sperm at all stages of spermatogenesis. Sertoli cells play several roles in spermatogenesis:

• They secrete endocrine hormones that help regulate spermatogenesis.
• They secrete substances that initiate meiosis.
• They concentrate testosterone (from Leydig cells), which is needed at high levels to maintain spermatogenesis.
• They phagocytize the extra cytoplasm that is shed from developing sperm cells.
• They secrete testicular fluid that helps carry sperm into the epididymis.
• They maintain a blood-testis barrier, so immune system cells cannot reach and attack the sperm.

Maturation in the Epididymis

Although the sperm produced in the testes have tails, they are not yet motile (able to “swim”). The non-motile sperm are transported to the epididymis in testicular fluid that is secreted by Sertoli cells with the help of peristaltic contractions. In the epididymis, the sperm gain motility, so they are capable of swimming up the female genital tract and reaching an ovum. The mature sperm are stored in the epididymis until ejaculation occurs.

Sperm are released from the body during ejaculation, which typically occurs during orgasm. Hundreds of millions of mature sperm — contained within a small amount of thick, whitish fluid called semen — are propelled from the penis during a normal ejaculation.

How Ejaculation Occurs

Ejaculation occurs when peristalsis of the muscle layers of the vas deferens and other accessory structures propel sperm from the epididymes, where mature sperm are stored. The muscle contractions force the sperm through the vas deferens and the ejaculatory ducts, and then out of the penis through the urethra. Due to the peristaltic action of the muscles, the ejaculation occurs in a series of spurts.

The Role of Semen

As sperm travel through the ejaculatory ducts during ejaculation, they mix with secretions from the seminal vesicles, prostate gland, and bulbourethral glands to form semen (see Figure 18.4.5 ). The average amount of semen per ejaculate is about 3.7 mL, which is a little less than a teaspoonful. Most of this volume of semen consists of glandular secretions, with the hundreds of millions of sperm cells actually contributing relatively little to the total volume.

The secretions in semen are important for the survival and motility of sperm. They provide a medium through which sperm can swim. They also include sperm-sustaining substances, such as high concentrations of the sugar fructose, which is the main source of energy for sperm. In addition, semen contains many alkaline substances that help neutralize the acidic environment in the female vagina. This protects the DNA in sperm from being denatured by acid, and prolongs the life of sperm in the female reproductive tract.

Besides providing a way for sperm to leave the body, the main role of the penis in reproduction is intromission, or depositing sperm in the vagina of the female reproductive tract. Intromission depends on the ability of the penis to become stiff and erect, a state referred to as an erection. The human penis, unlike that of most other mammals, contains no erectile bone. Instead, in order to reach its erect state, it relies entirely on engorgement with blood of its columns of spongy tissue. During sexual arousal, the arteries that supply blood to the penis dilate, allowing more blood to fill the spongy tissue. The now-engorged spongy tissue presses against and constricts the veins that carry blood away from the penis. As a result, more blood enters than leaves the penis, until a constant erectile size is achieved.

In addition to sperm, the penis also transports urine out of the body. These two functions cannot occur simultaneously. During an erection, the sphincters that prevent urine from leaving the bladder are controlled by centres in the brain so they cannot relax and allow urine to enter the urethra.

The final major function of the male reproductive system is the production of the male sex hormone testosterone. In mature males, this occurs mainly in the testes. Testosterone production is under the control of luteinizing hormone (LH) from the pituitary gland. LH stimulates Leydig cells in the testes to secrete testosterone.

Testosterone is important for male sexual development at puberty. It stimulates maturation of the male reproductive organs, as well as the development of secondary male sex characteristics (such as facial hair). Testosterone is also needed in mature males for normal spermatogenesis to be maintained in the testes. Follicle stimulating hormone (FSH) from the pituitary gland is also needed for spermatogenesis to occur, in part because it helps Sertoli cells in the testes concentrate testosterone to high enough levels to maintain sperm production. Testosterone is also needed for proper functioning of the prostate gland. In addition, testosterone plays a role in erection, allowing sperm to be deposited within the female reproductive tract.

If you’re a man and you use a laptop computer on your lap for long periods of time, you may be decreasing your fertility. The reason? A laptop computer generates considerable heat, and its proximity to the scrotum during typical use results in a significant rise in temperature inside the scrotum. Spermatogenesis is very sensitive to high temperatures, so it may be adversely affected by laptop computer use. If you want to avoid the potentially fertility-depressing effect of laptop computer use, you might want to consider using your laptop computer on a table or other surface rather than on your lap — at least when you log on for long computer sessions. Other activities that raise scrotal temperature and have the potential to reduce spermatogenesis including soaking in hot tubs, wearing tight clothing, and biking. Although the effects of short-term scrotal heating on fertility seem to be temporary, years of such heat exposure may cause irreversible effects on sperm production.

Attributions

Figure 18.4.1

Sperm-20051108 by Gilberto Santa Rosa from Rio de Janeiro, Brazil on Wikimedia Commons is used under a CC BY 2.0 (https://creativecommons.org/licenses/by/2.0) license.

Figure 18.4.2

Sperm Anatomy by Christinelmiller on Wikimedia Commons is used under a  CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0) license.

Figure 18.4.3

Spermatogenesis by OpenStax College is used and adapted by Christine Miller under a CC BY 3.0 (https://creativecommons.org/licenses/by/3.0) license.

Figure 18.4.4

Testis-cross-section by CK-12 Foundation is used under a CC BY-NC 3.0 (https://creativecommons.org/licenses/by-nc/3.0/) license.

Figure 18.4.5

Human_semen_in_a_petri_dish by Digitalkil on Wikimedia Commons is in the public domain (https://en.wikipedia.org/wiki/en:public_domain).

Figure 18.4.6

Laptop by logan-weaver-b76PEyeIptQ-unsplash [photo] by LOGAN WEAVER on Unsplash is used under the Unsplash License (https://unsplash.com/license).

References

Betts, J. G., Young, K.A., Wise, J.A., Johnson, E., Poe, B., Kruse, D.H., Korol, O., Johnson, J.E., Womble, M., DeSaix, P. (2013, June 19). Figure 27.5 Spermatogenesis [digital image].  In Anatomy and Physiology (Section 27.1). OpenStax. https://openstax.org/books/anatomy-and-physiology/pages/27-1-anatomy-and-physiology-of-the-male-reproductive-system

Brainard, J/ CK-12 Foundation. (2016). Figure 4 Cross-section of a testis and seminiferous tubules [digital image]. In CK-12 College Human Biology (Section 20.4) [online Flexbook]. CK12.org. https://www.ck12.org/book/ck-12-college-human-biology/section/20.4/

Janux. (2015, January 10). Human physiology - Reproduction: spermatogenesis. YouTube. https://www.youtube.com/watch?v=krSMZDsjLuU&feature=youtu.be

Seeker. (2014, June 16). How you're destroying your sperm! YouTube. https://www.youtube.com/watch?v=gNHSTa0Yct4&feature=youtu.be

 

Created by CK-12 Foundation/Adapted by Christine Miller

The banner in Figure 18.8.1 was carried in a 2014 march in Uganda as part of the celebration of Menstrual Hygiene Day. Menstrual Hygiene Day is an awareness day on May 28 of each year that aims to raise awareness worldwide about menstruation and menstrual hygiene. Maintaining good menstrual hygiene is difficult in developing countries like Uganda because of taboos on discussing menstruation and lack of availability of menstrual hygiene products. Poor menstrual hygiene, in turn, can lead to embarrassment, degradation, and reproductive health problems in females. May 28 was chosen as Menstrual Hygiene Day because of its symbolism. May is the fifth month of the year, and most women average five days of menstrual bleeding each month. The 28th day was chosen because the menstrual cycle averages about 28 days.

The menstrual cycle refers to natural changes that occur in the female reproductive system each month during the reproductive years. The cycle is necessary for the production of ova and the preparation of the uterus for pregnancy. It involves changes in both the ovaries and the uterus, and is controlled by pituitary and ovarian hormones. Day 1 of the cycle is the first day of the menstrual period, when bleeding from the uterus begins as the built-up endometrium lining the uterus is shed. The endometrium builds up again during the remainder of the cycle, only to be shed again during the beginning of the next cycle if pregnancy does not occur. In the ovaries, the menstrual cycle includes the development of a follicle, ovulation of a secondary oocyte, and then degeneration of the follicle if pregnancy does not occur. Both uterine and ovarian changes during the menstrual cycle are generally divided into three phases, although the phases are not the same in the two organs.

Menarche and Menopause

The female reproductive years are delineated by the start and stop of the menstrual cycle. The first menstrual period usually occurs around 12 or 13 years of age, an event that is known as menarche. There is considerable variation among individuals in the age at menarche. It may occasionally occur as early as eight years of age or as late as 16 years of age and still be considered normal. The average age is generally later in the developing world, and earlier in the developed world. This variation is thought to be largely attributable to nutritional differences.

The cessation of menstrual cycles at the end of a woman’s reproductive years is termed menopause. The average age of menopause is 52 years, but it may occur normally at any age between about 45 and 55 years of age. The age of menopause varies due to a variety of biological and environmental factors. It may occur earlier as a result of certain illnesses or medical treatments.

Variation in the Menstrual Cycle

The length of the menstrual cycle — as well as its phases — may vary considerably, not only among different women, but also from month to month for a given woman. The average length of time between the first day of one menstrual period and the first day of the next menstrual period is 28 days, but it may range from 21 days to 45 days. Cycles are considered regular when a woman’s longest and shortest cycles differ by less than eight days. The menstrual period itself is usually about five days long, but it may vary in length from about two days to seven days.

The events of the menstrual cycle that take place in the ovaries make up the ovarian cycle. It consists of changes that occur in the follicles of one of the ovaries. The ovarian cycle is divided into the following three phases: follicular phase, ovulation, and luteal phase. These phases are illustrated in Figure 18.8.2.

Follicular Phase

The follicular phase is the first phase of the ovarian cycle. It generally lasts about 12 to 14 days for a 28-day menstrual cycle. During this phase, several ovarian follicles are stimulated to begin maturing, but usually only one — called the Graafian follicle — matures completely so it is ready to release an egg. The other maturing follicles stop growing and disintegrate. Follicular development occurs because of a rise in the blood level of follicle stimulating hormone (FSH), which is secreted by the pituitary gland. The maturing follicle releases estrogen, the level of which rises throughout the follicular phase. You can see these and other changes in hormone levels that occur during the menstrual cycle in the following chart.

Ovulation

Ovulation is the second phase of the ovarian cycle. It usually occurs around day 14 of a 28-day menstrual cycle. During this phase, the Graafian follicle ruptures and releases its ovum. Ovulation is stimulated by a sudden rise in the blood level of luteinizing hormone (LH) from the pituitary gland. This is called the LH surge. You can see the LH surge in the top hormone graph in Figure 18.8.3. The LH surge generally starts around day 12 of the cycle and lasts for a day or two. The surge in LH is triggered by a continued rise in estrogen from the maturing follicle in the ovary. During the follicular phase, the rising estrogen level actually suppresses LH secretion by the pituitary gland. However, by the time the follicular phase is nearing its end, the level of estrogen reaches a threshold level above which this effect is reversed, and estrogen stimulates the release of a large amount of LH. The surge in LH matures the ovum and weakens the wall of the follicle, causing the fully developed follicle to release its secondary oocyte.

Luteal Phase

The luteal phase is the third and final phase of the ovarian cycle. It typically lasts about 14 days in a 28-day menstrual cycle. At the beginning of the luteal phase, FSH and LH cause the Graafian follicle that ovulated the egg to transform into a structure called a corpus luteum. The corpus luteum secretes progesterone, which in turn suppresses FSH and LH production by the pituitary gland and stimulates the continued buildup of the endometrium in the uterus. How this phase ends depends on whether or not the ovum has been fertilized.

• If fertilization has not occurred, the falling levels of FSH and LH during the luteal phase cause the corpus luteum to atrophy, so its production of progesterone declines. Without a high level of progesterone to maintain it, the endometrium starts to break down. By the end of the luteal phase, the endometrium can no longer be maintained, and the next menstrual cycle begins with the shedding of the endometrium (menses).
• If fertilization has occurred so a zygote forms and then divides to become a blastocyst, the outer layer of the blastocyst produces a hormone called human chorionic gonadotropin (HCG). This hormone is very similar to LH and preserves the corpus luteum. The corpus luteum can then continue to secrete progesterone to maintain the new pregnancy.

The events of the menstrual cycle that take place in the uterus make up the uterine cycle. This cycle consists of changes that occur mainly in the endometrium, which is the layer of tissue that lines the uterus. The uterine cycle is divided into the following three phases: menstruation, proliferative phase, and secretory phase. These phases are illustrated in Figure 18.8.4.

Menstruation

Menstruation (also called menstrual period or menses) is the first phase of the uterine cycle. It occurs if fertilization has not taken place during the preceding menstrual cycle. During menstruation, the endometrium of the uterus, which has built up during the preceding cycle, degenerates and is shed from the uterus, flowing through an opening in the cervix, and out through the external opening of the vagina. The average loss of blood during menstruation is about 35 mL (about 1 oz or 2 tablespoons). The flow of blood is often accompanied by uterine cramps, which may be severe in some women.

Proliferative Phase

The proliferative phase is the second phase of the uterine cycle. During this phase, estrogen secreted by cells of the maturing ovarian follicle causes the lining of the uterus to grow, or proliferate. Estrogen also stimulates the cervix of the uterus to secrete larger amounts of thinner mucus that can help sperm swim through the cervix and into the uterus, making fertilization more likely.

Secretory Phase

The secretory phase is the third and final phase of the uterine cycle. During this phase, progesterone produced by the corpus luteum in the ovary stimulates further changes in the endometrium so it is more receptive to implantation of a blastocyst. For example, progesterone increases blood flow to the uterus and promotes uterine secretions. It also decreases the contractility of smooth muscle tissue in the uterine wall.

 

Bringing it All Together

It is important to note that the pituitary gland, the ovaries and the uterus are all responsible for parts of the ovarian and uterine cycles.  The pituitary hormones, LH and FSH affect the ovarian cycle and its hormones.  The ovarian hormones, estrogen and progesterone affect the uterine cycle and also feedback on the pituitary gland.  Look at Figure 18.8.5 and look at what is happening on different days of the cycle in each of the sets of hormones, the ovarian cycle and the uterine cycle.

 

Attributions

Figure 18.8.1

WaterforPeople_Uganda by WaterforPeople_Uganda on Wikimedia Commons is used under a CC BY 2.0 (https://creativecommons.org/licenses/by/2.0) license.

Figure 18.8.2

Ovarian Cycle by CNX OpenStax on Wikimedia Commons is used and adapted under a CC BY 4.0 (https://creativecommons.org/licenses/by/4.0) license.

Figure 18.8.3

Figure_43_04_04 by CNX OpenStax on Wikimedia Commons is used under a CC BY 4.0 (https://creativecommons.org/licenses/by/4.0) license. (Original: modification of work by Mikael Häggström)

Figure 18.8.4

Ovarian and menstrual cycle by OpenStax College on Wikimedia Commons is used under a  CC BY 3.0 (https://creativecommons.org/licenses/by/3.0) license.

Figure 18.8.5

1000px-MenstrualCycle2_en.svg by Isometrik on Wikimedia Common is used under a CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0) license.

References

Betts, J. G., Young, K.A., Wise, J.A., Johnson, E., Poe, B., Kruse, D.H., Korol, O., Johnson, J.E., Womble, M., DeSaix, P. (2013, June 19). Figure 27.15 Hormone levels in ovarian and menstrual cycles [digital image].  In Anatomy and Physiology (Section 27.2). OpenStax. https://openstax.org/books/anatomy-and-physiology/pages/27-2-anatomy-and-physiology-of-the-female-reproductive-system

National Geographic. (2007, May 31). Girl's rite of passage | National Geographic. YouTube. https://www.youtube.com/watch?v=5B3Abpv0ysM&feature=youtu.be

OpenStax. (2016, May 27) Figure 4 Rising and falling hormone levels result in progression of the ovarian and menstrual cycles [digital image]. In Open Stax, Biology (Section 43.4). OpenStax CNX. https://cnx.org/contents/GFy_h8cu@10.53:Ha3dnFEx@6/Hormonal-Control-of-Human-Reproduction

TED-Ed. (2015, October 19). Why do women have periods? YouTube.  https://www.youtube.com/watch?v=cjbgZwgdY7Q&feature=youtu.be

Created by CK-12 Foundation/Adapted by Christine Miller

Figure 18.10.1 Families all have something in common.

What do all these families (Figure 18.10.1) have in common?  They were born!  Every person on this planet was conceived, carried in utero and then born.  While families come in all shapes, sizes and styles, we all came into existence in the same way.  Virtually all human societies past and present — value having children. Indeed, for many people, parenthood is an important life goal. Unfortunately, some people are unable to achieve that goal because of infertility.

Infertility is the inability of a sexually mature adult to reproduce by natural means. For scientific and medical purposes, infertility is generally defined as the failure to achieve a successful pregnancy after at least one year of regular, unprotected sexual intercourse. Infertility may be primary or secondary. Primary infertility applies to cases in which an individual has never achieved a successful pregnancy. Secondary infertility applies to cases in which an individual has had at least one successful pregnancy, but fails to achieve another after trying for at least a year. Infertility is a common problem. The government of Canada reported that in 2019, 16% of Canadian couples experience infertility, a number which has doubled since the 1980s. If you look around at the couples you know, that means that almost 1 in 6 of them are having issues with fertility.

Pregnancy is the result of a multi-step process. In order for a normal pregnancy to occur, a woman must release an ovum from one of her ovaries, the ovum must go through an oviduct, a man’s sperm must fertilize the ovum as it passes through the oviduct, and then the resulting zygote must implant in the uterus. If there is a problem with any of these steps, infertility can result.

A couple’s infertility may be due to a problem with either the male or the female partner. As shown in the circle graph below (Figure 18.10.2), about 40% of infertility cases are due to female infertility, and about 30% are due to male infertility. The remaining 30% of cases are due to a combination of male and female problems or unknown causes.

Causes of Male Infertility

Male infertility occurs when there are no, or too few, sperm, or when the sperm are not healthy and motile and cannot travel through the female reproductive tract to fertilize an egg. A common cause of inadequate numbers or motility of sperm is varicocele, which is enlargement of blood vessels in the scrotum. This may raise the temperature of the testes and adversely affect sperm production. In other cases, there is no problem with the sperm, but there is a blockage in the male reproductive tract that prevents the sperm from being ejaculated.

Factors that increase a man’s risk of infertility include heavy alcohol use, drug abuse, cigarette smoking, exposure to environmental toxins (such as pesticides or lead), certain medications, serious diseases (such as kidney disease), and radiation or chemotherapy for cancer. Another risk factor is advancing age. Male fertility normally peaks in the mid-twenties and gradually declines after about age 40, although it may never actually drop to zero.

Causes of Female Infertility

Female infertility generally occurs due to one of two problems: failure to produce viable ova by the ovaries, or structural problems in the oviducts or uterus. The most common cause of female infertility is a problem with ovulation. Without ovulation, there are no ova to be fertilized. Anovulatory cycles (menstrual cycles in which ovulation does not occur) may be associated with no or irregular menstrual periods, but even regular menstrual periods may be anovulatory for a variety of reasons. The most common cause of anovulatory cycles is polycystic ovary syndrome (PCOS), which causes hormone imbalances that can interfere with normal ovulation. Another relatively common cause of anovulation is primary ovarian insufficiency. In this condition, the ovaries stop working normally and producing viable eggs at a relatively early age, generally before the age of 40.

Structural problems with the oviducts or uterus are less common causes of female infertility. The oviducts may be blocked as a result of endometriosis. Another possible cause is pelvic inflammatory disease, which occurs when sexually transmitted infections spread to the oviducts or other female reproductive organs (see Figure 18.10.3). The infection may lead to scarring and blockage of the oviducts. If an ovum is produced and the oviducts are functioning — and a woman has a condition such as uterine fibroids — implantation in the uterus may not be possible. Uterine fibroids are non-cancerous clumps of tissue and muscle that form on the walls of the uterus.

Factors that increase a woman’s risk of infertility include tobacco smoking, excessive use of alcohol, stress, poor diet, strenuous athletic training, and being overweight or underweight. Advanced age is even more problematic for females than males. Female fertility normally peaks in the mid-twenties, and continuously declines after age 30 and until menopause around the age of 52, after which the ovary no longer releases eggs. About 1/3 of couples in which the woman is over age 35 have fertility problems. In older women, more cycles are likely to be anovulatory, and the eggs may not be as healthy.

Diagnosing Causes of Infertility

Diagnosing the cause(s) of a couple’s infertility often requires testing both the man and the woman for potential problems. In men, the semen is likely to be examined for the number, shape, and motility of sperm. If problems are found with sperm, further studies are likely to be done, such as medical imaging to look for structural problems with the testes or ducts.

In women, the first step is most often determining whether ovulation is occurring. This can be done at home by carefully monitoring body temperature (it rises slightly around the time of ovulation) or using a home ovulation test kit, which is available over the counter at most drugstores. Whether or not ovulation is occurring can also be detected with blood tests or ultrasound imaging of the ovaries. If ovulation is occurring normally, then the next step may be an X-ray of the oviducts and uterus to see if there are any blockages or other structural problems. Another approach to examining the female reproductive tract for potential problems is laparoscopy. In this surgical procedure, a tiny camera is inserted into the woman’s abdomen through a small incision. This allows the doctor to directly inspect the reproductive organs.

Infertility often can be treated successfully. The type of treatment depends on the cause of infertility.

Treating Male Infertility

Medical problems that interfere with sperm production may be treated with medications or other interventions that may lead to the resumption of normal sperm production. If, for example, an infection is interfering with sperm production, then antibiotics that clear up the infection may resolve the problem. If there is a blockage in the male reproductive tract that prevents the ejaculation of sperm, surgery may be able to remove the blockage. Alternatively, the man’s sperm may be removed from his body and then used for artificial insemination of his partner. In this procedure, the sperm are injected into the woman’s reproductive tract.

Treating Female Infertility

In females, it may be possible to correct blocked Fallopian tubes or uterine fibroids with surgery. Ovulation problems, on the other hand, are usually treated with hormones that act either on the pituitary gland or on the ovaries. Hormonal treatments that stimulate ovulation often result in more than one egg being ovulated at a time, thus increasing the chances of a woman having twins, triplets, or even higher multiple births. Multiple fetuses are at greater risk of being born too early or having health and developmental problems. The mother is also at greater risk of complications arising during pregnancy. Therefore, the possibility of multiple fetuses should be weighed in making a decision about this type of infertility treatment.

Assisted Reproductive Technology

Some cases of infertility are treated with assisted reproductive technology (ART). This is a collection of medical procedures in which ova are removed from the woman’s body and sperm are taken from the man’s body to be manipulated in ways that increase the chances of fertilization occurring. The eggs and sperm may be injected into one of the woman’s oviducts for fertilization to take place in vivo (in the body). More commonly, however, the eggs and sperm are mixed together outside the body so fertilization takes place in vitro (in a test tube or dish in a lab). The latter approach is illustrated in Figure 18.10.4. With in vitro fertilization, the fertilized eggs may be allowed to develop into embryos before being placed in the woman’s uterus.

ART has about a 40% chance of leading to a live birth in women under the age of 35, but only about a 20%t chance of success in women over the age of 35. Some studies have found a higher-than-average risk of birth defects in children produced by ART procedures, but this may be due to the generally higher ages of the parent — not the technologies used.

Other Approaches

Other approaches for certain causes of infertility include the use of a surrogate mother, a gestational carrier, or sperm donation.

• A surrogate mother is a woman who agrees to become pregnant using the man’s sperm and her own egg. The child, who will be the biological offspring of the surrogate and the male partner, is given up at birth for adoption by the couple. Surrogacy might be selected by women with no eggs or unhealthy eggs. A woman who carries a mutant gene for a serious genetic disorder might choose this option to ensure that the defective gene is not passed on to the offspring.
• A gestational carrier is a woman who agrees to receive a transplanted embryo from a couple and carry it to term. The child, who will be the biological offspring of the couple, is given to the parents at birth. A gestational carrier might be used by women who have normal ovulation but no uterus, or who cannot safely carry a fetus to term because of a serious health problem (such as kidney disease or cancer).
• Sperm donation is the use of sperm from a fertile man (generally through artificial insemination) for cases in which the male partner in a couple is infertile, or in which a woman seeks to become pregnant without a male partner. A lesbian couple may use donated sperm to enable one of them to become pregnant and have a child. Sperm can be obtained from a sperm bank, which buys and stores sperm for artificial insemination, or a male friend or other individual may donate sperm to a specific woman.

For people who have a strong desire for children of their own, infertility may lead to deep disappointment and depression. Individuals who are infertile may even feel biologically inadequate. Partners in infertile couples may argue and feel resentment toward each other, and married couples may get divorced because of infertility. Infertility treatments — especially ART procedures — are generally time-consuming and expensive. The high cost of the treatments can put them out of financial reach of many couples.

Ethical Concerns

Some people question whether the allocation of medical resources to infertility treatments is justified, and whether the resources could be better used in other ways. The status of embryos that are created in vitro and then not used for a pregnancy is another source of debate. Some people oppose their destruction on religious grounds, and couples may sometimes argue about what should be done with their extra embryos. Ethical issues are also raised by procedures that increase the chances of multiple births, because of the medical and developmental risks associated with multiple births.

Infertility in Developing Countries

Infertility is an under-appreciated problem in the poorer nations of the world, because of assumptions about overpopulation problems and high birth rates in developing countries. In fact, infertility is at least as great a problem in developing as in developed countries. High rates of health problems and inadequate health care in the poorer nations increase the risk of infertility. At the same time, infertility treatments are usually not available — or are far too expensive — for the vast majority of people who may need them. In addition, in many developing countries, the production of children is highly valued. Children may be needed for family income generation and economic security of the elderly. It is not uncommon for infertility to lead to social stigmatization, psychological problems, and abandonment by spouses.

Attributions

Figure 18.10.1

• Gay Pride Parade NYC 2013 - Happy Family by Bob Jagendorf on Flickr is used under a CC BY-NC 2.0 (https://creativecommons.org/licenses/by-nc/2.0/) license.
• #beaches #summer #family #blue #water by Jove Duero on Unsplash is used under the Unsplash License (https://unsplash.com/license).
• Photograph of five men near outdoor by Dollar Gill on Unsplash is used under the Unsplash License (https://unsplash.com/license). 
• Família by Laercio Cavalcanti on Unsplash is used under the Unsplash License (https://unsplash.com/license). 
• Happiness 🙂 by Ashwini Chaudhary on Unsplash is used under the Unsplash License (https://unsplash.com/license). 

Figure 18.10.2

Causes of infertility in Canada by Christine Miller is in the Public Domain (https://creativecommons.org/publicdomain/mark/1.0/).

Figure 18.10.3

1024px-Blausen_0719_PelvicInflammatoryDisease by BruceBlaus on Wikimedia Commons is used under a CC BY 3.0 (https://creativecommons.org/licenses/by/3.0) license. 

Figure 18.10.4

1024px-Blausen_0060_AssistedReproductiveTechnology by BruceBlaus on Wikimedia Commons is used under a CC BY 3.0 (https://creativecommons.org/licenses/by/3.0) license. 

References

Blausen.com Staff. (2014). Medical gallery of Blausen Medical 2014. WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.010. ISSN 2002-4436.

David Pakman Show. (2015, September 1). Smoking marijuana may lower sperm count by 33%. YouTube. https://www.youtube.com/watch?v=iqA8uAjvEdM

Fertility Specialist Sydney. (2014, April 11). ivf embryo developing over 5 days by fertility Dr Raewyn Teirney. YouTube. https://www.youtube.com/watch?v=V6-v4eF9dyA&t=5s

Public Health Agency of Canada. (2019, May 28). Fertility. Government of Canada. https://www.canada.ca/en/public-health/services/fertility/fertility.html

TED-Ed. (2015, May 7). How in vitro fertilization (IVF) works - Nassim Assefi and Brian A. Levine.  YouTube. https://www.youtube.com/watch?v=P27waC05Hdk&t=4s

TEDx Talks. (2014, June 26). A journey through infertility -- over terror's edge | Camille Preston | TEDxBeaconStreet. YouTube. https://www.youtube.com/watch?v=6BBmMtVfZ4Y&t=2s

TEDx Talks. (2016, November 15). Homosexuality: It's about survival - not sex | James O'Keefe | TEDxTallaght. YouTube. https://www.youtube.com/watch?v=4Khn_z9FPmU&t=1s

Science is a distinctive way of gaining knowledge about the natural world that starts with a question and then tries to answer the question using evidence and logic. It is an exciting exploration of all the whys and hows that any curious person might ask about the world. You can be part of that exploration! Besides your curiosity, all you need is a basic understanding of how scientists think and how science is done. In this concept, you'll learn how to think like a scientist.

Thinking Like a Scientist

Thinking like a scientist rests on certain underlying assumptions. Scientists assume that:

 

Nature Is Understandable

Scientists think of nature as a single system controlled by natural laws. By discovering natural laws, scientists strive to increase their understanding of the natural world. Laws of nature are expressed as scientific laws. A scientific law is a statement that describes what always happens under certain conditions in nature.

Scientific Ideas Are Open to Change

Science is both a process and body of knowledge. Scientific knowledge is generated through systematic processes, such as observation and experimentation. Scientists are always testing and revising their ideas, and as new observations are made, existing ideas may be challenged. Ideas may be replaced with new ideas that better fit the facts, but more often, existing ideas are simply revised. Through many new discoveries over time, scientists gradually build an increasingly accurate and detailed understanding of the natural world.

Scientific Knowledge May Be Long Lasting

Many scientific ideas have stood the test of time. About 200 years ago, the scientist John Dalton proposed atomic theory — the theory that all matter is made of tiny particles called atoms. This theory is still valid today. During the two centuries since the theory was first proposed, scientists have learned a lot more about atoms and the even smaller particles that compose them. Nonetheless, the idea that all matter consists of atoms remains valid. There are many other examples of basic scientific ideas that have been tested repeatedly and proven sound. You will learn about many of them as you study human biology.

Not All Questions Can be Answered by Science

Science rests on evidence and logic, and evidence comes from observations. Therefore, science deals only with things that can be observed. An observation is anything that is detected through human senses or with instruments or measuring devices that extend human senses. Things that cannot be observed or measured by current means — such as supernatural beings or events — are outside the bounds of science. Consider these two questions about life on Earth:

• Did life on Earth evolve over time?
• How did life on Earth originate?

The first question can be answered by science on the basis of scientific evidence (such as fossils and logical arguments). The second question could be a matter of belief, but no evidence can be gathered to support or refute it. Therefore, it is outside the realm of science.