17.4 Innate Immune System
Paper Cut
It’s just a paper cut, but the break in your skin could provide an easy way for to enter your body. If were to enter through the cut and infect the wound, your innate immune system would quickly respond with a dizzying array of general defenses.
What Is the Innate Immune System?
The is a subset of the human immune system that produces rapid, but non-specific responses to pathogens. Innate responses are generic, rather than tailored to a particular pathogen. The innate system responds in the same general way to every pathogen it encounters. Although the innate immune system provides immediate and rapid defenses against pathogens, it does not confer long-lasting immunity to them. In most organisms, the innate immune system is the dominant system of host defense. Other than most vertebrates (including humans), the innate immune system is the only system of host defense.
In humans, the innate immune system includes surface barriers, inflammation, the complement system, and a variety of cellular responses. Surface barriers of various types generally keep most pathogens out of the body. If these barriers fail, then other innate defenses are triggered. The triggering event is usually the identification of pathogens by pattern-recognition receptors on cells of the innate immune system. These receptors recognize molecules that are broadly shared by pathogens, but distinguishable from host molecules. Alternatively, the other innate defenses may be triggered when damaged, injured, or stressed cells send out alarm signals, many of which are recognized by the same receptors as those that recognize pathogens.
Barriers to Pathogens
The body’s first line of defense consists of three different types of barriers that keep most pathogens out of body tissues. The types of barriers are mechanical, chemical, and biological barriers.
Mechanical Barriers
are the first line of defense against pathogens, and they physically block pathogens from entering the body. The is the most important mechanical barrier. In fact, it is the single most important defense the body has. The outer layer of skin — the — is tough, and very difficult for pathogens to penetrate. It consists of dead cells that are constantly shed from the body surface, a process that helps remove bacteria and other infectious agents that have adhered to the skin. The epidermis also lacks blood vessels and is usually lacking moisture, so it does not provide a suitable environment for most pathogens. — which is an accessory organ of the skin — also helps keep out pathogens. Hairs inside the nose may trap larger pathogens and other particles in the air before they can enter the airways of the respiratory system (see Figure 17.4.2).
provide a mechanical barrier to pathogens and other particles at body openings. These membranes also line the respiratory, gastrointestinal, urinary, and reproductive tracts. Mucous membranes secrete mucus, which is a slimy and somewhat sticky substance that traps pathogens. Many mucous membranes also have hair-like that sweep mucus and trapped pathogens toward body openings, where they can be removed from the body. When you sneeze or cough, mucus and pathogens are mechanically ejected from the nose and throat, as you can see in Figure 17.4.3. A sneeze can travel as fast as 160 Km/hr (about 99 mi/hour) and expel as many as 100,000 droplets into the air around you (a good reason to cover your sneezes!). Other mechanical defenses include tears, which wash pathogens from the eyes, and urine, which flushes pathogens out of the urinary tract.
Chemical Barriers
Chemical barriers also protect against infection by pathogens. They destroy pathogens on the outer body surface, at body openings, and on inner body linings. Sweat, mucus, tears, saliva, and breastmilk all contain antimicrobial substances (such as the enzyme lysozyme) that kill pathogens, especially bacteria. Sebaceous glands in the dermis of the skin secrete acids that form a very fine, slightly acidic film on the surface of the skin. This film acts as a barrier to bacteria, viruses, and other potential contaminants that might penetrate the skin. Urine and vaginal secretions are also too acidic for many pathogens to endure. Semen contains zinc — which most pathogens cannot tolerate — as well as defensins, which are antimicrobial proteins that act mainly by disrupting bacterial cell membranes. In the stomach, stomach acid and digestive enzymes called proteases (which break down proteins) kill most of the pathogens that enter the gastrointestinal tract in food or water.
Biological Barriers
Biological barriers are living organisms that help protect the body from pathogens. Trillions of harmless bacteria normally live on the human skin and in the urinary, reproductive, and gastrointestinal tracts. These bacteria use up food and surface space that help prevent pathogenic bacteria from colonizing the body. Some of these harmless bacteria also secrete substances that change the conditions of their environment, making it less hospitable to potentially harmful bacteria. They may release toxins or change the pH, for example. All of these effects of harmless bacteria reduce the chances that pathogenic microorganisms will be able to reach sufficient numbers and cause illness.
Inflammation
If pathogens manage to breach the barriers protecting the body, one of the first active responses of the innate immune system kicks in. This response is . The main function of inflammation is to establish a physical barrier against the spread of infection. It also eliminates the initial cause of cell injury, clears out dead cells and tissues damaged from the original insult and the inflammatory process, and initiates tissue repair. Inflammation is often a response to infection by pathogens, but there are other possible causes, including burns, frostbite, and exposure to toxins.
The signs and symptoms of inflammation include redness, swelling, warmth, pain, and frequently some loss of function. These symptoms are caused by increased blood flow into infected tissue, and a number of other processes, illustrated in Figure 17.4.4.
Inflammation is triggered by chemicals such as and ,which are released by injured or infected cells, or by immune system cells such as macrophages (described below) that are already present in tissues. These chemicals cause capillaries to dilate and become leaky, increasing blood flow to the infected area and allowing blood to enter the tissues. Pathogen-destroying leukocytes and tissue-repairing proteins migrate into tissue spaces from the bloodstream to attack pathogens and repair their damage. Cytokines also promote , which is migration to the site of infection by pathogen-destroying leukocytes. Some cytokines have anti-viral effects. They may shut down protein synthesis in host cells, which viruses need in order to survive and replicate.
See the video “The inflammatory response” by Neural Academy to learn about inflammatory response in more detail:
The inflammatory response, Neural Academy, 2019.
Complement System
The is a complex biochemical mechanism named for its ability to “complement” the killing of pathogens by , which are produced as part of an adaptive immune response. The complement system consists of more than two dozen proteins normally found in the blood and synthesized in the . The proteins usually circulate as non-functional precursor molecules until activated.
As shown in Figure 17.4.5, when the first protein in the complement series is activated —typically by the binding of an antibody to an on a pathogen — it sets in motion a domino effect. Each component takes its turn in a precise chain of steps known as the complement cascade. The end product is a cylinder that punctures a hole in the pathogen’s cell membrane. This allows fluids and molecules to flow in and out of the cell, which swells and bursts.
Cellular Responses
Cellular responses of the innate immune system involve a variety of different types of . Many of these leukocytes circulate in the blood and act like independent, single-celled organisms, searching out and destroying pathogens in the human host. These and other immune cells of the innate system identify pathogens or debris, and then help to eliminate them in some way. One way is by .
Phagocytosis
is an important feature of innate immunity that is performed by cells classified as phagocytes. In the process of phagocytosis, phagocytes engulf and digest pathogens or other harmful particles. Phagocytes generally patrol the body searching for pathogens, but they can also be called to specific locations by the release of when occurs. Some phagocytes reside permanently in certain tissues.
As shown in Figure 17.4.6, when a pathogen such as a bacterium is encountered by a phagocyte, the phagocyte extends a portion of its plasma membrane, wrapping the membrane around the pathogen until it is enveloped. Once inside the phagocyte, the pathogen becomes enclosed within an intracellular vesicle called a phagosome. The phagosome then fuses with another vesicle called a , forming a phagolysosome. Digestive enzymes and acids from the lysosome kill and digest the pathogen in the phagolysosome. The final step of phagocytosis is excretion of soluble debris from the destroyed pathogen through .
Types of leukocytes that kill pathogens by phagocytosis include neutrophils, macrophages, and dendritic cells. You can see illustrations of these and other leukocytes involved in innate immune responses in Figure 17.4.7.
Neutrophils
are leukocytes that travel throughout the body in the blood. They are usually the first immune cells to arrive at the site of an infection. They are the most numerous types of phagocytes, and they normally make up at least half of the total circulating leukocytes. The of a normal healthy adult produces more than 100 billion neutrophils per day. During acute inflammation, more than ten times that many neutrophils may be produced each day. Many neutrophils are needed to fight infections, because after a neutrophil phagocytizes just a few pathogens, it generally dies.
Macrophages
s are large phagocytic leukocytes that develop from monocytes. Macrophages spend much of their time within the in body tissues. They are the most efficient phagocytes, and they can phagocytize substantial numbers of pathogens or other cells. Macrophages are also versatile cells that produce a wide array of chemicals — including enzymes, complement proteins, and s — in addition to their phagocytic action. As phagocytes, macrophages act as scavengers that rid tissues of worn-out cells and other debris, as well as pathogens. In addition, macrophages act as antigen-presenting cells that activate the .
Dendritic Cells
Like macrophages, develop from monocytes. They reside in tissues that have contact with the external environment, so they are located mainly in the skin, nose, lungs, stomach, and intestines. Besides engulfing and digesting pathogens, dendritic cells also act as antigen-presenting cells that trigger adaptive immune responses.
Eosinophils
are non-phagocytic leukocytes that are related to . They specialize in defending against . They are very effective in killing large parasites (such as worms) by secreting a range of highly-toxic substances when activated. Eosinophils may become overactive and cause or .
Basophils
are non-phagocytic leukocytes that are also related to neutrophils. They are the least numerous of all white blood cells. Basophils secrete two types of chemicals that aid in body defenses: and heparin. Histamines are responsible for dilating blood vessels and increasing their permeability in inflammation. inhibits blood clotting, and also promotes the movement of leukocytes into an area of infection.
Mast Cells
are non-phagocytic leukocytes that help initiate by secreting . In some people, histamines trigger , as well as inflammation. Mast cells may also secrete chemicals that help defend against parasites.
Natural Killer Cells
are in the subset of leukocytes called , which are produced by the lymphatic system. Natural killer cells destroy ous or -infected host cells, although they do not directly attack invading pathogens. Natural killer cells recognize these host cells by a condition they exhibit called “missing self.” Cells with missing self have abnormally low levels of cell-surface proteins of the , which normally identify body cells as self.
Innate Immune Evasion
Many pathogens have evolved mechanisms that allow them to evade human hosts’ innate immune systems. Some of these mechanisms include:
- Invading host cells to replicate so they are “hidden” from the immune system. The bacterium that causes tuberculosis uses this mechanism.
- Forming a protective capsule around themselves to avoid being destroyed by immune system cells. This defense occurs in bacteria, such as Salmonella species.
- Mimicking host cells so the immune system does not recognize them as foreign. Some species of Staphylococcus bacteria use this mechanism.
- Directly killing phagocytes. This ability evolved in several species of bacteria, including the species that causes anthrax.
- Producing molecules that prevent the formation of interferons, which are immune chemicals that fight viruses. Some influenza viruses have this capability.
- Forming complex biofilms that provide protection from the cells and proteins of the immune system. This characterizes some species of and fungi. You can see an example of a bacterial biofilm on teeth in Figure 17.4.8.
17.4 Summary
- The is a subset of the human immune system that produces rapid, but non-specific responses to pathogens. Unlike the , the innate system does not confer . The innate immune system includes surface barriers, , the , and a variety of cellular responses.
- The body’s first line of defense consists of three different types of barriers that keep most pathogens out of body tissues. The types of barriers are mechanical, chemical, and biological barriers.
- Mechanical barriers — which include the , , and fluids such as tears and — physically block pathogens from entering the body. Chemical barriers — such as enzymes in , , and — kill pathogens on body surfaces. Biological barriers are harmless bacteria that use up food and space so pathogenic bacteria cannot colonize the body.
- If pathogens breach protective barriers, occurs. This creates a physical barrier against the spread of infection, and repairs tissue damage. Inflammation is triggered by chemicals such as and , and it causes swelling, redness, and warmth.
- The is a complex biochemical mechanism that helps kill pathogens. Once activated, the complement system consists of more than two dozen proteins that lead to disruption of the cell membrane of pathogens and bursting of the cells.
- Cellular responses of the innate immune system involve various types of . For example, , , and phagocytize pathogens. and release chemicals that trigger inflammation. destroy cancerous or virus-infected cells, and eosinophils kill parasites.
- Many pathogens have evolved mechanisms that help them evade the innate immune system. For example, some pathogens form a protective capsule around themselves, and some mimic host cells so the immune system does not recognize them as foreign.
17.4 Review Questions
- What is the innate immune system?
- Identify the body’s first line of defense.
- What are biological barriers? How do they protect the body?
- State the purposes of inflammation. What triggers inflammation, and what signs and symptoms does it cause?
- Define the complement system. How does it help destroy pathogens?
- Describe two ways that pathogens can evade the innate immune system.
- What are the ways in which phagocytes can encounter pathogens in the body?
- Describe two different ways in which enzymes play a role in the innate immune response.
17.4 Explore More
How mucus keeps us healthy – Katharina Ribbeck, TED-Ed, 2015.
Human Physiology – Innate Immune System, Janux, 2015.
Myriam Sidibe: The simple power of handwashing, TED, 2014.
Everything You Didn’t Want To Know About Snot, Gross Science, 2017.
Cough Grosser Than Sneeze? | Curiosity – World’s Dirtiest Man, Discovery, 2011.
Attributions
Figure 17.4.1
Oww_Papercut_14365 by Laurence Facun on Wikimedia Commons is used under a CC BY 2.0 (https://creativecommons.org/licenses/by/2.0) license.
Figure 17.4.2
hairy-nose by Piotr Siedlecki on publicdomainpictures.net is used under a CC0 1.0 Universal Public Domain Dedication (http://creativecommons.org/publicdomain/zero/1.0/) license.
Figure 17.4.3
1024px-Sneeze by James Gathany/ CDC Public Health Image library (PHIL) ID# 11162 on Wikimedia Commons is in the public domain (https://en.wikipedia.org/wiki/public_domain).
Figure 17.4.4
OSC_Microbio_17_06_Erythema by CNX OpenStax on Wikimedia Commons is used under a CC BY 4.0 (https://creativecommons.org/licenses/by/4.0) license.
Figure 17.4.5
2212_Complement_Cascade_and_Function by OpenStax College on Wikimedia Commons is used under a CC BY 3.0 (https://creativecommons.org/licenses/by/3.0) license.
Figure 17.4.6
512px-Phagocytosis2 by Graham Colm at English Wikipedia on Wikimedia Commons is used under a CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0) license.
Figure 17.4.7
Innate_Immune_cells.svg by Fred the Oyster on Wikimedia Commons is in the public domain (https://en.wikipedia.org/wiki/Public_domain).
Figure 17.4.8
1024px-Gingivitis-before-and-after-3 by Onetimeuseaccount on Wikimedia Commons is used under a CC0 1.0 Universal Public Domain Dedication (http://creativecommons.org/publicdomain/zero/1.0/) 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 21.13 Complement cascade and function [digital image]. In Anatomy and Physiology (Section 21.2). OpenStax. https://openstax.org/books/anatomy-and-physiology/pages/21-2-barrier-defenses-and-the-innate-immune-response
Discovery. (2011, October 27). Cough grosser than sneeze? | Curiosity – World’s dirtiest man. YouTube. https://www.youtube.com/watch?v=dy1D3d1FBcw&feature=youtu.be
Gross Science. (2017, January 31). Everything you didn’t want to know about snot. YouTube. https://www.youtube.com/watch?v=shEPwQPQG4I&feature=youtu.be
Janux. (2015, January 10). Human physiology – Innate immune system. YouTube. https://www.youtube.com/watch?v=sYjtMP67vyk&feature=youtu.be
Mayo Clinic Staff. (n.d.). Anthrax [online article]. MayoClinic.org. https://www.mayoclinic.org/diseases-conditions/anthrax/symptoms-causes/syc-20356203
Mayo Clinic Staff. (n.d.). Influenza (flu) [online article]. MayoClinic.org. https://www.mayoclinic.org/diseases-conditions/flu/symptoms-causes/syc-20351719
Mayo Clinic Staff. (n.d.). Salmonella infection [online article]. MayoClinic.org. https://www.mayoclinic.org/diseases-conditions/salmonella/symptoms-causes/syc-20355329
Mayo Clinic Staff. (n.d.). Staph infection [online article]. MayoClinic.org. https://www.mayoclinic.org/diseases-conditions/staph-infections/multimedia/staph-infection/img-20008600
Mayo Clinic Staff. (n.d.). Tuberculosis [online article]. MayoClinic.org. https://www.mayoclinic.org/diseases-conditions/tuberculosis/symptoms-causes/syc-20351250
OpenStax. (2016, November 11). Figure 17.23 A typical case of acute inflammation at the site of a skin wound – Erythema [digital image]. In OpenStax, Microbiology (Section 17.5). https://openstax.org/details/books/microbiology?Bookdetails
TED. (2014, October 14). Myriam Sidibe: The simple power of handwashing. YouTube. https://www.youtube.com/watch?v=c64M1tZyWPM&feature=youtu.be
TED-Ed. (2015, November 5). How mucus keeps us healthy – Katharina Ribbeck. YouTube. https://www.youtube.com/watch?v=WW4skW6gucU&feature=youtu.be
A microorganism which causes disease.
Created by CK-12 Foundation/Adapted by Christine Miller
Case Study: Flight Risk
Nineteen-year-old Malcolm is about to take his first plane flight. Shortly after he boards the plane and sits down, a man in his late sixties sits next to him in the aisle seat. About half an hour after the plane takes off, the pilot announces that she is turning the seat belt light off, and that it is safe to move around the cabin.
The man in the aisle seat — who has introduced himself to Malcolm as Willie — immediately unbuckles his seat belt and paces up and down the aisle a few times before returning to his seat. After about 45 minutes, Willie gets up again, walks some more, then sits back down and does some foot and leg exercises. After the third time Willie gets up and paces the aisles, Malcolm asks him whether he is walking so much to accumulate steps on a pedometer or fitness tracking device. Willie laughs and says no. He is actually trying to do something even more important for his health — prevent a blood clot from forming in his legs.
Willie explains that he has a chronic condition: . Although it sounds scary, his condition is currently well-managed, and he is able to lead a relatively normal lifestyle. However, it does put him at risk of developing other serious health conditions, such as deep vein thrombosis (DVT), which is when a blood clot occurs in the deep veins, usually in the legs. Air travel — and other situations where a person has to sit for a long period of time — increases the risk of DVT. Willie’s doctor said that he is healthy enough to fly, but that he should walk frequently and do leg exercises to help avoid a blood clot.
As you read this chapter, you will learn about the heart, blood vessels, and blood that make up the cardiovascular system, as well as disorders of the cardiovascular system, such as heart failure. At the end of the chapter you will learn more about why DVT occurs, why Willie has to take extra precautions when he flies, and what can be done to lower the risk of DVT and its potentially deadly consequences.
Chapter Overview: Cardiovascular System
In this chapter, you will learn about the cardiovascular system, which transports substances throughout the body. Specifically, you will learn about:
- The major components of the : the heart, blood vessels, and blood.
- The functions of the cardiovascular system, including transporting needed substances (such as oxygen and nutrients) to the cells of the body, and picking up waste products.
- How blood is oxygenated through the pulmonary circulation, which transports blood between the heart and lungs.
- How blood is circulated throughout the body through the systemic circulation.
- The components of blood — including plasma, red blood cells, white blood cells, and platelets — and their specific functions.
- Types of blood vessels — including arteries, veins, and capillaries — and their functions, similarities, and differences.
- The structure of the heart, how it pumps blood, and how contractions of the heart are controlled.
- What blood pressure is and how it is regulated.
- Blood disorders, including anemia, HIV, and leukemia.
- Cardiovascular diseases (including heart attack, stroke, and angina), and the risk factors and precursors — such as high blood pressure and atherosclerosis — that contribute to them.
As you read the chapter, think about the following questions:
- What is heart failure?Why do you think it increases the risk of DVT?
- What is a blood clot? What are possible health consequences of blood clots?
- Why do you think sitting for long periods of time increases the risk of DVT? Why does walking and exercising the legs help reduce this risk?
Attribution
Figure 14.1.1
aircraft-1583871_1920 [photo] by olivier89 from Pixabay is used under the Pixabay License (https://pixabay.com/de/service/license/).
Image shows a diagram of the negative feedback loop governing thyroid gland function. In the absence of sufficient levels of thyroid hormones, the hypothalamus will secrete TRH, which stimulates the pituitary gland to secrete TSH, which stimulates the thyroid gland to make thyroid hormones. Sufficient blood levels of thyroid hormone inhibit the hypothalamus from secreting TRH, halting the pathway, until thyroid hormone level sdrop again
Created by CK-12 Foundation/Adapted by Christine Miller
Case Study: Wearing His Heart on His Sleeve
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.
Figure 10.1.2 Examples of henna art.
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.
Chapter 10 Overview: Integumentary System
In this chapter you will learn about the structure and functions of the integumentary system, along with its relationships to culture, evolution, and health. Specifically, you will learn about:
- The functions of the organs of the integumentary system — the skin, hair, and nails — including protecting the body, helping to regulate homeostasis, and sensing and interacting with the external world.
- The two main layers of the skin: the thinner outer layer (called the epidermis) and the thicker inner layer (called the dermis).
- The cells and layers of the epidermis and their functions, including synthesizing vitamin D and protecting the body against injury, pathogens, UV light exposure, and water loss.
- The composition of epidermal cells and how the epidermis grows.
- The composition and layers of the dermis and their functions, including cushioning other tissues, regulating body temperature, sensing the environment, and excreting wastes.
- The specialized structures in the dermis, which include sweat and sebaceous (oil) glands, hair follicles, and sensory receptors that detect touch, temperature, and pain.
- The structure and biological functions of hair, which include retaining body heat, detecting sensory stimuli, and protecting the body against UV light, pathogens, and small particles.
- How hair grows, how variations in hair colour and texture arise, and hypotheses about the evolution of hair in humans.
- The sociocultural roles of hair, including its expression of characteristics like sex and age, as well as cultural identity and social cues.
- The structure and functions of nails, which includes protecting the fingers and toes, enhancing the detection of sensory stimuli, and acting as tools.
- How nails grow and how they can reflect and affect our health.
- Skin cancer — which is the most common form of cancer — and its types and risk factors.
As you read the chapter and learn more about the skin, think about the following questions:
- Why do you think real tattoos are permanent, but mehndi is not?
- Why do you think mehndi might come out darker on the palms of the hands and soles of the feet than on other areas of the skin?
- What do you think are some of the health concerns about tattoos?
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/).
visible part of a nail that is external to the skin
The outer layer of skin that consists mainly of epithelial cells and lacks nerve endings, blood vessels, and other structures.
a colorless cell that circulates in the blood and body fluids and is involved in counteracting foreign substances and disease; a white (blood) cell. There are several types, all amoeboid cells with a nucleus, including lymphocytes, granulocytes, monocytes, and macrophages.
The space occurring between two or more membranes. In cell biology, it's most commonly described as the region between the inner membrane and the outer membrane of a mitochondrion or a chloroplast.
Image shows an operating room. There are several surgeons in gowns, masks and gloves. They are operating on a patient.
Image shows a man jogging in the forest. His shirt is wet with sweat.
Image shows a diagram of the layers of the epidermis. The outermost layer is the stratum corneum, below that is the stratum lucidum, below that the stratum granulosum, below that the stratum spinosum, below that the stratum basale, and then a basement membrane which connects the dermis to the epidermis.
Image shows a scraped knee. It is bleeding slightly.
Image shows a
Image shows a pictomicrograph of staphylococcus.
An antibody, also known as an immunoglobulin, is a large, Y-shaped protein produced mainly by plasma cells that is used by the immune system to neutralize pathogens such as pathogenic bacteria and viruses.
Created by CK-12 Foundation/Adapted by Christine Miller
Doing the ‘Fly
The swimmer in the Figure 13.3.1 photo is doing the butterfly stroke, a swimming style that requires the swimmer to carefully control his breathing so it is coordinated with his swimming movements. Breathing is the process of moving air into and out of the lungs, which are the organs in which gas exchange takes place between the atmosphere and the body. Breathing is also called , and it is one of two parts of the life-sustaining process of respiration. The other part is gas exchange. Before you can understand how breathing is controlled, you need to know how breathing occurs.
How Breathing Occurs
Breathing is a two-step process that includes drawing air into the lungs, or inhaling, and letting air out of the lungs, or exhaling. Both processes are illustrated in Figure 13.3.2.
Inhaling
Inhaling is an active process that results mainly from contraction of a muscle called the diaphragm, shown in Figure 13.3.2. The is a large, dome-shaped muscle below the lungs that separates the (chest) and cavities. When the diaphragm contracts it moves down causing the thoracic cavity to expand, and the contents of the abdomen to be pushed downward. Other muscles — such as intercostal muscles between the ribs — also contribute to the process of , especially when inhalation is forced, as when taking a deep breath. These muscles help increase thoracic volume by expanding the ribs outward. The increase in thoracic volume creates a decrease in thoracic air pressure. With the chest expanded, there is lower air pressure inside the lungs than outside the body, so outside air flows into the lungs via the respiratory tract according the the pressure gradient (high pressure flows to lower pressure).
Exhaling
Exhaling involves the opposite series of events. The diaphragm relaxes, so it moves upward and decreases the volume of the thorax. Air pressure inside the lungs increases, so it is higher than the air pressure outside the lungs. , unlike inhalation, is typically a passive process that occurs mainly due to the elasticity of the lungs. With the change in air pressure, the lungs contract to their pre-inflated size, forcing out the air they contain in the process. Air flows out of the lungs, similar to the way air rushes out of a balloon when it is released. If exhalation is forced, internal intercostal and abdominal muscles may help move the air out of the lungs.
Control of Breathing
Breathing is one of the few vital bodily functions that can be controlled consciously, as well as unconsciously. Think about using your breath to blow up a balloon. You take a long, deep breath, and then you exhale the air as forcibly as you can into the balloon. Both the inhalation and exhalation are consciously controlled.
Conscious Control of Breathing
You can control your breathing by holding your breath, slowing your breathing, or , which is breathing more quickly and shallowly than necessary. You can also exhale or inhale more forcefully or deeply than usual. Conscious control of breathing is common in many activities besides blowing up balloons, including swimming, speech training, singing, playing many different musical instruments (Figure 13.3.3), and doing yoga, to name just a few.
There are limits on the conscious control of breathing. For example, it is not possible for a healthy person to voluntarily stop breathing indefinitely. Before long, there is an irrepressible urge to breathe. If you were able to stop breathing for a long enough time, you would lose consciousness. The same thing would happen if you were to hyperventilate for too long. Once you lose consciousness so you can no longer exert conscious control over your breathing, control of breathing takes over.
Unconscious Control of Breathing
Unconscious breathing is controlled by in the and of the brainstem (see Figure 13.3.4). The respiratory centers automatically and continuously regulate the rate of breathing based on the body’s needs. These are determined mainly by blood acidity, or . When you exercise, for example, carbon dioxide levels increase in the blood, because of increased cellular respiration by muscle cells. The carbon dioxide reacts with water in the blood to produce carbonic acid, making the blood more acidic, so pH falls. The drop in pH is detected by in the medulla. Blood levels of oxygen and carbon dioxide, in addition to pH, are also detected by chemoreceptors in major arteries, which send the “data” to the respiratory centers. The latter respond by sending nerve impulses to the , “telling” it to contract more quickly so the rate of breathing speeds up. With faster breathing, more carbon dioxide is released into the air from the blood, and blood pH returns to the normal range.
The opposite events occur when the level of carbon dioxide in the blood becomes too low and blood pH rises. This may occur with involuntary hyperventilation, which can happen in panic attacks, episodes of severe pain, asthma attacks, and many other situations. When you hyperventilate, you blow off a lot of carbon dioxide, leading to a drop in blood levels of carbon dioxide. The blood becomes more basic (alkaline), causing its pH to rise.
Nasal vs. Mouth Breathing
Nasal breathing is breathing through the nose rather than the mouth, and it is generally considered to be superior to mouth breathing. The hair-lined nasal passages do a better job of filtering particles out of the air before it moves deeper into the respiratory tract. The nasal passages are also better at warming and moistening the air, so nasal breathing is especially advantageous in the winter when the air is cold and dry. In addition, the smaller diameter of the nasal passages creates greater pressure in the lungs during exhalation. This slows the emptying of the lungs, giving them more time to extract oxygen from the air.
Feature: Myth vs. Reality
Drowning is defined as respiratory impairment from being in or under a liquid. It is further classified according to its outcome into: death, ongoing health problems, or no ongoing health problems (full recovery). Four hundred Canadians die annually from drowning, and drowning is one of the leading causes of death in children under the age of five. There are some potentially dangerous myths about drowning, and knowing what they are might save your life or the life of a loved one, especially a child.
Myth | Reality |
---|---|
"People drown when they aspirate water into their lungs." | Generally, in the early stages of drowning, very little water enters the lungs. A small amount of water entering the trachea causes a muscular spasm in the larynx that seals the airway and prevents the passage of water into the lungs. This spasm is likely to last until unconsciousness occurs. |
"You can tell when someone is drowning because they will shout for help and wave their arms to attract attention." | The muscular spasm that seals the airway prevents the passage of air, as well as water, so a person who is drowning is unable to shout or call for help. In addition, instinctive reactions that occur in the final minute or so before a drowning person sinks under the water may look similar to calm, safe behavior. The head is likely to be low in the water, tilted back, with the mouth open. The person may have uncontrolled movements of the arms and legs, but they are unlikely to be visible above the water. |
"It is too late to save a person who is unconscious in the water." | An unconscious person rescued with an airway still sealed from the muscular spasm of the larynx stands a good chance of full recovery if they start receiving CPR within minutes. Without water in the lungs, CPR is much more effective. Even if cardiac arrest has occurred so the heart is no longer beating, there is still a chance of recovery. The longer the brain goes without oxygen, however, the more likely brain cells are to die. Brain death is likely after about six minutes without oxygen, except in exceptional circumstances, such as young people drowning in very cold water. There are examples of children surviving, apparently without lasting ill effects, for as long as an hour in cold water. Rescuers retrieving a child from cold water should attempt resuscitation even after a protracted period of immersion. |
"If someone is drowning, you should start administering CPR immediately, even before you try to get the person out of the water." | Removing a drowning person from the water is the first priority, because CPR is ineffective in the water. The goal should be to bring the person to stable ground as quickly as possible and then to start CPR. |
"You are unlikely to drown unless you are in water over your head." | Depending on circumstances, people have drowned in as little as 30 mm (about 1 ½ in.) of water. Inebriated people or those under the influence of drugs, for example, have been known to have drowned in puddles. Hundreds of children have drowned in the water in toilets, bathtubs, basins, showers, pails, and buckets (see Figure 13.3.5). |
13.3 Summary
- Breathing, or , is the two-step process of drawing air into the lungs (inhaling) and letting air out of the lungs (exhaling). is an active process that results mainly from contraction of a muscle called the diaphragm. is typically a passive process that occurs mainly due to the elasticity of the lungs when the diaphragm relaxes.
- Breathing is one of the few vital bodily functions that can be controlled consciously, as well as unconsciously. Conscious control of breathing is common in many activities, including swimming and singing. There are limits on the conscious control of breathing, however. If you try to hold your breath, for example, you will soon have an irrepressible urge to breathe.
- Unconscious breathing is controlled by respiratory centers in the and of the brainstem. They respond to variations in blood by either increasing or decreasing the rate of breathing as needed to return the pH level to the normal range.
- Nasal breathing is generally considered to be superior to mouth breathing because it does a better job of filtering, warming, and moistening incoming air. It also results in slower emptying of the lungs, which allows more oxygen to be extracted from the air.
- Drowning is a major cause of death in Canada, in particular in children under the age of five. It is important to supervise small children when they are playing in, around, or with water.
13.3 Review Questions
- Define breathing.
- Give examples of activities in which breathing is consciously controlled.
- Explain how unconscious breathing is controlled.
- Young children sometimes threaten to hold their breath until they get something they want. Why is this an idle threat?
- Why is nasal breathing generally considered superior to mouth breathing?
- Give one example of a situation that would cause blood pH to rise excessively. Explain why this occurs.
13.3 Explore More
https://www.youtube.com/watch?v=Kl4cU9sG_08
How breathing works - Nirvair Kaur, TED-Ed, 2012.
https://www.youtube.com/watch?v=yDtKBXOEsoM
How do ventilators work? - Alex Gendler, TED-Ed, 2020.
https://www.youtube.com/watch?v=XFnGhrC_3Gs&feature=emb_logo
How I held my breath for 17 minutes | David Blaine, TED, 2010.
https://www.youtube.com/watch?v=Vca6DyFqt4c&feature=emb_logo
The Ultimate Relaxation Technique: How To Practice Diaphragmatic Breathing For Beginners, Kai Simon, 2015.
Attributions
Figure 13.3.1
US_Marines_butterfly_stroke by Cpl. Jasper Schwartz from U.S. Marine Corps on Wikimedia Commons is in the public domain (https://en.wikipedia.org/wiki/Public_domain).
Figure 13.3.2
Inhale Exhale/Breathing cycle by Siyavula Education on Flickr is used under a CC BY 2.0 (https://creativecommons.org/licenses/by/2.0/) license.
Figure 13.3.3
Trumpet/ Frenchmen Street [photo] by Morgan Petroski on Unsplash is used under the Unsplash License (https://unsplash.com/license).
Figure 13.3.4
Respiratory_Centers_of_the_Brain by OpenStax College on Wikimedia Commons is used under a CC BY 3.0 (https://creativecommons.org/licenses/by/3.0) license.
Figure 13.3.5
Lily & Ava in the Kiddie Pool by mob mob on Flickr is used under a CC BY-NC 2.0 (https://creativecommons.org/licenses/by-nc/2.0/) 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 22.20 Respiratory centers of the brain [digital image]. In Anatomy and Physiology (Section 22.3). OpenStax. https://openstax.org/books/anatomy-and-physiology/pages/22-3-the-process-of-breathing
Kai Simon. (2015, January 11). The ultimate relaxation technique: How to practice diaphragmatic breathing for beginners. YouTube. https://www.youtube.com/watch?v=Vca6DyFqt4c&feature=youtu.be
TED. (2010, January 19). How I held my breath for 17 minutes | David Blaine. YouTube. https://www.youtube.com/watch?v=XFnGhrC_3Gs&feature=youtu.be
TED-Ed. (2012, October 4). How breathing works - Nirvair Kaur. YouTube. https://www.youtube.com/watch?v=Kl4cU9sG_08&feature=youtu.be
TED-Ed. (2020, May 21). How do ventilators work? - Alex Gendler. YouTube. https://www.youtube.com/watch?v=yDtKBXOEsoM&feature=youtu.be
The process by which information from a gene is used in the synthesis of a functional protein.
a colorless cell that circulates in the blood and body fluids and is involved in counteracting foreign substances and disease; a white (blood) cell. There are several types, all amoeboid cells with a nucleus, including lymphocytes, granulocytes, monocytes, and macrophages.
Image shows a diagram of how alzheimer's progresses. In preclinical AD, just a small portion of the brain is affected. More of the brain and more areas of the brain are affected in mild to moderate AD. In severe AD, most of the brain is affected.
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. occurs when the outer layer of the skin is damaged by 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 . 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 .
What is the Epidermis?
The is the outer of the two main layers of the . The inner layer is the . 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 that line the mouth, anus, urethra, and vagina.
Structure of the Epidermis
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 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 -producing epithelial cells called . 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 . These cells produce the pigment melanin that protects the dermis from UV light.
- About one per cent of epidermal cells are . These are immune system cells that detect and fight pathogens entering the skin.
- Less than one per cent of epidermal cells are , 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 is the innermost (or deepest) layer of the epidermis. It is separated from the dermis by a membrane called the . The stratum basale contains stem cells — called — which divide to form all the 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. and 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 . 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 .
Stratum Granulosum
The next layer above the stratum spinosum is the . In this layer, keratinocytes have become nearly filled with , giving their cytoplasm a granular appearance. 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 and , 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 . 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 . 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.
Functions of the Epidermis
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
that penetrates the epidermis can damage epidermal cells. In particular, it can cause mutations in that lead to the development of skin , 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.
s in the stratum basale of the epidermis contain small organelles called , which produce, store, and transport the dark brown pigment . As melanosomes become full of melanin, they move into thin extensions of the melanocytes. From there, the melanosomes are transferred to 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 . 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 , 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 . 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 (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.
What Gives Skin Its Colour?
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 is present in the epidermis and gives skin a yellowish tint, especially in skin with low levels of melanin.
- 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.
Bacteria on Skin
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.
Feature: Myth vs. Reality
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. |
10.3 Summary
- The is the outer of the two main layers of the skin. It is very thin, but has a complex structure.
- Cell types in the epidermis include that produce and make up 90 per cent of epidermal cells, that produce , that fight in the skin, and that respond to light touch.
- The epidermis in most parts of the body consists of four distinct layers. A fifth layer occurs only in the epidermis of the palms of the hands and soles of the feet.
- The innermost layer of the epidermis is the , which contains stem cells that divide to form new keratinocytes. The next layer is the , which is the thickest layer and contains Langerhans cells and spiny keratinocytes. This is followed by the , in which keratinocytes are filling with keratin and starting to die. The is next, but only on the palms and soles. It consists of translucent dead keratinocytes. The outermost layer is the , which consists of flat, dead, tightly packed keratinocytes that form a tough, waterproof barrier for the rest of the epidermis.
- Functions of the epidermis include protecting underlying tissues from physical damage and pathogens. Melanin in the epidermis absorbs and protects underlying tissues from . The epidermis also prevents loss of water from the body and synthesizes vitamin D.
- Melanin is the main pigment that determines the colour of human skin. The pigments carotene and hemoglobin, however, also contribute to skin colour, especially in skin with low levels of melanin.
- The surface of healthy skin normally is covered by vast numbers of representing about one thousand species from 19 phyla. Different areas of the body provide diverse habitats for skin microorganisms. Usually, microorganisms on the skin keep each other in check unless their balance is disturbed.
10.3 Review Questions
- What is the epidermis?
- Identify the types of cells in the epidermis.
- Describe the layers of the epidermis.
- State one function of each of the four epidermal layers found all over the body.
- Explain three ways the epidermis protects the body.
- What makes the skin waterproof?
- Why is the selective permeability of the epidermis both a benefit and a risk?
- How is vitamin D synthesized in the epidermis?
- Identify three pigments that impart colour to skin.
- Describe bacteria that normally reside on the skin, and explain why they do not usually cause infections.
- Explain why the keratinocytes at the surface of the epidermis are dead, while keratinocytes located deeper in the epidermis are still alive.
- Which layer of the epidermis contains keratinocytes that have begun to die?
- Explain why our skin is not permanently damaged if we rub off some of the surface layer by using a rough washcloth.
10.3 Explore More
https://www.youtube.com/watch?v=27lMmdmy-b8
Jonathan Eisen: Meet your microbes, TED, 2015.
https://www.youtube.com/watch?v=9AcQXnOscQ8
Why Do We Blush?, SciShow, 2014.
https://www.youtube.com/watch?v=_r4c2NT4naQ
The science of skin colour - Angela Koine Flynn, TED-Ed, 2016.
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
As per caption.
A type of immune cell that is one of the first cell types to travel to the site of an infection. Neutrophils help fight infection by ingesting microorganisms and releasing enzymes that kill the microorganisms. A neutrophil is a type of white blood cell, a type of granulocyte, and a type of phagocyte.
Created by CK-12 Foundation/Adapted by Christine Miller
Steady as She Goes
This device (Figure 7.8.1) looks simple, but it controls a complex system that keeps a home at a steady temperature — it's a thermostat. The device shows the current temperature in the room, and also allows the occupant to set the thermostat to the desired temperature. A thermostat is a commonly cited model of how living systems — including the human body— maintain a steady state called homeostasis.
What Is Homeostasis?
is the condition in which a system (such as the human body) is maintained in a more or less steady state. It is the job of , , , and throughout the body to maintain many different variables within narrow ranges compatible with life. Keeping a stable internal environment requires continually monitoring the internal environment and constantly making adjustments to keep things in balance.
Set Point and Normal Range
For any given variable, such as body temperature or blood glucose level, there is a particular that is the physiological optimum value. The set point for human body temperature, for example, is about 37 degrees C (98.6 degrees F). As the body works to maintain for temperature or any other internal variable, the value typically fluctuates around the set point. Such fluctuations are normal, as long as they do not become too extreme. The spread of values within which such fluctuations are considered insignificant is called the . In the case of body temperature, for example, the normal range for an adult is about 36.5 to 37.5 degrees C (97.7 to 99.5 degrees F).
A good analogy for set point, normal range, and maintenance of homeostasis is driving. When you are driving a vehicle on the road, you are supposed to drive in the centre of your lane — this is analogous to the . Sometimes, you are not driving in the exact centre of the lane, but you are still within your lines, so you are in the equivalent of the . However, if you were to get too close to the centre line or the shoulder of the road, you would take action to correct your position. You'd move left if you were too close to the shoulder, or right if too close to the centre line — which is analogous to our next concept, to maintain .
Maintaining Homeostasis
is normally maintained in the human body by an extremely complex balancing act. Regardless of the variable being kept within its normal range, maintaining homeostasis requires at least four interacting components: stimulus, sensor, control centre, and effector.
- The is provided by the variable being regulated. Generally, the stimulus indicates that the value of the variable has moved away from the set point or has left the normal range.
- The monitors the values of the variable and sends data on it to the control centre.
- The matches the data with normal values. If the value is not at the set point or is outside the normal range, the control centre sends a signal to the effector.
- The is an organ, gland, muscle, or other structure that acts on the signal from the control centre to move the variable back toward the set point.
Each of these components is illustrated in Figure 7.8.2. The diagram on the left is a general model showing how the components interact to maintain homeostasis. The diagram on the right shows the example of body temperature. From the diagrams, you can see that maintaining homeostasis involves feedback, which is data that feeds back to control a response. Feedback may be negative (as in the example below) or positive. All the feedback mechanisms that maintain homeostasis use . Biological examples of positive feedback are much less common.
Negative Feedback
In a , feedback serves to reduce an excessive response and keep a variable within the . Two processes controlled by negative feedback are body temperature regulation and control of blood glucose.
Body Temperature
Body temperature regulation involves , whether it lowers the temperature or raises it, as shown in Figure 7.8.3 and explained in the text that follows.
Cooling Down
The human body’s temperature regulatory centre is the in the brain. When the hypothalamus receives data from sensors in the skin and brain that body temperature is higher than the , it sets into motion the following responses:
- Blood vessels in the skin dilate (vasodilation) to allow more blood from the warm body core to flow close to the surface of the body, so heat can be radiated into the environment.
- As blood flow to the skin increases, sweat glands in the skin are activated to increase their output of sweat (diaphoresis). When the sweat evaporates from the skin surface into the surrounding air, it takes heat with it.
- Breathing becomes deeper, and the person may breathe through the mouth instead of the nasal passages. This increases heat loss from the lungs.
Heating Up
When the brain’s temperature regulatory centre receives data that body temperature is lower than the set point, it sets into motion the following responses:
- Blood vessels in the skin contract (vasoconstriction) to prevent blood from flowing close to the surface of the body, which reduces heat loss from the surface.
- As temperature falls lower, random signals to skeletal muscles are triggered, causing them to contract. This causes shivering, which generates a small amount of heat.
- The may be stimulated by the brain (via the pituitary gland) to secrete more thyroid hormone. This hormone increases metabolic activity and heat production in cells throughout the body.
- The may also be stimulated to secrete the hormone . This hormone causes the breakdown of glycogen (the carbohydrate used for energy storage in animals) to , which can be used as an energy source. This catabolic chemical process is , or heat producing.
Blood Glucose
In controlling the blood glucose level, certain endocrine cells in the pancreas (called alpha and beta cells) detect the level of glucose in the blood. They then respond appropriately to keep the level of blood glucose within the normal range.
- If the blood glucose level rises above the normal range, pancreatic beta cells release the hormone insulin into the bloodstream. Insulin signals cells to take up the excess glucose from the blood until the level of blood glucose decreases to the normal range.
- If the blood glucose level falls below the normal range, pancreatic alpha cells release the hormone glucagon into the bloodstream. Glucagon signals cells to break down stored glycogen to glucose and release the glucose into the blood until the level of blood glucose increases to the normal range.
https://www.youtube.com/watch?v=Iz0Q9nTZCw4
Homeostasis and Negative/Positive Feedback, Amoeba Sisters, 2017.
Positive Feedback
In a , feedback serves to intensify a response until an end point is reached. Examples of processes controlled by positive feedback in the human body include blood clotting and childbirth.
Blood Clotting
When a wound causes bleeding, the body responds with a positive feedback loop to clot the blood and stop blood loss. Substances released by the injured blood vessel wall begin the process of blood clotting. Platelets in the blood start to cling to the injured site and release chemicals that attract additional platelets. As the platelets continue to amass, more of the chemicals are released and more platelets are attracted to the site of the clot. The positive feedback accelerates the process of clotting until the clot is large enough to stop the bleeding.
Childbirth
Figure 7.8.6 shows the positive feedback loop that controls childbirth. The process normally begins when the head of the infant pushes against the cervix. This stimulates nerve impulses, which travel from the cervix to the hypothalamus in the brain. In response, the hypothalamus sends the hormone to the pituitary gland, which secretes it into the bloodstream so it can be carried to the uterus. Oxytocin stimulates uterine contractions, which push the baby harder against the cervix. In response, the cervix starts to dilate in preparation for the passage of the baby. This cycle of positive feedback continues, with increasing levels of oxytocin, stronger uterine contractions, and wider dilation of the cervix until the baby is pushed through the birth canal and out of the body. At that point, the cervix is no longer stimulated to send nerve impulses to the brain, and the entire process stops.
Normal childbirth is driven by a positive feedback loop. Positive feedback causes an increasing deviation from the normal state to a fixed end point, rather than a return to a normal set point as in homeostasis.
When Homeostasis Fails
Homeostatic mechanisms work continuously to maintain stable conditions in the human body. Sometimes, however, the mechanisms fail. When they do, may result, in which cells may not get everything they need or toxic wastes may accumulate in the body. If homeostasis is not restored, the imbalance may lead to disease — or even death. is an example of a disease caused by homeostatic imbalance. In the case of diabetes, blood glucose levels are no longer regulated and may be dangerously high. Medical intervention can help restore homeostasis and possibly prevent permanent damage to the organism.
Normal aging may bring about a reduction in the efficiency of the body’s control systems, which makes the body more susceptible to disease. Older people, for example, may have a harder time regulating their body temperature. This is one reason they are more likely than younger people to develop serious heat-induced illnesses, such as heat stroke.
Feature: My Human Body
is diagnosed in people who have abnormally high levels of blood glucose after fasting for at least 12 hours. A fasting level of blood glucose below 100 is normal. A level between 100 and 125 places you in the pre-diabetes category, and a level higher than 125 results in a diagnosis of diabetes.
Of the two types of diabetes, is the most common, accounting for about 90 per cent of all cases of diabetes in the United States. Type 2 diabetes typically starts after the age of 40. However, because of the dramatic increase in recent decades in obesity in younger people, the age at which type 2 diabetes is diagnosed has fallen. Even children are now being diagnosed with type 2 diabetes. Today, about 3 million Canadians (8.1% of total population) are living with diabetes.
You may at some point have your blood glucose level tested during a routine medical exam. If your blood glucose level indicates that you have diabetes, it may come as a shock to you because you may not have any symptoms of the disease. You are not alone, because as many as one in four diabetics do not know they have the disease. Once the diagnosis of diabetes sinks in, you may be devastated by the news. Diabetes can lead to heart attacks, strokes, blindness, kidney failure, nerve damage, and loss of toes or feet. The risk of death in adults with diabetes is 50 per cent greater than it is in adults without diabetes, and diabetes is the seventh leading cause of death of adults. In addition, controlling diabetes usually requires frequent blood glucose testing, watching what and when you eat, and taking medications or even insulin injections. All of this may seem overwhelming.
The good news is that changing your lifestyle may stop the progression of type 2 diabetes or even reverse it. By adopting healthier habits, you may be able to keep your blood glucose level within the normal range without medications or insulin. Here’s how:
- Lose weight. Any weight loss is beneficial. Losing as little as seven per cent of your weight may be all that is needed to stop diabetes in its tracks. It is especially important to eliminate excess weight around your waist.
- Exercise regularly. You should try to exercise for at least 30 minutes, five days a week. This will not only lower your blood sugar and help your insulin work better, but it will also lower your blood pressure and improve your heart health. Another bonus of exercise is that it will help you lose weight by increasing your basal metabolic rate.
- Adopt a healthy diet. Decrease your consumption of refined carbohydrates, such as sweets and sugary drinks. Increase your intake of fibre-rich foods, such as fruits, vegetables, and whole grains. About one-quarter of each meal should consist of high-protein foods, such as fish, chicken, dairy products, legumes, or nuts.
- Control stress. Stress can increase your blood glucose and also raise your blood pressure and risk of heart disease. When you feel stressed out, do breathing exercises or take a brisk walk or jog. Try to replace stressful thoughts with more calming ones.
- Establish a support system. Enlist the help and support of loved ones, as well as medical professionals, such as a nutritionist and diabetes educator. Having a support system will help ensure that you are on the path to wellness, and that you can stick to your plan.
7.8 Summary
- is the condition in which a system (such as the human body) is maintained in a more or less steady state. It is the job of cells, tissues, organs, and organ systems throughout the body to maintain homeostasis.
- For any given variable, such as body temperature, there is a particular that is the physiological optimum value. The spread of values around the set point that is considered insignificant is called the .
- Homeostasis is generally maintained by a that includes a , , , and . Negative feedback serves to reduce an excessive response and to keep a variable within the normal range. Negative feedback loops control body temperature and the blood glucose level.
- are not common in biological systems. Positive feedback serves to intensify a response until an end point is reached. Positive feedback loops control blood clotting and childbirth.
- Sometimes homeostatic mechanisms fail, resulting in . Diabetes is an example of a disease caused by homeostatic imbalance. Aging can bring about a reduction in the efficiency of the body’s control system, which makes the elderly more susceptible to disease.
7.8 Review Questions
- Compare and contrast negative and positive feedback loops.
- Explain how negative feedback controls body temperature.
- Give two examples of physiological processes controlled by positive feedback loops.
- During breastfeeding, the stimulus of the baby sucking on the nipple increases the amount of milk produced by the mother. The more sucking, the more milk is usually produced. Is this an example of negative or positive feedback? Explain your answer. What do you think might be the evolutionary benefit of the milk production regulation mechanism you described?
- Explain why homeostasis is regulated by negative feedback loops, rather than positive feedback loops.
- The level of a sex hormone, testosterone (T), is controlled by negative feedback. Another hormone, gonadotropin-releasing hormone (GnRH), is released by the hypothalamus of the brain, which triggers the pituitary gland to release luteinizing hormone (LH). LH stimulates the gonads to produce T. When there is too much T in the bloodstream, it feeds back on the hypothalamus, causing it to produce less GnRH. While this does not describe all the feedback loops involved in regulating T, answer the following questions about this particular feedback loop.
- What is the stimulus in this system? Explain your answer.
- What is the control centre in this system? Explain your answer.
- In this system, is the pituitary considered the stimulus, sensor, control centre, or effector? Explain your answer.
7.8 Explore More
https://www.youtube.com/watch?v=LSgEJSlk6W4
Homeostasis - What Is Homeostasis - What Is Set Point For Homeostasis - Homeostasis In The Human Body, Whats Up Dude, 2017.
https://www.youtube.com/watch?v=XMsJ-3qRVJM
Attributions
Figure 7.8.1
Nest_Thermostat by Amanitamano on Wikimedia Commons is used under a CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0/deed.en) license.
Figure 7.8.2
Negative_Feedback_Loops by OpenStax on Wikimedia Commons is used under a CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/deed.en) license.
Figure 7.8.3
Body Temperature Homeostasis by OpenStax College, Biology is used under a CC BY 4.0 license.
Figure 7.8.4
Homeostasis_of_blood_sugar by Christinelmiller on Wikimedia Commons is used under a CC0 1.0 Universal Public Domain Dedication (https://creativecommons.org/publicdomain/zero/1.0/deed.en) license.
Figure 7.8.5
Positive_Feedback_Diagram_Blood_Clotting by Elliottuttle on Wikimedia Commons is used under a CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0) license.
Figure 7.8.6
Pregnancy-Positive_Feedback by OpenStax on Wikimedia Commons is used under a CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/deed.en) license.
References
Amoeba Sisters. (2017, September 7). Homeostasis and negative/positive feedback. YouTube. https://www.youtube.com/watch?v=Iz0Q9nTZCw4&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 1.10 Negative feedback loop [digital image/ diagram]. In Anatomy and Physiology (Section 1.5). OpenStax. https://openstax.org/books/anatomy-and-physiology/pages/1-5-homeostasis
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 1.11 Positive feedback loop
Cognito. (2018, December 18). GCSE Biology - Homeostasis #38. YouTube. https://www.youtube.com/watch?v=XMsJ-3qRVJM&feature=youtu.be
Mayo Clinic Staff. (n.d.). Type 2 diabetes [online article]. MayoClinic.org. https://www.mayoclinic.org/diseases-conditions/type-2-diabetes/symptoms-causes/syc-20351193
OpenStax CNX. (2016, March 23). Figure 4 The body is able to regulate temperature in response to signals from the nervous system [digital image]. In OpenStax, Biology (Section 33.3). https://cnx.org/contents/GFy_h8cu@10.8:BP24ZReh@7/Homeostasis
Whats Up Dude. (2017, September 20). Homeostasis - What is homeostasis - What is set point for homeostasis - Homeostasis in the human body. YouTube. https://www.youtube.com/watch?v=LSgEJSlk6W4&feature=youtu.be
As per caption.
Pills from Pee
The medication pictured in Figure 9.3.1 with the brand name Progynon was a drug used to control the effects of menopause in women. The pills first appeared in 1928 and contained the human sex hormone estrogen. Estrogen secretion declines in women around the time of menopause and may cause symptoms like mood swings and hot flashes. The pills were supposed to ease the symptoms by supplementing in the body. The manufacturer of Progynon obtained estrogen for the pills from the urine of pregnant women, because it was a cheap source of the hormone. Progynon is still used today to treat menopausal symptoms. Although the drug has been improved over the years, it still contains estrogen, which is an example of an endocrine hormone.
How Do Endocrine Hormones Work?
Endocrine hormones like estrogen are messenger molecules secreted by endocrine glands into the bloodstream. They travel throughout the body in the circulation. Although they reach virtually every cell in the body in this way, each hormone affects only certain cells, called target cells. A is the type of cell on which a hormone has an effect. A target cell is affected by a particular hormone because it has receptor proteins — either on the cell surface or within the cell — that are specific to that hormone. An endocrine hormone travels through the bloodstream until it finds a target cell with a matching receptor to which it can bind. When the hormone binds to the receptor, it causes changes within the cell. The manner in which it changes the cell depends on whether the hormone is a steroid hormone or a non-steroid hormone.
Steroid Hormones
A (such as estrogen) is made of . It is fat soluble, so it can diffuse across a target cell’s , which is also made of lipids. Once inside the cell, a steroid hormone binds with receptor proteins in the . As you can see in Figure 9.3.2, the steroid hormone and its receptor form a complex — called a steroid complex — which moves into the , where it influences the expression of genes. Examples of steroid hormones include , which is secreted by the , and sex hormones, which are secreted by the .
Non-Steroid Hormones
A is made of . It is not fat soluble, so it cannot diffuse across the plasma membrane of a target cell. Instead, it binds to a receptor protein on the cell membrane. In the Figure 9.3.3 diagram, you can see that the binding of the hormone with the receptor activates an in the cell membrane. The enzyme then stimulates another molecule, called the second messenger, which influences processes inside the cell. Most endocrine hormones are non-steroid hormones. Examples include and , both produced by the .
Regulation of Endocrine Hormones
Endocrine hormones regulate many body processes, but what regulates the secretion of endocrine hormones? Most endocrine hormones are controlled by s. A feedback mechanism is a loop in which a product feeds back to control its own production. Feedback loops may be either negative or positive.
- Most endocrine hormones are regulated by loops. Negative feedback keeps the concentration of a hormone within a relatively narrow range, and maintains .
- Very few endocrine hormones are regulated by loops. Positive feedback causes the concentration of a hormone to become increasingly higher.
Regulation by Negative Feedback
A loop controls the synthesis and secretion of hormones by the . This loop includes the and , in addition to the thyroid, as shown in the diagram (Figure 9.3.4). When the levels of thyroid hormones circulating in the blood fall too low, the hypothalamus secretes (TRH). This hormone travels directly to the pituitary gland through the thin stalk connecting the two structures. In the pituitary gland, TRH stimulates the pituitary to secrete (TSH). TSH, in turn, travels through the bloodstream to the thyroid gland, and stimulates it to secrete thyroid hormones. This continues until the blood levels of thyroid hormones are high enough. At that point, the thyroid hormones feed back to stop the hypothalamus from secreting TRH and the pituitary from secreting TSH. Without the stimulation of TSH, the thyroid gland stops secreting its hormones. Eventually, the levels of thyroid hormones in the blood start to fall too low again. When that happens, the hypothalamus releases TRH, and the loop repeats.
Regulation by Positive Feedback
is a non-steroid endocrine hormone secreted by the . One of the functions of prolactin is to stimulate a nursing mother’s to produce milk. The regulation of prolactin in the mother is controlled by a that involves the , , , and . Positive feedback begins when a baby suckles on the mother’s nipple. Nerve impulses from the nipple reach the hypothalamus, which stimulates the pituitary gland to secrete prolactin. Prolactin travels in the blood to the mammary glands and stimulates them to produce milk. The release of milk causes the baby to continue suckling, which causes more prolactin to be secreted and more milk to be produced. The positive feedback loop continues until the baby stops suckling at the breast.
Feature: Myth vs. Reality
are synthetic versions of the naturally occurring male sex hormone . Male hormones have androgenic (or masculinizing) effects, but they also have anabolic (or muscle-building) effects. The anabolic effects are the reason that synthetic steroids are used by athletes. In addition to building muscles, they also accelerate the development of and , increase endurance so athletes can train harder and longer, and speed up muscle recovery. Unfortunately, these benefits of steroid use come with costs. If you ever consider taking anabolic steroids to build muscles and improve athletic performance, consider the following myths and corresponding realities.
Myth |
Reality |
"Steroids are safe." | Steroid use may cause several serious side effects. Prolonged use may increase the risk of liver cancer, heart disease, and high blood pressure. |
"Steroids will not stunt your growth." | Teens who take steroids before they have finished growing in height may have their growth stunted so they remain shorter throughout life than they would otherwise have been. Such stunting occurs because steroids increase the rate at which skeletal maturity is reached. Once skeletal maturity occurs, additional growth in height is impossible. |
"Steroids do not cause drug dependency." | Steroid use may cause dependency, as evidenced by the negative effects of stopping steroid use. These negative effects may include insomnia, fatigue, and depressed mood, among others. |
"There is no such thing as 'roid rage.'" | Steroid use has been shown to increase aggressiveness in some people. It has also been implicated in a number of violent acts committed by people who had not demonstrated violent tendencies until they started using steroids. |
"Only males use steroids." | Although steroid use is more common in males than females, some females also use steroids. They use them to build muscle and improve physical performance, generally either for athletic competition or for self-defense. |
9.3 Summary
- Endocrine hormones are messenger molecules secreted by endocrine glands into the bloodstream. They travel throughout the body but affect only certain cells, called , which have receptors specific to particular hormones.
- such as are endocrine hormones made of that cross and bind to receptors inside target cells. The hormone-receptor complexes then move into the , where they influence .
- (such as ) are endocrine hormones made of that bind to receptors on the surface of target cells. This activates an in the plasma membrane, and the enzyme controls a second messenger molecule, which influences cell processes.
- Most endocrine hormones are controlled by in which rising levels of a hormone feed back to stop its own production — and vice-versa. For example, a negative feedback loop controls production of hormones. The loop includes the , , and thyroid gland.
- Only a few endocrine hormones are controlled by , in which rising levels of a hormone feed back to stimulate continued production of the hormone. , the pituitary hormone that stimulates milk production by mammary glands, is controlled by a positive feedback loop. The loop includes the , hypothalamus, pituitary gland, and .
9.3 Review Questions
- Explain how steroid hormones influence target cells.
- How do non-steroid hormones affect target cells?
- Compare and contrast negative and positive feedback loops.
- Outline the way feedback controls the production of thyroid hormones.
- Describe the feedback mechanism that controls milk production by the mammary glands.
- People with a condition called hyperthyroidism produce too much thyroid hormone. What do you think this does to the level of TSH? Explain your answer.
- Which is more likely to maintain homeostasis— negative feedback or positive feedback? Explain your answer.
- Does testosterone bind to receptors on the plasma membrane of target cells or in the cytoplasm of target cells? Explain your answer.
9.3 Explore More
https://www.youtube.com/watch?v=WVrlHH14q3o&feature=emb_logo
Great Glands - Your Endocrine System: CrashCourse Biology #33, CrashCourse, 2012.
https://www.youtube.com/watch?v=qXaDDa3FB5Q&feature=emb_logo
National Geographic | Benefits and Side Effects of Steroids Use 2015, 24 Physic.
Attributions
Figure 9.3.1
L0058274 Glass bottle for ‘Progynon’ pills, United Kingdom, 1928-1948 by Wellcome Collection gallery (2018-03-29)/ Science Museum, London on Wikimedia Commons is used under a CC-BY-4.0 (https://creativecommons.org/licenses/by/4.0/) license.
Figure 9.3.2
Regulation_of_gene_expression_by_steroid_hormone_receptor.svg by Ali Zifan on Wikimedia Commons is used under a CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/deed.en) license.
Figure 9.3.3
Non-steroid hormone pathway by CK-12 Foundation, Biology for High School is used under a CC BY-NC 3.0 (https://creativecommons.org/licenses/by-nc/3.0/) license.
Figure 9.3.4
Thyroid Negative Feedback Loop by CK-12 Foundation, College Human Biology is used under a CC BY-NC 3.0 (https://creativecommons.org/licenses/by-nc/3.0/) license.
©CK-12 Foundation Licensed under • Terms of Use • Attribution
Figure 9.3.5
Lactation Positive Feedback Loop by Christinelmiller on Wikimedia Commons is used under a CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/deed.en) license.
References
24 Physic. (2015,July 19). National Geographic | Benefits and side effects of steroids use 2015. YouTube. https://www.youtube.com/watch?v=qXaDDa3FB5Q&feature=youtu.be
Brainard, J/ CK-12 Foundation. (2016, August 15). Figure 4 Thyroid negative feedback loop [digital image]. In CK-12 College Human Biology (Section 11.3 Endocrine hormones). CK12.org. https://www.ck12.org/book/ck-12-human-biology/section/11.3/
CK-12 Foundation. (2019, March 5). Figure 3 A non-steroid hormone binds with a receptor on the plasma membrane of a target cell [digital image]. In Flexbook 2.0: CK-12 Biology For High School (Section 13.21 Hormone). CK12. https://flexbooks.ck12.org/cbook/ck-12-biology-flexbook-2.0/section/13.21/primary/lesson/hormones-bio
CrashCourse. (2012, September 10). Great glands - Your endocrine system: CrashCourse Biology #33. YouTube. https://www.youtube.com/watch?v=WVrlHH14q3o&feature=youtu.be
TED-Ed. (2018, June 21). How do your hormones work? - Emma Bryce. YouTube. https://www.youtube.com/watch?v=-SPRPkLoKp8&feature=youtu.be
Image shows a diagram of the types and locations of sensory receptors in the dermis.
There are free nerve endings towards the exterior of the dermis, Merkle cells and Meissners corpuscles are embedded just below the free nerve endings. Ruffini corpuscles and lamellated corpuscles are present deeper in the dermal tissue.
A type of immune cell that has granules (small particles) with enzymes that are released during infections, allergic reactions, and asthma. An eosinophil is a type of white blood cell and a type of granulocyte.
Goose Bumps
No doubt you’ve experienced the tiny, hair-raising skin bumps called goose bumps, like those you see in Figure 10.4.1. They happen when you feel chilly. Do you know what causes goose bumps, or why they pop up when you are cold? The answers to these questions involve the layer of skin known as the dermis.
What is the Dermis?
The is the inner of the two major layers that make up the skin, the outer layer being the . The dermis consists mainly of . It also contains most skin structures, such as and blood vessels. The dermis is anchored to the tissues below it by flexible collagen bundles that permit most areas of the skin to move freely over subcutaneous (“below the skin”) tissues. Functions of the dermis include cushioning subcutaneous tissues, regulating body temperature, sensing the environment, and excreting wastes.
Anatomy of the Dermis
The basic anatomy of the dermis is a matrix, or sort of scaffolding, composed of connective tissues. These tissues include collagen fibres — which provide toughness — and elastin fibres, which provide elasticity. Surrounding these fibres, the matrix also includes a gel-like substance made of proteins. The tissues of the matrix give the dermis both strength and flexibility.
The dermis is divided into two layers: the papillary layer and the reticular layer. Both layers are shown in Figure 10.4.2 below and described in the text that follows.
Papillary Layer
The is the upper layer of the dermis, just below the that connects the dermis to the epidermis above it. The papillary layer is the thinner of the two dermal layers. It is composed mainly of loosely arranged collagen fibres. The papillary layer is named for its fingerlike projections — or papillae — that extend upward into the epidermis. The papillae contain capillaries and sensory touch receptors.
The papillae give the dermis a bumpy surface that interlocks with the epidermis above it, strengthening the connection between the two layers of skin. On the palms and soles, the papillae create epidermal ridges. Epidermal ridges on the fingers are commonly called fingerprints (see Figure 10.4.3). Fingerprints are genetically determined, so no two people (other than identical twins) have exactly the same fingerprint pattern. Therefore, fingerprints can be used as a means of identification, for example, at crime scenes. Fingerprints were much more commonly used forensically before DNA analysis was introduced for this purpose.
Reticular Layer
The is the lower layer of the dermis, located below the papillary layer. It is the thicker of the two dermal layers. It is composed of densely woven collagen and elastin fibres. These protein fibres give the dermis its properties of strength and elasticity. This layer of the dermis cushions subcutaneous tissues of the body from stress and strain. The reticular layer of the dermis also contains most of the structures in the dermis, such as and hair .
Structures in the Dermis
Both papillary and reticular layers of the dermis contain numerous , which make the skin the body’s primary sensory organ for the sense of . Both dermal layers also contain blood vessels. They provide nutrients to remove wastes from dermal cells, as well as cells in the lowest layer of the epidermis, the . The circulatory components of the dermis are shown in Figure 10.4.4 below.
Glands
Glands in the reticular layer of the dermis include sweat glands and sebaceous (oil) glands. Both are exocrine glands, which are glands that release their secretions through ducts to nearby body surfaces. The diagram in Figure 10.4.5 shows these glands, as well as several other structures in the dermis.
Sweat Glands
produce the fluid called sweat, which contains mainly water and salts. The glands have ducts that carry the sweat to hair follicles, or to the surface of the skin. There are two different types of sweat glands: eccrine glands and apocrine glands.
- occur in skin all over the body. Their ducts empty through tiny openings called pores onto the skin surface. These sweat glands are involved in temperature regulation.
- are larger than eccrine glands, and occur only in the skin of the armpits and groin. The ducts of apocrine glands empty into hair follicles, and then the sweat travels along hairs to reach the surface. Apocrine glands are inactive until puberty, at which point they start producing an oily sweat that is consumed by bacteria living on the skin. The digestion of apocrine sweat by bacteria causes body odor.
Sebaceous Glands
s are exocrine glands that produce a thick, fatty substance called sebum. is secreted into hair follicles and makes its way to the skin surface along hairs. It waterproofs the hair and skin, and helps prevent them from drying out. Sebum also has antibacterial properties, so it inhibits the growth of microorganisms on the skin. Sebaceous glands are found in every part of the skin — except for the palms of the hands and soles of the feet, where hair does not grow.
Hair Follicles
s are the structures where hairs originate (see the diagram above). Hairs grow out of follicles, pass through the epidermis, and exit at the surface of the skin. Associated with each hair follicle is a sebaceous gland, which secretes sebum that coats and waterproofs the hair. Each follicle also has a bed of capillaries, a nerve ending, and a tiny muscle called an .
Functions of the Dermis
The main functions of the dermis are regulating body temperature, enabling the sense of touch, and eliminating wastes from the body.
Temperature Regulation
Several structures in the reticular layer of the dermis are involved in regulating body temperature. For example, when body temperature rises, the of the sends nerve signals to sweat glands, causing them to release sweat. An adult can sweat up to four litres an hour. As the sweat evaporates from the surface of the body, it uses energy in the form of body heat, thus cooling the body. The hypothalamus also causes dilation of blood vessels in the dermis when body temperature rises. This allows more blood to flow through the skin, bringing body heat to the surface, where it can radiate into the environment.
When the body is too cool, sweat glands stop producing sweat, and blood vessels in the skin constrict, thus conserving body heat. The arrector pili muscles also contract, moving hair follicles and lifting hair shafts. This results in more air being trapped under the hairs to insulate the surface of the skin. These contractions of arrector pili muscles are the cause of goose bumps.
Sensing the Environment
Sensory receptors in the dermis are mainly responsible for the body’s tactile senses. The receptors detect such tactile stimuli as warm or cold temperature, shape, texture, pressure, vibration, and pain. They send nerve impulses to the brain, which interprets and responds to the sensory information. Sensory receptors in the dermis can be classified on the basis of the type of touch stimulus they sense. s sense mechanical forces such as pressure, roughness, vibration, and stretching. s sense variations in temperature that are above or below body temperature. s sense painful stimuli. Figure 10.4.6 shows several specific kinds of tactile receptors in the dermis. Each kind of receptor senses one or more types of touch stimuli.
- Free nerve endings sense pain and temperature variations.
- Merkel cells sense light touch, shapes, and textures.
- Meissner’s corpuscles sense light touch.
- Pacinian corpuscles sense pressure and vibration.
- Ruffini corpuscles sense stretching and sustained pressure.
Excreting Wastes
The sweat released by is one way the body excretes waste products. Sweat contains excess water, salts (electrolytes), and other waste products that the body must get rid of to maintain . The most common electrolytes in sweat are sodium and chloride. Potassium, calcium, and magnesium electrolytes may be excreted in sweat, as well. When these electrolytes reach high levels in the blood, more are excreted in sweat. This helps to bring their blood levels back into balance. Besides electrolytes, sweat contains small amounts of waste products from , including ammonia and urea. Sweat may also contain alcohol in someone who has been drinking alcoholic beverages.
Feature: My Human Body
is the most common skin disorder in the Canada. At least 20% of Canadians have acne at any given time and it affects approximately 90% of adolescents (as in Figure 10.4.7). Although acne occurs most commonly in teens and young adults, but it can occur at any age. Even newborn babies can get acne.
The main sign of acne is the appearance of pimples (pustules) on the skin, like those in the photo above. Other signs of acne may include whiteheads, blackheads, nodules, and other lesions. Besides the face, acne can appear on the back, chest, neck, shoulders, upper arms, and buttocks. Acne can permanently scar the skin, especially if it isn’t treated appropriately. Besides its physical effects on the skin, acne can also lead to low self-esteem and depression.
Acne is caused by clogged, sebum-filled pores that provide a perfect environment for the growth of bacteria. The bacteria cause infection, and the immune system responds with inflammation. Inflammation, in turn, causes swelling and redness, and may be associated with the formation of pus. If the inflammation goes deep into the skin, it may form an acne nodule.
Mild acne often responds well to treatment with over-the-counter (OTC) products containing benzoyl peroxide or salicylic acid. Treatment with these products may take a month or two to clear up the acne. Once the skin clears, treatment generally needs to continue for some time to prevent future breakouts.
If acne fails to respond to OTC products, nodules develop, or acne is affecting self-esteem, a visit to a dermatologist is in order. A dermatologist can determine which treatment is best for a given patient. A dermatologist can also prescribe prescription medications (which are likely to be more effective than OTC products) and provide other medical treatments, such as laser light therapies or chemical peels.
What can you do to maintain healthy skin and prevent or reduce acne? Dermatologists recommend the following tips:
- Wash affected or acne-prone skin (such as the face) twice a day, and after sweating.
- Use your fingertips to apply a gentle, non-abrasive cleanser. Avoid scrubbing, which can make acne worse.
- Use only alcohol-free products and avoid any products that irritate the skin, such as harsh astringents or exfoliants.
- Rinse with lukewarm water, and avoid using very hot or cold water.
- Shampoo your hair regularly.
- Do not pick, pop, or squeeze acne. If you do, it will take longer to heal and is more likely to scar.
- Keep your hands off your face. Avoid touching your skin throughout the day.
- Stay out of the sun and tanning beds. Some acne medications make your skin very sensitive to UV light.
10.4 Summary
- The is the inner and thicker of the two major layers that make up the skin. It consists mainly of a matrix of s that provide strength and stretch. It also contains almost all skin structures, including and s.
- The dermis has two layers. The upper has papillae extending upward into the epidermis and loose connective tissues. The lower has denser connective tissues and structures, such as glands and hair follicles. Glands in the dermis include eccrine and apocrine sweat glands and sebaceous glands. Hair are structures where hairs originate.
- Functions of the dermis include cushioning subcutaneous tissues, regulating body temperature, sensing the environment, and excreting wastes. The dense connective tissues of the dermis provide cushioning. The dermis regulates body temperature mainly by sweating and by vasodilation or vasoconstriction. The many tactile sensory receptors in the dermis make it the main organ for the sense of touch. Wastes excreted in sweat include excess water, electrolytes, and certain metabolic wastes.
10.4 Review Questions
- What is the dermis?
- Describe the basic anatomy of the dermis.
- Compare and contrast the papillary and reticular layers of the dermis.
- What causes epidermal ridges, and why can they be used to identify individuals?
- Name the two types of sweat glands in the dermis, and explain how they differ.
- What is the function of sebaceous glands?
- Describe the structures associated with hair follicles.
- Explain how the dermis helps regulate body temperature.
- Identify three specific kinds of tactile receptors in the dermis, along with the type of stimuli they sense.
- How does the dermis excrete wastes? What waste products does it excrete?
- What are subcutaneous tissues? Which layer of the dermis provides cushioning for subcutaneous tissues? Why does this layer provide most of the cushioning, instead of the other layer?
- For each of the functions listed below, describe which structure within the dermis carries it out.
- Brings nutrients to and removes wastes from dermal and lower epidermal cells
- Causes hairs to move
- Detects painful stimuli on the skin
10.4 Explore More
https://www.youtube.com/watch?v=FX-FwK0IIrE
How do you get rid of acne? SciShow, 2016.
https://www.youtube.com/watch?v=VcHQWMAClhQ&feature=emb_logo
When You Can't Scratch Away An Itch, Seeker, 2013.
Attributions
Figure 10.4.1
Goose_bumps by EverJean on Wikimedia Commons is used under a CC BY 2.0 (https://creativecommons.org/licenses/by/2.0) license.
Figure 10.4.2
Layers_of_the_Dermis by OpenStax College on Wikimedia Commons is used under a CC BY 3.0 (https://creativecommons.org/licenses/by/3.0) license.
Figure 10.4.3
Fingerprint_detail_on_male_finger_in_Třebíč,_Třebíč_District by Frettie on Wikimedia Commons is used under a CC BY 3.0 (https://creativecommons.org/licenses/by/3.0) license.
Figure 10.4.4
Blausen_0802_Skin_Dermal Circulation by BruceBlaus on Wikimedia commons is used under a CC BY 3.0 (https://creativecommons.org/licenses/by/3.0) license.
Figure 10.4.5
Anatomy_The_Skin_-_NCI_Visuals_Online by Don Bliss (artist) / National Cancer Institute (National Institutes of Health, with the ID 4604) is in the public domain (https://en.wikipedia.org/wiki/public_domain).
Figure 10.4.6
Blausen_0809_Skin_TactileReceptors by BruceBlaus on Wikimedia commons is used under a CC BY 3.0 (https://creativecommons.org/licenses/by/3.0) license.
Figure 10.4.7
Akne-jugend by Ellywa on Wikimedia Commons is released into the public domain (https://en.wikipedia.org/wiki/public_domain). (No machine-readable author provided. Ellywa assumed, based on copyright claims).
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 5.7 Layers of the dermis [digital image]. In Anatomy and Physiology (Section 5.1 Layers of the skin). OpenStax. https://openstax.org/books/anatomy-and-physiology/pages/5-1-layers-of-the-skin
Blausen.com staff. (2014). Medical gallery of Blausen Medical 2014. WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.010. ISSN 2002-4436.
SciShow. (2016, October 26). How do you get rid of acne? YouTube. https://www.youtube.com/watch?v=FX-FwK0IIrE
Seeker. (2013, October 26). When you can't scratch away an itch. YouTube. https://www.youtube.com/watch?v=VcHQWMAClhQ&feature=emb_logo
Image shows a bowl of kidney beans.
A type of immune cell that has granules (small particles) with enzymes that are released during allergic reactions and asthma. A basophil is a type of white blood cell and a type of granulocyte.
Image shows a diagram of a hair follicle. The follicle is embedded deep in the dermis. The hair follicle has an associated sebaceous gland which secretes oils, and an arrector pili muscle, which would cause goosebumps if activated.
Image shows a picture of a child with very curly hair.
A type of immune cell that has granules (small particles) with enzymes that can kill tumor cells or cells infected with a virus. A natural killer cell is a type of white blood cell.
Image shows a diagram of the hormones secreted by the thyroid gland, and how it is both controlled by and acting upon in a negative feedback the hypothalamus and the anterior pituitary gland.
A group of diseases involving abnormal cell growth with the potential to invade or spread to other parts of the body.
Image shows a diagram of the brain highlighting the region containing the hypothalamus and pituitary gland.
Image shows a photograph of a cotton-top tamarin monkey, which displays the straight hair characteristic of non-human primates.
Figure 7.4.1 Construction — It's important to have the right materials for the job.
The Right Material for the Job
Building a house is a big job and one that requires a lot of different materials for specific purposes. As you can see in Figure 7.4.1, many different types of materials are used to build a complete house, but each type of material fulfills certain functions. You wouldn't use insulation to cover your roof, and you wouldn't use lumber to wire your home. Just as a builder chooses the appropriate materials to build each aspect of a home (wires for electrical, lumber for framing, shingles for roofing), your body uses the right cells for each type of role. When many cells work together to perform a specific function, this is termed a .
Tissues
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
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. 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. 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 , 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:
- 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.
- 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.
- Fluid connective tissue: is characterized by a fluid matrix and includes both blood and lymph.
Fibrous Connective Tissue
Fibrous connective tissue contains cells called . 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 () or cells that create cartilage ().
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 . 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
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 .
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 , which conduct the electrical signals used to "tell" the chambers of the heart when to contract.
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
Neuroglia can be understood as support staff for the neuron. The neurons have such an important job, they need cells to bring them nutrients, take away cell waste, and build their mylein sheath. There are many types of neuroglia, which are categorized based on their function and/or their location in the nervous system. Neuroglia outnumber neurons by as much as 50 to 1, and are much smaller. See the diagram in 7.4.17 to compare the size and number of neurons and neuroglia.
Try out this memory game to test your tissues knowledge:
7.4 Summary
- Tissues are made up of cells working together.
- There are four main types of tissues: epithelial, connective, muscular and nervous.
- Epithelial tissue makes up the linings and coverings of the body and is characterized by having a free surface and a basement membrane. Types of epithelial tissue are distinguished by shape of cell (squamous, cuboidal or columnar) and layering (simple, stratified, pseudostratified and transitional). Different epithelial tissues can carry out diffusion, secretion, absorption, and/or protection depending on their particular cell shape and layering.
- Connective tissue provides structure and support for the body and is characterized as a scattering of cells in a non-cellular matrix. There are three main categories of connective tissue, each characterized by a particular type of matrix:
- Fibrous connective tissue contains protein fibres. Both loose and dense fibrous connective tissue belong in this category.
- Supportive connective tissue contains a very solid matrix, and includes both bone and cartilage.
- Fluid connective tissue contains cells in a fluid matrix with the two types of blood and lymph.
- Muscular tissue's defining feature is that it is contractile. There are three types of muscular tissue: skeletal muscle which is found attached to the skeleton for voluntary movement, smooth muscle which moves substances through body tubes, and cardiac muscle which moves blood through the heart.
- Nervous tissue contains specialized cells called neurons which can conduct electrical impulses. Also found in nervous tissue are neuroglia, which support neurons by providing nutrients, removing wastes, and creating myelin sheath.
7.4 Review Questions
- Define the term tissue.
- If a part of the body needed a lining that was both protective, but still able to absorb nutrients, what would be the best type of epithelial tissue to use?
- Where do you find skeletal muscle? Smooth muscle? Cardiac muscle?
- What are some of the functions of neuroglia?
7.4 Explore More
https://www.youtube.com/watch?v=O0ZvbPak4ck
Types of Human Body Tissue, MoomooMath and Science, 2017.
https://www.youtube.com/watch?v=uHbn7wLN_3k
How to 3D print human tissue - Taneka Jones, TED-Ed, 2019.
https://www.youtube.com/watch?v=1Qfmkd6C8u8
How bones make blood - Melody Smith, TED-Ed, 2020.
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
Image shows a microscopic view of the structure of spongy bone. It is an irregular lattice of bone and open space, which typically houses bone marrow and blood vessels.
Image shows the difference in morphology between a sickle cell and a normal red blood cell. The normal red blood cells are shaped like danishes, while the sickle cells are shaped like bananas
Image shows a photograph of an Amish man. His hairstyle and beard with no mustache is evidence that he is married.