12.6 Disorders of the Muscular System
Pain in the Neck
Spending hours each day looking down at hand-held devices is a pain in the neck — literally. The weight of the head bending forward can put a lot of strain on neck muscles, and muscle injuries can be very painful. Neck pain is one of the most common of all complaints that bring people to the doctor’s office. In any given year, about one in five adults will suffer from neck pain. That’s a lot of pains in the neck! Not all of them are due to muscular disorders, but many of them are. Muscular disorders, in turn, generally fall into two general categories: and .
Musculoskeletal Disorders
are injuries that occur in muscles or associated tissues (such as ) because of biomechanical stresses. They may be caused by sudden exertion, over-exertion, repetitive motions, or long periods of maintaining awkward positions. Musculoskeletal disorders are often work- or sports-related, and generally just one or a few muscles are affected. They can often be treated successfully, and full recovery can be very likely. The disorders include muscle strains, tendinitis, and carpal tunnel syndrome.
Muscle Strain
A is an injury in which muscle fibres tear as a result of overstretching. A muscle strain is also commonly called a pulled muscle or torn muscle. (Strains are often confused with sprains, which are similar injuries to ligaments.) Depending on the degree of injury to muscle fibres, a muscle strain can range from mildly to extremely painful. Besides pain, typical symptoms include stiffness and bruising in the area of the strained muscle. The photo here shows a large bruise caused by a hamstring muscle strain. Hamstring strains are very common in track and field athletes. In sprinters, for example, about one third of injuries are hamstring injuries. Having a previous hamstring injury puts an athlete at increased risk for having another one.
Proper first aid for a muscle strain includes five steps, which are represented by the acronym PRICE. The PRICE steps should be followed for several days after the injury. The five steps are:
- Protection: Apply soft padding to the strained muscle to minimize impact with objects that might cause further damage.
- Rest: Rest the muscle to accelerate healing and reduce the potential for re-injury.
- Ice: Apply ice for 20 minutes at a time every two hours to reduce swelling and pain.
- Compression: Apply a stretchy bandage to the strained muscle to reduce swelling.
- Elevation: Keep the strained muscle elevated to reduce the chance of blood pooling in the muscle.
Non-steroidal anti-inflammatory drugs (NSAIDs, such as ibuprofen) can help reduce inflammation and relieve pain. Because such drugs interfere with blood clotting, however, they should be taken only after bleeding in the muscle has stopped — not immediately after the injury occurs. For severe muscle strains, professional medical care may be needed.
Tendinitis
is inflammation of a tendon that occurs when it is over-extended or worked too hard without rest. Tendons that are commonly affected include those in the ankle, knee, shoulder, and elbow. The affected tendon depends on the type of use that causes the inflammation. Rock climbers tend to develop tendinitis in their fingers, while basketball players are more likely to develop tendinitis in the knees, to name a few examples.
Symptoms of tendinitis may include aching, sharp pain, a burning sensation, or joint stiffness. In some cases, swelling occurs around the inflamed tendon, and the area feels hot and looks red. Treatment includes the PRICE guidelines listed above, as well as the use of NSAIDs to further reduce inflammation and pain. Although symptoms should show improvement within a few days of treatment, full recovery may take several months. A gradual return to exercise or other use of the affected tendon is recommended. Physical or occupational therapy may speed the return to normal activity levels.
Carpal Tunnel Syndrome
is a common biomechanical problem that occurs in the wrist when the median nerve becomes compressed between carpal bones (see Figure 12.6.3). This may occur due to repetitive use of the wrist, a tumor, or trauma to the wrist. Two-thirds of cases are work-related. Computer work, work with vibrating tools, and work that requires a strong grip all increase one’s risk of developing this problem. Carpal tunnel syndrome occurs more often in women than men. Other risk factors include obesity, pregnancy, and arthritis. Genetics may also play a role.
Compression of the median nerve results in inadequate nervous stimulation of the muscles in the thumb and first two fingers of the hand. The main symptoms are pain, numbness, and tingling in these digits. Sometimes, symptoms can be improved by wearing a wrist splint or receiving corticosteroid injections. Surgery to cut the carpal ligament reduces pressure on the median nerve and is generally more effective than nonsurgical treatment. Recurrence of carpal tunnel syndrome after surgery is rare. Without treatment, on the other hand, the lack of nervous stimulation by the median nerve may eventually cause the affected muscles of the hand to weaken and waste away.
Neuromuscular Disorders
are systemic disorders that occur because of problems with the nervous control of muscle contractions, or with the muscle cells themselves. These disorders are often due to faulty genes and not due to biomechanical stresses. Other system-wide problems, such as abnormal immune systemno post responses, may also be involved in neuromuscular disorders.
Unlike musculoskeletal disorders, neuromuscular disorders generally affect most or all of the muscles in the body. The disorders also tend to be progressive and incurable. However, in most cases, treatment is available to slow the disease progression or to lessen the symptoms. Examples of neuromuscular disorders include muscular dystrophy, myasthenia gravis, and Parkinson’s disease.
Muscular Dystrophy
is a genetic disorder caused by defective proteins in muscle cells. It is characterized by progressive skeletal muscle weakness and death of muscle cells and tissues. Muscles become increasingly unable to contract in response to nervous stimulation. Approximately 50,000 Canadians suffer from muscular dystrophy.
There are at least nine major types of muscular dystrophy caused by different gene mutations. Some of the mutations cause autosomal or autosomal disorders, and some cause disorders. The most common type of childhood muscular dystrophy is Duchenne muscular dystrophy, which is due to a mutation in a recessive gene on the X chromosome. As an X-linked recessive disorder, Duchenne muscular dystrophy occurs almost exclusively in males.
Different types of muscular dystrophy affect different major muscle groups. In Duchenne muscular dystrophy, the lower limbs are affected. Signs of the disorder usually become apparent when a child starts walking. Difficulty walking becomes progressively worse through childhood. By the time a child is ten, braces may be needed for walking — and walking may no longer even by possible by age 12. The lifespan of someone with muscular dystrophy is likely to be shorter than normal because of the disease, ranging from 15 to 45 years.
In some cases, physical therapy, drug therapy, or orthopedic surgery may improve some of the signs and symptoms of muscular dystrophy. However, at present, there is no known cure for the disorder. Research is ongoing to find a cure, with financial support provided by such sources as Muscular Dystrophy Canada (MDC) (see Figure 12.6.4). MDC is a non-profit organization dedicated to curing muscular dystrophy by funding worldwide research.
Myasthenia Gravis
is a genetic neuromuscular disorder characterized by fluctuating muscle weakness and fatigue. It occurs more commonly in women, and generally begins between the ages of 20 and 40. The initial symptom of myasthenia gravis is painless muscle weakness, generally in muscles around the eye (see photos in Figure 12.6.5). The disease then progresses to muscles elsewhere in the body, eventually involving most of the muscles. Swallowing and chewing may become difficult as the disease progresses, and speech may become slow and slurred. In more advanced cases, myasthenia crises may occur, during which the muscles that control breathing may be affected. Emergency medical care to provide assisted ventilation is required to sustain life. A myasthenia gravis crisis may be triggered by various stressors, such as infection, fever, or stress.
Most commonly, myasthenia gravis is caused by immune system antibodies blocking receptors on muscle cells, as well as the actual loss of acetylcholine receptors. Acetylcholine is the main neurotransmitter used by to carry their signals to the they control. With acetylcholine blocked or the receptors lost, muscle cells fail to receive nervous stimulation to contract. Treatment of myasthenia gravis may include medications to counter the effects of the mutant gene or to suppress the immune system.
Parkinson’s Disease
is a degenerative disorder of the central nervous system that mainly affects the muscular system and movement. Four motor signs and symptoms are considered defining in Parkinson’s disease: muscle tremor (shaking), muscle rigidity, slowness of movement, and postural instability. Tremor is the most common and obvious symptom, and it most often occurs in a limb that is at rest, so it disappears during sleep or when the patient moves the limb voluntarily. Difficulty walking eventually develops, and dementia is common in the advanced stages of the disease. Depression is common, as well.
See the video “Neurology – Topic 14 – Parkinsons disease – examining a patient” by UCD Medicine, of a physician examining a patient living with advanced Parkinson’s disease:
Neurology – Topic 14 – Parkinsons disease – examining a patient, UCD Medicine, 2013.
Parkinson’s disease is more common in older people, with most cases being diagnosed after the age of 50. Often, the disease occurs for no known reason. Cases like this are called primary Parkinson’s disease. Sometimes, the disease has a known or suspected cause, such as exposure to toxins in pesticides, or repeated head trauma. In this case, it is called secondary Parkinson’s disease.
Regardless of the cause, the motor symptoms of the disease result from the death of in the midbrain. The cause of cell death is not fully understood, but it appears to involve the buildup in the brain of protein structures called . Early in the course of the illness, medications can be prescribed to help reduce the motor disturbances. As the disease progresses, however, the medications become ineffective. They also cause a negative side effect of involuntary writhing movements.
Feature: Human Biology in the News
On June 3, 2016, media all over the world exploded with news of the death of Muhammad Ali at the age of 74. The world champion boxer and Olympic gold medalist died that day of complications of a respiratory infection, but the underlying cause was Parkinson’s disease. Ali was diagnosed with Parkinson’s in 1984 when he was only 42 years old. Doctors attributed his disease to repeated head trauma from boxing.
In the days following Ali’s death, the news was full of stories and images from milestones in the athlete’s life, both before and after his diagnosis with Parkinson’s disease. Sadly, the news coverage also provided an overview of his gradual decline as the disease progressed. Ali was pictured in 1996 lighting the flame at the Summer Olympics in Atlanta; however, in 2012, Ali had to be helped to his feet by his wife just to stand before the flag
he was supposed to carry into the stadium. He was unable to carry it because of the ravages of Parkinson’s disease.
Muhammad Ali retired from boxing in 1981 at the age of 39, but he didn’t retire from fighting. Up until the final year of his life, Ali was a passionate activist for peace and justice, and against war and racism. In 1998, he joined Michael J. Fox, who also has Parkinson’s disease, to raise awareness of and funding for research on Parkinson’s disease. In 2002, Fox and Ali made a joint appearance before Congress to present their case (see Figure 12.6.6). In 2005, Ali received the Presidential Medal of Freedom for the many achievements and contributions he made throughout his amazing life, in spite of Parkinson’s disease.
12.6 Summary
- are injuries that occur in muscles or associated tissues (such as tendons) because of biomechanical stresses. The disorders may be caused by sudden exertion, over-exertion, repetitive motions, and similar stresses.
- A is an injury in which muscle fibres tear as a result of overstretching. First aid for a muscle strain includes the five steps represented by the acronym PRICE (protection, rest, ice, compression, and elevation). Medications for inflammation and pain (such as NSAIDs) may also be used.
- is inflammation of a tendon that occurs when it is over-extended or worked too hard without rest. Tendinitis may also be treated with PRICE and NSAIDs.
- is a biomechanical problem that occurs in the wrist when the median nerve becomes compressed between carpal bones. It may occur with repetitive use, a tumor, or trauma to the wrist. It may cause pain, numbness, and eventually, if untreated, muscle wasting in the thumb and first two fingers of the hand.
- are systemic disorders that occur because of problems with the nervous control of muscle contractions, or with the muscle cells themselves.
- is a genetic disorder caused by defective proteins in muscle cells. It is characterized by progressive skeletal muscle weakness and death of muscle tissues.
- is a genetic neuromuscular disorder characterized by fluctuating muscle weakness and fatigue. More muscles are affected, and muscles become increasingly weakened as the disorder progresses. Myasthenia gravis most often occurs because immune system antibodies block acetylcholine receptors on muscle cells, and also because acetylcholine receptors are lost.
- is a degenerative disorder of the central nervous system that mainly affects the muscular system and movement. It occurs because of the death of neurons in the midbrain. Characteristic signs of the disorder are muscle tremor, muscle rigidity, slowness of movement, and postural instability. Dementia and depression also often characterize advanced stages of the disease.
12.6 Review Questions
- What are musculoskeletal disorders? What causes them?
- How does a muscle strain occur?
- Define tendinitis. Why does it occur?
- Identify first-aid steps for treating musculoskeletal disorders, such as muscle strains and tendinitis.
- Describe carpal tunnel syndrome and how it may be treated.
- Define neuromuscular disorders.
- Identify the cause and symptoms of muscular dystrophy.
- Outline the cause and progression of myasthenia gravis.
- What is Parkinson’s disease? List four characteristic signs of the disorder.
- What are the main differences between musculoskeletal disorders and neuromuscular disorders?
- Why is padding of a strained muscle part of the typical treatment?
- What are two tissues — other than muscle tissue — that can experience problems that result in muscular system disorders?
12.6 Explore More
TEDxAmericanRiviera – Dr. Eric Goodman – The Unexpected Physical Consequences Of Technology, TEDx Talks, 2011.
Deep Brain Stimulation Surgery to treat Parkinson’s Disease at Mount Sinai Hospital,
Mount Sinai Health System, 2013.
Why sitting is bad for you – Murat Dalkilinç, TED-Ed, 2015.
Attributions
Figure 12.6.1
Texting by derick-anies-hDJT_ERrB-w [photo] by Derick Anies on Unsplash is used under the Unsplash License (https://unsplash.com/license).
Figure 12.6.2
pulled_hamstring by Daniel.Cardenas on Wikimedia Commons is used under a CC BY 3.0 (https://creativecommons.org/licenses/by/3.0) license.
Figure 12.6.3
Blausen Carpal_Tunnel_Syndrome by BruceBlaus on Wikimedia Commons is used under a CC BY 3.0 (https://creativecommons.org/licenses/by/3.0) license.
Figure 12.6.4
Firefighters fundraise for muscular dystrophy by Sombilon Studios on Flickr is used under a CC BY-ND 2.0 (https://creativecommons.org/licenses/by-nd/2.0/) license.
Figure 12.6.5
Myasthenia_gravis_ptosis_reversal by Kurukumbi, M., Weir, R.L., Kalyanam, J. et al. on Wikimedia Commons is used under a CC BY 2.0 (https://creativecommons.org/licenses/by/2.0) license.
Figure 12.6.6
Ali and Fox Parkinsons Disease from the Office of Senator Debbie Stabenow is in the public domain (https://en.wikipedia.org/wiki/Public_domain).
References
Blausen.com Staff. (2014). Medical gallery of Blausen Medical 2014. WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.010. ISSN 2002-4436.
Kurukumbi, M., Weir, R.L., Kalyanam, J., Nasim, M., Jayam-Trouth, A. (2008, 25 July). Rare association of thymoma, myasthenia gravis and sarcoidosis : a case report. Journal of Medical Case Reports, 2:245. doi:10.1186/1752-1947-2-245
TED-Ed. (2015, March 5). Why sitting is bad for you – Murat Dalkilinç. YouTube. https://www.youtube.com/watch?v=wUEl8KrMz14&feature=youtu.be
TEDx Talks. (2011, December 3). TEDxAmericanRiviera – Dr. Eric Goodman – The unexpected physical consequences of technology. YouTube. https://www.youtube.com/watch?v=BZcZenvWBlg&feature=youtu.be
UCD Medicine. (2013, January 10). Neurology – Topic 14 – Parkinsons disease – examining a patient. YouTube. https://www.youtube.com/watch?v=sJqKvajUC3k&feature=youtu.be
Created by CK-12 Foundation/ Adapted by Christine Miller
In the Blink of an Eye
As you drive into a parking lot, a boy on a skateboard suddenly flies in front of your car across your field of vision. You see the boy in the nick of time and react immediately. You slam on the brakes and steer sharply to the right — all in the blink of an eye. You avoid a collision, but just barely. You’re shaken up, but thankful that no one was hurt. How did you respond so quickly? Rapid responses like this are controlled by your nervous system.
Overview of the Nervous System
The , illustrated in the sketch below, is the human organ system that coordinates all of the body’s and actions, by transmitting electrical signals to and from different parts of the body. Specifically, the nervous system extracts information from the internal and external environments, using . Usually, it then sends signals encoding this information to the , which processes the information to determine an appropriate response. Finally, the brain sends signals to muscles, , or glands to bring about the response. In the example above, your eyes detected the boy, the information traveled to your brain, and your brain told your body to act so as to avoid a collision.
Signals of the Nervous System
The signals sent by the nervous system are electrical signals called , and they are transmitted by special nervous system cells called (or nerve cells), like the one in Figure 8.2.3. Long projections (called axons) from neurons carry nerve impulses directly to specific target cells. A cell that receives nerve impulses from a neuron (typically a muscle or a gland) may be excited to perform a function, inhibited from carrying out an action, or otherwise controlled. In this way, the information transmitted by the nervous system is specific to particular cells and is transmitted very rapidly. In fact, the fastest nerve impulses travel at speeds greater than 100 metres per second! Compare this to the chemical messages carried by the hormones that are secreted into the blood by endocrine glands. These hormonal messages are “broadcast” to all the cells of the body, and they can travel only as quickly as the blood flows through the cardiovascular system.
This simple model of a nerve cell shows part of its long axon which carries nerve impulses to other cells. The multiple shorter projections are called dendrites, and they receive nerve impulses from other cells.
Organization of the Nervous System
As you might predict, the human is very complex. It has multiple divisions, beginning with its two main parts, the (CNS) and the (PNS), as shown in the diagram below (Figure 8.2.4). The CNS includes the and , and the PNS consists mainly of , which are bundles of from neurons. The nerves of the PNS connect the CNS to the rest of the body.
The PNS can be further subdivided into two divisions, known as the and s (Figure 8.2.5). These divisions control different types of functions, and they often interact with the CNS to carry out these functions. The somatic nervous system controls activities that are under control, such as turning a steering wheel. The autonomic nervous system controls activities that are , such as digesting a meal. The autonomic nervous system has three main divisions: the (which controls the fight-or-flight response during emergencies), the (which controls the routine “housekeeping” functions of the body at other times), and the (which provides local control of the digestive system).
8.2 Summary
- The is the human organ system that coordinates all of the body’s and actions, by transmitting signals to and from different parts of the body.
- The nervous system has two major divisions, called the (CNS) and the (PNS). The CNS includes the and , and the PNS consists mainly of that connect the CNS with the rest of the body.
- The PNS can be subdivided into two major divisions: the and the .The somatic system controls activities that are under voluntary control. The autonomic system controls activities that are not under voluntary control. The autonomic nervous system is further divided into the (which controls the fight-or-flight response), the (which controls most routine involuntary responses), and the (which provides local control of the digestive system).
- Electrical signals sent by the nervous system are called . They are transmitted by special cells called . Nerve impulses can travel to specific target cells very rapidly.
8.2 Review Questions
- List the general steps through which the nervous system generates an appropriate response to information from the internal and external environments.
- What are neurons?
- Compare and contrast the central and peripheral nervous systems.
- Which major division of the peripheral nervous system allows you to walk to class? Which major division of the peripheral nervous system controls your heart rate?
- Identify the functions of the three main divisions of the autonomic nervous system.
- What is an axon, and what is its function?
- Define nerve impulses.
- Explain generally how the brain and spinal cord can interact with and control the rest of the body.
- How are nerves and neurons related?
- What type of information from the outside environment do you think is detected by sensory receptors in your ears?
8.2 Explore More
https://www.youtube.com/watch?v=qPix_X-9t7E
The Nervous System, Part 1: Crash Course A&P #8, CrashCourse, 2015.
https://www.youtube.com/watch?v=Nsxw5_Iz7mY&feature=emb_logo
Engineering the Human Nervous System: Megan Moynahan at TEDxBrussels,
TEDx Talks, 2013.
Attributions
Figure 8.2.1
Skateboard_1613 by Autoria propia on Wikimedia Commons is released into the public domain (https://en.wikipedia.org/wiki/Public_domain) (Derivative work of this file: SkateboardinDog.jpg)
Figure 8.2.2
Nervous_system_diagram.svg by The Emirr on Wikimedia Commons is used under a CC BY 3.0 (https://creativecommons.org/licenses/by/3.0/deed.en) license.
Figure 8.2.3
MultipolarNeuron by BruceBlaus on Wikimedia Commons is used under a CC BY 3.0 (https://creativecommons.org/licenses/by/3.0/deed.en) license.
Figure 8.2.4
Overview_of_Nervous_System by OpenStax on Wikimedia Commons is used under a CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/deed.en) license.
Figure 8.2.5
Divisions of the Nervous System by CK-12 Foundation is used under the CC BY-NC 3.0 (https://creativecommons.org/licenses/by-nc/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 12.2 Central and peripheral nervous system [digital image]. In Anatomy and Physiology (Section 12.1). OpenStax. https://openstax.org/books/anatomy-and-physiology/pages/12-1-basic-structure-and-function-of-the-nervous-system
Brainard, J/ CK-12 Foundation. (2016). Figure 5 [digital image]. In CK-12 College Human Biology (Section 10.2) [online Flexbook]. CK12.org. https://www.ck12.org/book/ck-12-college-human-biology/section/10.2/
CrashCourse. (2015, February 23). The nervous system, Part 1: Crash Course A&P #8. YouTube. https://www.youtube.com/watch?v=qPix_X-9t7E&feature=youtu.be
TEDx Talks. (2013, November 3). Engineering the human nervous system: Megan Moynahan at TEDxBrussels. YouTube. https://www.youtube.com/watch?v=Nsxw5_Iz7mY&feature=youtu.be
Defining Science
is a distinctive way of gaining knowledge about the natural world that starts with a question and then tries to answer the question using evidence and logic. It is an exciting exploration of all the whys and hows that any curious person might ask about the world. You can be part of that exploration! Besides your curiosity, all you need is a basic understanding of how scientists think and how is done. In this concept, you'll learn how to think like a scientist.
Thinking Like a Scientist
Thinking like a scientist rests on certain underlying assumptions. Scientists assume that:
Nature Is Understandable
Scientists think of nature as a single system controlled by natural laws. By discovering natural laws, scientists strive to increase their understanding of the natural world. Laws of nature are expressed as scientific laws. A is a statement that describes what always happens under certain conditions in nature.
Scientific Ideas Are Open to Change
Science is both a process and body of knowledge. Scientific knowledge is generated through systematic processes, such as and experimentation. Scientists are always testing and revising their ideas, and as new observations are made, existing ideas may be challenged. Ideas may be replaced with new ideas that better fit the facts, but more often, existing ideas are simply revised. Through many new discoveries over time, scientists gradually build an increasingly accurate and detailed understanding of the natural world.
Scientific Knowledge May Be Long Lasting
Many scientific ideas have stood the test of time. About 200 years ago, the scientist John Dalton proposed atomic theory — the theory that all matter is made of tiny particles called atoms. This theory is still valid today. During the two centuries since the theory was first proposed, scientists have learned a lot more about atoms and the even smaller particles that compose them. Nonetheless, the idea that all matter consists of atoms remains valid. There are many other examples of basic scientific ideas that have been tested repeatedly and proven sound. You will learn about many of them as you study human biology.
Not All Questions Can be Answered by Science
rests on evidence and logic, and evidence comes from observations. Therefore, science deals only with things that can be observed. An is anything that is detected through human senses or with instruments or measuring devices that extend human senses. Things that cannot be observed or measured by current means — such as supernatural beings or events — are outside the bounds of science. Consider these two questions about life on Earth:
- Did life on Earth evolve over time?
- How did life on Earth originate?
The first question can be answered by science on the basis of scientific evidence (such as fossils and logical arguments). The second question could be a matter of belief, but no evidence can be gathered to support or refute it. Therefore, it is outside the realm of science.
1.3 Summary
- Science is a distinctive way of gaining knowledge about the natural world that tries to answer questions using evidence and logic.
- Scientists assume that nature can be understood through systematic study.
- Scientific ideas are open to revision.
- Sound scientific ideas withstand the test of time.
- Science cannot provide answers to all of our questions.
1.3 Review Questions
- Define science.
- What is the general goal of science?
- Identify four basic assumptions that scientists make when they study the natural world.
- Do observations in science have to be made by the naked eye? Can you think of a way in which scientists might be able to make observations about something they cannot directly see?
- If something cannot be observed, can it be tested scientifically? Explain your reasoning.
- Scientific knowledge builds upon itself. Give an example of a scientific idea from the reading where the initial idea developed further as science advanced.
- Discuss this statement: “Scientific ideas are always changing, so they can't be trusted.” Do you think this is true?
- Why do you think that scientific knowledge expands as technology becomes more advanced?
1.3 Explore More
https://youtu.be/3nAETHZTObk
Nature of Science with the Ameoba Sisters, 2019.
References
Amoeba Sisters. (2019, Jun 6). Nature of science with Ameoba Sisters. YouTube. https://www.youtube.com/watch?v=3nAETHZTObk
Wikipedia contributors. (2020, July 25). John Dalton. In Wikipedia. https://en.wikipedia.org/w/index.php?title=John_Dalton&oldid=969425891
"Doing" Science
Science is as much about doing as knowing. Scientists are always trying to learn more and gain a better understanding of the natural world. There are basic methods of gaining knowledge that are common to all of science. At the heart of science is the scientific investigation. A is a systematic approach to answering questions about the physical and natural world. Scientific investigations can be observational — for example, observing a cell under a microscope and recording detailed descriptions. Other scientific investigations are experimental — for example, treating a cell with a drug while recording changes in the behavior of the cell.
The flow chart below shows the typical steps followed in an experimental scientific investigation. The series of steps shown in the flow chart is frequently referred to as the . Science textbooks often present this simple, linear "recipe" for a scientific investigation. This is an oversimplification of how science is actually done, but it does highlight the basic plan and purpose of an experimental scientific investigation: testing ideas with evidence. Each of the steps in the flow chart is discussed in greater detail below.
is actually a complex endeavor that cannot be reduced to a single, linear sequence of steps, like the instructions on a package of cake mix. Real science is nonlinear, iterative (repetitive), creative, unpredictable, and exciting. Scientists often undertake the steps of an investigation in a different sequence, or they repeat the same steps many times as they gain more information and develop new ideas. Scientific investigations often raise new questions as old ones are answered. Successive investigations may address the same questions, but at ever deeper levels. Alternatively, an investigation might lead to an unexpected observation that sparks a new question and takes the research in a completely different direction.
Knowing how scientists "do" science can help you in your everyday life, even if you aren't a scientist. Some steps of the scientific process — such as asking questions and evaluating evidence — can be applied to answering real-life questions and solving practical problems.
Making Observations
Testing an idea typically begins with observations. An is anything that is detected through human senses or with instruments or measuring devices that enhance human senses. We usually think of observations as things we see with our eyes, but we can also make observations with our sense of touch, smell, taste, or hearing. In addition, we can extend and improve our own senses with instruments such as thermometers and microscopes. Other instruments can be used to sense things that human senses cannot detect at all, such as ultraviolet light or radio waves.
Sometimes, chance observations lead to important scientific discoveries. One such observation was made by the Scottish biologist Alexander Fleming (pictured below) in the 1920s. Fleming's name may sound familiar to you because he is famous for a major discovery. Fleming had been growing a certain type of bacteria on glass plates in his lab when he noticed that one of the plates was contaminated with mold. On closer examination, Fleming observed that the area around the mold was free of bacteria.
Asking Questions
Observations often lead to interesting questions. This is especially true if the observer is thinking like a scientist. Having scientific training and knowledge is also useful. Relevant background knowledge and logical thinking help make sense of observations so the observer can form particularly salient questions. Fleming, for example, wondered whether the mold — or some substance it produced — had killed bacteria on the plate. Fortunately for us, Fleming didn't just throw out the mold-contaminated plate. Instead, he investigated his question and in so doing, discovered the antibiotic penicillin.
Hypothesis Formation
Typically, the next step in a scientific investigation is to form a hypothesis. A is a possible answer to a scientific question. But it isn’t just any answer. A hypothesis must be based on scientific knowledge. In other words, it shouldn't be at odds with what is already known about the natural world. A hypothesis also must be logical, and it is beneficial if the hypothesis is relatively simple. In addition, to be useful in science, a hypothesis must be testable and . In other words, it must be possible to subject the hypothesis to a test that generates evidence for or against it. It must also be possible to make observations that would disprove the hypothesis if it really is false.
For example, Fleming's hypothesis might have been: “A particular kind of bacteria growing on a plate will die when exposed to a particular kind of mold.” The hypothesis is logical and based directly on observations. The hypothesis is also simple, involving just one type each of mold and bacteria growing on a plate. In addition, hypotheses are subject to "if/then" conditions. Thus, Fleming might have stated, "If a certain type of mold is introduced to a particular kind of bacteria growing on a plate, then the bacteria will die." This makes the hypothesis easy to test and ensures that it is falsifiable. If the bacteria were to grow in the presence of the mold, it would disprove the hypothesis (assuming the hypothesis is really false).
Hypothesis Testing
Hypothesis testing is at the heart of the scientific method. How would Fleming test his hypothesis? He would gather relevant data as evidence. is any type of data that may be used to test a hypothesis. (singular, datum) are essentially just observations. The observations may be measurements in an experiment or just something the researcher notices. Testing a hypothesis then involves using the data to answer two basic questions:
- If my hypothesis is true, what would I expect to observe?
- Does what I actually observe match what I expected to observe?
A hypothesis is supported if the actual observations (data) match the expected observations. A hypothesis is refuted if the actual observations differ from the expected observations.
The scientific method is employed by scientists around the world, but it is not always conducted in the order above. Sometimes, hypothesis are formulated before observations are collected; sometimes observations are made before hypothesis are created. Regardless, it is important that scientists record their procedures carefully, allowing others to reproduce and verify the experimental data and results. After many experiments provide results supporting a hypothesis, the hypothesis becomes a . Theories remain theories forever, and are constantly being retested with every experiment and observation. Theories can never become fact or .
In science, a law is a mathematical relationship that exists between observations under a given set of conditions. There is a fundamental difference between observations of the physical world and explanations of the nature of the physical world. Hypotheses and theories are explanations, whereas laws and measurements are observational
1.4 Summary
- The scientific method consists of making observations, formulating a hypothesis, testing the hypothesis with new observations, making a new hypothesis if the new observations contradict the old hypothesis, or continuing to test the hypothesis if the observations agree.
- A hypothesis is a tentative explanation that can be tested by further observation.
- A theory is a hypothesis that has been supported with repeated testing.
- A scientific law is a statement that summarizes the results of many observations.
- Experimental data must be verified by reproduction from other scientists.
- Theories must agree with all observations made on the phenomenon under study.
- Theories are continually tested, forever.
1.4 Review Questions
1.4 Explore More
https://www.youtube.com/watch?v=F8UFGu2M2gM
How simple ideas lead to scientific discoveries, TED-Ed, 2012.
Attributions
Figure 1.4.1
The Scientific Method (simple), by Thebiologyprimer on Wikimedia Commons is used under a CC0 1.0 Universal Public Domain Dedication license (https://creativecommons.org/publicdomain/zero/1.0/deed.en).
Figure 1.4.2
Anatomy Bone Bones Check Doctor Examine Film, by rawpixel on Pixabay, used under the Pixabay License (https://pixabay.com/de/service/license/).
Figure 1.4.3
Penicillin Past, Present and Future- the Development and Production of Penicillin, England, 1944, by Ministry of Information Photo Division Photographer. This photograph was scanned and released by the Imperial War Museum on the IWM Non Commercial Licence. It is now in the public domain (https://en.wikipedia.org/wiki/Public_domain).
References
TED-Ed. (2012, Mar 13). How simple ideas lead to scientific discoveries. YouTube. https://www.youtube.com/watch?v=F8UFGu2M2gM
Wikipedia contributors. (2020, July 7). Alexander Fleming. In Wikipedia. https://en.wikipedia.org/w/index.php?title=Alexander_Fleming&oldid=966489433
Body cavity that fills the lower half of the trunk and holds the kidneys and the digestive and reproductive organs.
Refers to the relationship between two versions of a gene. Individuals receive two versions of each gene, known as alleles, from each parent. If the alleles of a gene are different, one allele will be expressed; it is the dominant gene. The effect of the other allele, called recessive, is masked.
Image shows a diagram comparison of a healthy lung and a lung with pneumonia. A healthy alveoli contains air for gas exchange. An alveoli suffering from pneumonia is filled with fluid (a mix of pus, bacteria and blood cells) and air cannot enter because the space is already filled by the fluid.
An organic chemical that functions in the brain and body of many types of animals (and humans) as a neurotransmitter—a chemical message released by nerve cells to send signals to other cells, such as neurons, muscle cells and gland cells.
Created by CK-12 Foundation/Adapted by Christine Miller
Case Study Conclusion: Trying to Conceive
The woman in Figure 18.12.1 is holding a home pregnancy test. The two pink lines in the middle are the type of result that Alicia and Victor are desperately hoping to see themselves one day — a positive pregnancy test. In the beginning of the chapter you learned that Alicia and Victor have been actively trying to get pregnant for a year, which, as you now know, is the time frame necessary for infertility to be diagnosed.
Alicia and Victor tried having on day 14 of her to optimize their chances of having his meet her . Why might this not be successful, even if they do not have fertility problems? Although the average menstrual cycle is 28 days, with occurring around day 14, many women vary widely from these averages (either consistently or variably) from month to month. Recall, for example, that menstrual cycles may vary from 21 to 45 days in length, and a woman’s cycle is considered to be regular if it varies within as many as eight days from shortest to longest cycle. This variability means that ovulation often does not occur on or around day 14, particularly if the woman has significantly shorter, longer, or irregular cycles — like Alicia does. Therefore, by aiming for day 14 without knowing when Alicia is actually ovulating, they may not be successful in helping Victor’s sperm encounter Alicia’s egg.
Lack of ovulation entirely can also cause variability in menstrual cycle length. As you have learned, the regulation of the menstrual cycle depends on an interplay of from the and , including and from the pituitary and and from the ovary — specifically from the which surrounds the maturing egg and becomes the after ovulation. Shifts in these hormones and processes can affect ovulation and menstrual cycle length. This is why Alicia was concerned about her long and irregular menstrual cycles. If they are a sign that she is not ovulating, that could be the reason why she is having trouble getting pregnant.
In order to get a better idea of whether Alicia is ovulating, Dr. Bashir recommended that she take her basal body temperature (BBT) each morning before getting out of bed, and track it throughout her menstrual cycle. As you have learned, BBT typically rises slightly and stays high after ovulation. While tracking BBT is not a particularly effective form of contraception, since the temperature rise occurs only after ovulation, it can be a good way to see whether a woman is ovulating at all. Although not every woman will see a clear shift in BBT after ovulation, it is a relatively easy way to start assessing a woman’s fertility and is used as part of a more comprehensive fertility assessment by some physicians.
Dr. Bashir also recommended that Alicia use a home ovulation predictor kit. This is another relatively cheap and easy way to assess ovulation. Most ovulation predictor kits work by detecting the hormone LH in urine using test strips, like the ones shown in Figure 18.12.2. Why can this predict ovulation? Think about what you have learned about how ovulation is triggered. Rising levels of estrogen from the maturing follicle in the ovary causes a surge in the level of LH secreted from the pituitary gland, which triggers ovulation. This surge in LH can be detected by the home kit, which compares the level of LH in a woman’s urine to that of a control on the strip. After the LH surge is detected, ovulation will typically occur within one to two days.
By tracking her BBT and using the ovulation predictor kit, Alicia has learned that she is most likely ovulating, but not in every cycle, and sometimes she ovulates much later than day 14. Because frequent anovulatory cycles can be a sign of an underlying hormonal disorder, such as polycystic ovary syndrome (PCOS) or problems with the pituitary or other glands that regulate the reproductive system, Dr. Bashir orders blood tests for Alicia and sets up an appointment for a physical exam.
However, because Alicia is sometimes ovulating, the problem may not lie solely with her. Recall that infertility occurs in similar proportions in men and women, and can be due to problems in both partners. This is why it is generally recommended that both partners get assessed for fertility issues when they are having trouble getting pregnant after a year of trying.
Therefore, Victor proceeds with the analysis that Dr. Bashir recommended. In this process, the man provides a semen sample by ejaculating into a cup or special condom, and the semen is then examined under a microscope. The semen is then checked for sperm number, shape, and motility. Sperm with an abnormal shape or trouble moving will likely have trouble reaching and fertilizing an egg. A low number of sperm will also reduce the chances of conception. In this way, semen analysis can provide insight into the possible underlying causes of infertility. For instance, a low sperm count could indicate problems in sperm production or a blockage in the male reproductive tract that is preventing sperm from being emitted from the penis. Further testing would have to be done to distinguish between these two possible causes.
Victor had been worried that past injuries to his testes may have affected his fertility. You may remember the testes are where sperm are produced, and because they are external to the body, they are vulnerable to injury. In addition to physical damage to the testes and other parts of the male reproductive tract, a testicular injury could potentially cause the creation of antibodies against a man’s own sperm. As you have learned, lining the are tightly packed so that the developing sperm are normally well-separated from the body’s immune system. However, in the case of an injury, this barrier can be breached, which can cause the creation of antisperm antibodies. These antibodies can hamper fertility by killing the sperm, or otherwise interfering with their ability to move or fertilize an egg. When infertility is due to such antibodies, it is called “immune infertility.”
Victor’s semen analysis shows that he has normal numbers of healthy sperm. Dr. Bashir recommends that while they investigate whether Alicia has an underlying medical issue, she continue to track her BBT and use ovulation predictor kits to try to pinpoint when she is ovulating. She recommends that once Alicia sees an LH surge, the couple try to have intercourse within three days to maximize their chances of conception. If Alicia is found to have a medical problem that is inhibiting ovulation, depending on what it is, they may either address the problem directly, or she can take medication that stimulates ovulation, such as clomiphene citrate (often sold under the brand name Clomid). This medication works by increasing the amount of FSH secreted by the pituitary.
Fortunately, tracking ovulation at home and timing intercourse appropriately was all Alicia and Victor needed to do to finally get pregnant! After their experience, they, like you, now have a much deeper understanding of the intricacies of the reproductive system and the complex biology that is involved in the making of a new human organism.
Chapter 18 Summary
In this chapter, you learned about the male and female reproductive systems. Specifically, you learned that:
- The reproductive system is the human organ system responsible for the production and of and, in females, the carrying of a .
- Both male and female reproductive systems have organs called ( in males, in females) that produce gametes ( or ) and sex hormones (such as in males and in females). Sex hormones are endocrine hormones that control prenatal development of sex organs, sexual maturation at , and reproduction after puberty.
- The reproductive system is the only organ system that is significantly different between males and females. A Y-chromosome gene called SRY is responsible for undifferentiated embryonic tissues developing into a male reproductive system. Without a Y chromosome, the undifferentiated embryonic tissues develop into a female reproductive system.
- Male and female reproductive systems are different at birth, but immature and nonfunctioning. Maturation of the reproductive system occurs during puberty when hormones from the and pituitary gland stimulate the gonads to produce sex hormones again. The sex hormones, in turn, cause the physical changes experienced during puberty.
- Male reproductive system organs include the testes, , , , , and .
-
- The two testes are sperm- and testosterone-producing male gonads. They are contained within the , a pouch that hangs down behind the penis. The testes are filled with hundreds of tiny, tightly coiled seminiferous tubules, where sperm are produced. The tubules contain sperm in different stages of development, as well as Sertoli cells, which secrete substances needed for sperm production. Between the tubules are , which secrete testosterone.
- The two epididymides are contained within the scrotum. Each epididymis is a tightly coiled tubule where sperm mature and are stored until they leave the body during an .
- The two vas deferens are long, thin tubes that run from the scrotum up into the . During ejaculation, each vas deferens carries sperm from one of the epididymides to one of the pair of ejaculatory ducts.
- The two seminal vesicles are glands within the pelvis that secrete fluid through ducts into the junction of each vas deferens and ejaculatory duct. This alkaline fluid makes up about 70% of semen, the sperm-containing fluid that leaves the penis during ejaculation. Semen contains substances and nutrients that sperm need to survive and “swim” in the female reproductive tract.
- The prostate gland is located just below the seminal vesicles and surrounds the urethra and its junction with the ejaculatory ducts. The prostate secretes an alkaline fluid that makes up close to 30% of semen. Prostate fluid contains a high concentration of zinc, which sperm need to be healthy and motile.
- The ejaculatory ducts form where the vas deferens joins with the ducts of the seminal vesicles in the prostate gland. They connect the vas deferens with the urethra. The ejaculatory ducts carry sperm from the vas deferens, and secretions from the seminal vesicles and prostate gland that together form semen.
- The paired are located just below the prostate gland. They secrete a tiny amount of fluid into semen. The secretions help lubricate the urethra and neutralize any acidic urine it may contain.
- The penis is the external male organ that has the reproductive function of , which is delivering sperm to the female reproductive tract. The penis also serves as the organ that excretes urine. The urethra passes through the penis and carries urine or semen out of the body. Internally, the penis consists largely of columns of spongy tissue that can fill with blood and make the penis stiff and erect. This is necessary for so intromission can occur.
- Parts of a mature sperm include the , , , and . The process of producing sperm is called . This normally starts during puberty and continues uninterrupted until death.
-
- Spermatogenesis occurs in the seminiferous tubules in the testes, and requires high concentrations of testosterone. Sertoli cells in the testes play many roles in spermatogenesis, including concentrating testosterone under the influence of follicle stimulating hormone (FSH) from the pituitary gland.
- Spermatogenesis begins with a stem cell called a , which undergoes to produce a primary spermatocyte. The primary spermatocyte undergoes meiosis I to produce haploid secondary spermatocytes, and these cells in turn, undergo meiosis II to produce spermatids. After the spermatids grow a tail and undergo other changes, they become sperm.
- Before sperm are able to “swim,” they must mature in the epididymis. The mature sperm are then stored in the epididymis until ejaculation occurs.
- Ejaculation is the process in which semen is propelled by in the vas deferens and ejaculatory ducts from the urethra in the penis.
- Leydig cells in the testes secrete testosterone under the control of luteinizing hormone (LH) from the pituitary gland. Testosterone is needed for male sexual development at puberty and to maintain normal spermatogenesis after puberty. It also plays a role in prostate function and the ability of the penis to become erect.
- Disorders of the male reproductive system include (ED), , , and.
-
- ED is a disorder characterized by the regular and repeated inability of a sexually mature male to obtain and maintain an erection. ED is a common disorder that occurs when normal blood flow to the penis is disturbed or there are problems with the nervous control of penile engorgement or arousal.
-
-
- Possible physiological causes of ED include aging, illness, drug use, tobacco smoking, and obesity, among others. Possible psychological causes of ED include stress, performance anxiety, and mental disorders.
- Treatments for ED may include lifestyle changes, such as stopping smoking and adopting a healthier diet and regular exercise. However, the first-line treatment is prescription drugs such as Viagra® or Cialis® that increase blood flow to the penis. Vacuum pumps or penile implants may be used to treat ED if other types of treatment fail.
- Epididymitis is inflammation of the epididymis. It is a common disorder, especially in young men. It may be acute or chronic and is often caused by a bacterial infection. Treatments may include antibiotics, anti-inflammatory drugs, and painkillers. Treatment is important to prevent the possible spread of infection, permanent damage to the epididymis or testes, and even infertility.
- Prostate cancer is the most common type of cancer in men and the second leading cause of cancer death in men. If there are symptoms, they typically involve urination, such as frequent or painful urination. Risk factors for prostate cancer include older age, family history, a high-meat diet, and sedentary lifestyle, among others.
-
-
-
- Prostate cancer may be detected by a physical exam or a high level of prostate-specific antigen (PSA) in the blood, but a biopsy is required for a definitive diagnosis. Prostate cancer is typically diagnosed relatively late in life, and is usually slow growing, so no treatment may be necessary. In younger patients or those with faster-growing tumors, treatment is likely to include surgery to remove the prostate, followed by chemotherapy and/or radiation therapy.
- Testicular cancer, or cancer of the testes, is the most common cancer in males between the ages of 20 and 39 years. It is more common in males of European than African ancestry. A lump or swelling in one testis, fluid in the scrotum, and testicular pain or tenderness are possible signs and symptoms of testicular cancer.
-
-
-
- Testicular cancer can be diagnosed by a physical exam and diagnostic tests, such as ultrasound or blood tests. Testicular cancer has one of the highest cure rates of all cancers. It is typically treated with surgery to remove the affected testis, and this may be followed by radiation therapy, and/or chemotherapy. Normal male reproductive functions are still possible after one testis is removed, if the remaining testis is healthy.
-
- The female reproductive system is made up of internal and external organs that function to produce haploid female gametes called ova, secrete female sex hormones (such as estrogen), and carry and give birth to a .
- Female reproductive system organs include the , , , , , and .
-
- The vagina is an elastic, muscular canal that can accommodate the penis. It is where sperm are usually ejaculated during sexual intercourse. The vagina is also the birth canal, and it channels the flow of menstrual blood from the uterus. A healthy vagina has a balance of and an acidic .
- The uterus is a muscular organ above the vagina where a fetus develops. Its muscular walls contract to push out the fetus during childbirth. The is the neck of the uterus that extends down into the vagina. It contains a canal connecting the vagina and uterus for sperm or an infant to pass through. The innermost layer of the uterus, the , thickens each month in preparation for an embryo but is shed in the following menstrual period if fertilization does not occur.
- The oviducts extend from the uterus to the ovaries. Waving at the ovary ends of the oviducts guide ovulated ova into the tubes where fertilization may occur as the ova travel to the uterus. and peristalsis help eggs move through the tubes. Tubular fluid helps nourish sperm as they swim up the tubes toward eggs.
- The ovaries are gonads that produce eggs and secrete sex hormones including estrogen. Nests of cells called follicles in the ovarian cortex are the functional units of ovaries. Each follicle surrounds an immature ovum. At birth, a baby girl’s ovaries contain at least a million eggs, and they will not produce any more during her lifetime. One egg matures and is typically ovulated each month during a woman’s reproductive years.
- The is a general term for external female reproductive organs. The vulva includes the , two pairs of , and openings for the and vagina. Secretions from Bartholin’s glands near the vaginal opening lubricate the vulva.
- The are technically not reproductive organs, but their produce milk to feed an infant after birth. Milk drains through ducts and sacs and out through the nipple when a baby sucks.
- is the process of producing eggs in the ovaries of a female fetus. Oogenesis begins when a diploid oogonium divides by mitosis to produce a diploid primary . The primary oocyte begins meiosis I and then remains at this stage in an immature ovarian follicle until after birth.
- After puberty, one follicle a month matures and its primary oocyte completes meiosis I to produce a secondary oocyte, which begins meiosis II. During ovulation, the mature follicle bursts open and the secondary oocyte leaves the ovary and enters a oviducts.
- While a follicle is maturing in an ovary each month, the endometrium in the uterus is building up to prepare for an . Around the time of ovulation, cervical mucus becomes thinner and more alkaline to help sperm reach the secondary oocyte.
- If the secondary oocyte is fertilized by a sperm, it quickly completes meiosis II and forms a , which will continue through the oviducts. The zygote will go through multiple cell divisions before reaching and implanting in the uterus. If the secondary oocyte is not fertilized, it will not complete meiosis II, and will soon disintegrate.
- is the carrying of one or more offspring from fertilization until birth. The maternal organism must provide all the nutrients and other substances needed by the developing offspring, and also remove its wastes. She should also avoid exposures that could potentially damage the offspring, especially early in the pregnancy when organ systems are developing.
-
- The average duration of pregnancy is 40 weeks (from the first day of the last menstrual period) and is divided into three trimesters of about three months each. Each trimester is associated with certain events and conditions that a pregnant woman may expect, such as morning sickness during the first trimester, feeling fetal movements for the first time during the second trimester, and rapid weight gain in both fetus and mother during the third trimester.
- , which is the general term for the birth process, usually begins around the time the amniotic sac breaks and its fluid leaks out. Labour occurs in three stages: dilation of the cervix, birth of the baby, and delivery of the placenta (afterbirth).
- The physiological function of female breasts is , or the production of breast milk to feed an infant. Sucking on the breast by the infant stimulates the release of the hypothalamic hormone from the posterior pituitary, which causes the flow of milk. The release of milk stimulates the baby to continue sucking, which in turn keeps the milk flowing. This is one of the few examples of in the human organism.
- The ovaries produce female sex hormones, including estrogen and . Estrogen is responsible for sexual maturation and secondary sex characteristics at puberty. It is also needed to help regulate the menstrual cycle and ovulation after puberty until menopause. Progesterone prepares the uterus for pregnancy each month during the menstrual cycle, and helps maintain the pregnancy if fertilization occurs.
- The menstrual cycle refers to natural changes that occur in the female reproductive system each month during the reproductive years, except when a woman is pregnant. The cycle is necessary for the production of ova and the preparation of the uterus for pregnancy. It involves changes in both the ovaries and uterus and is controlled by pituitary hormones (FSH and LH) and ovarian hormones (estrogen and progesterone).
-
- The female reproductive period is delineated by , or the first menstrual period, which usually occurs around age 12 or 13; and by , or the cessation of menstrual periods, which typically occurs around age 52. A typical menstrual cycle averages 28 days in length but may vary normally from 21 to 45 days. The average menstrual period is five days long, but may vary normally from two to seven days. These variations in the menstrual cycle may occur both between women and within individual women from month to month.
- The events of the menstrual cycle that take place in the ovaries make up the . It includes the , when a follicle and its ovum mature due to rising levels of FSH; ovulation, when the ovum is released from the ovary due to a rise in estrogen and a surge in LH; and the , when the follicle is transformed into a structure called a that secretes progesterone. In a 28-day menstrual cycle, the follicular and luteal phases typically average about two weeks in length, with ovulation generally occurring around day 14 of the cycle.
- The events of the menstrual cycle that take place in the uterus make up the . It includes , which generally occurs on days 1 to 5 of the cycle and involves shedding of endometrial tissue that built up during the preceding cycle; the , during which the endometrium builds up again until ovulation occurs; and the , which follows ovulation and during which the endometrium secretes substances and undergoes other changes that prepare it to receive an .
- Disorders of the female reproductive system include , , and .
-
- Cervical cancer occurs when cells of the cervix grow abnormally and develop the ability to invade nearby tissues, or spread to other parts of the body. Worldwide, cervical cancer is the second-most common type of cancer in females and the fourth-most common cause of cancer death in females. Early on, cervical cancer often has no symptoms; later, symptoms such as abnormal vaginal bleeding and pain are likely.
-
-
- Most cases of cervical cancer occur because of infection with , so the HPV vaccine is expected to greatly reduce the incidence of the disease. Other risk factors include smoking and a weakened immune system. A can diagnose cervical cancer at an early stage. Where Pap smears are done routinely, cervical cancer death rates have fallen dramatically. Treatment of cervical cancer generally includes surgery, which may be followed by radiation therapy or chemotherapy.
- Vaginitis is inflammation of the vagina. A discharge is likely, and there may be itching and pain. About 90% of cases of vaginitis are caused by infection with , typically by the yeast Candida albicans. A minority of cases are caused by irritants or allergens in products such as soaps, spermicides, or douches.
-
-
-
- Diagnosis of vaginitis may be based on characteristics of the discharge, which can be examined microscopically or cultured. Treatment of vaginitis depends on the cause, and is usually an oral or topical anti-fungal or antibiotic medication.
- Endometriosis is a disease in which endometrial tissue grows outside the uterus. This tissue may bleed during the menstrual period and cause inflammation, pain, and scarring. The main symptom of endometriosis is pelvic pain, which may be severe. Endometriosis may also lead to infertility.
-
-
-
- Endometriosis is thought to have multiple causes, including genetic mutations. Retrograde menstruation may be the immediate cause of endometrial tissue escaping the uterus and entering the pelvic cavity. Endometriosis is usually treated with surgery to remove the abnormal tissue and medication for pain. If surgery is more conservative than hysterectomy, endometriosis may recur.
-
- is the inability of a sexually mature adult to reproduce by natural means. It is defined scientifically and medically as the failure to achieve a successful pregnancy after at least one year of regular, unprotected sexual intercourse.
- About 40% of infertility in couples is due to female infertility, and another 30% is due to male infertility. In the remaining cases, a couple’s infertility is due to problems in both partners or to unknown causes.
- Male infertility occurs when there are no or too few healthy, motile sperm. This may be caused by problems with spermatogenesis or by blockage of the male reproductive tract that prevents sperm from being ejaculated. Risk factors for male infertility include heavy alcohol use, smoking, certain medications, and advancing age, to name just a few.
- Female infertility occurs due to failure to produce viable ova by the ovaries or structural problems in the oviducts or uterus. Polycystic ovary syndrome is the most common cause of failure to produce viable eggs. Endometriosis and uterine fibroids are possible causes of structural problems in the oviducts and uterus. Risk factors for female infertility include smoking, stress, poor diet, and older age, among others.
- Diagnosing the cause(s) of a couple’s infertility generally requires testing both the man and the woman for potential problems. For men, semen is likely to be examined for adequate numbers of healthy, motile sperm. For women, signs of ovulation are monitored, for example, with an ovulation test kit or ultrasound of the ovaries. For both partners, the reproductive tract may be medically imaged to look for blockages or other abnormalities.
-
- Treatments for infertility depend on the cause. For example, if a medical problem is interfering with sperm production, medication may resolve the underlying problem so sperm production is restored. Blockages in either the male or the female reproductive tract can often be treated surgically. If there are problems with ovulation, hormonal treatments may stimulate ovulation.
- Some cases of infertility are treated with . This is a collection of medical procedures in which eggs and sperm are taken from the couple and manipulated in a lab to increase the chances of fertilization occurring and an embryo forming. Other approaches for certain causes of infertility include the use of a surrogate mother, gestational carrier, or sperm donation.
- Infertility can negatively impact a couple socially and psychologically, and it may be a major cause of marital friction or even divorce. Infertility treatments may raise ethical issues relating to the costs of the procedures and the status of embryos that are created in vitro but not used for pregnancy. Infertility is an under-appreciated problem in developing countries where birth rates are high and children have high economic as well as social value. In these countries, poor health care is likely to lead to more problems with infertility and fewer options for treatment.
- More than half of all fertile couples worldwide use contraception (birth control), which is any method or device used to prevent pregnancy. Different methods of contraception vary in their effectiveness, typically expressed as the failure rate, or the percentage of women who become pregnant using a given method during the first year of use. For most methods, the failure rate with typical use is much higher than the failure rate with perfect use.
- Types of birth control methods include , hormonal methods, intrauterine devices, behavioural methods, and . Except for sterilization, all of the methods are reversible.
-
- Barrier methods are devices that block sperm from entering the uterus. They include condoms and diaphragms. Of all birth control methods, only condoms can also prevent the spread of sexually transmitted infections.
- Hormonal methods involve the administration of hormones to prevent ovulation. Hormones can be administered in various ways, such as in an injection, through a skin patch, or, most commonly, in birth control pills. There are two types of birth control pills: those that contain estrogen and progesterone, and those that contain only progesterone. Both types are equally effective, but they have different potential side effects.
- An intrauterine device (IUD) is a small T-shaped plastic structure containing copper or a hormone that is inserted into the uterus by a physician and left in place for months or even years. It is highly effective even with typical use, but it does have some risks, such as increased menstrual bleeding and, rarely, perforation of the uterus.
- Behavioural methods involve regulating the timing or method of intercourse to prevent introduction of sperm into the female reproductive tract, either altogether or when an egg may be present. In fertility awareness methods, unprotected intercourse is avoided during the most fertile days of the cycle as estimated by basal body temperature or the characteristics of cervical mucus. In withdrawal, the penis is withdrawn from the vagina before ejaculation occurs. Behavioural methods are the least effective methods of contraception.
- Sterilization is the most effective contraceptive method, but it requires a surgical procedure and may be irreversible. Male sterility is usually achieved with a vasectomy, in which the vas deferens are clamped or cut to prevent sperm from being ejaculated in semen. Female sterility is usually achieved with a tubal ligation, in which the oviducts are clamped or cut to prevent sperm from reaching and fertilizing eggs.
- Emergency contraception is any form of contraception that is used after unprotected vaginal intercourse. One method is the “morning after” pill, which is a high-dose birth control pill that can be taken up to five days after unprotected sex. Another method is an IUD, which can be inserted up to five days after unprotected sex.
In this chapter, you learned how the male and female reproductive systems work together to produce a zygote. In the next chapter, you will learn about how the human organism grows and develops throughout life — from a zygote all the way through old age.
Chapter 18 Review
- Which glands produce the non-sperm fluids that make up semen? What is the rough percentage of each fluid in semen?
- What is one reason why semen's alkalinity assists in reproduction?
- What are three things that pass through the cervical canal of females, going in either direction?
- Other than where the cancer originates, what is one difference between prostate and testicular cancer?
- If a woman is checking her basal body temperature each morning as a form of contraception, and today is day 12 of her menstrual cycle and her basal body temperature is still low, is it safe for her to have unprotected sexual intercourse today? Why or why not?
- Where is a diaphragm placed? How does it work to prevent pregnancy?
- Why are the testes located outside of the body?
- Why is it important to properly diagnose the causative agent when a woman has vaginitis?
- Describe two ways in which sperm can move through the male and/or female reproductive tracts.
Attributions
Figure 18.12.1
Pregnancy test/ Dos rayitas by Esparta Palma on Flickr is used under a CC BY 2.0 (https://creativecommons.org/licenses/by/2.0/) license.
Figure 18.12.2
1024px-Ovulatietest by Sapp on Wikimedia Commons is in the public domain (https://en.wikipedia.org/wiki/Public_domain).
Figure 18.12.3
Sperm Count by CK-12 Foundation is used under a CC BY-NC 3.0 (https://creativecommons.org/licenses/by-nc/3.0/) license.
References
Brainard, J/ CK-12 Foundation. (2016). Figure 3 Normal vs. low sperm count [digital image]. In CK-12 College Human Biology (Section 20.12) [online Flexbook]. CK12.org. https://www.ck12.org/book/ck-12-college-human-biology/section/20.12/
Created by: CK-12/Adapted by Christine Miller
Figure 6.7.1 Men from Maasai Mara, Kenya.
Built for Heat
These tall, slender men live near the equator in Kenya, East Africa — one of the hottest regions of the world. These and the other people of his tribal group, called the Maasai, are among the tallest, most linear people on the planet. Their body build is thought to be an adaptation to their climate, which is hot year-round.
Climate Extremes
refers to the average weather conditions in a region over a long period of time. One of the main determinants of climate is temperature. Both hot and cold temperatures are serious environmental stresses on the human body.
In the cold, there is risk of , which is a dangerous decrease in core body temperature. The normal temperature of the human body is 37 degrees C (98.6 degrees F). Hypothermia sets in when body temperature drops to 34.4 degrees C (94 degrees F). If body temperature falls below 29.4 degrees C (85 degrees F), it starts to cool very rapidly because the body’s temperature regulation mechanism starts to fail.
The opposite problem occurs in the heat, where the risk is , which is a dangerous increase in core body temperature. If human body temperature rises above about 40.6 degrees C (105 degrees F), hyperthermia may become life threatening. If a temperature this high persists more than a few days, it generally damages the brain and other internal organs, leading to death.
Human Adaptation to Heat and Cold
Humans are the most widespread species on the planet, and they have lived in extreme climates for tens of thousands of years. As a result, many human populations have had to cope with extreme temperatures for hundreds of generations, which has forced them to develop genetic adaptations to these climate extremes.
The size and proportions of the human body may play an important role in how well an individual is able to handle hot or cold temperatures. In general, people with a tall, slender build, like the Maasai man pictured in Figure 6.7.1, are well adapted to heat, whereas people with a short, stocky build (like the Indigenous North American Inuit pictured in Figure 6.7.2) are well adapted to cold. These relationships between body build and climate were first noticed in other animal species in the 1800s by biologists Carl Bergmann and Joel Allen. These scientists formulated what are now known as Bergmann’s and Allen’s rules.
Figure 6.7.2 Indigenous North American Inuit.
Bergmann’s Rule
states that within a broadly distributed taxonomic group, populations or species of larger size are found in colder environments, whereas populations or species of smaller size are found in warmer environments. Bergmann’s rule has been shown to generally apply to widespread species of mammals and birds, although there are also many exceptions to the rule.
What explains Bergmann’s rule? Larger animals have a lower surface area to volume ratio than smaller animals, which is illustrated in Table 6.7.1 for a simple shape, a cube. From the table, you can see how the surface area to volume ratio of a cube decreases dramatically as the size of the cube increases. Because heat is lost through the surface of the body, an animal with a smaller surface area to volume ratio radiates less body heat per unit of mass. The larger body mass also allows the animal to generate more heat. A larger animal has more cells, so it can produce more body heat as a byproduct of cellular metabolism. Both of these factors allow a larger animal to stay warmer in a cold climate.
Table 6.7.1
Relationship of Surface Area to Volume in Cubes of Different Sizes
Relationship of Surface Area to Volume in Cubes of Different Sizes | |||
Side of Cube (cm) | Surface Area of Cube (cm2) | Volume of Cube (cm3) | Surface Area:Volume Ratio |
2 | 24 | 8 | 3:1 |
4 | 96 | 64 | 3:2 |
6 | 216 | 216 | 3:3 |
12 | 864 | 1728 | 3:6 |
20 | 2400 | 8000 | 3:10 |
Warmer climates impose the opposite problem: body heat generated by metabolism needs to be dissipated quickly rather than stored within the body. Smaller animals have a higher surface area to volume ratio that maximizes heat loss through the surface of the body and helps cool the body. With less mass and fewer cells, smaller animals also generate less heat due to cellular metabolism.
Anthropologists have found that many human populations tend to follow Bergmann’s rule. For example, a study of 100 human populations in the 1950s found a strong negative correlation between mean body mass and average yearly temperature. In other words, higher body mass was generally found in colder places, and lower body mass was generally found in hotter places.
There are also exceptions to the rule, in part because we use cultural responses to temper environmental stresses so we do not need to change genetically or physiologically in order to cope. Humans, for example, use clothing and heated buildings to stay warm in cold climates, which tends to counter the effects of natural selection changing human body shape in cold climates.
Allen’s Rule
is a corollary of Bergmann’s rule. It states that animals living in hotter climates generally have longer extremities (such as limbs, tails, snouts, and ears) than closely related animals living in colder climates. The explanation for Allen’s rule is similar to the rationale behind Bergmann’s rule. Longer extremities maximize an animal’s surface area, allowing greater heat loss through the surface of the body. Therefore, having long extremities is adaptive in hot climates where the main challenge is dissipating body heat.
Anthropologists have noted that, in populations that have lived in tropical regions for long periods of time, the limbs of people tend to be longer in proportion to overall body height. The Maasai man pictured in Figure 6.7.1 is a clear example. His exceptionally long limbs — like those of other members of his population — are optimally proportioned for the hot climate in Kenya. The shorter-limbed body proportions of the Inuit people (Figure 6.7.2) suit them well for their cold climate. Marked differences in limb length have also been observed in related populations that have lived for long periods of time at different altitudes. High altitudes have colder climates than lower altitudes and — consistent with Allen's rule —people tend to have shorter limbs at higher altitudes.
Other Human Responses to Heat
Humans exhibit several other responses to high temperatures that are generally considered either short-term physiological responses or examples of longer-term acclimatization.
Sweating and Humidity
Because humans are basically tropical animals, we generally have an easier time dealing with excessive heat than excessive cold. Evaporation of sweat is the main way we cool the body. The dancer in Figure 6.7.4 is sweating copiously while working out in a hot environment. Why does sweating cool us? When sweat evaporates from the skin, it requires heat. The heat comes from the surface of the body, resulting in evaporative cooling.
How well we can deal with high air temperatures depends in large part on the humidity of the air. We have a harder time losing excess body heat when the humidity is high because our sweat does not evaporate as well as it does when the humidity is low. Instead, the sweat stays on the skin, making us feel clammy and warmer than we would feel if the humidity were lower. If the air is dry, on the other hand, sweat evaporates readily, and we feel more comfortable. For this reason, we are able to tolerate higher temperatures when the humidity is low. This is the basis of the common aphorism, “It’s not the heat, but the humidity.”
The heat index (HI) is a number that combines air temperature and relative humidity to indicate how hot the air feels due to the humidity. The heat index is also called "apparent temperature.” Figure 6.7.5 shows the heat index at different combinations of air temperature and relative humidity. As you can see, when the humidity is very high, even a 90-degree F (32 degrees C) temperature can be very dangerous.
Acclimatization to Heat
If humidity is low, evaporation of sweat can be an effective way to keep the body from overheating. However, the loss of water and salts in sweat can also be dangerous. In very hot conditions, an adult may lose up to four litres of sweat per hour and up to 14 litres per day. Such water losses may cause severe dehydration if the water is not replaced by drinking much more than usual. The loss of salts may also upset the normal salt balance in the body, which can be dangerous. Becoming acclimatized to heat by gradually increasing the exposure time to high temperatures — particularly while exercising or doing physical work — can reduce the risk of these effects.
It may take up to 14 days to attain maximum heat acclimatization. As the body becomes acclimatized, sweat output increases, and sweating begins sooner. The salt content of the sweat also declines, as does the output of urine. These and other physiological changes help the body lose heat through the evaporation of sweat, while maintaining the proper balance of salts and fluids in the body. There may also be increased blood flow to the body surface through the widening of blood vessels near the skin. This is called . This brings more heat from the body core to the skin, and from there it may be radiated out into the environment.
Becoming acclimatized to heat allows one to safely perform more exercise or work in the heat. It also helps prevent heat-related illnesses by reducing strain on the body. Heat-related illnesses — from least to most serious — include heat cramps, heat exhaustion, and heat stroke.
- Heat cramps are muscle spasms caused by loss of water and salts. They often follow prolonged sweating brought on by over-exertion in hot weather.
- Heat exhaustion is a condition in which over-heating of the body causes dizziness, headache, profuse sweating, rapid heartbeat. and other symptoms. Without prompt treatment, heat exhaustion can lead to heat stroke.
- Heat stroke is potentially life threatening and a medical emergency. Heat stroke results from prolonged exposure to high temperatures, usually in combination with dehydration. It leads to failure of the body's temperature control system and is diagnosed when the core body temperature exceeds 105 degrees F (40 degrees C). Symptoms may include nausea, seizures, confusion, disorientation, and coma.
Acclimatization to heat, like other types of acclimatization, is a reversible process. Just as quickly as heat acclimatization occurs, the physiological changes fade away in the absence of heat exposure. The body returns to its baseline state within a week or two of no longer exercising or working at high temperatures.
Other Human Responses to Cold
Besides genetic difference in body build, there are two major ways the human body can respond to the cold. One way is by producing more body heat, and the other way is by conserving more body heat. An immediate response to cooling of the body is shivering. This is an involuntary and simultaneous contraction of many tiny muscles in the body. These muscle contractions generate a small amount of heat. Another early response to cold temperature is a narrowing of blood vessels near the skin. This is called . This helps to shunt blood away from the body surface so more heat is held at the body core. The skin cools down and radiates less heat into the environment.
Hunting Response
At temperatures below freezing, vasoconstriction can be dangerous if it lasts too long. The extremities become too cold because of lack of blood flow, and cold injury (such as frostbite) may occur. is tissue destruction that occurs when tissue freezes. You can see a mild-to-moderate case of frostbite of the fingers in Figure 6.7.7. If frostbite is severe, it may lead to gangrene and amputation of the affected extremities.
The body counters the possibility of cold injury with a reaction called the . This is a process of alternating vasoconstriction and vasodilation in extremities exposed to cold. About five to ten minutes after the start of cold exposure, the blood vessels in the extremities suddenly dilate, which increases blood flow and subsequently the temperature of the extremities. This is soon followed by another phase of vasoconstriction, and then the process repeats.
The hunting response occurs in most people, but several factors may influence the strength of the response. People who live or work regularly in cold environments show an increased hunting response. Through acclimatization, however, tropical residents can develop an increased response, which is indistinguishable from that of arctic residents. Genetic factors may play a role in the hunting response, but this is uncertain because it is difficult to differentiate between adaptation and acclimatization.
Persistent Vasoconstriction
Where temperatures rarely fall below freezing but are repeatedly very chilly, the hunting response may not occur. Instead, vasoconstriction may persist to keep heat within the body at the expense of cooling the skin. As long as the temperature stays above freezing, cold injury (such as frostbite) will not occur. This type of response has been shown to occur in indigenous desert dwellers in southern Africa and Australia, where the temperature is hot during the day and very cold at night. People in these populations also tend to deposit fat around the organs in their chest and abdomen. The fat serves as insulation, protecting vital structures from the cold.
High-Fat Diet
Besides shivering, another way to increase body heat is to raise the basal metabolic rate. The (BMR) is the amount of energy that a person needs to keep the body functioning at rest. The higher the BMR, the more heat the body generates, even without exercise or physical labor. The BMR can be increased by consuming large quantities of high-calorie fatty foods. People living in very cold subarctic regions, including the Inuit, traditionally ate whale and seal blubber and other high-fat foods, which helped them maintain a high BMR and stay warm.
Figure 6.7.8 Whale and seal blubber (mainly on abundant ring seals) is an important part of the traditional Inuit diet.
Feature: Human Biology in the News
Too many news stories report young children being seriously injured or dying from heat stroke in hot vehicles. On average, 38 children die in hot vehicles each year from heat-related deaths after being trapped inside. Most often, this happens by accident, when a parent or caregiver unknowingly leaves a sleeping child in a car. In other cases, children get into cars on their own, and then cannot get out again.
A child’s thermoregulatory system is not as efficient as that of an adult, and a child’s body temperature may increase as much as five times faster. This makes children prime candidates for heat stroke. A motor vehicle is also easily heated by direct sun. The windows of the vehicle allow solar radiation to pass through and heat up objects inside. A dark-coloured dashboard or seat may quickly reach a temperature of more than 180 degrees F (82 degrees C)! These hot surfaces can just as quickly heat the adjacent air, rapidly increasing the temperature of the air trapped inside the vehicle.
Here are several simple tips that parents and caregivers can follow to prevent heat stroke tragedies:
- Never leave children alone in or around cars — not even for a minute.
- Always open the back door and check the back seat before leaving your vehicle to be sure no child has been left behind.
- Put something you will need, such as your cell phone or handbag, in the back seat so you will have to open the back door to retrieve it whenever you park the car.
- Keep a large stuffed animal in the child's car seat, and when the child is placed in the car seat, put the stuffed animal in the front passenger seat as a visual reminder that the child is in the back.
- Make sure you have a strict policy in place with everyone involved in the care of your child that you should always be called whenever your child does not show up at daycare or school as scheduled.
- Keep vehicles locked at all times, even in driveways and garages. Ask home visitors, child care providers, and neighbors to do the same.
- Keep car keys and remote vehicle openers out of reach of children.
- If a child is missing, immediately check the inside passenger compartments and trunks of all vehicles in the area. Check vehicles even if they are locked, because a child may lock a vehicle after entering and not be able to unlock it again to get out.
- If you see a child alone in a vehicle, call 911 immediately. If the child seems hot or sick, get them out of the vehicle as quickly as possible.
- Pay for gas at the pump and use drive-throughs at the bank, pharmacy, or wherever else they are available.
6.7 Summary
- Both hot and cold temperatures are serious environmental stresses on the human body. In the cold, there is risk of , which is a dangerous decrease in core body temperature. In the heat, there is risk of, which is a dangerous increase in core body temperature.
- According to , body size tends to be negatively correlated with temperature, because larger body size increases heat production and decreases heat loss. The opposite holds true for small body size. Bergmann’s rule applies to many human populations that are hot- or cold-adapted.
- According to , the length of body extremities is positively correlated with temperature, because longer extremities are better at dissipating excess body heat. The opposite applies to shorter extremities. Allen’s rule applies to relative limb lengths in many human populations that have adapted to heat or cold.
- Sweating is the primary way that humans lose body heat. The evaporation of sweat from the skin cools the body. This only works well when the relative humidity is fairly low. At high relative humidity, sweat does not readily evaporate to cool us down. The heat index (HI) indicates how hot it feels due to the humidity.
- Gradually working longer and harder in the heat can bring about heat acclimatization, in which the body has improved responses to heat stress. For example, sweating starts earlier, sweat contains less salt, and vasodilation brings heat to the surface to help cool the body. Full acclimatization takes up to 14 days and reverses just as quickly when the heat stress is removed.
- The human body can respond to cold by producing more heat (by shivering or increasing the basal metabolic rate) or by conserving heat (by vasoconstriction at the body surface or a layer of fat-insulating internal organs).
- At temperatures below freezing, the hunting response occurs to prevent cold injury, such as frostbite. This is a process of alternating vasoconstriction and vasodilation in extremities that are exposed to dangerous cold. Where temperatures are repeatedly cold but rarely below freezing, the hunting response may not occur, and the skin may remain cold due to vasoconstriction alone.
6.7 Review Questions
- Compare and contrast hypothermia and hyperthermia.
- State Bergmann’s and Allen’s rules.
- How do the Maasai and Inuit match the predictions based on Bergmann’s and Allen’s rules?
- Explain how sweating cools the body.
- What is the heat index?
- Relate the heat index to evaporative cooling of the body.
- Identify three heat-related illnesses, from least to most serious.
- How does heat acclimatization occur?
- State two major ways the human body can respond to the cold, and give an example of each.
- Explain how and why the hunting response occurs.
- Define basal metabolic rate.
- How does a high-fat diet help prevent hypothermia?
- Explain why frostbite most commonly occurs in the extremities, such as the fingers and toes.
6.7 Explore More
https://www.youtube.com/watch?v=PpHM4DfPZQU
What happens when you get heat stroke? - Douglas J. Casa, TED-Ed, 2014.
https://youtu.be/_Ifq73REJiM
Hailstones' Inupiaq Traditions | Life Below Zero, National Geographic, 2014.
https://www.youtube.com/watch?v=1L7EI0vKVuU
How An Igloo Keeps You Warm, It's Okay To Be Smart, 2017.
https://www.youtube.com/watch?v=fctH_1NuqCQ&t=
Why do we sweat? - John Murnan, TED-Ed, 2018.
https://www.youtube.com/watch?time_continue=44&v=j3sY67aGFXY&feature=emb_logo
Wim Hof Method, Wim Hof, 2011.
Attributions
Figure 6.7.1
- Maasai warrior by Ninaras on Wikimedia Commons is used under a CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/deed.en) license.
- Smiling man from Maasai Mara, Kenya by Sneha on Unsplash is used under the Unsplash License (https://unsplash.com/license).
Figure 6.7.2
- Inuit-Kleidung women by Ansgar Walk on Wikimedia Commons is used under a CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0/deed.lv) license.
- Kulusuk, Tunumiit Inuit couple by Arian Zwegers on Wikimedia Commons is used under a CC BY 2.0 (https://creativecommons.org/licenses/by/2.0/deed.en) license.
-
Inuit girls by Susan van Gelder on Flickr is used under a CC BY-NC-SA 2.0 (https://creativecommons.org/licenses/by-nc-sa/2.0/) license.
Figure 6.7.3
Bergmann’s_rule_-_Canis_lupus by Dhaval Vargiya at Yellowstone National Park on Wikimedia Commons is in the public domain (https://en.wikipedia.org/wiki/Public_domain).
Figure 6.7.4
Sweating [photo] by Avi Richards on Unsplash is used under the Unsplash License (https://unsplash.com/license).
Figure 6.7.5
Heat_Index by U.S. National Oceanic and Atmospheric Administration (NOAA) on Wikimedia Commons is in the public domain (https://en.wikipedia.org/wiki/Public_domain).
Figure 6.7.6
Thirsty [photo] by Dylan Alcock on Unsplash is used under the Unsplash License (https://unsplash.com/license).
Figure 6.7.7
Frostbitten_hands by Winky from Oxford, UK on Wikimedia Commons is used under a CC BY 2.0 (https://creativecommons.org/licenses/by/2.0/deed.en) license.
Figure 6.7.8
- Ringed seal preparation by Ansgar Walk on Wikimedia Commons is used under a CC BY-SA 2.5 (https://creativecommons.org/licenses/by-sa/2.5/deed.en) license.
- Butchering a narwhal by Spencer & Carole on Flickr is used under a CC BY-NC-SA 2.0 (https://creativecommons.org/licenses/by-nc-sa/2.0/) license.
- Inuit children playing while the family is on seal hunt, by GRID-Arendal on Flickr is used under a CC BY-NC-SA 2.0 (https://creativecommons.org/licenses/by-nc-sa/2.0/) license.
References
It's Okay To Be Smart. (2017, January 9). How an igloo keeps you warm. YouTube. https://www.youtube.com/watch?v=1L7EI0vKVuU&feature=youtu.be
National Geographic. (2014, April 7). Hailstones' Inupiaq traditions | Life below zero. YouTube. https://www.youtube.com/watch?v=_Ifq73REJiM&feature=youtu.be
TED-Ed. (2014, July 21). What happens when you get heat stroke? - Douglas J. Casa. YouTube. https://www.youtube.com/watch?v=PpHM4DfPZQU&feature=youtu.be
TED-Ed. (2018, May 15). Why do we sweat? - John Murnan. YouTube. https://www.youtube.com/watch?v=fctH_1NuqCQ&feature=youtu.be
Wim Hof. (2011, June 19). Wim Hof Method. YouTube. https://www.youtube.com/watch?v=j3sY67aGFXY&feature=youtu.be
Created by CK-12 Foundation/Adapted by Christine Miller
Stroller Moms
These moms (Figure 12.5.1) are setting a great example for their children by engaging in physical exercise. Adopting a habit of regular physical exercise is one of the most important ways to maintain fitness and good health. From higher self-esteem to a healthier heart, physical exercise can have a positive effect on virtually all aspects of health, including physical, mental, and emotional health.
What Is Physical Exercise?
is any bodily activity that enhances or maintains physical fitness and overall health and wellness. We generally think of physical exercise as activities that are undertaken for the main purpose of improving physical fitness and health. However, physical activities that are undertaken for other purposes may also count as physical exercise. Scrubbing a floor, raking a lawn, or playing active games with young children or a pet are all activities that can have fitness and health benefits, even though they generally are not done mainly for this purpose.
How much physical exercise should people get? In the Canada, both the Canadian Food Guide and the Canadian Society for Exercise Physiology recommend that every child and adult who is able should participate in moderate exercise for a minimum of 60 minutes a day. This might include walking, swimming, and/or household or yard work.
Types of Physical Exercise
Physical exercise can be classified into three types, depending on the effects it has on the body: aerobic exercise, anaerobic exercise, and flexibility exercise. Many specific examples of physical exercise (including playing soccer and rock climbing) can be classified as more than one type.
Aerobic Exercise
is any physical activity in which muscles are used at well below their maximum contraction strength, but for long periods of time. Aerobic exercise uses a relatively high percentage of slow-twitch muscle fibres that consume a large amount of oxygen. The main goal of aerobic exercise is to increase cardiovascular endurance, although it can have many other benefits, including muscle toning. Examples of aerobic exercise include cycling, swimming, brisk walking, jumping rope, rowing, hiking, tennis, and kayaking as shown in Figure 12.5.2 .
Anaerobic Exercise
is any physical activity in which muscles are used at close to their maximum contraction strength, but for relatively short periods of time. Anaerobic exercise uses a relatively high percentage of fast-twitch muscle fibres that consume a small amount of oxygen. Goals of anaerobic exercise include building and strengthening muscles, as well as improving bone strength, balance, and coordination. Examples of anaerobic exercise include push-ups, lunges, sprinting, interval training, resistance training, and weight training (such as biceps curls with a dumbbell, as pictured in Figure 12.5.3).
Flexibility Exercise
is any physical activity that stretches and lengthens muscles. Goals of flexibility exercise include increasing joint flexibility, keeping muscles limber, and improving the range of motion, all of which can reduce the risk of injury. Examples of flexibility exercise include stretching, yoga (as in Figure 12.5.4), and tai chi.
Health Benefits of Physical Exercise
Many studies have shown that physical exercise is positively correlated with a diversity of health benefits. Some of these benefits include maintaining physical fitness, losing weight and maintaining a healthy weight, regulating digestive health, building and maintaining healthy bone density, increasing muscle strength, improving joint mobility, strengthening the immune system, boosting cognitive ability, and promoting psychological well-being. Some studies have also found a significant positive correlation between exercise and both quality of life and life expectancy. People who participate in moderate to high levels of physical activity have been shown to have lower mortality rates than people of the same ages who are not physically active and daily exercise has been shown to increase life expectancy up to an average of five years.
The underlying physiological mechanisms explaining why exercise has these positive health benefits are not completely understood. However, developing research suggests that many of the benefits of exercise may come about because of the role of skeletal muscles as endocrine organs. Contracting muscles release hormones called , which promote tissue repair and the growth of new tissue. Myokines also have anti-inflammatory effects, which, in turn, reduce the risk of developing inflammatory diseases. Exercise also reduces levels of , the adrenal cortex stress hormone that may cause many health problems — both physical and mental — at sustained high levels.
Cardiovascular Benefits of Physical Exercise
The beneficial effects of exercise on the cardiovascular system are well documented. Physical inactivity has been identified as a risk factor for the development of coronary artery disease. There is also a direct correlation between physical inactivity and cardiovascular disease mortality. Physical exercise, in contrast, has been demonstrated to reduce several risk factors for cardiovascular disease, including (high blood pressure), “bad” cholesterol (low-density lipoproteins), high total cholesterol, and excess body weight. Physical exercise has also been shown to increase “good” cholesterol (high-density lipoproteins), insulin sensitivity, the mechanical efficiency of the heart, and exercise tolerance, which is the ability to perform physical activity without undue stress and fatigue.
Cognitive Benefits of Physical Exercise
Physical exercise has been shown to help protect people from developing , such as dementia. A 30-year study of almost 2,400 men found that those who exercised regularly had a 59 per cent reduction in when compared with those who did not exercise. Similarly, a review of cognitive enrichment therapies for the elderly found that physical activity — in particular, aerobic exercise — can enhance the cognitive function of older adults. Anecdotal evidence suggests that frequent exercise may even help reverse alcohol-induced brain damage. There are several possible reasons why exercise is so beneficial for the brain. Physical exercise:
- Increases blood flow and oxygen availability to the brain.
- Increases growth factors that promote new brain cells and new neuronal pathways in the brain.
- Increases levels of neurotransmitters (such as serotonin), which increase memory retention, information processing, and cognition.
Mental Health Benefits of Physical Exercise
Numerous studies suggest that regular works as well as pharmaceutical antidepressants in treating mild-to-moderate . A possible reason for this effect is that exercise increases the biosynthesis of at least three that may act as . The euphoric effect of exercise is well known. Distance runners may refer to it as “runner’s high,” and people who participate in crew (as in Figure 12.5.5) may refer to it as “rower’s high.” Because of these effects, health care providers often promote the use of aerobic exercise as a treatment for depression.
Additional mental health benefits of physical exercise include reducing stress, improving body image, and promoting positive self-esteem. Conversely, there is evidence to suggest that being sedentary is associated with increased risk of anxiety.
Sleep Benefits of Physical Exercise
A recent review of published scientific research suggests that exercise generally improves sleep for most people, and helps sleep disorders, such as . In fact, exercise is the most recommended alternative to sleeping pills for people with insomnia. For sleep benefits, the optimum time to exercise may be four to eight hours before bedtime, although exercise at any time of day seems to be beneficial. The only possible exception is heavy exercise undertaken shortly before bedtime, which may actually interfere with sleep.
Other Benefits of Physical Exercise
Some studies suggest that physical activity may benefit the immune systemno post. For example, moderate exercise has been found to be associated with a decreased incidence of upper respiratory tract infections. Evidence from many studies has found a correlation between physical exercise and reduced death rates from , specifically breast cancer and colon cancer. Physical exercise has also been shown to reduce the risk of and .
Variation in Responses to Physical Exercise
Not everyone benefits equally from physical exercise. When participating in , most people will have a moderate increase in their endurance, but some people will as much as double their endurance. Some people, on the other hand, will show little or no increase in endurance from aerobic exercise. Genetic differences in and skeletal muscle fibres may play a role in these different results. People with more slow-twitch fibres may be able to develop greater endurance, because these muscle fibres have more capillaries, , and than fast-twitch fibres. As a result, slow-twitch fibres can carry more oxygen and sustain aerobic activity for a longer period of time than fast-twitch fibres. Studies show that endurance athletes (like the marathoner pictured in Figure 12.5.6) generally do tend to have a higher proportion of slow-twitch fibres than other people.
There is also great variation in individual responses to muscle building as a result of anaerobic exercise. Some people have a much greater capacity to increase muscle size and strength, whereas other people never develop large muscles, no matter how much they exercise them. People who have more fast-twitch than slow-twitch muscle fibres may be able to develop bigger, stronger muscles, because fast-twitch muscle fibres contribute more to muscle strength and have greater potential to increase in mass. Evidence suggests that athletes who excel at power activities (such as throwing and jumping) tend to have a higher proportion of fast-twitch fibres than do endurance athletes.
Can You “Overdose” on Physical Exercise?
Is it possible to exercise too much? Can too much exercise be harmful? Evidence suggests that some adverse effects may occur if exercise is extremely intense and the body is not given proper rest between exercise sessions. Athletes who train for multiple marathons have been shown to develop scarring of the heart and heart rhythm abnormalities. Doing too much exercise without prior conditioning also increases the risk of injuries to muscles and joints. Damage to muscles due to overexertion is often seen in new military recruits (see Figure 12.5.7). Too much exercise in females may cause amenorrhea, which is a cessation of menstrual periods. When this occurs, it generally indicates that a woman is pushing her body too hard.
Many people develop delayed onset muscle soreness (DOMS), which is pain or discomfort in muscles that is felt one to three days after exercising, and generally subsides two or three days later. DOMS was once thought to be caused by the buildup of lactic acid in the muscles. Lactic acid is a product of in muscle tissues. However, lactic acid disperses fairly rapidly, so it is unlikely to explain pain experienced several days after exercise. The current theory is that DOMS is caused by tiny tears in muscle fibres, which occur when muscles are used at too high a level of intensity.
Feature: My Human Body
Most people know that exercise is important for good health, and it’s easy to find endless advice about exercise programs and fitness plans. What is not so easy to find is the motivation to start exercising — and to stick with it. This is the main reason why so many people fail to get regular exercise. Practical concerns like a busy schedule and bad weather can certainly make exercising more of a challenge, but the biggest barriers to adopting a regular exercise routine are mental. If you want to exercise but find yourself making excuses or getting discouraged and giving up, here are some tips that may help you get started and stay moving:
- Avoid an all-or-nothing point of view. Don’t think you need to spend hours sweating at the gym or training for a marathon to get healthy. Even a little bit of exercise is better than nothing at all. Start out with ten or 15 minutes of moderate activity each day. Taking a walk around your neighborhood is a great way to begin! From there, gradually increase the amount of time until you are exercising to at least 30 minutes a day, five days a week. Make it a routine.
- Be kind to yourself, and reinforce positive behaviors with rewards. Don’t be down on yourself because you are overweight or out of shape. Don’t beat yourself up because of a supposed lack of willpower. Instead, look at any past failures as opportunities to learn and do better. When you do achieve even small exercise goals, treat yourself to something special. Did you just complete your first workout? Reward yourself with a relaxing bath or other treat.
- Don’t make excuses for not exercising. Common complaints include being too busy or tired or not athletic enough. Such excuses are not valid reasons to avoid exercising, and they will sabotage any plans to improve your fitness. If you can’t find a 30-minute period to work out, try to find ten minutes, three times a day. If you’re feeling tired, know that exercise can actually reduce fatigue and boost your energy level. If you feel clumsy and uncoordinated, remind yourself that you don’t need to be athletic to take a walk or engage in vigorous house or yard work.
- Find an activity that you truly enjoy doing. Don’t think you have to lift weights or run on a treadmill to exercise your muscles. If you find such activities boring or unpleasant, you won’t stick with them. Any activity that increases your heart rate and uses large muscles can provide a workout, especially if you’re not in the habit of exercising, so find something you like to do. Do you like to dance? Put on some music and dance up a sweat! Do you enjoy gardening? Get out in the yard and dig up some dirt! Still not interested? Try an activity-based video game, such as Wii or Kinect. You may find it so much fun that it doesn’t seem like exercise until you realize you’ve worked up a sweat.
- Make yourself accountable. Tell friends and family members that you’re going to start exercising. You’ll be letting them — as well as yourself — down if you don’t follow through. Some people find that keeping an exercise log to track their progress is a good way to be accountable and stick to an exercise program. Perhaps the best way to keep at it is to find an exercise partner. If you’ve got someone waiting to exercise with you, you will be less likely to make excuses for not exercising.
- Add more physical activity to your daily life. You don’t need to follow a structured exercise program to increase your activity level. Do your house or yard work briskly for a workout. Park your car further than necessary from work or the mall, and walk the extra distance. If you live close enough, leave the car at home and walk to and from your destination. Rather than taking elevators or escalators, walk up and down stairs. When you take breaks at work, take a walk instead of sitting. Every time a commercial comes on while you’re watching TV, take a quick exercise break — run in place or do some curls with hand weights.
12.5 Summary
- is any bodily activity that enhances or maintains physical fitness and overall health. Activities such as household chores may count as physical exercise, even if they are not done for their health benefits. Current recommendations for adults are 30 minutes a day of moderate exercise.
- is any physical activity that uses muscles at less than their maximum contraction strength, but for long periods of time. This type of exercise uses a relatively high percentage of that consume large amounts of oxygen. Aerobic exercises increase cardiovascular endurance and include cycling and brisk walking.
- is any physical activity that uses muscles at close to their maximum contraction strength, but for short periods of time. This type of exercise uses a relatively high percentage of that consume small amounts of oxygen. Anaerobic exercises increase muscle and bone mass and strength, and they include push-ups and sprinting.
- is any physical activity that stretches and lengthens muscles, thereby improving range of motion and reducing risk of injury. Examples include stretching and yoga.
- Many studies have shown that physical exercise is positively correlated with a diversity of physical, mental, and emotional health benefits. Physical exercise also increases quality of life and life expectancy.
- Many of the benefits of exercise may come about because contracting muscles release hormones called , which promote tissue repair and growth and have anti-inflammatory effects.
- Physical exercise can reduce risk factors for cardiovascular disease, including hypertension and excess body weight. Physical exercise can also increase factors associated with cardiovascular health, such as mechanical efficiency of the heart.
- Physical exercise has been shown to offer protection from and other cognitive problems, perhaps because it increases blood flow or in the , among other potential effects.
- Numerous studies suggest that regular aerobic exercise works as well as pharmaceutical antidepressants in treating mild-to-moderate depression, possibly because it increases synthesis of natural euphoriants in the brain.
- Research shows that physical exercise generally improves sleep for most people and helps sleep disorders, such as insomnia. Other health benefits of physical exercise include better immune system function and reduced risk of type 2 diabetes and obesity.
- There is great variation in individual responses to exercise, partly due to genetic differences in proportions of slow-twitch and fast-twitch skeletal muscle fibres. People with more slow-twitch fibres may be able to develop greater endurance from aerobic exercise, whereas people with more fast-twitch fibres may be able to develop greater muscle size and strength from anaerobic exercise.
- Some adverse effects may occur if exercise is extremely intense and the body is not given proper rest between exercise sessions. Many people who overwork their muscles develop delayed onset muscle soreness (DOMS), which may be caused by tiny tears in muscle fibres.
12.5 Review Questions
- How do we define physical exercise?
- What are current recommendations for physical exercise for adults?
- Define flexibility exercise, and state its benefits. What are two examples of flexibility exercises?
- In general, how does physical exercise affect health, quality of life, and longevity?
- What mechanism may underlie many of the general health benefits of physical exercise?
- Relate physical exercise to cardiovascular disease risk.
- What may explain the positive benefits of physical exercise on cognition?
- How does physical exercise compare with antidepressant drugs in the treatment of depression?
- Identify several other health benefits of physical exercise.
- Explain how genetics may influence the way individuals respond to physical exercise.
- Can too much physical exercise be harmful?
12.5 Explore More
https://www.youtube.com/watch?v=hmFQqjMF_f0
How playing sports benefits your body ... and your brain - Leah Lagos and Jaspal Ricky Singh, TED-Ed, 2016.
https://www.youtube.com/watch?v=rLsimrBoYXc&t=12s
The surprising reason our muscles get tired - Christian Moro, TED-Ed, 2019.
https://youtu.be/2tM1LFFxeKg
What makes muscles grow? - Jeffrey Siegel, TED-Ed, 2015.
https://www.youtube.com/watch?v=QeIrdqU0o9s
Why some people find exercise harder than others | Emily Balcetis, TED, 2014.
Attributions
Figure 12.5.1
stroller fit by Serge Melki from Indianapolis, USA on Wikimedia Commons is used under a CC BY 2.0 (https://creativecommons.org/licenses/by/2.0) license.
Figure 12.5.2
Children kayaking young sport by Hagerty Ryan, USFWS on Pixnio is used under a public domain (CC0) Certification (https://creativecommons.org/licenses/publicdomain/).
Figure 12.5.3
Bicep curls [photo] by Senior Airman Jarrod Grammel from U.S. Moody Air Force Base is released into the public domain (https://en.wikipedia.org/wiki/Public_domain).
Figure 12.5.4
Flexibility exercise by carl-barcelo-nqUHQkuVj3c [photo] by Carl Barcelo on Unsplash is used under the Unsplash License (https://unsplash.com/license).
Figure 12.5.5
Canadian women’s double scull silver Rio 2016 by Gerhard Pratt on Flickr is used under a CC BY-NC 2.0 (https://creativecommons.org/licenses/by-nc/2.0/) license.
Figure 12.5.6
Toronto Marathon 2012 by Marc Roberts on Flickr is used under a CC BY-NC-SA 2.0 (https://creativecommons.org/licenses/by-nc-sa/2.0/) license.
Figure 12.5.7
Muscle damage in military recruits by Lance Cpl. Bridget M. Keane from the United States Marine Corps Recruit Depot is in the public domain (https://en.wikipedia.org/wiki/Public_domain).
References
Elwood, P., Galante, J., Pickering, J., Palmer, S., Bayer, A., Ben-Shlomo, Y., Longley, M., & Gallacher, J. (2013). Healthy lifestyles reduce the incidence of chronic diseases and dementia: evidence from the Caerphilly cohort study. PloS one, 8(12), e81877. https://doi.org/10.1371/journal.pone.0081877
Mayo Clinic Staff. (n.d.). Amenorrhea [online article]. MayoClinic.org. https://www.mayoclinic.org/diseases-conditions/amenorrhea/symptoms-causes/syc-20369299#
Mayo Clinic Staff. (n.d.). Coronary artery disease [online article]. MayoClinic.org. https://www.mayoclinic.org/diseases-conditions/coronary-artery-disease/symptoms-causes/syc-20350613
TED-Ed. (2016, June 28). How playing sports benefits your body ... and your brain - Leah Lagos and Jaspal Ricky Singh. YouTube. https://www.youtube.com/watch?v=hmFQqjMF_f0&feature=youtu.be
TED-Ed. (2019, April 18). The surprising reason our muscles get tired - Christian Moro. YouTube. https://www.youtube.com/watch?v=rLsimrBoYXc&feature=youtu.be
TED-Ed. (2015, November 3). What makes muscles grow? - Jeffrey Siegel. YouTube https://www.youtube.com/watch?v=2tM1LFFxeKg&feature=youtu.be
TED. (2014, November 14). Why some people find exercise harder than others | Emily Balcetis, YouTube. https://www.youtube.com/watch?v=QeIrdqU0o9s&feature=youtu.be
Wikipedia contributors. (2020, August 1). Delayed onset muscle soreness. In Wikipedia. https://en.wikipedia.org/w/index.php?title=Delayed_onset_muscle_soreness&oldid=970682631