The female sex hormone secreted mainly by the ovaries.

A chemical reaction that releases energy through light or heat.

A type of immune cell that has granules (small particles) with enzymes that can kill tumor cells or cells infected with a virus. A natural killer cell is a type of white blood cell.

A substance that is insoluble in water. Examples include fats, oils and cholesterol. Lipids are made from monomers such as glycerol and fatty acids.

A semi-permeable lipid bilayer that separates the interior of all cells from their surroundings.

The jellylike material that makes up much of a cell inside the cell membrane, and, in eukaryotic cells, surrounds the nucleus. The organelles of eukaryotic cells, such as mitochondria, the endoplasmic reticulum, and (in green plants) chloroplasts, are contained in the cytoplasm.

A central organelle containing hereditary material.

A glucocorticoid hormone produced by the cortex of the adrenal gland that is released in response to stress and also helps control metabolic rate, suppression of the immune system, and other functions

one of a pair of glands located on top of the kidneys that secretes hormones such as cortisol and adrenaline

A type of disease in which cells of the central nervous system stop working or die. Neurodegenerative disorders usually get worse over time and have no cure. They may be genetic or be caused by a tumor or stroke.

Any reaction which requires or absorbs energy from its surroundings, usually in the form of heat.

Amino acids are organic compounds that combine to form proteins.

Biological molecules that lower amount the energy required for a reaction to occur.

A peptide hormone, produced by alpha cells of the pancreas. It works to raise the concentration of glucose and fatty acids in the bloodstream, and is considered to be the main catabolic hormone of the body. It is also used as a medication to treat a number of health conditions.

Created by CK-12 Foundation/Adapted by Christine Miller

Nineteen-year-old Malcolm is about to take his first plane flight. Shortly after he boards the plane and sits down, a man in his late sixties sits next to him in the aisle seat. About half an hour after the plane takes off, the pilot announces that she is turning the seat belt light off, and that it is safe to move around the cabin.

The man in the aisle seat — who has introduced himself to Malcolm as Willie — immediately unbuckles his seat belt and paces up and down the aisle a few times before returning to his seat. After about 45 minutes, Willie gets up again, walks some more, then sits back down and does some foot and leg exercises. After the third time Willie gets up and paces the aisles, Malcolm asks him whether he is walking so much to accumulate steps on a pedometer or fitness tracking device. Willie laughs and says no. He is actually trying to do something even more important for his health — prevent a blood clot from forming in his legs.

Willie explains that he has a chronic condition: heart failure. Although it sounds scary, his condition is currently well-managed, and he is able to lead a relatively normal lifestyle. However, it does put him at risk of developing other serious health conditions, such as deep vein thrombosis (DVT), which is when a blood clot occurs in the deep veins, usually in the legs. Air travel — and other situations where a person has to sit for a long period of time — increases the risk of DVT. Willie’s doctor said that he is healthy enough to fly, but that he should walk frequently and do leg exercises to help avoid a blood clot.

As you read this chapter, you will learn about the heart, blood vessels, and blood that make up the cardiovascular system, as well as disorders of the cardiovascular system, such as heart failure. At the end of the chapter you will learn more about why DVT occurs, why Willie has to take extra precautions when he flies, and what can be done to lower the risk of DVT and its potentially deadly consequences.

Attribution

Figure 14.1.1

aircraft-1583871_1920 [photo] by olivier89 from Pixabay is used under the Pixabay License (https://pixabay.com/de/service/license/).

Image shows a diagram of Killer T Cell funtion. An infected cell displays a pathogen antigen on an MHC. The Killer T Cell interacts with the MHC and in response produces perforin ( a protein that pokes holes in cell membranes) and granzymes (proteins that instruct a cell to carry out programmed cell death). The infected cell dies from the combination of these substances, and as it dies, so does the pathogen inside the infected cell. The Killer T Cell is free to move on and find and destroy other infected cells.

A loop system wherein the system responds to a perturbation. The response may be in the same direction (as in positive feedback) or in the opposite direction (as in negative feedback). A feedback mechanism may be observed at the level of cells, organisms, ecosystems, or the biosphere.

Created by CK-12 Foundation/Adapted by Christine Miller

One Piano, Four Hands

Did you ever see two people play the same piano? How do they coordinate all the movements of their own fingers — let alone synchronize them with those of their partner? The peripheral nervous system plays an important part in this challenge.

The peripheral nervous system (PNS) consists of all the nervous tissue that lies outside of the central nervous system (CNS). The main function of the PNS is to connect the CNS to the rest of the organism. It serves as a communication relay, going back and forth between the CNS and muscles, organs, and glands throughout the body.

The PNS is mostly made up of cable-like bundles of axons called nerves, as well as clusters of neuronal cell bodies called ganglia (singular, ganglion). Nerves are generally classified as sensory, motor, or mixed nerves based on the direction in which they carry nerve impulses.

• Sensory nervesno post transmit information from sensory receptors in the body to the CNS. Sensory nerves are also called afferent nerves. You can see an example in the figure below.
• Motor nerves transmit information from the CNS to muscles, organs, and glands. Motor nerves are also called efferent nerves. You can see one in the figure below.
• Mixed nerves contain both sensory and motor neurons, so they can transmit information in both directions. They have both afferent and efferent functions.

 

The PNS is divided into two major systems, called the autonomic nervous system and the somatic nervous system. In the diagram below, the autonomic system is shown on the left, and the somatic system on the right. Both systems of the PNS interact with the CNS and include sensory and motor neurons, but they use different circuits of nerves and ganglia.

Somatic Nervous System

The somatic nervous system primarily senses the external environment and controls voluntary activities about which decisions and commands come from the cerebral cortex of the brain. When you feel too warm, for example, you decide to turn on the air conditioner. As you walk across the room to the thermostat, you are using your somatic nervous system. In general, the somatic nervous system is responsible for all of your conscious perceptions of the outside world, as well as all of the voluntary motor activities you perform in response. Whether it’s playing a piano, driving a car, or playing basketball, you can thank your somatic nervous system for making it possible.

Somatic sensory and motor information is transmitted through 12 pairs of cranial nerves and 31 pairs of spinal nerves. Cranial nerves are in the head and neck and connect directly to the brain. Sensory components of cranial nerves transmit information about smells, tastes, light, sounds, and body position. Motor components of cranial nerves control skeletal muscles of the face, tongue, eyeballs, throat, head, and shoulders. Motor components of cranial nerves also control the salivary glands and swallowing. Four of the 12 cranial nerves participate in both sensory and motor functions as mixed nerves, having both sensory and motor neurons.

Spinal nerves emanate from the spinal column between vertebrae. All of the spinal nerves are mixed nerves, containing both sensory and motor neurons. The areas of skin innervated by the 31 pairs of spinal nerves are shown in the figure below. These include sensory nerves in the skin that sense pressure, temperature, vibrations, and pain. Other sensory nerves are in the muscles, and they sense stretching and tension. Spinal nerves also include motor nerves that stimulate skeletal muscles to contract, allowing for voluntary body movements.

Autonomic Nervous System

The autonomic nervous system primarily senses the internal environment and controls involuntary activities. It is responsible for monitoring conditions in the internal environment and bringing about appropriate changes in them. In general, the autonomic nervous system is responsible for all the activities that go on inside your body without your conscious awareness or voluntary participation.

Structurally, the autonomic nervous system consists of sensory and motor nerves that run between the CNS (especially the hypothalamus in the brain), internal organs (such as the heart, lungs, and digestive organs), and glands (such as the pancreas and sweat glands). Sensory neurons in the autonomic system detect internal body conditions and send messages to the brain. Motor nerves in the autonomic system affect appropriate responses by controlling contractions of smooth or cardiac muscle, or glandular tissue. For example, when sensory nerves of the autonomic system detect a rise in body temperature, motor nerves signal smooth muscles in blood vessels near the body surface to undergo vasodilation, and the sweat glands in the skin to secrete more sweat to cool the body.

The autonomic nervous system, in turn, has three subdivisions: the sympathetic division, parasympathetic division, and enteric division. The first two subdivisions of the autonomic system are summarized in the figure below. Both affect the same organs and glands, but they generally do so in opposite ways.

• The sympathetic division controls the fight-or-flight response. Changes occur in organs and glands throughout the body that prepare the body to fight or flee in response to a perceived danger. For example, the heart rate speeds up, air passages in the lungs become wider, more blood flows to the skeletal muscles, and the digestive system temporarily shuts down.
• The parasympathetic division returns the body to normal after the fight-or-flight response has occurred. For example, it slows down the heart rate, narrows air passages in the lungs, reduces blood flow to the skeletal muscles, and stimulates the digestive system to start working again. The parasympathetic division also maintains internal homeostasis of the body at other times.
• The enteric division is made up of nerve fibres that supply the organs of the digestive system. This division allows for the local control of many digestive functions.

Disorders of the Peripheral Nervous System

Unlike the CNS — which is protected by bones, meninges, and cerebrospinal fluid — the PNS has no such protections. The PNS also has no blood-brain barrier to protect it from toxins and pathogens in the blood. Therefore, the PNS is more subject to injury and disease than is the CNS. Causes of nerve injury include diabetes, infectious diseases (such as shingles), and poisoning by toxins (such as heavy metals). PNS disorders often have symptoms like loss of feeling, tingling, burning sensations, or muscle weakness. If a traumatic injury results in a nerve being transected (cut all the way through), it may regenerate, but this is a very slow process and may take many months.

Two other diseases of the PNS are Guillain-Barre syndrome and Charcot-Marie-Tooth disease.

• Guillain-Barre syndrome is a rare disease in which the immune system attacks nerves of the PNS, leading to muscle weakness and even paralysis. The exact cause of Guillain-Barre syndrome is unknown, but it often occurs after a viral or bacterial infection. There is no known cure for the syndrome, but most people eventually make a full recovery. Recovery can be slow, however, lasting anywhere from several weeks to several years.
• Charcot-Marie-Tooth disease is a hereditary disorder of the nerves, and one of the most common inherited neurological disorders. It affects predominantly the nerves in the feet and legs, and often in the hands and arms, as well. The disease is characterized by loss of muscle tissue and sense of touch. It is presently incurable.

The autonomic nervous system is considered to be involuntary because it doesn't require conscious input. However, it is possible to exert some voluntary control over it. People who practice yoga or other so-called mind-body techniques, for example, can reduce their heart rate and certain other autonomic functions. Slowing down these otherwise involuntary responses is a good way to relieve stress and reduce the wear-and-tear that stress can place on the body. Such techniques may also be useful for controlling post-traumatic stress disorder and chronic pain. Three types of integrative practices for these purposes are breathing exercises, body-based tension modulation exercises, and mindfulness techniques.

Breathing exercises can help control the rapid, shallow breathing that often occurs when you are anxious or under stress. These exercises can be learned quickly, and they provide immediate feelings of relief. Specific breathing exercises include paced breath, diaphragmatic breathing, and Breathe2Relax or Chill Zone on MindShift™ CBT, which are downloadable breathing practice mobile applications, or "Apps". Try syncing your breathing with Eric Klassen's "Triangle breathing, 1 minute" video:

https://www.youtube.com/watch?v=u9Q8D6n-3qw

Triangle breathing, 1 minute, Erin Klassen, 2015.

Body-based tension modulation exercises include yoga postures (also known as “asanas”) and tension manipulation exercises. The latter include the Trauma/Tension Release Exercise (TRE) and the Trauma Resiliency Model (TRM). Watch this video for a brief — but informative — introduction to the TRE program:

https://www.youtube.com/watch?v=67R974D8swM&feature=youtu.be

TRE® : Tension and Trauma Releasing Exercises, an Introduction with Jessica Schaffer, Jessica Schaffer Nervous System RESET, 2015.

Mindfulness techniques have been shown to reduce symptoms of depression, as well as those of anxiety and stress. They have also been shown to be useful for pain management and performance enhancement. Specific mindfulness programs include Mindfulness Based Stress Reduction (MBSR) and Mindfulness Mind-Fitness Training (MMFT). You can learn more about MBSR by watching the video below.

https://www.youtube.com/watch?v=0TA7P-iCCcY&feature=youtu.be

Mindfulness-Based Stress Reduction (UMass Medical School, Center for Mindfulness), Palouse Mindfulness, 2017.

Attributions

Figure 8.6.1

Kid’s piant duet by PJMixer on Flickr is used under a CC BY-NC-ND 2.0 (https://creativecommons.org/licenses/by-nc-nd/2.0/) license.

Figure 8.6.2

Nervous_system_diagram by ¤~Persian Poet Gal  on Wikimedia Commons is released into the public domain (https://en.wikipedia.org/wiki/Public_domain).

Figure 8.6.3

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

Figure 8.6.4

Autonomic and Somatic Nervous System by Christinelmiller on Wikimedia Commons is used under a CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0) license.

Figure 8.6.5

Dermatoms.svg by Ralf Stephan (mailto:ralf@ark.in-berlin.de) on Wikimedia Commons is released into the public domain (https://en.wikipedia.org/wiki/Public_domain).

Figure 8.6.6

The_Autonomic_Nervous_System by Geo-Science-International on Wikimedia Commons is used and adapted by Christine Miller under a CC0 1.0 Universal
Public Domain Dedication license (https://creativecommons.org/publicdomain/zero/1.0/).

References

Erin Klassen. (2015, December 15). Triangle breathing, 1 minute. YouTube. https://www.youtube.com/watch?v=u9Q8D6n-3qw&feature=youtu.be

Jessica Schaffer Nervous System RESET. (2015, January 15). TRE® : Tension and trauma releasing exercises, an Introduction with Jessica Schaffer. YouTube. https://www.youtube.com/watch?v=67R974D8swM&feature=youtu.be

Mayo Clinic Staff. (n.d.). Charcot-Marie-Tooth disease [online article]. MayoClinic.org. https://www.mayoclinic.org/diseases-conditions/charcot-marie-tooth-disease/symptoms-causes/syc-20350517

Mayo Clinic Staff. (n.d.). Diabetes [online article]. MayoClinic.org. https://www.mayoclinic.org/diseases-conditions/diabetes/symptoms-causes/syc-20371444

Mayo Clinic Staff. (n.d.). Guillain-Barre syndrome [online article]. MayoClinic.org. https://www.mayoclinic.org/diseases-conditions/guillain-barre-syndrome/symptoms-causes/syc-20362793

Mayo Clinic Staff. (n.d.). Shingles [online article]. MayoClinic.org. https://www.mayoclinic.org/diseases-conditions/shingles/symptoms-causes/syc-20353054

Mayo Clinic Staff. (n.d.). Stroke [online article]. MayoClinic.org. https://www.mayoclinic.org/diseases-conditions/stroke/symptoms-causes/syc-20350113

Palouse Mindfulness. (2017, March 25).  Mindfulness-based stress reduction (UMass Medical School, Center for Mindfulness), YouTube. https://www.youtube.com/watch?v=0TA7P-iCCcY&feature=youtu.be

Plethrons, (2015, March 23). Phantom limbs explained. YouTube. https://www.youtube.com/watch?v=ySIDMU2cy0Y&feature=youtu.be

Reactions. (2015, December 1). Why do hot peppers cause pain? YouTube. https://www.youtube.com/watch?v=73yo5nJne6c&feature=youtu.be

 

 

 

The ability of an organism to maintain constant internal conditions despite external changes.

Created by CK-12 Foundation/Adapted by Christine Miller

This adorable nursing infant (Figure 9.4.1) is part of a positive feedback loop. When he suckles on the nipple, it sends nerve impulses to his mother’s hypothalamus. Those nerve impulses “tell” her pituitary gland to release the hormone prolactin into her bloodstream. Prolactin travels to the mammary glands in the breasts and stimulates milk production, which motivates the infant to keep suckling.

The pituitary gland is the master gland of the endocrine system, which is the system of glands that secrete hormones into the bloodstream. Endocrine hormones control virtually all physiological processes. They control growth, sexual maturation, reproduction, body temperature, blood pressure, and metabolism. The pituitary gland is considered the master gland of the endocrine system, because it controls the rest of the endocrine system. Many pituitary hormones either promote or inhibit hormone secretion by other endocrine glands.

The pituitary gland is about the size of a pea. It protrudes from the bottom of the hypothalamus at the base of the inner brain (see Figure 9.4.2). The pituitary is connected to the hypothalamus by a thin stalk (called the infundibulum). Blood vessels and nerves in the stalk allow direct connections between the hypothalamus and pituitary gland.

Anterior Lobe

The anterior pituitary is the lobe is at the front of the pituitary gland. It synthesizes and releases hormones into the blood. Table 9.4.1 shows some of the endocrine hormones released by the anterior pituitary, including their targets and effects.

Table 9.4.1

Endocrine Hormones Released by the Anterior Pituitary, and Their Targets and Effects.

Anterior Pituitary Hormone	Target	Effect
Adrenocorticotropic hormone (ACTH)	Adrenal glands	Stimulates the cortex of each adrenal gland to secrete its hormones.
Thyroid-stimulating hormone (TSH)	Thyroid gland	Stimulates the thyroid gland to secrete thyroid hormone.
Growth hormone (GH)	Body cells	Stimulates body cells to synthesize proteins and grow.
Follicle-stimulating hormone (FSH)	Ovaries, testes	Stimulates the ovaries to develop mature eggs.  stimulates the testes to produce sperm.
Luteinizing hormone (LH)	Ovaries, testes	Stimulates the ovaries and testes to secrete sex hormones; stimulates the ovaries to release eggs.
Prolactin (PRL)	Mammary glands	Stimulates the mammary glands to produce milk.

The anterior pituitary gland is regulated mainly by hormones from the hypothalamus. The hypothalamus secretes hormones (called releasing hormones and inhibiting hormones) that travel through capillaries directly to the anterior lobe of the pituitary gland. The hormones stimulate the anterior pituitary to either release or stop releasing particular pituitary hormones. Several of these hypothalamic hormones and their effects on the anterior pituitary are shown in the table below.

Table 9.4.2

Hypothalamic Hormones and Their Effects on the Anterior Pituitary

Hypothalamic Hormone	Effect on Anterior Pituitary
Thyrotropin releasing hormone (TRH)	Release of thyroid stimulating hormone (TSH)
Corticotropin releasing hormone (CRH)	Release of adrenocorticotropic hormone (ACTH)
Gonadotropin releasing hormone (GnRH)	Release of follicle-stimulating hormone (FSH) and luteinizing hormone (LH)
Growth hormone releasing hormone (GHRH)	Release of growth hormone (GH)
Growth hormone inhibiting hormone (GHIH) (Somatostatin)	Stopping of growth hormone release
Prolactin releasing hormone (PRH)	Release of prolactin
Prolactin inhibiting hormone (PIH) (Dopamine)	Stopping of prolactin release

Posterior Lobe

The posterior pituitary is the lobe is at the back of the pituitary gland. This lobe does not synthesize any hormones. Instead, the posterior lobe stores hormones that come from the hypothalamus along the axons of nerves connecting the two structures (also shown in Figure 9.4.2). The posterior pituitary then secretes the hormones into the bloodstream as needed. Hypothalamic hormones secreted by the posterior pituitary include vasopressin and oxytocin.

• Vasopressin (also called antidiuretic hormone, or ADH) helps maintain homeostasis in body water. It stimulates the kidneys to conserve water by producing more concentrated urine. Specifically, vasopressin targets ducts in the kidneys and makes them more permeable to water. This allows more water to be resorbed by the body, rather than excreted in urine.
• Oxytocin (OXY) targets cells in the uterus to stimulate uterine contractions, as in childbirth. It also targets cells in the breasts of a nursing mother to stimulate the letdown of milk.

Attributions

Figure 9.4.1

Breastfeeding by Petr Kratochvil  on Wikimedia Commons is used under a CC0 1.0 Universal
Public Domain Dedication (https://creativecommons.org/publicdomain/zero/1.0/deed.en) license.

Figure 9.4.2

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

References

Betts, J. G., Young, K.A., Wise, J.A., Johnson, E., Poe, B., Kruse, D.H., Korol, O., Johnson, J.E., Womble, M., DeSaix, P. (2013, June 19). Figure 17.7 Hypothalamus–pituitary complex [digital image]. In Anatomy and Physiology (Section 17.3). OpenStax. https://openstax.org/books/anatomy-and-physiology/pages/17-3-the-pituitary-gland-and-hypothalamus

Swedish. (2012, April 19). Common pituitary diseases. YouTube. https://www.youtube.com/watch?v=jUKQFkmBuww&feature=youtu.be

UCSF Neurosurgery. (2015, May 13). Diagnosing and treating pituitary tumors - California Center for Pituitary Disorders at UCSF. YouTube. https://www.youtube.com/watch?v=v41AJGP-XmI&feature=youtu.be

Image shows a diagram of Thrombocytes in their normal state and activated.
Thrombocytes (platelets) are typically ovoid during normal circulation, but when activated become super fibrous. The not activated platelets look like very smooth and the activated platelets look like sea anemones- lots of little projects sticking out of their surface.

Created by: CK-12/Adapted by Christine Miller

Oh, the Agony!

Wearing braces can be very uncomfortable, but it is usually worth it. Braces and other orthodontic treatments can re-align the teeth and jaws to improve bite and appearance. Braces can change the position of the teeth and the shape of the jaws because the human body is malleable. Many phenotypic traits — even those that have a strong genetic basis — can be molded by the environment. Changing the phenotype in response to the environment is just one of several ways we respond to environmental stress.

There are four different types of responses that humans may make to cope with environmental stress:

1. Adaptation
2. Developmental adjustment
3. Acclimatization
4. Cultural responses

The first three types of responses are biological in nature, and the fourth type is cultural. Only adaptation involves genetic change and occurs at the level of the population or species. The other three responses do not require genetic change, and they occur at the individual level.

An adaptation is a genetically-based trait that has evolved because it helps living things survive and reproduce in a given environment. Adaptations generally evolve in a population over many generations in response to stresses that last for a long period of time. Adaptations come about through natural selection. Those individuals who inherit a trait that confers an advantage in coping with an environmental stress are likely to live longer and reproduce more. As a result, more of their genes pass on to the next generation. Over many generations, the genes and the trait they control become more frequent in the population.

A Classic Example: Hemoglobin S and Malaria

Probably the most frequently-cited example of a genetic adaptation to an environmental stress is sickle cell trait. As you read in the previous section, people with sickle cell trait have one abnormal allele (S) and one normal allele (A) for hemoglobin, the red blood cell protein that carries oxygen in the blood. Sickle cell trait is an adaptation to the environmental stress of malaria, because people with the trait have resistance to this parasitic disease. In areas where malaria is endemic (present year-round), the sickle cell trait and its allele have evolved to relatively high frequencies.  It is a classic example of natural selection favoring heterozygotes for a gene with two alleles. This type of selection keeps both alleles at relatively high frequencies in a population.

To Taste or Not to Taste

Another example of an adaptation in humans is the ability to taste bitter compounds. Plants produce a variety of toxic compounds in order to protect themselves from being eaten, and these toxic compounds often have a bitter taste. The ability to taste bitter compounds is thought to have evolved as an adaptation, because it prevented people from eating poisonous plants. Humans have many different genes that code for bitter taste receptors, allowing us to taste a wide variety of bitter compounds.

A harmless bitter compound called phenylthiocarbamide (PTC) is not found naturally in plants, but it is similar to toxic bitter compounds that are found in plants. Humans' ability to taste this harmless substance has been tested in many different populations. In virtually every population studied, there are some people who can taste PTC (called tasters), and some people who cannot taste PTC, (called nontasters). The ratio of tasters to non-tasters varies among populations, but on average, 75 per cent of people can taste PTC and 25 per cent cannot.

Like many scientific discoveries, human variation in PTC-taster status was discovered by chance. Around 1930, a chemist named Arthur Fox was working with powdered PTC in his lab. Some of the powder accidentally blew into the air. Another lab worker noticed that the powdered PTC tasted bitter, but Fox couldn't detect any taste at all. Fox wondered how to explain this difference in PTC-tasting ability. Geneticists soon determined that PTC-taster status is controlled by a single gene with two common alleles, usually represented by the letters T and t. The T allele encodes a chemical receptor protein (found in taste buds on the tongue, as illustrated in Figure 6.4.2) that can strongly bind to PTC. The other allele, t, encodes a version of the receptor protein that cannot bind as strongly to PTC. The particular combination of these two alleles that a person inherits determines whether the person finds PTC to taste very bitter (TT), somewhat bitter (Tt), or not bitter at all (tt).

 

If the ability to taste bitter compounds is advantageous, why does every human population studied contain a significant percentage of people who are nontasters? Why has the nontasting allele been preserved in human populations at all? Some scientists hypothesize that the nontaster allele actually confers the ability to taste some other, yet-to-be identified, bitter compound in plants. People who inherit both alleles would presumably be able to taste a wider range of bitter compounds, so they would have the greatest ability to avoid plant toxins. In other words, the heterozygote genotype for the taster gene would be the most fit and favored by natural selection.

Most people no longer have to worry whether the plants they eat contain toxins. The produce you grow in your garden or buy at the supermarket consists of known varieties that are safe to eat. However, natural selection may still be at work in human populations for the PTC-taster gene, because PTC tasters may be more sensitive than nontasters to bitter compounds in tobacco and vegetables in the cabbage family (that is, cruciferous vegetables, such as the broccoli, cauliflower, and cabbage pictured in Figure 6.4.3).

• People who find PTC to taste very bitter are less likely to smoke tobacco, presumably because tobacco smoke has a stronger bitter taste to these individuals. In this case, selection would favor taster genotypes, because tasters would be more likely to avoid smoking and its serious health risks.
• Strong tasters find cruciferous vegetables to taste bitter. As a result, they may avoid eating these vegetables (and perhaps other foods, as well), presumably resulting in a diet that is less varied and nutritious. In this scenario, natural selection might work against taster genotypes.

Figure 6.4.3 Cruciferous vegetables.

It takes a relatively long time for genetic change in response to environmental stress to produce a population with adaptations. Fortunately, we can adjust to some environmental stresses more quickly by changing in nongenetic ways. One type of nongenetic response to stress is developmental adjustment. This refers to phenotypic change that occurs during development in infancy or childhood, and that may persist into adulthood. This type of change may be irreversible by adulthood.

Phenotypic Plasticity

Developmental adjustment is possible because humans have a high degree of phenotypic plasticity, which is the ability to alter the phenotype in response to changes in the environment. Phenotypic plasticity allows us to respond to changes that occur within our lifetime, and it is particularly important for species (like our own) that have a long generation time. With long generations, evolution of genetic adaptations may occur too slowly to keep up with changing environmental stresses.

Developmental Adjustment and Cultural Practices

Developmental adjustment may be the result of naturally occurring environmental stresses or cultural practices, including medical or dental treatments. Like our example at the beginning of this section, using braces to change the shape of the jaw and the position of the teeth is an example of a dental practice that brings about a developmental adjustment. Another example of developmental adjustment is the use of a back brace to treat scoliosis (see images in Figure 6.4.4). Scoliosis is an abnormal curvature from side to side in the spine. If the problem is not too severe, a brace, if worn correctly, should prevent the curvature from worsening as a child grows, although it cannot straighten a curve that is already present. Surgery may be required to do that.

Developmental Adjustment and Nutritional Stress

An important example of developmental adjustment that results from a naturally occurring environmental stress is the cessation of physical growth that occurs in children who are under nutritional stress. Children who lack adequate food to fuel both growth and basic metabolic processes are likely to slow down in their growth rate — or even to stop growing entirely. Shunting all available calories and nutrients into essential life functions may keep the child alive at the expense of increasing body size.

Table 6.4.1 shows the effects of inadequate diet on children's' growth in several countries worldwide. For each country, the table gives the prevalence of stunting in children under the age of five. Children are considered stunted if their height is at least two standard deviations below the median height for their age in an international reference population.

Table 6.4.1

Percentage of Stunting in Young Children in Selected Countries (2011-2015)

Percentage of Stunting in Young Children in Selected Countries (2011-2015)
Country	Per cent of Children Under Age 5 with Stunting
United States	2.1
Turkey	9.5
Mexico	13.6
Thailand	16.3
Iraq	22.6
Philippines	33.6
Pakistan	45.0
Papua New Guinea	49.5

After a growth slow-down occurs and if adequate food becomes available, a child may be able to make up the loss of growth. If food is plentiful, the child may grow more rapidly than normal until the original, genetically-determined growth trajectory is reached. If the inadequate diet persists, however, the failure of growth may become chronic, and the child may never reach his or her full potential adult size.

Phenotypic plasticity of body size in response to dietary change has been observed in successive generations within populations. For example, children in Japan were taller, on average, in each successive generation after the end of World War II. Boys aged 14-15 years old in 1986 were an average of about 18 cm (7 in.) taller than boys of the same age in 1959, a generation earlier. This is a highly significant difference, and it occurred too quickly to be accounted for by genetic change. Instead, the increase in height is a developmental adjustment, thought to be largely attributable to changes in the Japanese diet since World War II. During this period, there was an increase in the amount of animal protein and fat, as well as in the total calories consumed.

Other responses to environmental stress are reversible and not permanent, whether they occur in childhood or adulthood. The development of reversible changes to environmental stress is called acclimatization. Acclimatization generally develops over a relatively short period of time. It may take just a few days or weeks to attain a maximum response to a stress. When the stress is no longer present, the acclimatized state declines, and the body returns to its normal baseline state. Generally, the shorter the time for acclimatization to occur, the more quickly the condition is reversed when the environmental stress is removed.

Acclimatization to UV Light

A common example of acclimatization is tanning of the skin (see Figure 6.4.5). This occurs in many people in response to exposure to ultraviolet radiation from the sun. Special pigment cells in the skin, called melanocytes, produce more of the brown pigment melanin when exposed to sunlight. The melanin collects near the surface of the skin where it absorbs UV radiation so it cannot penetrate and potentially damage deeper skin structures. Tanning is a reversible change in the phenotype that helps the body deal temporarily with the environmental stress of high levels of UV radiation. When the skin is no longer exposed to the sun’s rays, the tan fades, generally over a period of a few weeks or months.

Figure 6.4.5 Tanning of the skin occurs in many people in response to exposure to ultraviolet radiation from the sun.

Another common example of acclimatization occurs in response to heat. Changes that occur with heat acclimatization include increased sweat output and earlier onset of sweat production, which helps the body stay cool because evaporation of sweat takes heat from the body’s surface in a process called evaporative cooling. It generally takes a couple of weeks for maximum heat acclimatization to come about by gradually working out harder and longer at high air temperatures. The changes that occur with acclimatization just as quickly subside when the body is no longer exposed to excessive heat.

Acclimatization to High Altitude

Short term acclimatization to high altitude occurs as a response to low levels of oxygen in the blood.  This reduced level of oxygen is detected by carotid bodies, which will trigger in increase in breathing and heart rate.  Over a period of weeks the body will compensate by increasing red blood cell production, thereby improving the oxygen-carrying capacity of the blood.  This is why mountaineers wishing to climb to the peak of Mount Everest must complete the full climb in portions; it is recommended that climbers spend 2-3 days acclimatizing for every 600 metres of elevation increase.  In addition, the higher to altitude, the longer it make take to acclimatize; climbers are advised to spend 4-5 days acclimatizing at base camp (whether the base camp in Nepal or China) before completing the final leg of the climb to the peak.  The concentration of red blood cells gradually decreases to normal levels once a climber returns to their normal elevation.

More than any other species, humans respond to environmental stresses with learned behaviors and technology. These cultural responses allow us to change our environments to control stresses, rather than changing our bodies genetically or physiologically to cope with the stresses. Even archaic humans responded to some environmental stresses in this way. For example, Neanderthals used shelters, fires, and animal hides as clothing to stay warm in the cold climate in Europe during the last ice age. Today, we use more sophisticated technologies to stay warm in cold climates while retaining our essentially tropical-animal anatomy and physiology. We also use technology (such as furnaces and air conditioners) to avoid temperature stress and stay comfortable in hot or cold climates.

 

Attributions

Figure 6.4.1

Free_Awesome_Girl_With_Braces_Close_Up by D. Sharon Pruitt from Hill Air Force Base, Utah, USA on Wikimedia Commons is used under a  CC BY 2.0 (https://creativecommons.org/licenses/by/2.0/deed.en) license.

Figure 6.4.2

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

Figure 6.4.3

• White cauliflower on brown wooden chopping board by Louis Hansel @shotsoflouis on Unsplash is used under the Unsplash License (https://unsplash.com/license).
• Broccoli on wooden chopping board by Louis Hansel @shotsoflouis on Unsplash is used under the Unsplash License (https://unsplash.com/license).
• Green cabbage close up by Craig Dimmick on Unsplash is used under the Unsplash License (https://unsplash.com/license).
• Cabbage hybrid/ brussel sprouts by Solstice Hannan on Unsplash is used under the Unsplash License (https://unsplash.com/license).
• Kale by Laura Johnston on Unsplash is used under the Unsplash License (https://unsplash.com/license).
• Tiny bok choy at the Asian market by Jodie Morgan on Unsplash is used under the Unsplash License (https://unsplash.com/license).

Figure 6.4.4

Scoliosis_patient_in_cheneau_brace_correcting_from_56_to_27_deg by Weiss H.R. from Scoliosis Journal/BioMed Central Ltd. on Wikimedia Commons is used under a CC BY 2.0 (https://creativecommons.org/licenses/by/2.0) license.

Figure 6.4.5

• Tan Lines by k.steudel on Flickr is used under a CC BY 2.0 (https://creativecommons.org/licenses/by/2.0/) license.
• Twin tan lines (all sizes) by Quinn Dombrowski on Flickr is used under a CC BY-SA 2.0 (https://creativecommons.org/licenses/by-sa/2.0/) license.
• Wedding ring tan line by Quinn Dombrowski on Flickr is used under a CC BY-SA 2.0 (https://creativecommons.org/licenses/by-sa/2.0/) license.
• Tan by Evil Erin on Flickr is used under a CC BY 2.0 (https://creativecommons.org/licenses/by/2.0/) license.

Figure 6.4.6

Nepalese base camp by Mark Horrell on Flickr is used under a CC BY-NC-SA 2.0 (https://creativecommons.org/licenses/by-nc-sa/2.0/) license.

References

TED-Ed. (2016, October 4). Could we survive prolonged space travel? - Lisa Nip. YouTube. https://www.youtube.com/watch?v=upp9-w6GPhU&feature=youtu.be

TED-Ed. (2019, May 6). How this disease changes the shape of your cells - Amber M. Yates. YouTube. https://www.youtube.com/watch?v=hRnrIpUMyZQ&feature=youtu.be

Weiss, H. (2007). Is there a body of evidence for the treatment of patients with Adolescent Idiopathic Scoliosis (AIS)? [Figure 2 - digital photograph],  Scoliosis, 2(19). https://doi.org/10.1186/1748-7161-2-19

Created by: CK-12/Adapted by Christine Miller

Giving the Gift of Life

Did you ever donate blood? If you did, then you probably know that your blood type is an important factor in blood transfusions. People vary in the type of blood they inherit, and this determines which type(s) of blood they can safely receive in a transfusion. Do you know your blood type?

Blood is composed of cells suspended in a liquid called plasma. There are three types of cells in blood: red blood cells, which carry oxygen; white blood cells, which fight infections and other threats; and platelets, which are cell fragments that help blood clot. Blood type (or blood group) is a genetic characteristic associated with the presence or absence of certain molecules, called antigens, on the surface of red blood cells. These molecules may help maintain the integrity of the cell membrane, act as receptors, or have other biological functions. A blood group system refers to all of the gene(s), alleles, and possible genotypes and phenotypes that exist for a particular set of blood type antigens. Human blood group systems include the well-known ABO and Rhesus (Rh) systems, as well as at least 33 others that are less well known.

Antigens and Antibodies

Antigens — such as those on the red blood cells — are molecules that the immune system identifies as either self (produced by your own body) or non-self (not produced by your own body). Blood group antigens may be proteinsno post, carbohydrates, glycoproteins (proteins attached to chains of sugars), or glycolipids (lipids attached to chains of sugars), depending on the particular blood group system. If antigens are identified as non-self, the immune system responds by forming antibodies that are specific to the non-self antigens. Antibodies are large, Y-shaped proteins produced by the immune system that recognize and bind to non-self antigens. The analogy of a lock and key is often used to represent how an antibody and antigen fit together, as shown in the illustration below (Figure 6.5.2). When antibodies bind to antigens, it marks them for destruction by other immune system cells. Non-self antigens may enter your body on pathogens (such as bacteria or viruses), on foods, or on red blood cells in a blood transfusion from someone with a different blood type than your own. The last way is virtually impossible nowadays because of effective blood typing and screening protocols.

Genetics of Blood Type

An individual’s blood type depends on which alleles for a blood group system were inherited from their parents. Generally, blood type is controlled by alleles for a single gene, or for two or more very closely linked genes. Closely linked genes are almost always inherited together, because there is little or no recombination between them. Like other genetic traits, a person’s blood type is generally fixed for life, but there are rare instances in which blood type can change. This could happen, for example, if an individual receives a bone marrow transplant to treat a disease, such as leukemia. If the bone marrow comes from a donor who has a different blood type, the patient’s blood type may eventually convert to the donor’s blood type, because red blood cells are produced in bone marrow.

The ABO blood group system is the best known human blood group system. Antigens in this system are glycoproteins. These antigens are shown in the list below. There are four common blood types for the ABO system:

1. Type A, in which only the A antigen is present.
2. Type B, in which only the B antigen is present.
3. Type AB, in which both the A and B antigens are present.
4. Type O, in which neither the A nor the B antigen is present.

Genetics of the ABO System

The ABO blood group system is controlled by a single gene on chromosome 9. There are three common alleles for the gene, often represented by the letters A , B , and O. With three alleles, there are six possible genotypes for ABO blood group. Alleles A and B, however, are both dominant to allele O and codominant to each other. This results in just four possible phenotypes (blood types) for the ABO system. These genotypes and phenotypes are shown in Table 6.5.1.

Table 6.5.1

ABO Blood Group System: Genotypes and Phenotypes

ABO Blood Group System
Genotype	Phenotype (Blood Type, or Group)
AA	A
AO	A
BB	B
BO	B
OO	O
AB	AB

The diagram below (Figure 6.5.3) shows an example of how ABO blood type is inherited. In this particular example, the father has blood type A (genotype AO) and the mother has blood type B (genotype BO). This mating type can produce children with each of the four possible ABO phenotypes, although in any given family, not all phenotypes may be present in the children.

The ABO system is the most important blood group system in blood transfusions. If red blood cells containing a particular ABO antigen are transfused into a person who lacks that antigen, the person’s immune system will recognize the antigen on the red blood cells as non-self. Antibodies specific to that antigen will attack the red blood cells, causing them to agglutinate (or clump) and break apart. If a unit of incompatible blood were to be accidentally transfused into a patient, a severe reaction (called acute hemolytic transfusion reaction) is likely to occur, in which many red blood cells are destroyed. This may result in kidney failure, shock, and even death. Fortunately, such medical accidents virtually never occur today.

These antibodies are often spontaneously produced in the first years of life, after exposure to common microorganisms in the environment that have antigens similar to blood antigens. Specifically, a person with type A blood will produce anti-B antibodies, while a person with type B blood will produce anti-A antibodies. A person with type AB blood does not produce either antibody, while a person with type O blood produces both anti-A and anti-B antibodies. Once the antibodies have been produced, they circulate in the plasma. The relationship between ABO red blood cell antigens and plasma antibodies is shown in Figure 6.5.4.

 

Which blood types are compatible and which are not? Type O blood contains both anti-A and anti-B antibodies, so people with type O blood can only receive type O blood. However, they can donate blood to people of any ABO blood type, which is why individuals with type O blood are called universal donors. Type AB blood contains neither anti-A nor anti-B antibodies, so people with type AB blood can receive blood from people of any ABO blood type. That’s why individuals with type AB blood are called universal recipients. They can donate blood, however, only to people who also have type AB blood. These and other relationships between blood types of donors and recipients are summarized in the simple diagram to the right.

Geographic Distribution of ABO Blood Groups

The frequencies of blood groups for the ABO system vary around the world. You can see how the A and B alleles and the blood group O are distributed geographically on the maps in Figure 6.5.6.

• Worldwide, B is the rarest ABO allele, so type B blood is the least common ABO blood type. Only about 16 per cent of all people have the B allele. Its highest frequency is in Asia. Its lowest frequency is among the indigenous people of Australia and the Americas.
• The A allele is somewhat more common around the world than the B allele, so type A blood is also more common than type B blood. The highest frequencies of the A allele are in Australian Aborigines, the Lapps (Sami) of Northern Scandinavia, and Blackfoot Native Americans in North America. The allele is nearly absent among Native Americans in Central and South America.
• The O allele is the most common ABO allele around the world, and type O blood is the most common ABO blood type. Almost two-thirds of people have at least one copy of the O allele. It is especially common in Native Americans in Central and South America, where it reaches frequencies close to 100 per cent. It also has relatively high frequencies in Australian Aborigines and Western Europeans. Its frequencies are lowest in Eastern Europe and Central Asia.

Evolution of the ABO Blood Group System

The geographic distribution of ABO blood type alleles provides indirect evidence for the evolutionary history of these alleles. Evolutionary biologists hypothesize that the allele for blood type A evolved first, followed by the allele for blood type O, and then by the allele for blood type B. This chronology accounts for the percentages of people worldwide with each blood group, and is also consistent with known patterns of early population movements.

The evolutionary forces of founder effect and genetic drift have no doubt played a significant role in the current distribution of ABO blood types worldwide. Geographic variation in ABO blood groups is also likely to be influenced by natural selection, because different blood types are thought to vary in their susceptibility to certain diseases. For example:

• People with type O blood may be more susceptible to cholera and plague. They are also more likely to develop gastrointestinal ulcers.
• People with type A blood may be more susceptible to smallpox and more likely to develop certain cancers.
• People with types A, B, and AB blood appear to be less likely to form blood clots that can cause strokes. However, early in our history, the ability of blood to form clots — which appears greater in people with type O blood — may have been a survival advantage.
• Perhaps the greatest natural selective force associated with ABO blood types is malaria. There is considerable evidence to suggest that people with type O blood are somewhat resistant to malaria, giving them a selective advantage where malaria is endemic.

Another well-known blood group system is the Rhesus (Rh) blood group system. The Rhesus system has dozens of different antigens, but only five main antigens (called D, C, c, E, and e). The major Rhesus antigen is the D antigen. People with the D antigen are called Rh positive (Rh+), and people who lack the D antigen are called Rh negative (Rh-). Rhesus antigens are thought to play a role in transporting ions across cell membranes by acting as channel proteins.

The Rhesus blood group system is controlled by two linked genes on chromosome 1. One gene, called RHD, produces a single antigen, antigen D. The other gene, called RHCE, produces the other four relatively common Rhesus antigens (C, c, E, and e), depending on which alleles for this gene are inherited.

Rhesus Blood Group and Transfusions

After the ABO system, the Rhesus system is the second most important blood group system in blood transfusions. The D antigen is the one most likely to provoke an immune response in people who lack the antigen. People who have the D antigen (Rh+) can be safely transfused with either Rh+ or Rh- blood, whereas people who lack the D antigen (Rh-) can be safely transfused only with Rh- blood.

Unlike anti-A and anti-B antibodies to ABO antigens, anti-D antibodies for the Rhesus system are not usually produced by sensitization to environmental substances. People who lack the D antigen (Rh-), however, may produce anti-D antibodies if exposed to Rh+ blood. This may happen accidentally in a blood transfusion, although this is extremely unlikely today. It may also happen during pregnancy with an Rh+ fetus if some of the fetal blood cells pass into the mother’s blood circulation.

Hemolytic Disease of the Newborn

If a woman who is Rh- is carrying an Rh+ fetus, the fetus may be at risk. This is especially likely if the mother has formed anti-D antibodies during a prior pregnancy because of a mixing of maternal and fetal blood during childbirth. Unlike antibodies against ABO antigens, antibodies against the Rhesus D antigen can cross the placenta and enter the blood of the fetus. This may cause hemolytic disease of the newborn (HDN), also called erythroblastosis fetalis, an illness in which fetal red blood cells are destroyed by maternal antibodies, causing anemia. This illness may range from mild to severe. If it is severe, it may cause brain damage and is sometimes fatal for the fetus or newborn. Fortunately, HDN can be prevented by preventing the formation of anti-D antibodies in the Rh- mother. This is achieved by injecting the mother with a medication called Rho(D) immune globulin.

Geographic Distribution of Rhesus Blood Types

The majority of people worldwide are Rh+, but there is regional variation in this blood group system, as there is with the ABO system. The aboriginal inhabitants of the Americas and Australia originally had very close to 100 per cent Rh+ blood. The frequency of the Rh+ blood type is also very high in African populations, at about 97 to 99 per cent. In East Asia, the frequency of Rh+ is slightly lower, at about 93 to 99 per cent. Europeans have the lowest frequency of the Rh+ blood type at about 83 to 85 per cent.

What explains the population variation in Rhesus blood types? Prior to the advent of modern medicine, Rh+ positive children conceived by Rh- women were at risk of fetal or newborn death or impairment from HDN. This was an enigma, because presumably, natural selection would work to remove the rarer phenotype (Rh-) from populations. However, the frequency of this phenotype is relatively high in many populations.

Recent studies have found evidence that natural selection may actually favor heterozygotes for the Rhesus D antigen. The selective agent in this case is thought to be toxoplasmosis, a parasitic disease caused by the protozoan Toxoplasma gondii, which is very common worldwide. You can see a life cycle diagram of the parasite in Figure 6.5.7. Infection by this parasite often causes no symptoms at all, or it may cause flu-like symptoms for a few days or weeks. Exposure to the parasite has been linked, however, to increased risk of mental disorders (such as schizophrenia), neurological disorders (such as Alzheimer’s), and other neurological problems, including delayed reaction times. One study found that people who tested positive for antibodies to the parasite were more than twice as likely to be involved in traffic accidents.

 

 

 

Attributes

Figure 6.5.1

Following the Blood Donation Trail by EJ Hersom/ USA Department of Defense is in the public domain. [Disclaimer: The appearance of U.S. Department of Defense (DoD) visual information does not imply or constitute DoD endorsement.]

Figure 6.5.2

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

Figure 6.5.3

ABO system codominance.svg, adapted by YassineMrabet (original "Codominant" image from US National Library of Medicine) on Wikimedia Commons is in the public domain (https://en.wikipedia.org/wiki/Public_domain).

Figure 6.5.4

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

Figure 6.5.5

Blood Donor and recipient ABO by CK-12 Foundation is used under a CC BY-NC 3.0 (https://creativecommons.org/licenses/by-nc/3.0/) license.

Figure 6.5.6

• Map of Blood Group A by Muntuwandi at en.wikipedia on Wikimedia Commons is used under a CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0/) license.
• Map of Blood Group B by Muntuwandi at en.wikipedia on Wikimedia Commons is used under a CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0/) license.
• Map of Blood Group O by anthro palomar at en.wikipedia on Wikimedia Commons is used under a CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0/) license.

Figure 6.5.7

Toxoplasma_gondii_Life_cycle_PHIL_3421_lores by Alexander J. da Silva, PhD/Melanie Moser, Centers for Disease Control and Prevention's Public Health Image Library (PHIL#3421) on Wikimedia Commons is in the public domain (https://en.wikipedia.org/wiki/Public_domain).

Table 6.5.1 

ABO Blood Group System: Genotypes and Phenotypes was created by Christine Miller.

 

References

Dean, L. (2005). Chapter 4 Hemolytic disease of the newborn. In Blood Groups and Red Cell Antigens [Internet]. National Center for Biotechnology Information (US). https://www.ncbi.nlm.nih.gov/books/NBK2266/

Mayo Clinic Staff. (n.d.). Toxoplasmosis [online article]. MayoClinic.org. https://www.mayoclinic.org/diseases-conditions/toxoplasmosis/symptoms-causes/syc-20356249

MedlinePlus. (2019, January 29). Hemolytic transfusion reaction [online article]. U.S. National Library of Medicine. https://en.wikipedia.org/w/index.php?title=Chromosome_9&oldid=946440619

TED-Ed. (2015, June 29). Why do blood types matter? - Natalie S. Hodge. YouTube. https://www.youtube.com/watch?v=xfZhb6lmxjk&feature=youtu.be

TED-Ed. (2020, February 18). How do blood transfusions work? - Bill Schutt. YouTube. https://www.youtube.com/watch?v=qcZKbjYyOfE&feature=youtu.be

Wikipedia contributors. (2020, May 10). Chromosome 1. In Wikipedia.  https://en.wikipedia.org/w/index.php?title=Chromosome_1&oldid=955942444

Wikipedia contributors. (2020, March 20). Chromosome 9. In Wikipedia.  https://en.wikipedia.org/w/index.php?title=Chromosome_9&oldid=946440619

 

 

The minimum energy required to cause a reaction to occur.

A sequence of nucleotides in DNA or RNA that codes for a molecule that has a function.

Genes causing a trait or disorder which are present on the X sex determining chromosome.

Genes causing a trait or disorder which are present on the X sex determining chromosome.

The shape formed by two parallel lines that twist around each other.

A type of immune cell that has granules (small particles) with enzymes that are released during infections, allergic reactions, and asthma. An eosinophil is a type of white blood cell and a type of granulocyte.

A substance that takes part in and undergoes change during a chemical reaction.

A rigid organ that constitutes part of the vertebrate skeleton in animals.

A lipid. Cholesterol and its derivatives are important constituents of cell membranes and precursors of other steroid compounds, but a high proportion in the blood of low-density lipoprotein (which transports cholesterol to the tissues) is associated with an increased risk of coronary heart disease.

The process by which information from a gene is used in the synthesis of a functional protein.

Created by CK-12 Foundation/Adapted by Christine Miller

Seeing Is Believing

At first glance, Figure 8.7.1 appears to be just random dots of colour, but hidden within it is the three-dimensional shape of a bee. Can you see it among the dots? This figure is an example of a stereogram, which is a two-dimensional picture that, when viewed correctly, reveals a three-dimensional object. If you can’t see the hidden image, it doesn’t mean that there is anything wrong with your eyes. It’s all in how your brain interprets what your eyes are sensing. The eyes are special sensory organs, and vision is one of our special senses.

The human body has two basic types of senses, called special senses and general senses. Special senses have specialized sense organs that gather sensory information and change it into nerve impulses. Special senses include vision (for which the eyes are the specialized sense organs), hearing (ears), balance (ears), taste (tongue), and smell (nasal passages). General senses, in contrast, are all associated with the sense of touch. They lack special sense organs. Instead, sensory information about touch is gathered by the skin and other body tissues, all of which have important functions besides gathering sense information. Whether the senses are special or general, however, they all depend on cells called sensory receptors.

A sensory receptor is a specialized nerve cell that responds to a stimulus in the internal or external environment by generating a nerve impulse. The nerve impulse then travels along the sensory (afferent) nerve to the central nervous system for processing and to form a response.

There are several different types of sensory receptors that respond to different kinds of stimuli:

• Mechanoreceptors respond to mechanical forces, such as pressure, roughness, vibration, and stretching. Most mechanoreceptors are found in the skin and are needed for the sense of touch. Mechanoreceptors are also found in the inner ear, where they are needed for the senses of hearing and balance.
• Thermoreceptors respond to variations in temperature. They are found mostly in the skin and detect temperatures that are above or below body temperature.
• Nociceptors respond to potentially damaging stimuli, which are generally perceived as pain. They are found in internal organs, as well as on the surface of the body. Different nociceptors are activated depending on the particular stimulus. Some detect damaging heat or cold, others detect excessive pressure, and still others detect painful chemicals (such as very hot spices in food).
• Photoreceptors detect and respond to light. Most photoreceptors are found in the eyes and are needed for the sense of vision.
• Chemoreceptors respond to certain chemicals. They are found mainly in taste buds on the tongue — where they are needed for the sense of taste — and in nasal passages, where they are needed for the sense of smell.

Touch is the ability to sense pressure, vibration, temperature, pain, and other tactile stimuli. These types of stimuli are detected by mechanoreceptors, thermoreceptors, and nociceptors all over the body, most noticeably in the skin. These receptors are especially concentrated on the tongue, lips, face, palms of the hands, and soles of the feet. Various types of tactile receptors in the skin are shown in Figure 8.7.2.

Vision (or sight) is the ability to sense light and see. The eye is the special sensory organ that collects and focuses light and forms images. The eye, however, is not sufficient for us to see. The brain also plays a necessary role in vision.  Vision is our primary sense and more than 50 per cent of the cerebral cortex is devoted to processing visual information.  A person with normal colour vision can differentiate between hundreds of thousands of different colours, hues, and shades.

How the Eye Works

Figure 8.7.4 (below) shows the anatomy of the human eye in cross-section. The eye gathers and focuses light to form an image, and then changes the image to nerve impulses that travel to the brain. The eye's functions are summarized in the following steps.

1. Light passes first through the cornea, which is a clear outer layer that protects the eye and helps to focus the light by refracting (or bending) it.
2. Next, light enters the interior of the eye through an opening called the pupil. The size of this opening is controlled by the coloured part of the eye (called the iris), which adjusts the size based on the brightness of the light. The iris causes the pupil to narrow in bright light and widen in dim light.  Filling the space between the cornea and the iris is a semi-gelatinous fluid called aqueous humor and functions to maintain the shape of the eye.
3. The light then passes through the lens, which refracts the light even more and focuses it on the retina at the back of the eye, as an inverted image. Sitting behind the lens is a gelatinous fluid called vitreous humor, which functions to maintain the shape of the eye.
4. The retina contains two types of photoreceptors: rod and cone cells . Rod cells, which are found mainly in all areas of the retina other than the very center, are particularly sensitive to low levels of light. Cone cells, which are found mainly in the center of the retina, are sensitive to light of different colours, and allow colour vision. The rods and cones convert the light that strikes them to nerve impulses.
5. The nerve impulses from the rods and cones travel to the optic nerve via the optic disc (also known as the optic nerve), which is a circular area at the back of the eye where the optic nerve connects to the retina.

Colour Vision

Humans have colour vision because we have three types of cone cells:  blue, green and red.  Each of these types of cone cell detects a specific wavelength of light, for which they are named.  The combined stimulus  is then perceived as a specific colour, based on the ratio of the amount stimulus coming from each of the three types of cone cells.  Do you know what else uses these same three pieces of information to communicate colour?  Your computer monitor!  When working in a creative program, such as Paint, these three reference points of red (R), green (G), and blue (B), can be used to create any of the million colours the human eye can perceive, as illustrated in Figure 8.7.5. Take a look at each of the numerical values for red, green, and blue and what colour their combined values create:

Figure 8.7.5 RGB colours. 

Role of the Brain in Vision

The optic nerves from both eyes meet and cross just below the bottom of the hypothalamus in the brain. The information from both eyes is sent to the visual cortex in the occipital lobe of the cerebrum, which is part of the cerebral cortex. The visual cortex is the largest system in the human brain, and is responsible for processing visual images. It interprets messages from both eyes and “tells” us what we are seeing.

Vision Problems

Vision problems are very common. Two of the most common are myopia and hyperopia, and they often start in childhood or adolescence. Another common problem, called presbyopia, occurs in most people, beginning in middle adulthood. In all three conditions, the eyes fail to focus images correctly on the retina, resulting in blurred vision.

Myopia

Myopia (or nearsightedness) occurs when the light that comes into the eye does not directly focus on the retina, but in front of it, as shown in Figure 8.7.7. As a result, distant objects may appear out of focus, but the focus of close objects is not affected. Myopia may occur because the eyeball is elongated from front to back, or because the cornea is too curved. Myopia can be corrected with the use of corrective lenses, either eyeglasses or contact lenses. Myopia can also be corrected by refractive surgery performed with a laser.

Hyperopia

Hyperopia (or farsightedness) happens when the light coming into the eye does not directly focus on the retina but behind it, as shown in Figure 8.7.8. This causes close objects to appear out of focus, but does not affect the focus of distant objects. Hyperopia may occur because the eyeball is too short from front to back, or because the lens is not curved enough. Hyperopia can be corrected through the use of corrective lenses or laser surgery.

Presbyopia

Presbyopia is a vision problem associated with aging, in which the eye gradually loses its ability to focus on close objects. The precise origin of presbyopia is not known for certain, but evidence suggests that the lens may become less elastic with age, causing the muscles that control the lens to lose power as people grow older. The first signs of presbyopia — eyestrain, difficulty seeing in dim light, problems focusing on small objects and fine print — are usually first noticed between the ages of 40 and 50. Most older people with this problem use corrective lenses to focus on close objects, because surgical procedures to correct presbyopia have not been as successful as those for myopia and hyperopia.

Hearing is the ability to sense sound waves, and the ear is the organ that senses sound. Sound waves enter the ear through the ear canal and travel to the eardrum (see the diagram of the ear Figure 8.7.9). The sound waves strike the eardrum, and make it vibrate. The vibrations then travel through the three tiny bones (incus, malleus and stapes) of the middle ear, which amplify the vibrations. From the middle ear, the vibrations pass to the cochlea in the inner ear. The cochlea is a coiled tube filled with liquid. The liquid moves in response to the vibrations, causing tiny hair cells(which are mechanoreceptors) lining the cochlea to bend. In response, the hair cells send nerve impulses to the auditory nerve, which carries the impulses to the brain. The brain interprets the impulses and “tells” us what we are hearing.

The ears are also responsible for the sense of balance. Balance is the ability to sense and maintain an appropriate body position. The semicircular canals inside the ear (see the figure above) contain fluid that moves when the head changes position. Tiny hairs lining the semicircular canals sense movement of the fluid. In response, they send nerve impulses to the vestibular nerve, which carries the impulses to the brain. The brain interprets the impulses and sends messages to the peripheral nervous system, which triggers contractions of skeletal muscles as needed to maintain balance.

Taste and smellno post are both abilities to sense chemicals, so both taste and olfactory (odor) receptors are chemoreceptors. Both types of chemoreceptors send nerve impulses to the brain along sensory nerves, and the brain “tells” us what we are tasting or smelling.

Taste receptors are found in tiny bumps on the tongue called taste budsno post.You can see a diagram of a taste receptor cell and related structures in Figure 8.7.10. Taste receptor cells make contact with chemicals in food through tiny openings called taste pores. When certain chemicals bind with taste receptor cells, it generates nerve impulses that travel through afferent nerves to the CNS. There are separate taste receptors for sweet, salty, sour, bitter, and meaty tastes. The meaty — or savory — taste is called umami.

The most common cause of blindness in the Western hemisphere is age-related macular degeneration (AMD). Approximately 1.4 million people in Canada have this type of blindness, and 196 million people are affected worldwide and is expected to increase to 288 millions people by the year 2040. At present, there is no cure for AMD. The disease occurs with the death of a layer of cells called retinal pigment epithelium, which normally provides nutrients and other support to the macula of the eye. The macula is an oval-shaped pigmented area near the center of the retina that is specialized for high visual acuity and has the retina’s greatest concentration of cones. When the epithelial cells die and the macula is no longer supported or nourished, the macula also starts to die. Patients experience a black spot in the center of their vision, and as the disease progresses, the black spot grows outward. Patients eventually lose the ability to read and even to recognize familiar faces before developing total blindness.

In 2016, a landmark surgery was performed as a trial on a patient with severe AMD. In the first ever operation of its kind, Dr. Pete Coffey of the University of London implanted a tiny patch of cells behind the retina in each of the patient’s eyes. The cells were retinal pigmented epithelial cells that had been grown in a lab from stem cells, which are undifferentiated cells that can develop into other cell types. Within six months of the operation, the new cells were still surviving, and the doctor was hopeful that the patient’s vision loss would stop and even be reversed. At that point, several other operations had already been planned to test the new procedure. If these cases are a success, Dr. Coffey predicts that the surgery will become as routine as cataract surgery, and that it will prevent millions of patients from losing their vision.

Attributions

Figure 8.7.1

Bee Stereogram by Be Mosaic on Flickr is used under a CC BY-NC-SA 2.0 (https://creativecommons.org/licenses/by-nc-sa/2.0/) license.

Figure 8.7.2

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

Figure 8.7.3

Macro shot photograph of someone's right eye [photo] by Jordan Whitfield on Unsplash is used under the Unsplash License (https://unsplash.com/license).

Figure 8.7.4

EyeAnatomy_01 by BruceBlaus on Wikimedia Commons is used under a CC BY 3.0 (https://creativecommons.org/licenses/by/3.0) license.

Figure 8.7.5

RGB colours [screenshots] from Microsoft Paint.

Figure 8.7.6

Through the reading glasses [photo] by Dmitry Ratushny on Unsplash is used under the Unsplash License (https://unsplash.com/license).

Figure 8.7.7

Myopia_Diagram by National Eye Institute/ National Institutes of Health on Wikimedia Commons is used under a CC BY 2.0 (https://creativecommons.org/licenses/by/2.0) license.

Figure 8.7.8

Hyperopia by National Institute of Health/NIH on Wikimedia Commons is in the public domain (https://en.wikipedia.org/wiki/Public_domain).

Figure 8.7.9

AnatomyHumanEar by unknown author from Occupational Safety & Health Administration on Wikimedia Commons is in the public domain (https://en.wikipedia.org/wiki/Public_domain).

Figure 8.7.10

Taste_bud_2_eng.svg by Jonas Töle 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 8.7.11

Head_olfactory_nerve by Patrick.lynch, medical illustrator on Wikimedia Commons is used under a CC BY 2.5 (https://creativecommons.org/licenses/by/2.5/deed.en) license.

References

Age-Related Macular Degeneration. (n.d.). WebMD. https://www.webmd.com/eye-health/macular-degeneration/age-related-macular-degeneration-overview#3 (Reviewed by Alan Kozarsky, MD on October 26, 2019)

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

da Cruz, L., Fynes, K., Georgiadis, O. et al. (2018, March 19). Phase 1 clinical study of an embryonic stem cell–derived retinal pigment epithelium patch in age-related macular degeneration. Natural Biotechnology, 36, 328–337. https://doi.org/10.1038/nbt.4114

File:Eye Diagram without text.gif. (2018, February 9). Wikimedia Commons. https://commons.wikimedia.org/w/index.php?title=File:Eye_Diagram_without_text.gif&oldid=286008241 (original image from National Eye Institute - modified by User:Nordelch) [public domain (https://en.wikipedia.org/wiki/Public_domain)]

Occupational Health and Safety Administration. (n.d.). Figure 7. Anatomy of the human ear [diagram]. In OSHA Technical Manual (Section III, Chapter 5 - Noise). United States Department of Labour [online]. https://www.osha.gov/dts/osta/otm/new_noise/

Seeker. (2016, March 18). What is vertigo & why do we get it? YouTube. https://www.youtube.com/watch?v=UL8YSLhqa5U&feature=youtu.be

TED-Ed. (2013, June 10). What color is Tuesday? Exploring synesthesia - Richard E. Cytowic. YouTube. https://www.youtube.com/watch?v=rkRbebvoYqI&feature=youtu.be

TED-Ed. (2014, December 1). What are those floaty things in your eye? - Michael Mauser. YouTube. https://www.youtube.com/watch?v=Y6e_m9iq-4Q&feature=youtu.be

TED-Ed. (2016, August 25). How do animals see in the dark? - Anna Stöckl​. YouTube. https://www.youtube.com/watch?v=t3CjTU7TaNA&feature=youtu.be