Human Anatomy and Physiology

1. Anatomy is the study of the structure of the parts of the body. Physiology is more concerned with function.
2. The book begins by spending a lot of time on sexual anatomy and physiology. I am not going to spend as much, in part because a lot of technical information about meiosis, which is the way that gametes (the sperm and egg) are produced. While I will mention some of these topics, I am going to deal with some other bodily functions first. It seems out of place to talk about reproduction before talking about the systems that keep us alive, and upon which the reproductive system depends. It is possible for a human to live with a defective reproductive system, but not with a seriously defective circulatory (blood) or respiratory (breathing) systems!
3. The circulatory system is crucial to our well-being. Blood distributes oxygen, food (glucose), regulatory compounds (horomones), and disease fighting agents (white blood cells) throughout our bodies. It is used to carry cellular wastes away for disposal by the kidneys. It is also used to transport water and even heat within us.
4. Blood is about half water. Most of the rest are suspended cells.
5. Oxygen (from the air we breath) is carried to our cells by the red blood cells. These cells contain hemoglobin, an iron-based compound which, when oxygenated, it bright red. As the hemoglobin gives up oxygen, it becomes darker. Red blood cells (RBC's) look like center-filled Certs mints: little doughnuts that are not quite punched all the way through in the center. When functioning properly, RBC’s in humans lack a nucleus when mature. They are produced in the bone marrow, live for a few weeks or months, then are reabsorbed by the body.
6. There are a number of types of white blood cells. All are used to fight diseases by attacking foreign cells and other objects in our blood. Some try to eat and digest the invaders. Others produce antibodies which stick to the bad guys, and help mark them for destruction. "Invaders" can be bacteria, viruses, fungi, poisons, or even cancer cells.
7. A third solid component of blood is the platelets. These are small fragments of certain kinds of white blood cells. When a wound occurs, platelets accumulate and begin to disintegrate. The platelet pieces form fibers that catch red blood cells, thus causing clotting, scab formation, and the plugging of the wound.
8. The mechanical force driving blood through the body is of course the heart. High pressure blood leaves the heart and travels to organs in arteries. Once in the organs, the arteries subdivide again and again until they are small capillaries. These capillaries can be so small that the red blood cells have to go through single file.
9. Eventually the capillaries recombine to from larger vessels again. These are called veins, and take the de-oxygenated (purple-ish colored) blood back to the heart.
10. Actually, this is only part of the story. If we look at the heart (p. 713), we see that it has four chambers. The top two are called atria (singular = atrium), while the bottom two are ventricles. The small atrium/ventricle pair pumps blood to the lungs, where it picks up oxygen. It then gets passed to the larger atrium/ventricle pair to be pumped throughout the body.
11. Arteries are thicker than veins, because the blood pressure is higher there. Blood pressure is measured in "millimeters of mercury." This is how high a column of mercury would be pushed by heart pressure. Blood pressure is given as two numbers: the systolic blood pressure is the pressure right when the heart contracts (beats). The diastolic blood pressure is the pressure when the heart relaxes. The numbers are then paired, with the systolic pressure given first. "120/80" (said "one-twenty over eighty) is a good normal blood pressure for an adult.
12. Note that as arteries age (or get clogged with fat from bad eating habits!) they become less elastic and may "harden." Two bad things happen as a result: blood pressure goes up, because there is less "give" to the artery walls. Worse, these pressure spikes make it more likely that an artery may burst. If this happens in the brain, it is called a stroke.
13. Oxygen gets into the body through the mouth and nose. At the epiglottis the path divides. When the epiglottis is open (which it normally is), air then passes down the trachea, through the bronchial tubes, and into the lungs. (When we swallow, the epiglottis acts as a valve which closes momentarily, and allows food to pass the other way, to the esophagus and then to the stomach. More on that later.)
14. Air passes through the bronchial passages, which divide again and again until they end in tiny sacs that look sort of like grape clusters. These are alveoli, and are where oxygen passes into the book and carbon dioxide leaves. Much of this path is lined with muscle cells, which allow us to cough and sneeze, thus blowing out clogs and foreign material like dust. When malfunctioning, it causes asthma attacks—the muscles all constrict at once, making it hard to breathe. Breathing itself is caused by muscles in the chest causing the chest to expand and the diaphragm to move down, followed by relaxation.
15. That takes care of circulation and breathing. What about digestion?
16. Digestion actually starts in the mouth. When we chew on a piece of bread, our saliva contains salivary amylase, which starts breaking down starch. When we swallow, the chewed food passes through the epiglottis, down the esophagus and into the stomach. Here gastric juice made of acids and enzymes breaks the food down while the stomach grinds and churns.
17. From the stomach the digested food passes into the small intestine. Here the acids are neutralized. Also, the liver dumps bile in at this point, which starts breaking down fat. The nutrients from the digested food are also absorbed by in the small intestine. The actual absorption takes place by millions of tiny fingers sticking out of the intestinal wall called villi.
18. From there the material formerly known as food passes into the large intestine. Here several things occur. Water is reabsorbed into the body. Also, there are microorganisms living in our large intestine that consume left-over nutrients and produce vitamins (which are absorbed the large intestine) as a result. Finally, the waste material is dumped from the body. The time from start to finish is normally about 25 hours.
19. While on the topic of waste, we should also talk about the kidneys. The kidneys are responsible for cleaning the body’s blood supply. Each person has two kidneys, in the back just below the ribs. The actual filtering takes place in small units called nephrons. There are about 2.4 million nephrons in a normal kidney. When working correctly, the kidneys get rid of waste products like the urea, excess salt, many drugs, and alcohol. All the blood in the body is filtered through the kidneys in less than hour. (This is why people need to go the bathroom after drinking a lot of beer!)
20. If the kidneys are not working correctly, they can pass things that they should not... like glucose or blood. This can mean something is seriously wrong! The kidneys can also be poisoned by things like large amounts of dissolved heavy metals (like nickel, lead, copper, or zinc), drugs (such as Tylenol), and certain antibiotics. A person can live with only one kidney, but if both fail they will need to have dialysis to clean their blood.
21. We will now look at the internal control mechanisms for the body. Probably the most obvious of these is the nervous system. The nervous system is what is behind the five senses--special nerves receive input in the form of light, sound, touch, smell, and taste. These signals are then transmitted via nerve fibers to the brain, where they are processed. Finally, if a physical response is needed, the brain can activate muscles--again, by transmitting impulses through nerves.
22. How does a nerve cell actually work? We can see a picture of one (greatly enlarged!) on p. 730. The main nerve cell is looks a little like the palm of your hand. The "fingers" are called dendrites. They receive impulses, either from external stimulus or from other nerve cells. Once triggered, the signal moves down the long body of the cell (the axon) to the other end, where it triggers the next nerve cell in line (or a muscle). The axon is surrounded by Schwann cells, which act sort of like insulation around a wire.
23. The way the nerve signal travels is by electro-chemical means. The outside of the cell is surrounded by positive ions (sodium ions), while the inside has more negative ions. Left ot their own devices, the ions would equalize. However, the cell keeps the imbalance by actively pumping the positive ions out and the negative ions in. The result is a slight electrical potential between the inside and the outside of the cell (about 70 mV) which is measurable with a sensitive voltmeter.
24. When the cell triggers, however, the pumping stops and the ions equalize or depolarize. This is what makes the actual nerve signal go.
25. Right after the signal goes by, the cell immediately starts pumping the sodium ions back out again, repolarizing the cell.
26. Note that this takes a small, but significant, amount of time. If a cell has fired, it cannot fire again until it repolarizes, no matter how much stimulus is used.
27. Intensity of sensation is normally connected to how often the cell fires. Each firing is always of the same intensity. Low sensation = few firings. High sensation = many firings. Again, note that sensation can "max out" for a single cell--it can only fire so rapidly. After that, more stimulus does nothing.
28. Also, firing is all or nothing. The cell either fires or it does not. If an impulse falls below a certain threshold, nothing happens.
29. We can see that nervous impulses thus take a certain amount of time to travel. They are fast, but not instantaneous. In humans, they travel up to (200 mph/120 meters per sec.) from toe to brain.
30. Note that the gap between two nerve cells is not bridged by electrical means. Instead, it is done chemically. When the nerve impulse reaches the end of the cell, the little "pads" on the end release a neurotransmitter chemical. One very common one is acetylcholine. When this chemical reaches the dendrites of the next cell, it triggers that cell and the impulse is passed on.
31. Note that immediately after acetylcholine is released, it is quickly broken down by an enzyme called acetylecholineesterase. This keeps the gap between the two cells clear, so that the cells will quickly be ready again for an impulse to be passed on.
32. SO WHAT? It is good to understand how the nerve cells work because it helps us understand how things like caffeine, nicotine, and other stimulants work. With caffeine and nicotine, the nerves are sensitized so that that smaller than normal amounts of acetylecholine will trigger them, thus making them more sensitive to stimulus. You will feel more awake. Or, if you have too much, you will feel nervous and jumpy.
33. Of course, some depressants ("downers") work the opposite way: they make cells less sensitive to neurotransmitters, and the person may be less alert.
34. Note, too, that this system can be fouled up by disease, poison, or other means.
35. An electric shock causes the cells to all trigger at once, causing our muscles to clench up.
36. Some diseases can attack the sheath surrounding the axon and/or the Schwann cells. If this happens, the nerves cease to work correctly. This is what happens in multiple sclerosis.
37. Some poisons (nerve gases and pesticides such as Temik, and perhaps tetanus) work by preventing acetylcholinesterase from working. As a result, the muscles lock up--including the breathing muscles. We then suffocate.
38. On the other hand, other poisons work by blocking neurotransmitter function. If this happens, nerves like limp--including the breathing muscles. We then suffocate.
39. Illegal drugs often affect neurotransmission of nerve signals. Heroin, opium, and LSD all mimic naturally occurring neurotransmitters. Others (like cocaine) keep the neurotransmitters from being broken down. Methamphetamine causes neurotransmitters to be released in greater-than-normal amounts.
40. If drugs are used for a long period of time, the actual physical structure of the nerve cells in the brain can change. When this happens, the drug user becomes a drug addict. The point at which this occurs varies by drug and by individual.
41. We should spend a bit of time examining how our senses work.
42. Sight and vision of course begins with our eyes. (See picture on p. 734.) Light is focused by the lens in our eye onto the retina. The retina is where the light-sensitive layer. We have two types of light-sensitive cells: the rods and the cones.
43. Rods are more sensitive to light, but only work for black and white vision. (This is why we cannot see color well at night!)
44. Cones are less sensitive to light, but can see color. Actually, there are three types of cones: red-sensitive, blue-sensitive, and green-sensitive. When the signals are combined, they allow us to see different shades of color.
45. If one of our types of cones is defective, we end up at least partly colorblind. (About one in twelve males are at least partly colorblind.)
46. Cones and rods all contain a pigment called rhodopsin, which is made in part from vitamin A.
47. The ear allows us to hear. Sound is funneled in through the outer ear. The tympanum (also called the eardrum) separates the outer ear from the middle ear.
48. The middle ear is made of three bones that pass the vibrations from the eardrum to the inner ear.
49. The inner ear contains the cochlea, which looks like a small snail shell. This is where hearing actually occurs. Like a snail shell, the inside of the cochlea contains spiral passages that are lined with tiny hairs. Sound of different frequencies causes different hair cells to vibrate like tuning forks, allowing us tell pitch. The degree of vibration allows us to tell volume.
50. Loud sounds (like rock concerts, jack hammers, and gun shots) can cause the hairs to be sheared off, thus causing hearing loss. Hearing can also be damaged by an infection of the inner ear, certain drugs and antibiotics, and genetic disorders. However, in some cases a cochlear implant can help restore some measure of hearing.
51. Also, the fluid-filled semicircular canals are found in the inner ear. They actually do not have anything to do with hearing. Instead, they help us keep our balance.
52. Our sense of touch comes mainly through our skin. There are actually several different touch receptors. Some detect heat, others cold, others pressure, and others pain.
53. Differing amounts of receptors are found in different parts of our bodies. The tongue, for example, is better at pressure than it is at detecting hot and cold. Our fingers, lips, face, neck, feet, and genitals have the highest numbers of receptors, while the middle back has the least.
54. We normally have constant sensory input from our receptors. If our foot "goes to sleep," though, we see what it is like to lose input.
55. Our sense of smell is very sensitive. In some cases, we can detect as little as one molecule of an odor! Most of our smelling takes place in the olfactory epithelium in the nasal cavity.
56. We can detect thousands of different smells, but exactly how we do it is not well understood. We do know that our sense of smell fatigues rapidly; we get used to smells fairly quickly and then can no longer detect them after a few minutes.
57. Linked to our sense of smell is our sense of taste. While some animals can taste with their feet (house flies) or skin (sharks), most tasting by humans is done by the taste buds on our tongues, though there are others scattered around inside the mouth. The main things that we can taste are sweet, sour, salty, and bitter.
58. Sour is the easiest to understand, because it seems to basically be an acid detector. This is why acids normally taste sour.
59. Salty is triggered by sodium chloride ( = table salt), though similar chemicals (such as potassium chloride) may also be perceived as salty.
60. Numerous organic compounds are detected as "sweet." Table sugar is one, but so is Nutrasweet (the trade name for the organic called aspartame), and it is a completely different family of chemical!
61. Some lead compounds also taste sweet, which is why there used to be a problem with children eating lead paint chips! Also, the ancient Romans tended to store wine in lead containers to "sweeten" it--probably resulting in lead poisoning.
62. Numerous organic and inorganic compounds (such as metals) are detected as being bitter. Many of them are poisonous. This is probably why we tend to reject bitter things as foods, especially when the flavor is strong and unexpected. The main things that we routinely eat that are bitter are coffee, tea, and chocolate.
63. Note that "taste" is made up of more than just flavor. Temperature and texture can affect our perceptions of food. Hot coffee tastes different from cold coffee, in spite of the fact that they are chemically identical.
64. Interestingly, women seem to have more taste buds than men. Also, the number of taste buds in our mouths declines over time. This is probably why some foods become more palatable over time--the flavors are too strong for children.
65. The sense of taste can also be influenced by head injury, vitamin deficiency, smoking, and medications.
66. We thus conclude our quick survey of the senses and of the nervous system. There is, however, another control system in our bodies besides the nerves. This is the endocrine system.
67. The endocrine system is made of glands. These glands produce small amounts of chemicals that act as messengers within our bodies. These messenger molecules are carried throughout the body in our blood. However, only cells with the correct receptor sites on them can detect the chemical "message."
68. These chemical messages are usually called hormones. Well known hormones include Growth Stimulating Hormone (which makes us grow in height), testosterone (which promotes growth of hair and muscle, and the growth of the penis), and estrogen (which causes the breasts to grow, and also helps regulate the female menstrual cycle).
69. Possibly the most commonly known hormone is adrenaline. This is released when we are startled, scared, or angry, thus preparing us for "fight or flight." It accelerates the heart and increases blood glucose levels.