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Kansas School Naturalist

KSN - Vol 29, No 1 - I Didn't Know that! (Humans)

Volume 29, Number 1 - October 1982

I Didn't Know That! (Humans)

by Edward C. Rowe

PDF of Issue


Published by Emporia State University

Prepared and issued by The Division of Biology

Editor: Robert F. Clarke

Editorial Committee: Gilbert A. Leisman, Tom Eddy, Robert J. Boles, John Ransom

Online format by: Terri Weast

The Kansas School Naturalist is sent upon request, free of charge, to Kansas teachers, school board members and administrators, librarians, conservationists, youth leaders, and other adults interested in nature education. Back numbers are sent free as long as supply lasts, except Vol.5, No.3, Poisonous Snakes of Kansas. Copies of this issue may be obtained for 25 cents each postpaid. Send orders to The Kansas School Naturalist, Division of Biology, Emporia Kansas State College, Emporia, Kansas, 66801.

The Kansas School Naturalist is published in October, December, February, and April of each year by the Kansas State Teachers College, 1200 Commercial Street, Emporia, Kansas, 66801. Second-class postage paid at Emporia, Kansas.

"Statement required by the Act of August 12, 1970, Section 3685, Title 34, United States Code, showing Ownership, Management and Circulation." The Kansas School Naturalist is published in October, December, February, and April. Editorial Office and Publication Office at 1200 Commercial Street, Emporia, Kansas 66801. The Naturalist is edited and published by Emporia State University, Emporia, Kansas. Editor, Robert F. Clarke, Division of Biological Sciences


Dr. Edward C. Rowe is a Professor of Biology at Emporia State University. He has taught here since 1961. His major teaching area and interest is human anatomy and physiology. He is a pre-med advisor and has been instrumental in implementing our Emergency Medical Technology and the CPR programs.

I Didn't Know That! (Humans)

by Edward C. Rowe, Ph. D.

The amount of heat produced by a person while is about the same as the amount of heat produced by a 100 watt light bulb. Assuming we put you on an electrical generator connected to a bicycle, you would find it tiring to pedal long enough to keep a light bulb burning long enough to read  this issue of The Kansas School Naturalist.

The prize for the cell with the largest volume goes to the egg cell at the time it is released from the ovary. Although a human egg cell and a human sperm provide equivalent nuclear or genetic material, there is a great difference in the amount of cytoplasm in the two cells. In fact, the egg cell has about 15,000 times the volume of the sperm cell.

Each of us probably produces about 50,000 different proteins. The most important thing do is to act as enzymes or catalysts to speed up chemical reactions. We use dozens of different enzymes in digesting our food. We use thousands of different enzymes to speed up the many different chemical processes inside cells. Other specific proteins and their functions are: water proof keratin on the outside of the skin, tough collagen giving strength to our tendons, hemoglobin carrying our oxygen around, and several different protein hormones coordinating body processes.

In a typical young, lean, adult male, about 45% of his mass is muscle tissue and about 15% is fatty storage tissue. His female counterpart is about 35% muscle and about 25% fatty tissue.

female-male symbol diagram

A light-skinned person looks redder during vigorous exercise. The change in color is due to the way our body gets rid of the excess heat produced in our active muscles. More blood is allowed to flow into the vessels just under the skin surface. This means more heat is brought close to the surface where it can be radiated away. The skin looks redder because we can detect the increased flow of red blood through the slightly transparent skin.

The skin, and particularly the lips, of a light-skinned person look blue when that person has been in cold water for some time. The reason the lips "turn blue" is that the body protects itself from losing heat by reducing blood flow to the skin, which is the part of the body which most readily loses heat. The small amount of blood which is still flowing in the skin vessels gives up more of its oxygen than usual. Hemoglobin, the oxygen-carrying substance in our red blood cells, turns bluish-red when it gives up its oxygen, and we detect this color change through the slightly transparent skin.

The food we eat usually has some bacteria in it. The strong acid of the stomach kills most bacteria and prevents them from reproducing. Although the bacteria receive a severe set-back by the stomach, they slowly recover and begin to increase in numbers in the small intestine. By the time they reach the large intestine they are beginning to take over; bacteria make up about one-half of the material in the large intestine. Many of these bacteria are helpful to us; some, for in stance, produce needed vitamins.

The liver, which lies just below the diaphragm (just below the level of the rib cage) is the heaviest of the internal organs of the body, weighting in at about 4 pounds. The liver has many essential functions, including the following: 1) It is the body's general chemical factory. Other organs release their problem materials into the blood stream and the liver in many cases solves the problem. For instance, it handles lactic acid (which is made in fatigued muscles) and it breaks down alcohol absorbed by the digestive tract. For many other toxic chemical substances it either breaks them down or converts them to chemicals which can be more readily excreted by the kidney. 2) It produces important protein substances of the blood plasma, including the clotting proteins. 3) When there is a surplus of sugar available in the blood, the liver converts the excess into the storage carbohydrate known as glycogen. When sugar levels in the blood drop below normal, the liver releases glucose from the glycogen storage. 4) It destroys old red blood cells and recycles most of their chemical "parts" (iron and protein), and excretes only the small remainder that it can't use. The liver destroys homones, the chemical substances which regulate many body functions. The body has to be able to control the concentration of each hormone, and this is done by balancing the rate of production of the homone (by the gland which secretes it) against its rate of destruction by the liver.

The pancreas lies under the stomach and has a duct it with the small intestine. It is an important general-purpose source of enzymes. These are released into the small intestine, where they digest the complex foodstuffs. The pancreas' other function is secretion of hormones into the bloodstream. In diabetes the problem is that pancreatic cells produce inadequate amounts of the homone insulin, and the body can no control blood sugar concentrations.

If the interior of the small intestine were smooth, like the interior of a garden hose, it would have only about 2 1/2 square feet of surface. Since the amount of food we can absorb depends on the amount of surface available to absorb it, it is important to have a larger absorptive surface. This is accomplished by folding the interior of the small intestine into thousands of finger-shaped (villi), which increase the absorptive surface 30 fold to the equivalent of a flat 7 1/2 x 10 foot surface.

The small intestine is small in diameter only -about one inch - but 20 feet long. Since it lies entirely in our abdominal cavity, it is obvious that it coils back and forth several times in that cavity, much like a garden hose crammed into a small sack. Most of the chemical digestion of food (breakdown of larger molecules into smaller ones) occurs in the small intestine, as does most absorption (movement of molecules into the bloodstream).

The extremely delicate tissues lining the deepest air pockets (alveoli) of our lungs are fairly well protected from the dust and bacteria floating in the air we breathe. The airway passages leading into the lungs are lined with one type of cell, which secretes mucus, and another type, which has cilia. The mucus provides a sticky lining, which traps much of the bacteria and dust. The cilia, which resemble microscopic hairs, function like brooms to sweep the mucus and its load of dirt away from the alveoli. Inside the alveoli there is a final line of defense: amoeba-like cells patrol these air sacs to surround and destroy any foreign matter which gets this far. They do this by a process known as phagocytosis or intracellular digestion.

cartoon of the small intestine at work

The lung's defense mechanisms are not perfect and can be defeated. For instance, substances in tobacco smoke inhibit the movement of the cilia, so mucus and dust tend to pile up in the airway of smokers, to the point that the smoker has to cough to expell the mucus. (The cough is distinctive and is known as "smoker's hack.") Or, the environment may have so much dust that the cilia-mucus defense is overwhelmed; miners and city inhabitants have dirtier lungs than do people who breathe cleaner air of the countryside.

Even when we are inactive, we move about 6 liters (6 quarts) of air into and out of our lungs each minute. This adds up to about 13 cubic feet of air per hour or about 300 cubic feet per day.

The total volume in an adult is less than that in a basketball, but because the lungs are subdivided into about 600 million bubble-like chambers, or alveoli, each about the diameter of a pencil point and each surrounded by capillary blood vessels, the total surface across which oxygen can move from the air into the bloodstream is about the same as one side of a tennis court.

It's an oversimplification to say that we breathe in oxygen and breathe out carbon dioxide. The air we inhale is 21% oxygen and 0.04% carbon dioxide and the gas mixture we exhale is about 15% oxygen and 4% carbon dioxide. Mouth-to-mouth rescue of persons who have stopped breathing would be impossible if the exhaled air did nor retain this rather large amount of oxygen.

While we are digesting a meal, a large proportion of our blood is flowing to the digestive tract and relatively little is flowing to our muscles. The situation reverses when we are exercising: we reduce blood flow to the digestive tract and greatly increase blood to the active muscles. These shifts are made possible by small muscles in the walls of the arteries. When the muscles relax, more blood can flow in that artery.

In a marathon run of 26.2 miles, a runner will typically record an eight pound drop in weight. Only about one pound of this is fat utilized as energy source for the run. The remaining seven pounds is water lost through the lungs and through sweat.

In addition to arteries and veins, we have a third system of vessels, the lymph vessels. These drain excess fluid which would accumulate in our organs (since not quite all fluid delivered by arteries is drained by veins).

Bacteria could potentially use the lymph vessels as routes to invade the whole body, but lymph nodes or "glands" serve to block these routes. The lymph nodes are bean-shaped structures located along the lymph routes. The nodes are filled with white blood cells capable of destroying any bacteria moving through the lymph vessels.

Lymph nodes are found in large numbers in the neck, in the armpit, and in the groin region. If bacteria are invading through the lymph vessel system, the nodes fight back by increasing the number of defensive white blood cells. Sometimes the response is strong enough that we can see or feel the swollen nodes through the skin in these areas. Often these are called "kernels."

The most important vessels in the human body are the extremely small and thin-walled capillaries, since oxygen, nutrients, and wastes can move freely across the walls of the capillaries. Even though each capillary is only about one mm (1/25th inch) in length, there are so many capillaries that there is a total estimated 60,000 miles of capillaries in the whole body.

Heart muscle is especially well supplied with capillaries. In a cross section of heart muscle the same diameter as the lead in a wooden pencil, there are 10,000 capillaries.

The smallest capillaries of our body have about the same diameter as a red blood cell, so that red cells must be soft and flexible to get through the vessel system. With one complete circulation each minute and a lot of friction between blood cells and vessel walls, it is no wonder that red blood cells wear out! In fact, red cells survive about 120 days once they are released into the blood stream. Since 1/120th of the red cells are destroyed each day and there are about 25 trillion red cells in the body, about 200 billion are destroyed each day. (This works out to 2 or 3 million red cells being destroyed each second, but don't worry: you are producing new red cells at very nearly the same rate as you are destroying the old ones).

When physicians, nurses, and other medical personnel use a blood pressure cuff to measure your "blood pressure" they are actually getting a measure of the pressure of the blood within your arteries. This arterial blood pressure, which drives the blood through the capillaries and then through the vein back to the heart, is about two pounds per square inch.

The heart has its own natural, cellular pacemaker, which is responsible for making it beat rhythmically. It does NOT need to have impulses coming from the brain telling it, "Contract now!". (There are nerves from the brain to the heart, but their function is simply to cause the pacemaker tissue to speed up or slow down a little.)

cartoon of heart singing

A red blood cell makes one complete circuit of the whole vessel system in about one minute. The two most important seconds of this minute are the one second it spends in a lung capillary (where it picks up oxygen) and the one second it spends in a capillary somewhere else in the body (where it gives up oxygen).

Your heart is a hollow pump about the same size as your fist, and is made up mostly of muscle tissue. Although five or six liters (quarts) of blood are pumped through the heart chambers each minute, this blood doesn't get close enough to the muscle cells of the heart to supply them with oxygen and nutrients.

The heart muscle has its very own set of arteries, the coronary arteries, which branch extensively within the heart to provide a very rich capillary supply. In fact, each heart muscle cell is surround ed by several capillaries. About 5% of the blood pumped by the heart goes to the coronary arteries to provide oxygen and nutrients for the heart muscle.

The resting heart beat rate of a well-conditioned endurance athlete is usually much slower than that of an average person. Many marathon runners have resting pulse rates of 40 beats per minute or lower. In response to years of regular long-distance running, their hearts have become larger and stronger and more blood is pumped with each beat of their hearts.

Younger persons' hearts beat faster than hearts of adults. Here are some typical heart rates for children and adults:

Age - Beats per minute
Birth - 140
5 years - 100-110
Childhood - 90
Adult men - 68-76
Adult women - 74-80

An adult's heart beats about 70 times each minute, or 4200 times each hour, or about 10,000 times each day. The muscle of the heart has to get its rest between beats, because it can't quit, even for a few minutes.

The entire blood volume of a resting, non-exercising person is circulated about once each minute. During heavy exercise, a world-class athlete can circulate his whole blood volume in less than ten seconds.

There are about five million red blood cells per cubic millimeter of blood. The total number of red blood cells circulating in an adult human is about 25 trillion.

Red blood cells are so small that it would take more than 3000 of them lined up end to end to form a line one inch long. Or, if you stacked them, one on the top the other, it would take 10,000 of them to form a stack one inch high.

Red blood cells have the "biconcave disc" or "beanbag" shape shown in the drawing. They are quite unusual in humans because they lack a nucleus and many of the other "standard" structures found in most other cells. In fact , it can be argued that they are no longer true cells but just a cell membrane surrounding a large amount of hemoglobin, the red protein which carries oxygen from lungs to body tissues.

diagram of red blood cells

The ear drum of a human is about the same size as the head of a thumb tack. When sound waves hit the ear drum, they cause a series of three miniature bones to vibrate. The long dimension of the largest of these three bones is one-quarter inch.

Our inner ears not only give us our sense of hearing, they provide us with motion senses (sense of rotation and falling) and balance.

The total number of visual receptor cells in each eye (rod cells for night vision and cone cells for daylight vision) is 110 million. The total number of nerve cells in each optic nerve (which carries nerve impulses from the eye to the brain) is one million!

The eye in many ways resembles a camera. The retina is the thin layer at the back of the eye which contains the light-sensitive cells. It obviously serves the same function as the film in a camera, but the "film" is not all of the same type. A small part of the retina (the fovea, straight back from the lens) serves as the "fine grain, color film" portion of the retina. Whenever we wish to see an object in detail we have to aim the eye so that the image of interest falls on this special part of the retina. The rest of the retina is not as good at detecting detail or color, but makes up for these deficiencies by being more sensitive to extremely low light levels, such as when the only illumination is starlight or moonlight.

The human eye is appropriately referred to as an eyeball. It is essentially a sphere (almost exactly one inch in diameter), except on the front surface, where it bulges out slightly, and on the back surface, where its stem-like nerve carries visual information back to the brain.

If a nerve outside of the brain or spinal cord is cut or crushed, nerve cells within the nerve may regenerate. The rate of re-growth varies but 1 mm per day (about 1 inch per month) is not unusual.

You may find it surprising that you have cells in your body that are three feet or longer. The longest cells in your body are undoubtedly those which "turn on" the contractions in your toe muscles. Each of these nerve cells has a nucleus in the lumbar (lower back) region of your spinal cord and a long, microscopically thin fiber, or axon, which runs down the back of your leg, around your ankle, and ends finally in a small muscle in your toe. My cells of this type are 4 1/2 feet long.

The right side of the brain controls the left side of the body and the left side of the brian controls the right side of the body. In more than 90% of humans, the dominant speech control centers are located in the left side of the brain.

cartoon of the human brain

The brain makes up about 2% of the body weight, but under resting conditions it consumes about 20% of the oxygen used by the whole body. Proportional to its weight, it takes a lot more energy to run the brain than to run most other organs!

The brain and spinal cord are very delicate organs and would be readily injured if it were not for the fact that they float in a thin layer of clear cerebrospinal fluid, which provides a fluid cushion between nervous tissue and surrounding bones. As we bump into objects, the fluids "spread out" the forces and reduces injuries, much as the proposed automobile "air bags" would do.

The cerebrospinal fluid which surrounds our brain and spinal cord is constantly being replaced. In fact , the total volume is replaced about once every 4 or 5 hours.

The brain uses carbohydrate as its energy source (not fat or protein). Every hour of every day, whether we are asleep or awake, our brain needs 6 grams (about two level teaspoons) of the sugar, glucose, as a fuel. We lose consciousness very rapidly if our brain is without glucose for even a short period of time.

The human hand is a marvelous piece of "machinery." Although other animals have hands useful for grasping and manipulating objects, few have as movable a thumb as we do: because our thumb can touch each of other fingers, we can do extremely fine manipulations.

The control system for the hand, especially the thumb, is extremely well-developed in man. The region of the brain controlling the thumb is disproportionately large compared to other muscle control areas.

Tendons are the strong, living "cables" by which our muscles pull on our bones and move them. To lift a 30-pound weight off the table, your biceps must pull on its tendon with a force of 100 pounds or more. It is not unusual for the tension of this tendon to be 500 pounds or even greater.

cartoon of how the human arm works

The Achilles tendon, the strongest tendon in the body, can be felt as a thick cord above your heel. It is about one-third of an inch in diameter and constructed mainly of a very tough white fibrous protein. The Achilles tendon connects the calf muscles on the back on your lower leg to the back of your heel. When these calf muscles contract, they pull on the heel part of the foot and cause the toe to "point."

When a muscle is resting, most of the blood vessels in the muscle are "shut down" and blood is flowing in only a small proportion of the capillaries. During maximum exertion, blood can flow in all the capillaries and the volume of blood flow through the muscle can increase by about 40 times.

Bone has two important chemical substances, mineral calcium salts and a leathery protein called collagen. If you soak a bone for weeks or months in vinegar, the acid dissolves the calcium salts but leaves the protein collagen untouched. The remaining structure has the shape of the original bone but is now flexible enough to be tied in knots. The leather-like "bone" is still strong, but it has lost the mineral which gave it hardness.

Engineers have long known that you can get greater strength with less weight if you build with hollow tubes instead of solid rods. This is the reason bicycles are constructed of hollow metal tubes. The long bones of your anns and legs are hollow tubes of bone. An adult human skeleton weighs about 24 pounds, but it would have to weigh much more if all the bones were solid.

With each step you take, the neck of your thigh bone is exposed to a downward impact equivalent to 500 pounds of pressure. When we run, this narrow region of bone has to withstand about 1000 pounds with each step.

cartoon diagram of pressure on upper thigh

One way of measuring the strength of a material is to measure the amount of force needed to break it. By this measure, bone is about as strong as cast iron.

Cartilage ("gristle") is not as strong a support material as bone, but it has at least two advantages over bone: 1) It is more flexible than bone, and this is used to advantage in the human rib cage. Cartilage connects the ribs to the breast bone and flexibility of the cartilage per mits the breathing movements of the chest. 2) Bone is such a rigid tissue that it prevents its own growth. Wherever a bone is growing, a disc of cartilage within the bone is actually doing all the growing.

Another advantage of cartilage is that it moves on cartilage with very little friction. In the freely movable joints of the body (ankle, elbow, shoulder, etc.), the bone ends are capped with cartilage, and the cartilages are shaped to fit with each other closely. The body provides a small amount of lubricant (synovial fluid) between the two cartilage surfaces which reduces the friction to one-third the friction of melting ice moving on melting ice.

The biceps muscle (the large one on the front of your upper arm) has about 200,000 muscle cells or "fibers," and it is controlled by about 300 to 400 nerve cells. Each nerve cell has control over many muscle cells but each muscle cell receives impulses from only one nerve cell.

If we were to give a prize for the largest cell in the body, what kind of cell would win? One possible answer is the skeletal muscle fiber, which is truly huge compared to other types of cells. The biggest muscle fibers are a tenth of a millimeter in diameter (big enough to be visable) and at least three or four inches in length. But many people would disqualify skeletal muscle fibers because they are not typical cells; a fiber is surrounded by a single cell membrane but it contains dozens or even hundreds of nuclei. It's really more fair to consider it as a population of cells which have fused inside a common membrane.

The sartorius muscle, which runs diagonally across the front of the thigh from the outside of the hip to the inside of the knee, is the longest muscle in the body. In an adult it is about 20 inches long.

The outer surface of our skin is an extremely thin, dead layer of a waterproof protein known as keratin. It has the important factor of holding our body fluids in. It also keeps bacteria out of the body. Chemists have found that hairs, fingernails, and animal horns are all made of this same keratin.

The total number of cells in a human body is 100 trillion or more. Each of these cells is small; most are microscopic but a few are just large enough to be seen with the unaided eye.

Most humans produce a brown or black pigment known as melanin. The amount of melanin determines how light or dark our skin is. Blacks have large amounts of this pigment; Indians have less; whites have still less; and albinos have none at all. Melanin also provides color for our hair and eyes. Melanin protects the deeper layers of skin from damaging effects of ultraviolet radiation, but it is important that not quite all ultraviolet light is blocked; some ultraviolet has to penetrate if we are to make vitamin D, needed for normal calcium absorption and thus for normal skeletal strength.

A freckle is a darker spot of the protein melanin on a background of lighter skin.

cartoon of a person with a lot of freckles

A hair doesn't grow at its tip (the way a tree does), The part that is growing is the "root" of the hair, which is down in the deeper layers of the skin.

A hair can grown about one-third of a millimeter per day (or about half an inch per month). But not all of your hairs are actually growing at any one time; individual hairs rest for a while, have a growth spurt, then go back to resting.

The human body has from 2 million to 5 million sweat glands distributed over the surface of the skin. Under extremely hot circumstances, these glands can produce as much as 4 liters (4 quarts) of sweat per hour!

Although sweat may seem salty to us, especially at the end of a hot day, sweat is actually rather dilute when it is initially released to the skin surface. Its initial salt concentration is one-third that of blood (or even more dilute) and would hardly seem salty to us, except that the water in sweat evaporates, leaving behind more and more concentrated salts.

Its a basic fact of physics that large amounts of energy are absorbed in the process of converting liquid water into water vapor. The body makes use of this fact in regulating body temperature: The warmer we become, the more we sweat; the more we sweat, the more water evaporates; the more water evaporates, the more heat energy is absorbed from the skin surface, and from the air close to the skin. In other words, the greater the cooling of the surface and of the blood, the greater the cooling effect on the body.

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