From the heart, blood surges through the aorta to the arteries throughout the body. From the arteries, smaller vessels called arterioles branch out. From the arterioles, the blood flows to the smallest vessels, the capillaries. The capillaries carry the blood to the individual cells of the body, where oxygen and other chemicals are delivered and waste products are collected. The capillaries then connect with venules, which run into veins, which, in turn, flow into the venae cavae.
Except for the microscopic spaces in the cell walls of the capillaries, the blood flows through a sealed conduit of vessels that would total some 70,000 miles if placed end to end. The lining of the system is a membrane of cells called endothelium. The endothelium is so thin that, even when magnified a thousand times, it appears no wider than a chalk mark. But it forms the smooth inner layer of the heart and all the blood vessels.
The arteries have layers of smooth muscle cells, elastic fibers, and connective tissue. One layer of muscle cells is circular, and another layer of muscle fibers runs lengthwise. In the large arteries, which have thick walls, smaller arteries run through their layers of muscle and fibrous tissue. Nerve fibers also accompany the muscle fibers that control the size of the opening in the arteries. The arterioles are barely visible without a microscope.
The blood flow from the arteriole to the capillary is controlled by a circle of muscle fibers called a sphincter. The capillary system is so vast that if all of its vessels were opened at one time they could swallow up the body's entire supply of blood. Yet the individual capillaries are so small that the red blood cells have to pass through them in single file. The red blood cells are, in turn, so small that a line of 3,000 of them would fall a little short of an inch in length.
It has been estimated that the human body contains 70,000 miles of blood vessels. Of this total length, the largest percentage by far is capillaries. They could, as we pointed out, hold the body's entire blood supply (about five quarts). But only a part of the capillary system has blood flowing through it at any instant. The capillaries open and close first in one area, then another. Tissues that use more blood, such as the arm and leg muscles, contain a greater share of capillaries than tissues that are not as active.
The endothelial walls of a capillary are solid enough to keep the red cells from leaking out of the "closed circuit" of blood vessels. But white cells, chemicals, and small amounts of liquid can leave the bloodstream by squeezing through tiny openings between the cells that make up the walls of the capillary. The blood flow is very slow at this point. It would take a minute for the blood to go an inch of capillary tube. (Blood travels through the arteries at a speed of more than 40 miles an hour.) But during the slow trip through the capillaries, the food and oxygen needed by the body cells leave the tiny blood vessels and go into the tissues. A liquid called lymph that bathes the tissue cells also trickles through the capillary walls as do the ingredients needed for repair and maintenance of the tissues.
At the same time, the carbon dioxide and other waste products from the tissue cells enter the capillary and are carried away. The lymph seeps into a special set of capillaries which return it eventually to the blood through a special vein near the heart.
The dark, used blood in the upper part of the body can flow downhill from the capillaries to the heart. But what about the blood that has finished its trip through the capillaries in the hands and feet? Nature has provided for the need to push the used blood back up to the heart by two unique devices. One is the location of the veins between the skeletal muscles. The other is a system of valves that permits used blood to flow only toward the heart.
Good examples of these devices can be found in the legs. The veins in the legs seem to weave among the muscles so that when each muscle contracts it squeezes a vein. Squeezing the vein pushes the blood closer to the heart. The valves in the vein prevent the blood from flowing backwards when the muscle relaxes.
If we spend too much time standing or sitting in one position, the used blood from the lower part of the body does not get the push it needs to return to the heart. That is why we may experience fatigue or sluggishness after standing still for a long time or after a long ride. Varicose veins result from a failure of the used blood in the legs to return to the heart at the proper rate. The veins may lose some of their elasticity or the valves may fail to close properly. The used blood then accumulates and causes the veins to swell. Varicose veins occur frequently in the legs of people whose jobs require long periods of standing.
So far, we have discussed the "plumbing" of the circulatory system -- the pumps, the pipes, and the valves. Now let's look at the blood itself. When blood is taken from the body and studied in a laboratory, it can be separated into two main parts. A little more than half of the blood is a watery fluid called plasma. The remainder is composed of red blood cells, white blood cells, and platelets.
The red cells give blood its color. They are tiny disks, so small that you could hide 40 or 50 of them beneath the period at the end of this sentence. They are thinner in the center than along the edge -- which permits them to fold over when passing through an opening smaller than their own diameter. The human body contains nearly 25 trillion red cells. Each lasts only four months. They simply wear out and break up in the blood. New red cells are produced by the bone marrow at the rate of around one million a second, and they wear out at the same rate.
Red cells are colorless when they are first produced. The color is acquired just before the cells are released into the bloodstream. It comes from an iron pigment, which is combined with a protein. The chemical combination is called hemoglobin. It is the hemoglobin that carries the oxygen from the lungs to the capillaries, where it is released to the individual tissue cells. The hemoglobin also attracts carbon dioxide and carries it in the red blood cells to the lungs, where it is exhaled.
Because of the ability of hemoglobin to hold large quantities of oxygen, animals can be smaller than if the plasma itself had to carry the oxygen. Hemoglobin enables the blood to move up to 60 times as much oxygen as would be possible if the oxygen had to be dissolved in water or plasma. In other words, without hemoglobin, humans would need up to 300 quarts of blood --instead of just five -- in order to live on this planet. Even the muscles contain hemoglobin. It permits the muscles to build up a reserve of oxygen for the spurts of energy we need when running, playing, or working hard.
The oxygen is held very loosely by the hemoglobin. In simple terms, the hemoglobin acquires a lot of oxygen when oxygen is abundant -- as in the lungs after you inhale. And the hemoglobin gives up its oxygen when it reaches a region where oxygen is scarce -- as in the areas where the capillaries branch among the tissue cells. Carbon dioxide similarly is picked up by the hemoglobin where carbon dioxide is plentiful and released in the lungs, where it is comparatively scarce. Unfortunately, hemoglobin also is strongly attracted to carbon monoxide, the poison gas that comes from automobile exhaust and faulty furnaces. The carbon monoxide crowds the oxygen out of the hemoglobin, and the body tissues suffer from oxygen starvation.
When the number of red cells in the blood is too small or the red cells do not contain enough hemoglobin, the body is not able to get the proper amount of oxygen. The muscles and other tissues are not able to burn all of their supplies of fuel. The result is that the body can't get the energy it needs, and we call this condition anemia. If anemia is caused by a lack of hemoglobin, the shortage can be overcome by eating foods rich in iron, such as eggs and meat and particularly liver. Since hemoglobin is a protein, good quality protein foods are needed in the daily diet.
White cells are less common in the bloodstream, but they still number in the billions. The proportion is about one white blood cell for every 600 or 700 red blood cells. And there are several kinds of white cells. One kind, the granular leukocytes, develop in the bone marrow along with the red blood cells. Another kind, the lymphocytes, develop in the lymph nodes, tonsils, and adenoids. The leukocytes play an important role in defending the body against the invasion of bacteria. These white cells can move around and literally chase down the bacteria.
As we mentioned earlier, the white cells can even squeeze through tiny openings in the walls of capillaries. When they corner bacteria, they engulf and digest them. One result of the battle is pus -- a thick yellowish fluid composed of lymph, bacteria, and dead white cells.
Blood platelets are much smaller than red blood cells. They help blood to clot, or coagulate. When a blood vessel is cut, the platelets collect at the site of the injury.
Many of the platelets break into smaller pieces, and a complicated chemical process follows. Almost instantly, tiny threads appear from the platelets. The threads, called fibrin, form a tangled mass with the blood cells and platelets. This is the clot. The red blood cells give the clot its dark color.
The platelets do not perform the task of clotting by themselves. Chemicals in the injured tissue cells, the white cells, and the plasma are used by the body to form a clot.
The plasma is about 90 percent water. The remainder is made up of minerals, such as sodium, calcium, potassium, and phosphorus, plus enzymes, antibodies, fats, sugars, and the plasma proteins. The plasma proteins include fibrinogen (one of the clotting chemicals), albumin, and serum globulins. Some oxygen and carbon dioxide are dissolved in the plasma.
When blood is lost because of injury or disease, it can be replaced by transfusion. The blood is obtained from another person, but the blood of the donor must be compatible with that of the person receiving the transfusion. The main types of blood are classified as A, B, O, and AB. If the types are not compatible, the transfused blood cells may form clumps that can block the small vessels. It also is necessary to differentiate between Rh-positive and Rh- negative blood. If incompatible blood is given, serious consequences may result.
This article and accompanying test were prepared by BAPR member Nancy Patterson of Los Angeles, California. The article "The Blood" is reprinted by permission of the American Medical Association from The Human Machine, copyright 1979.
Test for "The Blood"
1. The Rh in the ``Rh factor'' stands for
A. Red hemoglobin
D. Rheumatic heart
2. The administration of the wrong type of blood in a transfusion causes
B. blood urea nitrogen
D. coagulated blood serum
3. When they are first produced, red cells are
4. In the lungs, oxygen is picked up by
5. Hemoglobin has an unfortunate affinity for
A. carbolic acid
B. carbon dioxide
C. carbon monoxide
6. Fibrinogen is
B. plasma protein
C. plasma carbohydrate
7. The red blood cell is also called the
8. Platelets are also called
9. Granular leukocytes develop in the
B. lymph nodes
D. bone marrow
10. The initials c.b.c. stand for
A. complete blood count
B. carbon dioxide combination
C. capillary blood control
D. collected bone cells
11. Lymphocytes develop in
B. bone marrow
C. varicose veins
12. What is removed from plasma to leave blood serum?
13. Carbon monoxide poisoning is actually
A. destruction of arteries
B. loss of leukocytes
C. loss of hemoglobin
D. deprivation of oxygen
14. Hemoglobin is a combination of
A. red cells and lymphocytes
B. carbon dioxide and plasma
C. iron pigment and a protein
D. potassium and phosphorus
15. Cytopenia means
A. diseased white cells
B. too many white cells
C. abnormally pointed cells
D. lack of blood cells
16. Blood flows in which order?
A. arteries - arterioles - capillaries - venules - veins
B. arterioles - arteries - venules - veins - capillaries
C. veins - venules - capillaries - arteries - arterioles
D. arteries - veins - arterioles - venules - capillaries
17. Bacteria are destroyed by
A. serum globulin
B. red blood cells
C. white blood cells
D. blood platelets
18. Clotting is the principal function of the
A. blood serum
C. blood albumin
19. White cells are
A. larger than red cells
B. more numerous than red cells
C. smaller than red cells
D. made of enzymes and antibodies
20. A varix is
A. a point of exchange
B. critical blood shortage
C. a distended vein
D. abnormal blood pressure
21. The veins in the legs are squeezed by
B. endothelial tissue
C. arterial action
22. Blood is prevented from flowing backward in the veins by
A. valve action
B. muscle relaxation
23. A basophil
A. is required for each platelet
B. stains readily with basic dyes
C. is the most numerous of all blood cells
D. has a basic triangular structure
24. The Latin word "sanguis" means
25. Presence of blood detected only by a microscope is
A. occult blood
B. splanchnic blood
C. venous blood
D. peripheral blood