Your Amazing Circulatory System
I AM your blood, and I travel through some 100,000 miles of blood vessels from the top of your head to the tip of your toes. I am really a very special transportation system. In me are red blood cells, white blood cells and platelets carried around in a fluid called plasma. The average person contains about six quarts of my fluid.
Just relax for a few minutes as we take you on a tour. My red blood cells would like to tell you something about their role in your amazing circulatory system.
Red Blood Cells
Do you know what we red blood cells look like? Well, we are shaped much like a doughnut without the hole; we are very small disks with indentations on both sides. We are colored a faint pink by an iron-containing substance called “hemoglobin.” However, when we are joined together by the millions in a drop of blood, we look red. Now, we are not going to dwell on that. We know that some of you get uneasy when you see us, so we try to stay out of sight. But we are your friends.
We are made in a “factory,” as it were, called red marrow. In the bones of your arms, legs, and so forth, your marrow is very busy turning out new supplies of us red cells. Every day about eight and a half trillion of us are made; and a similar number are destroyed. Did you know that all of that was going on inside you?
Our life-span is not very long—only about four months. What happens when we come to the close of our life? Well, we end up either in the liver or in the spleen. Here special cells called phagocytes (meaning “cells that devour”) are waiting for us. They engulf the old red cells as they move by in the blood. Then inside the phagocytes we are broken up. But our remains are treated with respect. About 85 percent of the iron in the hemoglobin is kept in your body to use in the making of hemoglobin for the new red cells. So only the 15 percent of iron that is used up needs to be replaced. You do that by eating liver, oatmeal, eggs, spinach—a diet that includes iron-containing foods.
Our main purpose is to carry oxygen from your lungs to the cells of your body. As you may know, when we go to the lungs, we unload our carbon dioxide and get a fresh supply of oxygen.
Since the number of us in your blood is determined to an extent by your needs for oxygen, some persons have more of us than others do. For example, an athlete has more red blood cells than an office worker. And people who live at high altitudes have more of us than those who live at sea level. You may have heard the familiar expression, “Your blood thins when you become accustomed to warm weather.” Actually, as far as we are concerned, the opposite is true! For we increase in number during warm weather and we thin out in numbers during cold weather.
There is a peculiarity that we have. You really ought to know about it because it might save your life. You see, we grab up a poisonous gas called “carbon monoxide” even more quickly than we do oxygen. If you start your car in the garage before you open the door, we red cells load up with carbon monoxide. This is bad for you, because then we cannot carry oxygen. Furthermore, the hemoglobin does not let go of that poisonous gas easily. So it is a long time before we affected red cells can do good work for you again. In the meantime, your body cells are deprived of the oxygen they need. The result is dangerous. It is as if a large number of us were taken out of circulation! If too many of us are affected, that is the finish—for us and for you! So please be careful.
Our companions, the white blood cells, also have something to tell you.
White Blood Cells
First, let us say that we are not really white but colorless, so that to see us under a microscope you have to add a special dye. Even though there are five kinds of us white blood cells, there is only one of us to about every 700 or so of the red cells. Let us tell you how we differ from red blood cells.
To us white blood cells, our companions, the red cells, are like rafts on a river; they cannot choose where they want to travel. They just go whatever way the river flows. But we are different. We can move anywhere! We even have the amazing ability to change shape! Because of this some of us can actually squeeze through the tiny spaces between the cells of the capillaries and move about among the body cells.
Now our ability to travel where we want to go is very important for you. You see, we can rush to any spot in your body to aid you in destroying certain germs. Say you cut yourself; as soon as bacteria get into the wound, we gather our forces and a battle begins. You do not hear any noise; you may even sleep through it. But we are on the job to take care of you. Each one of us can engulf many of the bacteria. Though most of the time we win the battle, there are casualties on our side too. As the engulfed bacteria are broken up, bacterial toxins or poisonous wastes are released. These toxins kill some of us white cells. But then other white cells arrive to carry on the struggle.
Now a word from our smaller associates, the blood platelets.
Blood Platelets
We may not look very impressive; we are small and colorless, but we are important to you. You see, we aid in the clotting of your blood. Without us, you could bleed to death. Did you know that?
Just how blood clots is not completely understood by your medical scientists. We keep it as our secret. But we will tell you this: As blood starts to escape from a cut in your body, we platelets stick to the rough edges of the wound and begin to dissolve. At the same time we release a substance that aids in forming a trap for the escaping red and white blood cells. As this net or trap contracts, a firm clot is produced. The system really works.
All of us in the bloodstream have high regard for the heart, and we would like to have you hear from him too.
The Heart
Physically speaking, I am your master pump. I am located in the chest a little to the left side and between the lungs. My two auricles receive blood from the veins and my two ventricles force blood through the arteries. I am about the size of your fist, have strong muscular walls, and continually pump blood to all the cells of the body and back again. In a healthy person, I contract from sixty to eighty times a minute. Try to open and close your hand that fast. How long can you last? Remember, I do it for a lifetime, with no days, nights or weekends off.
Men have spent a lot of time studying me. They still have much to learn. But one of them did make this observation:
“The work the heart does during the day is about equal to the energy expended by a man in climbing to the top of a mountain 3,600 feet high. Assuming that the man weighs about 150 pounds, this would be equal to an amount of energy sufficient to lift 90 tons to a height of three feet. The work of the left side is greater than that of the right, since the former has to drive the blood all over the body, while the latter has only to force it to the lungs, which are near by. For this reason the muscle walls of the right ventricle are much thinner than that of the left ventricle.”—Elements of Biology.
Every time I beat, the blood is forced into the arteries in a wave that can be felt in the wrist or neck by placing your finger over an artery. This rhythmic wave, as you know, is called the pulse.
There is much, much more to be said, but that is enough for this time. While you are getting acquainted with us, the blood vessels would like to tell you about their role.
Blood Vessels
We are mainly three kinds: arteries, veins and capillaries. You might compare us to three sizes of pipes. In fact, your amazing circulatory system is much like the water supply system of a big city with its variety of pipes. How is that? Well, water in the reservoir has to be pushed a long way to the city. Pumps do the pushing. The city needs a big pipe through which the water can travel. The big pipe branches off into smaller ones. Each of these smaller pipes may provide water for one street. Then smaller pipes lead from the street pipe into the houses. In your circulatory system, your pump is, of course, your heart, and we blood vessels are the pipes.
We arteries carry blood AWAY from the heart. The aorta is like a main pipe in a water supply system, since it is the biggest of your arteries. Starting directly from the heart, the aorta branches into smaller arteries that enter every organ of your body. The walls of us arteries are much thicker than the walls of veins. Inside we are very smooth when we are in good health, so blood can flow through quickly. We have no small valves as the veins have. But they will tell you about that.
Before we veins tell you about our valves, you may want to know why we have valves in the first place. This is because blood has to get back to the heart, and we veins do not push it back, for we have very little muscle. So what pushes your blood back to the heart?
Well, it is not entirely pressure, because that pressure is largely gone by the time blood reaches us veins. In fact, the pressure in us is only one thirtieth of the pressure that is in the arteries. This explains, incidentally, why a cut artery bleeds more vigorously than a cut vein. Blood in us veins needs help, then, to push it along. The ordinary muscles of your body and arms and legs provide that help, the muscle action forcing the blood in us toward your heart.
‘Why doesn’t the blood just fall back again when the muscles relax,’ did you ask? That is where our valves come in. You see, our valves are unique in that they consist of a small fold of tissue that lies closely pressed against the walls when the blood flows toward the heart. However, we form cuplike pockets and fill with blood when backward pressure is exerted. Thus we keep the flow of blood going in the proper direction. We like to keep our valves in good shape, but sometimes they weaken to the point that they no longer keep the blood from backtracking. Doctors then call us “varicose veins.”
You probably have noticed that when you sit still in one place for a long time you become uncomfortable. We are involved. You see, the flow of blood through us veins has been slowed down. But when you move about a little, you feel better, because the movement of your body and leg muscles speeds up the flow of blood in us. Keep that in mind and help us out.
If you don’t mind, we arteries would like another word: After we extend into all parts of the body, we generally divide into capillaries, which come into close contact with all the cells of the body and supply them with dissolved oxygen and foods. Now, just a word from those capillaries.
We are quite small. How small? So small that the red blood cells (millions of which are in a single drop of blood) have to pass through us in single file. However, what we lack in size we make up in numbers—we are so many that if all of us in a man were stretched out in a single line, we would measure almost 60,000 miles! Because of our great numbers we can easily carry nourishment and oxygen to the tissues and readily remove the waste.
So you can see now what the general pattern of blood circulation is: From the heart, blood goes through the arteries, then usually spreads out to us tiny capillaries; next, it moves into the veins and travels back to the heart again. We have been doing this for people for thousands of years, but this basic fact of circulation of the blood is something that was discovered by your medical scientists only some 350 years ago.
All of us hope that this brief tour of your amazing circulatory system will help you to understand our individual roles and to appreciate the marvel of the human body, that it is, as the Holy Bible says, “wonderfully made.”—Ps. 139:14.
[Diagram on page 13]
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A
B
C
A. Red blood cells. B. White blood cells. C. Blood platelets.