Circulation Of The Blood
( Originally Published 1920 )
The Blood and the Blood Vessels.—The food for the various organs of the body is taken in through the walls of the intestines, and the oxygen through the walls of the lungs. Somehow the food and the oxygen must be carried about the body to all the different organs; and you have learned in Chapter II that it is the blood which does this work.
If you could trace one of the tiny blood vessels in the lung, you would find that the stream it carries joins with another and flows into a larger vessel; and that joins with others and flows into a still larger one; and so on, until finally the combined stream from both the lungs pours into the heart. The heart is a hollow organ, about the size of the fist, which lies in the chest between the lungs (see Fig. 50). From the heart, the stream flows out again through a big blood vessel that measures nearly an inch across. This blood vessel branches into finer ones, which in turn branch into still finer vessels; and in these the stream flows at last to the most distant parts of the body, to the top of the head and the soles of the feet and the tips of the fingers (Fig. 51).
The blood vessels that carry the blood into the heart are called veins. Those by which the blood flows out from the heart are called arteries.
You know how fortunate a city is that is situated on the shore of a river, so that steamers can bring to it food and clothing, wood and coal, and the other things its people need. The blood serves the organs of the body as the river serves such a city, for through every organ the blood is constantly flowing as a stream of life, laden with the food and oxygen the body needs.
The blood serves another very important purpose, too, for it not only brings to the organs the oxygen and food, but it carries away the wastes which the organs are forming all the time and which they must get rid of, if the body is to keep well.
The Cells of the Blood.—You may wonder perhaps if there are any bodies in the blood stream corresponding to the ships which sail on a river and carry goods to the city on its banks. There are; and by using a microscope such as was described on page 102, we can see the ships that sail in the blood stream.
We cannot very well see the blood actually flowing through our own blood vessels even with a microscope; but it is possible to see the blood flowing in the vessels of certain animals which have a very thin, transparent skin. The foot of a frog, for example, has a very delicate skin between the toes; and if the frog's foot is held under a microscope, one can see a wonderful sight. The thin part of the foot between the toes is full of small blood vessels; and each of these, under the microscope, is seen to be crowded with little round discs, like those which are drawn in Fig. 52. They are not still and quiet, how-ever, as you see them in the picture, but are rushing past at a great speed and tumbling about in the stream, as they are carried along by the blood flow. These tiny ships are the red cells of the blood. It is the red cells which carry oxygen from the lungs to the other organs, and in the tissues exchange the oxygen for carbon dioxide. They carry the carbon dioxide to the lungs, where they give it up and take on a new cargo of oxygen in its place.
These red cells are so abundant in the blood that they give it the red color. The liquid in which they float is not red at all, but a yellowish straw-color.
There is another kind of cell in the blood stream which might be likened to a warship, since we have compared the red cells to merchant ships. These are larger and less numerous than the red cells. As they are whitish in color, they are called the white cells. They attack and destroy harmful things, like disease germs, which enter the blood. When we "get over" an attack of influenza or typhoid fever or a cold in the head or some other disease, it is largely because of the activity of these white cells in defending the body against its enemies.
The Work of the Heart.—It takes a great deal of force to drive the blood through these fine channels all over the body, and this force is furnished by the beat of the heart.
The heart is a hollow chamber with very heavy muscular walls. It is all the time expanding and contracting with a regular beat, which one can hear by putting the ear to the left' side of a person's chest. At each expansion blood is drawn in from the veins, and at each contraction it is forced out into the arteries.
The waves of pressure set up by the beating of the heart are carried all the way along the arteries. When the doctor puts his finger on the large artery in your wrist to feel your pulse, what he is really doing is to count the beats of the heart as they are recorded by these waves of pressure in-the artery.
Richard the Lion-heart.—The heart is a wonderful organ, beating all the time about once every second, and driving the life blood out to all parts of the body. It is such an important part of the body that we often speak of a man who is very good and noble as being "great-hearted," and of one who is very gentle as "kind-hearted."
There was once a king of England., Richard the First, who was so brave and such a great soldier that he was called "Cceur-de-Lion," which is the French for "the Lion-heart." He won this name because he was so brave that it was thought that he must have a heart something like that of the bravest and fiercest of beasts, the lion. You will read all about him some day, how he led his army into the Holy Land to try to reconquer Jerusalem from the Turks, how he performed many acts of personal bravery, and how on his return he was taken prisoner by an Austrian ruler.
For a long time no one knew where Richard was imprisoned; but according to one story, he was at last found by a faithful minstrel, named Blondel, who had been with him in the Holy Land. Blondel disguised himself and wandered all through Germany and Austria, singing one of the king's favorite songs under the walls of every castle. He hoped that when he got to the place where Richard was, the king would hear him and know that a friend was near. At last Blondel's patience was rewarded, for as he was singing at the foot of a tower, Richard's voice took up the next verse of the song in reply. Blondel carried home to England the news of where the king was, and through him Richard was restored to his throne and his country. Blondel must have been somewhat of a Lion-heart himself, if that kind of heart always goes with courage.
Harvey's Discovery of the Circulation of the Blood. —A long, long time after Richard Coeur-de-Lion reigned, there was another king of England, Charles the First, who had a court physician named William Harvey. It is to Harvey that we owe the discovery of how the blood circulates, for even the wisest people before Harvey's time did not know as much as you know about it—if you have studied this chapter carefully.
Scientific men three hundred years ago knew that blood flowed out through the arteries and in through the veins to the heart. But many of them thought that the blood which went out of the heart never came back, and that the blood which came in was being made fresh all the time from the water and food in the alimentary canal. Harvey was a man who thought things out for himself. As an illustration of his studious habits, it is said that one day when he was placed in charge of the king's children during a battle in which his royal master was engaged, he sat with them under a hedge calmly reading a book all the time the battle was raging. He did more than think and read, however.
He studied nature at first hand, and particularly the actual structure and behavior of the human body. He found out that the same blood forced out by the beat of the heart through the arteries comes back again to the heart through the veins, and is thus kept in a true and constant circulation.
How the Blood Supply to Different Organs is Regulated.—The various organs of the body need different amounts of blood at different times. When you are running or playing actively, for example, your muscles are working hard, and they need more oxygen than when they are at rest; they also make more carbon dioxide that must be carried away. In order to meet this need, the heart beats faster, so that the blood with its freight of oxygen will go faster through the muscles. This more rapid beat after active exercise can easily be measured by noting the increase in the rate of the pulse beat at the wrist.
It is not only the rate of the heart beat that varies. The blood vessels also adapt themselves to changing needs. The walls of the arteries are not stiff and hard but elastic, and in these walls there are tiny muscles which make the vessels smaller when they contract and larger when they expand. When a muscle or any other organ is active, the walls of the arteries in that particular part grow larger, so that more blood can flow through it. Think how wonderful is this arrangement by which the needs of all parts of the body are met without a thought or a care on your part.
The Body Temperature.—Another very important thing which the blood vessels do for us is to help to regulate the body temperature.
When you have been outdoors in winter, your hands and your cheeks often feel cold; but if a thermometer such as the doctor uses were placed in your mouth, it would register between 98° and 99° in January, just as it would in August. This is a very remarkable thing—that the body should keep its temperature just the same, whether the air around it is below zero or nearly too °. We can make machines, like the incubators used in raising chickens, which will do this; but they do not work nearly so perfectly as the human body does.
Have you ever wondered where the body gets its heat, in the first place?, You know, when you get into bed in winter, how cold the sheets are, and how nice and warm they get, after you have been under the covers a little while. All this heat has been formed in your body. As you have learned in Chapter VII, the heat of the body is made from the food. A great deal of this production of heat goes on in the muscles, and when the muscles are actively exercised, you are likely to get overheated.
How the Body Temperature is Regulated.—In order that the temperature of the body shall remain about the same all the time, the amount of heat given off from the body through the skin must just equal that which is formed inside the body. In cold weather, you might expect that the body would lose heat very rapidly and that the blood flowing through the skin would become chilled. The muscles in the blood vessels of the skin, however, take care that this shall not hap-pen. They contract and make these vessels so small that very little blood can pass through them, and so the heat of most of the blood is kept shut up in the inner parts of the body.
When one is in a very warm room, on the other hand, the body has hard work to get rid of its heat. Then all the blood vessels of the skin expand, so that as much blood as possible is allowed to flow through and be cooled by the outside air. The cheeks become flushed in a hot room, and the whole body becomes pink in a hot bath, because of this enlargement of the blood vessels of the skin.
There is another change that takes place when the air is warm, which helps a great deal in regulating the temperature of the body. All through the skin there are tiny organs called sweat-glands. They pour out sweat, or perspiration, as soon as there is danger that the body may become overheated. The moisture thus produced evaporates and cools the skin. You can see that the evaporation of moisture cools the skin, if you wet your finger and then hold it up in a breeze.
In sickness these arrangements are upset, and the temperature of the body often changes. If it goes very much above 99 degrees, we say the person has a fever. If one feels half-sick or out-of-sorts, it is a very good thing to have the temperature taken, by putting a special kind of thermometer in the mouth, to see if one has fever. Often the rise in temperature is the first sign that an attack of some disease is beginning.
Alcohol and Tobacco and the Circulation. The organs of circulation, the heart and the blood vessels, are among the most important of all the organs of the body, and anything which harms them will seriously injure the health of the body as a whole. Both the heart and the blood vessels are especially sensitive to the effect of any poisons taken into the body, for such poisons get into the blood and come into direct contact with their delicate walls. In old people, the walls of the arteries become hard and brittle and do not do their work well. The use of alcoholic drinks is likely to in-crease this hardening of the arteries, making people old before their time.
Smoking, particularly in young people, affects the heart and makes its action irregular. Boys and men who are training for athletic teams are never allowed to use either alcohol or tobacco.