The food that is absorbed from the intestines, and the oxygen that is taken in through the lungs, must in some way be carried to all parts of the body. It is the task of the blood, flowing through the circulatory system, to bring these necessary substances to muscles, skin, brain, and all the other organs. We may well speak of this fluid as “the life-blood.”
The living parts of the body are changing all the time, building up new tissue out of the food and “burning up” both living matter and lifeless fuel material by the action of oxygen. These activities, as we have seen, are constantly producing waste materialscarbon dioxide and other thingswhich, if they are not eliminated, poison the body. These waste products must be carried away from the tissues; and it is again the blood which does this work.
The Blood Vessels: Arteries, Veins, and Capillaries.The blood flows inside a system of tubes, the blood vessels, which have many branches and go to all parts of the body. These blood vessels may be compared to the streets of a city, the routes by which everything passes from one place to another. Through the streets, the grocery wagons and the milk wagons come to deliver food, and the garbage wagons and the ash carts take the waste away from the houses. In somewhat the same way, the blood flows through the blood vessels, carrying food to the tissues and taking away their waste products.
The system of blood vessels starts from the heart, passes through all parts of the body, and returns to the heart again. The blood vessels that run out from the heart are called arteries. The arteries subdivide, at their ends, into numerous tiny branches called capillaries, which form a fine network all over the body. Wherever you prick or cut yourself deeply, you are sure to strike a capillary, from which the blood will flow.
These capillaries unite to form larger vessels, the veins, which bring the blood back to the heart. The walls of the capillaries are much thinner than the walls of the blood vessels, and through these thin walls the food and other things in solution may pass to and from the tissues.
The Blood : Red Corpuscles and Plasma.The liquid part of the blood is not red, but a pale straw color; it is called the plasma. Its redness, as we see it, is due to great numbers of red corpuscles (kor’ pus’ls) floating in it. These red corpuscles are of a disc shape and so small that there are millions in each drop of blood. They are living cells, and contain a sub-stance called hemoglobin, which unites very readily with oxygen and carries most of the oxygen from the lungs to the capillaries. The red cells contain more of the element iron than most tissues. It is said that a French physiologist used to exhibit in his lecture room a lump of iron which had been extracted from great quantities of blood.
If a drop of blood is set aside for half an hour under an inverted tumbler, the substances in it will separate and you will see a little mass of red corpuscles, mixed with a peculiar fibrous substance, floating in a yellowish liquid, the plasma. This separation is called clotting or coagulation. The clot or mass of solid matter which forms in this way is what stops the flow of blood from a small cut in your finger.
The “flesh color” of the skin is due to the many fine blood vessels in it. When the blood contains too few red blood corpuscles, as in certain diseases or in cases of under-feeding, a person may become very pale in appearance. Such a condition is called anaemia (a ne mi a).
In addition to the red corpuscles, the blood contains a much smaller number of white corpuscles, having their own special duties, which will be discussed later.
The Lymph-.The arteries, veins, and capillaries, as we have seen, form practically a system of closed tubes, and the exchange of oxygen and food, of carbon dioxide and waste products, takes place through the delicate walls of the capillaries. How do the oxygen and food reach the tissue cells themselves, the living units which make up the tissues between the capillaries?
This final contact with the cells is brought about by means of another fluid called the lymph, which is found all through the body, bathing its tissues (see Fig. 47). A large part of the lymph is made up of blood plasma which has passed through the walls of the capillaries; and it also contains waste products given off from the cells. If you scrape the back of your hand a little, but not deeply enough to draw blood, a yellowish fluid oozes out; if you burn yourself, a yellowish fluid gathers in the blister. This is the lymph.
The lymph is not a stagnant fluid but is in constant, although very slow, motion. Plasma is always passing out through the capillary walls into each organ of the body, and there must be some way for it to get back to the blood system again. This it does by means of a second system of tubes or ducts, called the lymphatics, which collect the lymph from the various organs and carry it to larger and larger lymphatics, which finally discharge into the large veins opening into the heart.
The HeartDid you ever wonder why we speak of learning something “by heart” or why pictures of hearts on a valentine are a sign of affection? Learning and loving belong to the brain rather than to the heart, but the heart is such an essential organ of the body, and such an active one, that it has come to stand for life itself.
The heart is the most important part of the circulatory system, the pump which, by its rhythmic contraction, drives the blood through the blood vessels to all parts of the body. It is a “heart-shaped” mass of muscle, about the size of a man’s fist, lying between the lungs. You can feel it beating (expanding and contracting) all the time, night and day, if you put your hand on the left side of your chest.
The General Course of the Blood.The heart is divided into halves, and each half is divided into an upper part or chamber, the auricle, and a lower chamber, the ventricle. From the right half of the heart, a large artery, the pulmonary artery, carries the blood (which has just returned from the tissues and has given up its oxygen to them) to the lungs. There the blood gets a new supply of oxygen, and large veins bring it back from the lungs to the left side of the heart. The blood, rich in oxygen, now passes out from the left side of the heart by a large artery, the aorta (a or’ ta), nearly an inch in diameter. It goes through the smaller arteries to the capillaries, thence to the smaller veins, and is finally brought back through the large veins to the right side of the heart, where the cycle begins again.
If you follow out this course, you will see that in the arteries (except the pulmonary artery) and the capillaries the blood is rich in oxygen, which it is carrying to the tissues. In the veins, on the other hand (except the vein coming from the lungs), it has given up most of its oxygen and has become loaded with carbon dioxide.
Blood on its way out to the tissues, rich with its gifts of oxygen, is bright red, like the capillary blood which flows when you cut your finger. Blood on the way back, after giving up its oxygen, is of a darker purplish red, almost blue, as in the veins in the back of the hand.
The Valves of the Heart and Veins.The blood flows out through the arteries (not through the veins) when the heart contracts; and it flows in from the veins (not from the arteries) when the heart expands. This is due to the arrangement of the little flaps or valves located between the auricles and ventricles, and at the places where the arteries leave the heart (see Fig. 51) The valves of the first set open downward, so that when the heart expands they open and allow the blood to flow down into the ventricles. When the heart contracts, they close again and the valves into the arteries open upward, so that, with the contraction of the heart, the blood is pushed out into the arteries. When the heart expands, the valves at the arteries close again and the valves between auricle and ventricle open. In the veins, there are similar valves which allow the blood to flow toward the heart but not in the other direction.
The Working of the Circulation.The fact that the blood circulates from the heart to all parts of the body and back again, was discovered in 1621 by a famous Englishman named William Harvey. Harvey (15781667) was Court Physician to Charles I, and it is said that being placed in charge of the king’s children at the battle of Edgehill, he sat with them under a hedge calmly reading a book all through the fighting. Through his discoveries, he became the greatest figure in the history of physiology.
It seems curious to think that only three hundred years ago scientific people believed that the blood flowing into the heart by the veins all came from the water and food taken in through the alimentary canal, that the expansion of the heart sucked this in, and that some kind of suction exerted by the tissues drew it out through the arteries. The great volume of blood flowing into the heart should have shown that this explanation was absurd; but it was Harvey who first convinced physicians and physiologists that the blood flowing out through the arteries is the same blood which comes back again through the veinsin other words, that there is a true circulation.
This circulation of the blood is one of the most wonderful of the many wonderful things which take place within our bodies. The whole of the blood (in an adult man about twelve pints) may complete its round through the body in from twenty to thirty seconds. At that rate, it makes the circuit more than three thousand times a day. Night and day, year after year, “this river of life,” as one writer has said, “is impetuously rushing through every part of the body, by means of an elaborate network of canals.”
The chief force that carries on this work is the beat of the heart, which is produced by the contraction of its muscular walls. The blood vessels themselves, however, also play an active part in the process of circulation. Their walls, particularly those of the arteries, are elastic, and they press on the blood and help to drive it farther and farther into the smaller blood vessels. With a rubber bulb, filled with water and connected with a rubber tube, you can imitate this action. When you press on such a bulb, you force the water into the tube, making it bigger; but the tube itself tends to contract and press the water onward. Each squeeze on the bulb sends a wave of pressure along the tube. This is just what happens in the circulation. Each beat of the heart causes a wave of pressure to pass along the arteries.
By placing the finger gently on a large artery, such as the one in the wrist, you can feel these changes in pressure, which we call the pulse. Each pulse beat corresponds to a beat of the heart. In grown men there are about seventy pulse beats a minute, and in women about eighty. In children the pulse beats more rapidly.
How the Heart and Blood Vessels Meet the Changing Needs of the Body.When a muscle is working actively, it needs more food and oxygen and has more waste to get rid of than when it is at rest. The same thing is true of every part of the body.
If, for instance, a person who has been lying down rises to a standing position, the muscles have more work to do in holding the body erect, and the heart meets this need at once by beating a little faster. If you run or play hard at any game, your heart beats a good deal faster, to supply extra blood to the muscles. You can easily notice that your pulse rate increases during active exercise.
It is not only the heart beat, however, that varies to suit the changing needs of the body. The amount of blood which goes to any special organ depends largely on the contraction or expansion of the walls of the arteries leading to it. When more blood is wanted in a particular place, the muscles of the walls of the smaller arteries relax and their fine branches grow bigger, so that more blood reaches that particular organ. After a meal, for instance, the stomach and intestines are working hard and need more blood, and the blood vessels in those parts of the body expand to supply the need. It is hurtful to take exercise just after meals, because exercise draws the blood to the muscles and away from the digestive organs. It is because the blood is largely in the digestive organs, and the amount flowing through the brain is lessened, that we often feel sleepy after eating.
The way in which the blood vessels change in size is well illustrated by the act of blushing. Embarrassment causes a dilation of the blood vessels of the skin, and the face be-comes flushed, as a result.
The Temperature of the Body and How It is Controlled.When you go from a hot room into a cold one, your body feels colder. In reality, it is only the surface of the body which changes its temperature. As long as you are in good health, the temperature of the body below its surface stays very close to 98.6° Fahrenheit, whatever the temperature is outside. In the Tropics the air temperature may rise above 130°, and in certain factories, iron foundries, and glassworks it may for a time be even higher. In Arctic regions it may be nearly 100 below zero. Yet through a range of more than 200 difference of air temperature, the body temperature remains the same. At the North Pole with Peary or in the depths of the Brazilian forests with Roosevelt, the thermometer placed under the tongue of a man in good health would register close to 98.6°.
The higher animals, such as cats and dogs and birds, all maintain their bodies at a constant temperature in this way, and as the temperature is usually higher than that of the air about them, they are called warm-blooded animals. Fishes, frogs, insects, and the like are called cold-blooded, though what is really meant is, not that they are cold, but that they take on the temperature of their surroundings.
It is mainly by changes in the distribution of blood between the surface and the inner parts of the body that the general temperature of the human body is kept uniform. The life processes are constantly producing heat, and this heat is constantly given off from the surface of the body to the air about it. When the air is cold, the body will lose heat rapidly; but when it is warm, the body, in order to keep its temperature at 98.6°, must find some special way of giving off its surplus heat. This it does largely by an increase in the size of the skin blood vessels, so that more blood will go to the skin, to be exposed to the cooling influence of the air.
If you pass from the cold street into a hot room, your face becomes flushed. This is because the blood vessels in the skin have expanded so as to bring more blood to the surface to be cooled off. In a cold room, on the other hand, the blood in the skin would cool very rapidly and the vessels contract so as to prevent too much chilling.
The fact that the nerves control the blood vessels of the skin in this way was another of the discoveries which developed from the work of the French physiologist, Claude Bernard. He found that if the nerves running to a certain part of the body were injured, the part would become flushed and hot. It was finally shown that this heating was due to the expansion of the blood vessels, which let an excess of blood into the tissues, and that the vessels expand in this way whenever the muscles in their walls are not controlled by the nerves.
A second important aid in keeping the body coolwhich comes into play particularly when the atmosphere is hot is the sweat, or perspiration, which is poured out from tiny glands in the skin. When perspiration is produced very rapidly, it collects on the skin, but even when the skin does not feel moist, there may be perspiration forming and evaporating into the air just as fast as it is formed. The evaporation of this moisture makes the skin cooler, just as your finger feels cool if you wet it and hold it up in a breeze. This is the reason that we are so much more comfortable on a dry hot day than on a moist hot day, for the sweat cannot evaporate freely when there is much moisture in the air. Dogs and other animals sweat very little. On a hot day they can relieve themselves only by violent panting, which draws cool air into the lungs and increases evaporation from the surfaces of the lungs and of the nose and mouth passages. In our own case, when it grows hot outside, or when we exercise and produce a great deal of heat in the body, the secretion of sweat increases; when it grows cool, the secretion diminishes.
The Circulation and the Body Temperature in Illness.A good healthy circulation is one that meets all the changing conditions of life quickly and easily. When the heart, or the muscles in the walls of the blood vessels, or the nerves which control them are not working properly, we say a person has a poor circulation; such a person’s hands or feet may grow cold very easily, and he may also be very sensitive to heat.
In many illnesses, the complicated machinery of the circulatory system can no longer do its work successfully. In certain diseases the heart beats faster than it shouldin others, more slowly. In some diseases its beat is weak and irregular. The feeling of the pulse is often a great help to the physician in finding out what is wrong. Its rate, which is the same as the beat of the heart, can be observed, as well as the evenness of the beats and the amount of force with which the blood is pressing on the walls of the vessels. Of course, the pulse varies greatly even in perfect health as during exerciseand only a physician can tell whether or not a particular condition of the pulse indicates anything wrong.
In many kinds of illness the temperature machinery of the body is upset. The temperature may rise above 98.6° or fall below it. When it reaches 100° or more, the condition is known as fever. It is a good plan to have, in every family, a clinical thermometer. When any one, child or grown person, feels indisposed, the temperature should be taken; and if the thermometer records a temperature over 100°, the patient should stay indoors and the doctor should be sent for.
Effects of Alcohol upon the Circulation.People some-times take alcoholic drinks to warm themselves when they are chilly. What alcohol does in such a case is to dilate the skin blood vessels and make the person feel warmer be-cause the skin, where the nerves of feeling are, is warmer. But the presence of so much blood in the skin means a more rapid loss of heat and a consequent chilling, rather than a warming of the body as a whole. As we shall see later, alcohol is not really an effective stimulant.
The delicate machinery of the heart and blood vessels is easily and seriously injured by alcohol and other poisons. The heart muscles in persons who use alcoholic drinks are likely to become flabby and weak, so that they are not able to do their work when special strain is put upon them.
When people grow old, lime collects in the walls of the arteries and causes them to become hard and stiff, much as the bones become richer in mineral matter with advancing age. Such arteries, instead of being soft and elastic like a new rubber tube, become brittle like old dry rubber, and can no longer respond quickly to the needs of the body. Many of the diseases from which old people suffer are due to this “hardening of the arteries.” Alcoholic drinks, the poisons which are formed during an attack of a communicable disease, and the poisons which are absorbed from bacterial decay in the intestines, all help to injure the walls of the blood vessels and bring on hardening of the arteries long before normal old age should set in. When the arteries harden in this way, it takes more force to pump the blood through them. The heart is often weakened by such over-work, so that it is not able to withstand strains such as those put upon it by serious illness.
Effects of Tobacco upon the Circulation.Tobacco also exerts harmful effects on the heart and the circulation. Even the moderate use of tobacco causes shortness of breath, which is really due to poor heart action. When smokers have “poor wind “, it is because their hearts cannot pump the blood fast enough to supply the oxygen needed by their muscles, and the lungs have to do extra work to make good the deficiency. The disease called “tobacco heart” is another well-recognized result of smoking. In this disease the heart beats very weakly and irregularly.
It may easily be seen that such disturbances of the heart and circulation seriously affect the best working of the whole body. Experience shows that smokers are much less likely to be successful in athletic competitions than those who do not use tobacco. Professor F. J. Pack reports that of 93 smokers who tried for places on six different football teams, only 31 “made the team”; while of 117 non-smokers, 79 were successful. That is, only 33 per cent of the smokers secured places on the teams, as against 68 per cent of the non-smokers. Tobacco is, of course, always forbidden to athletes in training.
QUESTIONS FOR DISCUSSION AND REVIEW
1. By what process do digested foods get from the intestines to the tissues which they build or repair?
2. By what means are the waste products of the tissues carried away?
3. What are blood vessels? How many kinds of blood vessels can you name? How do they differ?
4. Compare the blood vessels with the streets of a city.
5. Imagine that you are looking at some blood through a microscope. What would you see?
6. What gives the red color to blood?
7. What part of the blood carries the oxygen?
8. What is the advantage of the clot which forms when blood comes in contact with the air?
9. Would you suppose that an anaemic person got as much oxygen from the air as a rosy person?
10. What is the special work of the lymph?
11. Where does the lymph come from and where does it go?
12. Are lymph and blood at all alike? How do they differ?
13. Why is the heart such an important organ?
14. Of what kind of tissue is the heart composed?
15. How is the heart divided?
16. Outline the course of the blood from the time it leaves the heart till it returns.
17. How does the blood in the aorta differ from that in the big veins emptying into the heart?
18. What prevents the blood from flowing backward when the heart contracts? Where else in the circulatory system do we find a similar arrangement? Do you know of any pieces of machinery which work in the same way?
19. Who discovered the true facts of the circulation of the blood? Of what value do you think this discovery has been?
20. What kind of walls do we find in the arteries? How do they help the work of the heart?
21. How many times a minute does the heart beat?
22. Count the pulse of one of your schoolmates, using the second hand of a watch. Have him run up and down a flight of stairs, and then count his pulse. Make him lie down a few minutes and count again. How do you explain the variation?
23. How is the amount of blood in the different parts of the body controlled?
24. Why is it difficult to study after a hearty dinner? Why is it dangerous to play a hard game of ball just after dinner?
25. What is the normal temperature of the body? What is a fever? Does the temperature ever go below normal?
26. How is the temperature of the body controlled so that it is the same in all climates?
27. What makes one’s face red after violent exercise? On a hot day?
28. What is the difference between a cold-blooded and a warm-blooded animal? Which is man?
29. Where does the heat of the body come from?
30. How does the body get rid of the extra heat?
31. In warm climates, people cool water bottles by covering them with a wet cloth and hanging them in the breeze. Ex-plain. Compare with your body.
32. Why do we drink more water in hot weather than in cold weather?
33. In what ways do alcohol and tobacco affect the circulation?