*Functions of the Capillaries.*-On account of the thinness of their walls, the capillaries are able to serve a twofold purpose in the body:

1. They admit materials into the blood vessels.

2. They allow materials to pa.s.s from the blood vessels to the surrounding tissues.

When it is remembered that the blood, as blood, does not escape from the blood vessels under normal conditions, the importance of the work of the capillaries is apparent. To serve its purpose as a carrier, there must be places where the blood can load up with the materials which it is to carry, and places also where these can be unloaded. Such places are supplied by the capillaries.

The capillaries also serve the purpose of spreading the blood out and of bringing it very near the individual cells in all parts of the body (Fig.

21).

*Functions of Arteries and Veins.*-While the capillaries provide the means whereby materials may both enter and leave the blood, the arteries and veins serve the general purpose of pa.s.sing the blood from one set of capillaries to another. Since pressure is necessary for moving the blood, these tubes must connect with the source of the pressure, which is the heart. In the arteries and veins the blood neither receives nor gives up material, but having received or given up material at one set of capillaries, it is then pushed through these tubes to where it can serve a similar purpose in another set of capillaries (Fig. 23).

*Divisions of the Circulation.*-Man, in common with all warm-blooded animals, has a double circulation, a fact which explains the double structure of his heart. The two divisions are known as the _pulmonary_ and the _systemic_ circulations. By the former the blood pa.s.ses from the right ventricle through the lungs, and is then returned to the left auricle; by the latter it pa.s.ses from the left ventricle through all parts of the body, returning to the right auricle.

The general plan of the circulation is indicated in Fig. 23. All the blood flows continuously through both circulations and pa.s.ses the various parts in the following order: right auricle, tricuspid valve, right ventricle, right semilunar valve, pulmonary artery and its branches, capillaries of the lungs, pulmonary veins, left auricle, mitral valve, left ventricle, left semilunar valve, aorta and its branches, systemic capillaries, the smaller veins, superior and inferior venae cavae, and then again into the right auricle.

In the pulmonary capillaries the blood gives up carbon dioxide and receives oxygen, changing from a dark red to a bright red color. In the systemic capillaries it gives up oxygen, receives carbon dioxide and other impurities, and changes back to a dark red color.

In addition to the two main divisions of the circulation, special circuits are found in various places. Such a circuit in the liver is called the _portal_ circulation, and another in the kidneys is termed the _renal_ circulation. To some extent the blood supply to the walls of the heart is also outside of the general movement; it is called the _coronary_ circulation.

[Fig. 23]

Fig. 23-*General scheme of the circulation*, showing places where the blood takes on and gives off materials. 1. Body in general. 2. Lungs. 3.

Kidneys. 4. Liver. 5. Organs of digestion. 6. Lymph ducts. 7. Pulmonary artery. 8. Aorta.

*Blood Pressure and Velocity.*-The blood, in obedience to physical laws, pa.s.ses continuously through the blood vessels, moving always from a place of greater to one of less pressure. Through the contraction of the ventricles, a relatively high pressure is maintained in the arteries nearest the heart.(21) This pressure diminishes rapidly in the small arteries, becomes comparatively slight in the capillaries, and falls practically to nothing in the veins. Near the heart in the superior and inferior venae cavae, the pressure at intervals is said to be _negative_.

This means that the blood from these veins is actually drawn into the right auricle by the expansion of the chest walls in breathing.(22)

The velocity of the blood is greatest in the arteries, less in the veins, and _much_ less in the capillaries than in either the arteries or the veins. The slower flow of the blood through the capillaries is accounted for by the fact that their united area is many times greater than that of the arteries which supply, or the veins which relieve, them. This allows the same quant.i.ty of blood, flowing through them in a given time, a wider channel and causes it to move more slowly. The time required for a complete circulation is less than one minute.

*Summary of Causes of Circulation.*-The chief factor in the circulation of the blood is, of course, the heart. The ventricles keep a pressure on the blood which is sufficient to force it through all the blood tubes and back to the auricles. The heart is aided in its work by the elasticity of the arteries, which keeps the blood under pressure while the ventricles are in a state of relaxation. It is also aided by the muscles and elastic tissue in all of the blood vessels. These, by keeping the blood vessels in a state of "tone," or so contracted that their capacity just equals the volume of the blood, enable pressure from the heart to be transmitted to all parts of the blood stream. A further aid to the circulation is found in the valves in the veins, which enable muscular contraction within the body, and variable pressure upon its surface, to drive the blood toward the heart. The heart is also aided to some extent by the movements of the chest walls in breathing. The organs Of circulation are under the control of the nervous system (Chapter XVIII).

HYGIENE OF THE CIRCULATION

*Care of the Heart.*-The heart, consisting largely of muscle, is subject to the laws of muscular exercise. It may be injured by over-exertion, but is strengthened by a moderate increase in its usual work.(23) It may even be subjected to great exertion without danger, if it be trained by gradually increasing its work. Such training, by giving the heart time to gain in size and strength, prepares it for tasks that could not at first be accomplished.

In taking up a new exercise requiring considerable exertion, precautions should be observed to prevent an overstrain of the heart. The heart of the amateur athlete, bicyclist, or mountain climber is frequently injured by attempting more than the previous training warrants. The new work should be taken up gradually, and feats requiring a large outlay of physical energy should be attempted only after long periods of training.

Since the heart is controlled by the nervous system, it frequently becomes irregular in its action through conditions that exhaust the nervous energy. Palpitations of the heart, the missing of beats, and pains in the heart region frequently arise from this cause. It is through their effect upon the nervous system that worry, overstudy, undue excitement, and dissipation cause disturbances of the heart. In all such cases the remedy lies in the removal of the cause. The nervous system should also be "toned up" through rest, plenty of sleep, and moderate exercise in the open air.

*Effect of Drugs.*-A number of substances cla.s.sed as drugs, mainly by their action on the nervous system, produce undesirable effects upon the organs of circulation. Unfortunately some of these are extensively used, alcohol being one of them. If taken in any but small quant.i.ties, alcohol is a disturbing factor in the circulation. It increases the rate of the heart beat and dilates the capillaries. Its effect upon the capillaries is shown by the "bloodshot" eye and the "red nose" of the hard drinker.

Another bad effect from the use of much alcohol is the weakening of the heart through the acc.u.mulation of fat around this organ and within the heart muscle. The use of alcohol also leads in many cases to a hardening of the walls of the arteries, such as occurs in old age. This effect makes the use of alcohol especially dangerous for those in advanced years.

Tobacco contains a drug, called nicotine, which has a bad effect upon the heart in at least two ways: 1. When the use of tobacco is begun in early life, it interferes with the growth of the heart, leading to its weakness in the adult. 2. When used in considerable quant.i.ty, by young or old, it causes a nervous condition both distressing and dangerous, known as "tobacco heart."

Tea and coffee contain a drug, called caffeine, which acts upon the nervous system and which may, on this account, interfere with the proper control of the heart. In some individuals the taking of a very small amount of either tea or coffee is sufficient to cause irregularities in the action of the heart. Tea is considered the milder of the two liquids and the one less liable to injure.

*Effect of Rheumatism.*-The disease which affects the heart more frequently than any other is rheumatism. This attacks the lining membrane, or endocardium, and causes, not infrequently, a shrinkage of the heart valves. The heart is thus rendered defective and, to perform its function in the body, must work harder than if it were in a normal condition.

Rheumatic attacks of the heart do most harm when they occur in early life-the period when the valves are the most easily affected. Any tendency toward rheumatism in children has, therefore, a serious significance and should receive the attention of the physician. Any one having a defective heart should avoid all forms of exercise that demand great exertion.

*Strengthening of the Blood Vessels.*-Disturbances of the circulation, causing too much blood to be sent to certain parts of the body and an insufficient amount to others, when resulting from slight causes, are usually due to weakness of the walls of the blood vessels, particularly of the muscular coat. Such weakness is frequently indicated by extreme sensitiveness to heat or cold and by a tendency to "catch cold." From a health standpoint the preservation of the normal muscular "tone" of the blood vessels is a problem of great importance. Though the muscles of the blood vessels cannot be exercised in the same manner as the voluntary muscles, they may be called actively into play through all the conditions that induce changes in the blood supply to different parts of the body.

The usual forms of physical exercise necessitate such changes and indirectly exercise the muscular coat. The exposure of the body to cold for short intervals, because of the changes in the circulation which this induces, also serves the same purpose. A cold bath taken with proper precautions is beneficial to the circulation of many and so also is a brisk walk on a frosty morning. Both indirectly exercise and strengthen the muscular coat of the blood vessels. On the other hand, too much time spent indoors, especially in overheated rooms, leads to a weakening of the muscular coat and should be avoided.

*Checking of Flow of Blood from Wounds.*-The loss of any considerable quant.i.ty of blood is such a serious matter that every one should know the simpler methods of checking its flow from wounds. In small wounds the flow is easily checked by binding cotton or linen fiber over the place. The absorbent cotton, sold in small packages at drug stores, is excellent for this purpose and should be kept in every home. A simple method of checking "nosebleed" is that of drawing air through the bleeding nostril, while the other nostril is compressed with the finger.(24) Another method is to "press with the finger (or insert a small roll of paper) under the lip against the base of the nose." (25) Where the bleeding is persistent, the nostril should be plugged with a small roll of clean cotton or paper. When this is done, the plug should not be removed too soon because of the likelihood of starting the flow afresh.

In dealing with large wounds the services of a physician are indispensable. But in waiting for the physician to arrive temporary aid must be rendered. The one who gives such aid should first decide whether an artery or a vein has been injured. This is easily determined by the nature of the blood stream, which is in jets, or spurts, from an artery, but flows steadily from a vein. If an artery is injured, the limb should be tightly bandaged on the side of the wound nearest the heart; if a vein, on the side farthest from the heart. In addition to this, the edges of the wound should be closed and covered with cotton fiber and the limb should be placed on a support above the level of the rest of the body. A large handkerchief makes a convenient bandage if properly applied. This should be folded diagonally and a knot tied in the middle. Opposite ends are then tied, making a loose-fitting loop around the limb. The knot is placed directly over the blood vessel to be compressed and a short stick inserted in the loop. The necessary pressure is then applied by twisting the handkerchief with the stick. Time must not be lost, however, in the preparation of a suitable bandage. The blood vessel should be compressed with the fingers while the bandage is being prepared.

*Summary.*-The blood, to serve as a transporting agent, must be kept continually moving through all parts of the body. The blood vessels hold the blood, supply the channels and force necessary for its circulation, and provide conditions which enable materials both to enter and to leave the blood stream. The heart is the chief factor in propelling the blood, although the muscles and the elastic tissue in the walls of the arteries and the valves in the veins are necessary aids in the process. In the capillaries the blood takes on and gives off materials, while the arteries and veins serve chiefly as tubes for conveying the blood from one system of capillaries to another.

*Exercises.*-1. Of what special value in the study of the body was the discovery of the circulation of the blood?

2. State the necessity for a circulating liquid in the body.

3. Show by a drawing the general plan of the heart, locating and naming the essential parts. Show also the connection of the large blood vessels with the cavities of the heart.

4. Compare the purpose served by the chordae tendineae to that served by doorstops (the strips against which the door strikes in closing).

5. Explain how the heart propels the blood. To what cla.s.s of pumps does it belong? What special work is performed by each of its divisions?

6. Define a valve. Of what use are the valves in the heart? In the veins?

7. By what means is pressure from contracting muscles in different parts of the body made to a.s.sist in the circulation?

8. Of what advantage is the elasticity of the arteries?

9. How is blood forced from the capillaries back to the heart?

10. Why should there be a difference in structure between the two sides of the heart?

11. Following Fig. 23, trace the blood through a complete circulation, naming all the divisions of the system in the order of the flow of the blood.

12. If the period of rest following the period of contraction of the heart be as long as the period of contraction, how many hours is the heart able to rest out of every twenty-four?

13. State the functions of the capillaries. Show how their structure adapts them to their work.

14. What kind of physical exercise tends to strengthen the heart? What forms of exercise tend to injure it? State the effects of alcohol and tobacco on the heart.

15. How may rheumatism injure the heart?

16. Give directions for checking the flow of blood from small and from large blood vessels.