Physiology and Hygiene for Secondary Schools. Francis M. Walters
the inferior vena cava and the veins of the brain, which are not compressed by movements of the body, do not have valves.
Purposes of the Muscular Coat.—The muscular coat, which is thicker in the arteries than in the veins and is more marked in small arteries than in large ones, serves two important purposes. In the first place it, together with the elastic tissue, keeps the capacity of the blood vessels equal to the volume of the blood. Since the blood vessels are capable of holding more blood than may be[pg 050] present at a given time in the body, there is a liability of empty spaces occurring in these tubes. Such spaces would seriously interfere with the circulation, since the heart pressure could not then reach all parts of the blood stream. This is prevented by the contracted state, or "tone," of the blood vessels, due to the muscular coat.
In the second place, the muscular coat serves the purpose of regulating the amount of blood which any given organ or part of the body receives. This it does by varying the caliber of the arteries going to the organ in question. To increase the blood supply, the muscular coat relaxes. The arteries are then dilated by the blood pressure from within so as to let through a larger quantity of blood. To diminish the supply, the muscle contracts, making the caliber of the arteries less, so that less blood can flow to this part of the body. Since the need of organs for blood varies with their activity, the muscular coat serves in this way a very necessary purpose.
Fig. 21—Diagram of network of capillaries between a very small artery and a very small vein. Shading indicates the change of color of the blood as it passes through the capillaries. S. Places between capillaries occupied by the cells.
Capillaries.—The capillaries consist of a network of minute blood vessels which connect the terminations of the smallest arteries with the beginnings of the smallest veins (Fig. 21). They have an average diameter of less than one two-thousandth of an inch (12 µ) and an average length of less than one twenty-fifth of an inch (1 millimeter). Their walls consist of a single [pg 051] coat which is continuous with the lining of the arteries and veins. This coat is formed of a single layer of thin, flat cells placed edge to edge (Fig. 22). With a few exceptions, the capillaries are found in great abundance in all parts of the body.
Fig. 22—Surface of capillary highly magnified, showing its coat of thin cells placed edge to edge.
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 pass 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 passing 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[pg 052] 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 passes from the right ventricle through the lungs, and is then returned to the left auricle; by the latter it passes 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 passes 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 venæ cavæ, 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—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, passes 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[pg 054] small arteries, becomes comparatively slight in the capillaries, and falls practically to nothing in the veins. Near the heart in the superior and inferior venæ cavæ, 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 quantity 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).
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HYGIENE OF THE CIRCULATION
Care of the