Physiology and Hygiene for Secondary Schools

by Francis M. Walters

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.