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 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 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