*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