From the ultimate branches of the arteries the blood passes into a close network of tubes, the capillaries, which in enormous numbers are distributed in the tissues and have walls so thin that they allow fluid and gaseous interchange between their contents and the fluid around them to take place.

The blood from the capillaries is then collected into a series of tubes, the veins, by which it is returned to the heart.

This circulation is maintained by means of a pumping organ or heart, which receives the blood from the veins and by the contraction of its powerful walls forces this into the arteries, the direction of flow being determined as in a pump, by a system of valves.

The waste products of cell life pass from the cells into the fluid about them, and are in part directly returned into the blood, but for the greater part pass into it indirectly through another set of vessels, the lymphatics.

These are thin-walled tubes which originate in the tissues, and in which there is a constant flow towards the heart, maintained by the constant but varying pressure of the tissue around them, the direction of flow being maintained by numerous valves.