The innumerable cell units of the body must have material to provide energy, and useless material which results from their activity must be removed.

A household might be almost as much embarrassed by the accumulation of garbage and ashes as by the absence of food and coal.
The food, which is taken into the alimentary canal and converted by the digestive fluids into material more directly adapted to the uses of cells, must be conveyed to them.

A supply of oxygen is essential for the life of the cells, and the supply which is given by respiration must be carried from the lungs to every cell of the body.
All this is effected by the circulation of the blood, which takes place in the system of branching closed tubes in which the blood remains (Fig.

11).

Certain of these tubes, the arteries, have strong and elastic walls and serve to convey and distribute the blood to the different organs and tissues.