It is essential in so complicated a structure as the body that some apparatus should exist to provide for the interchange of material.  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.  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.  The colorless fluid within these vessels is termed “lymph.”  At intervals along these tubes are small structures termed the lymph nodes, which essentially are filters, and strain out from the fluid substances which might work great injury if they passed into the blood.  Between the capillary vessels and the lymphatics is the tissue fluid, in which all the exchange takes place.  It is constantly added to by the blood, and returns fluid to the blood and lymph; it gives material to the cells and receives material from them.

[Illustration:  Fig. 11.—­A diagrammatic view of the blood vessels.  An artery (a) opens into a system of capillaries, (c) and after passing through these collects into a vein (b).  Notice that the capillaries connect with other vascular territories at numerous points (d).  If the artery (a) became closed the capillaries which it supplies could be filled by blood coming from other sources.]