The entire circulatory system is an elaborate device for providing each cell of a complex, multicellular organism with the type of exchange, diffusion is practiced by the simplest unicellular organisms. This exchange is accomplished at the level of the capillaries, the tiniest of all vessels. Here waste, nutrients, gases, and hormones are exchanged between the blood and body cells. Capillaries are finely adapted to their role of exchange their walls are only a single cells thick. Most nutrients, oxygen, and carbon dioxide diffuse readily through capillary cell membranes. Salts and small charged molecule move through fluid- filled spaces within the capillary cell membrane or between adjacent capillary cells.

The pressure within capillaries causes a continuous leakage of fluid from the blood plasma into the places surrounding the capillaries and tissues. This is known as the intestinal fluid and consists primarily of water in which are dissolved nutrients, hormones, gases, wastes, and small proteins from the blood. The large plasma proteins, red blood cells, and platelets are unable to leave the capillaries because of their size, although white blood cell can ooze through the openings between capillary cells. The exchange of materials between capillary blood and nearby cells occurs through this intestinal fluid, which bathes nearly all the cell of the body.

Capillaries are so narrow that red blood cells must pass through them in a single file. Consequently, all the blood is sure to pass very close to the capillary walls, where exchange occurs. In addition, capillaries are so numerous that no body is more the 100 micrometers from a capillary. These factors facilitate the exchange of materials by diffusion. It is estimated that the total length of capillaries in a human is over 80600 km, enough to encircle the globe twice! The speed of blood flow drops very quickly as blood is force through this narrow, almost endless network of capillaries, allowing more time for diffusion to occur.