|Capillaries are tiny blood vessels, (about 10 micrometers in diameter) that form part of the circulatory system. These tiny structures are located within the tissues of the body and transfer blood from the arteries, through the tissues and back to the veins.
Capillary blood vessel's walls are thin and narrow and the red blood cells can only travel through them in single file. Substances such as oxygen, carbon dioxide, nutrients and wastes are passed through the walls of the capillaries. This makes the capillaries the main area of exchange between the fluid (called lymph) bathing the body tissues and the blood.
Because they are so numerous and their diameter is so small, Capillary blood vessel's purpose is to provide a large surface area in order to maximize diffusion of oxygen and nutrients.