Capillaries are thin, fragile vessels embedded in bodily tissue that are just large enough to allow red blood cells to pass through them in a single file. The fine walls of the capillaries consist of a single layer of overlapping cells that allow the exchange of gases.

Oxygen, carbon dioxide, nutrients, and wastes are exchanged between the blood and the body tissues via capillaries. Capillaries are part of a large branching network of circulatory vessels and are the final destination of arterial blood flowing from the heart. Correspondingly, the capillaries are also the starting point for flow of venous blood returning to the heart. Located between the arterioles and the capillaries are precapillaries, transitional vessels possessing muscle fibers that enable them to contract. Therefore, the precapillaries are able to dictate the draining and filling of the capillary beds.

The Danish scientist Shack August Krogh was the first person to demonstrate that capillaries expand in order to meet blood demand. For many years prior to his discovery, it was mistakenly believed that blood pressure increased in order for more blood to flow through the vessels. However, Krogh was able to prove that muscles requiring additional blood supplies send signals that trigger the expansion of capillaries and was awarded with the Nobel Prize in 1920 for his efforts.