A capillary is an extremely small blood vessel located within the tissues of the body, that transports blood from arteries to veins. Capillaries are most abundant in tissues and organs that are metabolically active. For example, muscle tissues and the kidneys have a greater amount of capillary networks than do connective tissues.

Capillaries are so small that red blood cells can only travel through them in single file. Capillaries measure in size from about 5-10 microns in diameter. Capillary walls are thin and are composed of endothelium (a type of simple squamous epithelial tissue). Oxygen, carbon dioxide, nutrients and wastes are exchanged through the thin walls of the capillaries.

Capillaries play an important role in microcirculation. Microcirculation deals with the circulation of blood from the heart to arteries, to smaller arterioles, to capillaries, to venules, to veins and back to the heart.

The flow of blood in the capillaries is controlled by structures called precapillary sphincters. These structures are located between arterioles and capillaries and contain muscle fibers that allow them to contract. When the sphincters are open, blood flows freely to the capillary beds of body tissue. When the sphincters are closed, blood is not allowed to flow through the capillary beds. Fluid exchange between the capillaries and the body tissues takes place at the capillary bed.