Capillaries are the smallest blood vessels, usually about 5-10 um in diameter. They irrigate tissues by exchange of metabolites and gases across their walls. They receive blood from the arterial circulation and empty into the venous circulation. Continuous capillaries are the most common type consisting of a single endothelial cell wrapped around to join with itself and are found in bone, muscle, and nervous tissue. Fenestrated capillaries contain intracellular perforations (fenestrae) and are found in endocrine glands, intestinal villi, and kidney glomeruli. Discontinuous capillaries have intercellular gaps and are found in liver, bone marrow and spleen.