In anatomy and physiology, a duct is a tube leading from an exocrine gland or organ. It usually carries the gland's product to where it is used. An example is the vas deferens, which carries sperm to the ejaculatory ducts. Another example is tear ducts, which carry tears to the eye.
Ducts are lined by pavement-type endothelial cells. Outside the endothelium in blood and lymphatic vessels is a layer with no cells called the basement membrane. Outside that are the smooth muscle cells. The smooth muscle squeezes the contents along.