Venous valve

Many medium-sized veins, especially in the limbs, exhibit infoldings of the tunica interna that meet in the lumen to form venous valves directed toward the heart. Venous pressure is relatively low, so the return of blood to the heart against gravity requires assistance from the skeletal muscle pump and from these valves, which prevent backflow when the muscles relax. When surrounding skeletal muscles contract, they compress the veins and push blood through the valves, producing the propulsion of blood by muscular contractions. Tthis mechanism is known as the skeletal muscle pump.

Varicose veins result in part from valve failure (incompetence). Venous valves are absent from the smallest veins (venules) and are generally absent from the veins of the abdominal cavity and the brain. Many large veins, especially the major thoracic and abdominal veins, also lack valves, although there are exceptions.