Veins

Veins are blood vessels that carry blood toward the heart. In the systemic circulation they typically return deoxygenated blood from tissues, while pulmonary veins carry oxygenated blood from the lungs to the left atrium.

Vein walls are thinner and less muscular than arteries; their layers include the tunica intima, tunica media, and tunica adventitia, with the media being relatively thin. The lumen is often wide and collapsible, enabling veins to act as a reservoir for much of the blood in the circulation. Many veins contain one-way valves formed by the tunica intima, which help maintain unidirectional flow, especially in the limbs.

Veins are organized into superficial and deep networks. Deep veins accompany corresponding arteries and drain the tissues of the limbs and trunk, while superficial veins lie closer to the surface. Major systemic veins include the superior and inferior vena cavae, as well as the internal and external jugular veins, azygos system, and splenic, hepatic, and portal veins. Pulmonary veins return oxygenated blood from the lungs to the left atrium.

Functionally, veins serve as capacitance vessels and can hold a substantial portion of the blood volume at rest. Venous return is aided by skeletal muscle contraction, respiratory movements, and venous valves. Pathological conditions include varicose veins from valve failure, deep vein thrombosis, and portal hypertension with varices in the esophagus or stomach. The venous system also includes intracranial dural venous sinuses and, in the heart, the coronary veins.