Veins

Veins are the vessels that return blood to the heart. Most veins carry deoxygenated blood back from the body's tissues to the right side of the heart; the pulmonary veins are the exception, returning oxygenated blood from the lungs to the left atrium. Veins operate under much lower pressure than arteries and are remarkably expandable.

Wall Structure and Valves

Veins share the same three-layer plan as arteries (tunica intima, media, and adventitia), but the muscular tunica media is much thinner because veins do not need to withstand high pressure. As a result, vein walls are floppier and the vessels collapse easily when empty. Many veins, especially in the limbs, contain one-way valves, flap-like folds of the inner lining that prevent blood from flowing backward and help it move against gravity toward the heart.

Function

Veins serve two main roles. First, they are the return system, collecting blood from capillaries through small venules that merge into progressively larger veins and ultimately the superior and inferior vena cavae. Second, they act as the body's main capacitance (reservoir) vessels: at any moment veins hold roughly two-thirds of the total blood volume, and the body can shift this stored blood toward the heart when needed. Because venous pressure is low, blood return relies on help from the "skeletal muscle pump" (contracting muscles squeezing veins), the valves that prevent backflow, and pressure changes from breathing.

Clinical Relevance

When venous valves weaken, blood pools in the legs, producing varicose veins and chronic venous insufficiency. Blood clots forming in deep veins (deep vein thrombosis) can break loose and travel to the lungs as a pulmonary embolism, a potentially fatal event. Veins are also the usual access point for blood draws and intravenous medications. This information is educational and not a substitute for professional medical advice.