vein

A vein is a low-pressure pipe returning blood toward the heart—the ‘return road’ of the circulation. Compared with arteries, veins have thinner, floppier walls and a wider lumen, because the blood inside them is no longer under the strong push of the heartbeat.

Because venous pressure is low, especially in the legs where blood must travel uphill against gravity, many veins contain one-way valves that prevent backflow. Blood is nudged toward the heart largely by the squeezing of surrounding skeletal muscles (the ‘muscle pump’) and by changes in chest pressure during breathing. Veins also serve as the body’s main blood reservoir: at rest they hold the majority of the circulating blood volume and can release it when needed.

A caveat to the ‘veins carry deoxygenated blood’ rule: this holds across the systemic circulation, but the pulmonary veins carry freshly oxygenated blood from the lungs back to the left atrium. As with arteries, the defining feature of a vein is its direction—toward the heart—not its oxygen content.