Basic Anatomy and Physiology of Veins

Veins form the low-pressure, high-capacitance part of the circulatory system that returns blood to the heart. They carry deoxygenated blood from the body’s tissues back to the heart (via the systemic veins) and carry oxygenated blood from the lungs to the heart (via the pulmonary veins). Each vein has a wall of three layers – the tunica intima (endothelium), media (smooth muscle), and adventitia – similar to arteries but with much thinner, more compliant walls to accommodate larger volumes at low pressure.

Most veins, especially in the legs, contain bicuspid valves formed from folds of the endothelium. These one-way valves ensure antegrade (upward) flow and prevent backflow under gravity. Because veins are at low pressure, they rely on two main external forces to propel blood upward: skeletal muscle pumps (especially calf muscles, often called the “second heart”) and changes in intrathoracic pressure with breathing. Contraction of the calf muscle pump compresses deep veins and pushes blood towards the heart; as muscles relax, venous valves prevent reflux.

Venous anatomy is typically divided into superficial veins, deep veins, and perforator veins. In the limbs, deep veins run alongside arteries within the muscles and handle the bulk of venous return (e.g., the femoral, popliteal, and tibial veins in the legs). In the legs, deep veins contain about 80–90% of the blood returning to the heart. Superficial veins lie just under the skin (e.g., the great and small saphenous veins in the legs) and connect to deep veins via muscular perforators. Superficial veins are visible under the skin and carry blood more slowly since they lack a surrounding muscle pump. Perforator (connecting) veins traverse the deep fascia to link the superficial and deep systems; they also contain valves that close during muscle contraction to prevent blood from flowing back to the superficial compartment.

Together, these three subsystems maintain efficient venous return: muscle contractions compress deep veins and force blood centrally, venous valves close to prevent backflow, and perforators ensure blood from the skin surface drains into the deep system. Venous valves (present in both superficial and deep veins) are critical. Each valve has two leaflets that meet to block backward flow. The coordinated action of valves and calf muscle pumps maintains a low-pressure gradient toward the heart. Dysfunction (degeneration) of valve leaflets leads to venous reflux and hypertension, the fundamental problem in many venous diseases. Because veins operate at low pressure, they have less smooth muscle and elastic tissue than arteries. This makes their walls thinner and more distensible. The largest veins (like the inferior vena cava) have relatively thick adventitia for support, but overall veins are more collapsible than arteries. In summary, veins are flexible conduits with one-way valves and rely on muscle pumps and respiration to return blood; understanding this anatomy (as first detailed in classic texts) is key to grasping venous pathophysiology.