循環器

Veins carry deoxygenated blood from body tissues back to the heart for reoxygenation. They transport blood low in oxygen from all body regions through a network of hollow tubes. This continuous circulation maintains the body's overall fluid balance and ensures tissues receive oxygenated blood.

Veins have wider lumens measuring 0.5 mm to 3 cm in diameter and thinner walls than arteries. Their tunica interna is thin with minimal endothelium, the tunica media is poorly developed with fewer smooth muscle fibers, and they lack internal and external elastic laminae. The tunica externa is well-developed with longitudinally arranged collagen and elastic fibers.

Venous valves are thin folds of tunica interna that form flaplike cusps projecting into the lumen. These one-way valves prevent backflow of blood, ensuring it moves only toward the heart. They are particularly important in limb veins where blood must work against gravity.

Veins store approximately 60% of the body's total blood volume at rest, functioning as blood reservoirs. This stored blood can be mobilized when needed to maintain circulation and respond to physiological demands. The thin, distensible walls of veins allow them to accommodate varying volumes of blood.

Venous sinuses are specialized veins that appear flattened with only an extremely thin endothelial layer and lacking all other tunics. Examples include dural venous sinuses in the brain, which transport deoxygenated blood from the brain to the heart, and the coronary sinus of the heart.

When the body is overheated, veins dilate to allow more blood to flow near the skin surface, where heat is released to the environment. This vasodilation increases heat dissipation and helps maintain normal body temperature. Veins thus play an active role in thermoregulation alongside their primary function of returning blood to the heart.

The hepatic portal vein carries nutrient-rich blood from digestive organs to the liver for processing and metabolism. Although most veins carry deoxygenated blood, this specialized vein transports absorbed nutrients from the gastrointestinal tract. This allows the liver to process and distribute nutrients throughout the body.