Veins

Structure

Vein walls have the same three layers as arteries but with less smooth muscle and elastic tissue. Many veins, especially in the limbs, contain one-way valves that prevent backflow against gravity.

Function

Because venous pressure is low, return flow depends on the skeletal muscle pump, respiratory pressure changes, and venous valves. Smooth muscle tone helps adjust venous capacity and maintain cardiac filling.

Types

Superficial veins lie just under the skin and are visible on the limbs. Deep veins accompany arteries within muscles. Specialized portal veins, like the hepatic portal vein, carry blood between two capillary beds before returning to the heart.