ABSTRACT
The peripheral venous system functions both as a reservoir to hold extra blood and as a conduit to return blood from the periphery to the heart and lungs. Unlike arteries, which possess three well-defined layers, most veins are composed of a single tissue layer. Only the largest veins possess internal elastic membranes, and at best this layer is thin and unevenly distributed, providing little buttress against high internal pressures. The correct functioning of the venous system depends on a complex series of valves and pumps that are individually frail and prone to malfunction, yet the system as a whole performs remarkably well under extremely adverse conditions.
