Physiology and Pathophysiology of Venous Flow

Abstract

Veins provide heart filling flow with lower velocity and pressure than those in arteries. The right heart receives systemic venous blood and pumps blood into the pulmonary circulation that returns oxygenated blood into the left heart for its ejection at high velocity and pressure into the systemic circulation. Whereas systemic veins carry deoxygenated blood from cells to the right cardiac pump, oxygenated blood flows in pulmonary veins running to the left cardiac pump, although pulmonary veins receive a part of the systemic venous blood that is drained from the lung tissue. Usually, one or two veins run with an artery, collecting lymphatic vessel, and nerve packaged in a sheath. In the head, veins follow paths that differ from those of arteries. Whereas venous flow in the standing position in veins below the heart level is supported by the hydrostatic pressure, blood flow in veins situated above this level must struggle against the gravity effect. Veins constitute the major blood storage compartment. They accommodate blood volume changes by dilating and shrinking to possibly reach a collapsed state. Veins, into which blood pressure is relatively small, are usually more deformable than accompanying arteries subjected to the same external pressure. However, deep veins embedded into skeletal muscle are less deformable than superficial veins close to the skin. Although both types for a given merging generation have similar wall thickness, they behave as thick- and thin-walled conduit, respectively. However, deep veins embedded into skeletal muscle are less deformable than superficial veins close to the skin. Although both types for a given merging generation have similar wall thickness, they behave as stiff and and soft conduit, respectively. In other words, deep and superficial veins can be represented by thin-walled veins in a gel and air, respectively, the former being mush less collapsible than the latter. Compression stocking (or supportive hose) diverts superficial venous flow of legs to deep veins that are less subjected to chronic venous insufficiency, as it collapses superficial veins without deforming deep veins. Similar to arterial flow, venous flow is unsteady, especially in abdominal and thoracic veins that experience both breathing and cardiac pumping. In addition, veins of the inferior and superior limbs undergo more or less transient external compression by contracting skeletal muscles. During walking, venous valves prevent backflow to the feet, whereas muscles ensure an additional pumping that favors venous return.