Skeletal-muscle pump

The skeletal-muscle pump is a collection of skeletal muscles that aid the heart in the circulation of blood. It is especially important in increasing venous return to the heart, but may also play a role in arterial blood flow.

Venous return
During physical exercise, muscles contract and expand laterally. The intramuscular pressure exerted on the veins by the surrounding muscle pushes blood through the one-way valves of the veins, returning it to the heart. This pumping action keeps blood from pooling in the lower limbs, and individuals that stand still for extended periods of time can experience reduced venous return to the heart and low blood pressure (hypotension) leading to dizziness or fainting (syncope).

Arterial blood pumping
While the muscle pump clearly is important in aiding the return of blood to the heart from the lower limbs, whether the muscle pump plays a role in the control of muscle arterial blood flow is a topic of debate.

Between muscle contractions, intramuscular pressure transiently returns to a level below the venous blood pressure and blood from the capillary system refills the veins until the next contraction. It is postulated that this change in pressure may be great enough to actually create a vacuum that draws blood from the arterial side to the venous side. It is hypothesized that this vacuum during rhythmic contraction actually increases blood flow through the muscle and may be responsible for a portion of the increase in muscle blood flow immediately at the onset of activity. While this explanation is attractive because it would explain the readily observable tight coupling between muscle contraction and a rapid increase in muscle blood flow, recent evidence has emerged that cast doubts on this theory. Experiments have shown that a strong muscle contraction can occur without a corresponding increase in skeletal muscle blood flow. Given the proposed manner of action of the muscle pump to increase arterial blood flow, it would seem impossible for a muscle contraction and skeletal muscle hyperemia to be uncoupled.