- Skeletal-muscle pump
The skeletal-muscle pump is a collection of
skeletal muscle s that aid theheart in the circulation ofblood . It is especially important in increasingvenous 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 thevein s 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 reducedvenous return to the heart andlow blood pressure (hypotension) leading to dizziness orfainting (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. [Sheriff D. Point: The muscle pump raises muscle blood flow during locomotion. J Appl Physiol. 2005 Jul;99(1):371-2.] [Clifford PS, Hamann JJ, Valic Z, Buckwalter JB. Counterpoint: The muscle pump is not an important determinant of muscle blood flow during exercise. J Appl Physiol. 2005 Jul;99(1):372-4] 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. [Hamann, J. J., J. B. Buckwalter, and P. S. Clifford. Vasodilation is obligatory for contraction-induced hyperemia in canine skeletal muscle. Journal of Physiology (London). 557:1013-1020, 2004.] 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.
References
External links
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Medical College of Georgia article on the [http://www.lib.mcg.edu/edu/eshuphysio/program/section3/3ch10/s3c10_6.htm Skeletal-muscle pump]
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