Canadian journal of physiology and pharmacology
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Can. J. Physiol. Pharmacol. · Oct 2002
Theoretical analysis of the noncardiac limits to maximum exercise.
When right atrial pressure (Pra) is greater than zero (atmospheric pressure), cardiac output is determined by the intersection of two functions, cardiac function and return function, which is used here to mean the determinants of venous return. When Pra < or = 0, flow is only determined by circuit function. The objective of this analysis was to determine the potential changes in return function that need to occur to allow the maximum cardiac output during exercise when Pra < or = 0 or is constant. ⋯ The major conclusions are that, to achieve the high cardiac output that occurs at peak exercise, there need to be marked changes in the distribution of blood flow, recruitment of unstressed volume, and the venous resistance draining vascular beds. A consequence of the increase in peripheral flow is a marked increase in pressure in the veins of the working muscle. Muscle contractions are potentially a very important mechanism for transiently decreasing this pressure and preventing excessive filtration of plasma during exercise.