Peripheral limitations to exercise.

We present a computer simulation of a two-compartment model of the systemic circulation which demonstrates how this model can be used to understand the mechanism(s) for the maximal exercise cardiac output (Q). The model consists of two parallel vascular channels, the splanchnic channel (all blood draining through the hepatic veins) and the peripheral channel (all other vascular beds). The distinguishing characteristic of each channel is the product of its venous compliance and venous resistance. Model parameters for the human circulation were estimated from similar parameters obtained directly from animal experiments. "Exercise" was achieved by decreasing the compliance of both channels to 40% of their initial value and by redistributing the Q such that the fraction of Q perfusing the splanchnic channel fell from 38 to 5%, while that perfusing the peripheral channel (skeletal muscles) increased from 62 to 95%. These combined changes increased Q from 4.4 to 22.0 l X min-1 and suggest that maximal adjustments of the two-compartment model parameters lead to a prediction of a maximal Q that approaches the maximal Q usually obtained by humans during exercise.