Figure 6.

Schematic showing idealized distribution of blood flow at rest (left) and during maximum exercise in a healthy untrained young male subject (middle) and an elite endurance athlete (right). At rest, cardiac output is ∼5 l/min and rises to ∼20 l/min in the untrained subject during maximum exercise. Values of ∼40 l/min have been reported in elite endurance athletes. As cardiac output rises with exercise, brain blood flow remains constant (or increases slightly) while blood flow to the heart increases to meet the increased demands for myocardial blood flow that are primarily associated with exercise-induced increases in heart rate. Skeletal muscle blood flow increases dramatically, while blood flow to other tissues, especially the abdominal viscera and kidneys, is reduced. During heavy exercise, the vast increase in cardiac output is directed almost exclusively to contracting skeletal and cardiac muscles. Since maximum heart rate is similar in both young healthy subjects and elite athletes, the primary factor responsible for the very high cardiac outputs in the elite athlete is an extremely high stroke volume facilitated by a large compliant left ventricle. Ideas synthesized from Refs. 39, 67, 86, 91, 146, 292, 389–391, 394, 396.