Redistribution of regional and organ blood volume and effect on cardiac function in relation to upright exercise intensity in healthy human subjects

To determine the effect of relative exercise intensity on organ blood volume and its relation to cardiac function, changes in relative blood volume and cardiac function were monitored with radionuclide techniques in 14 healthy volunteers. After labeling the subject's red cells with technetium 99m, we acquired data at rest, zero-load cycling, and at 50%, 75%, and 100% of maximal oxygen uptake. From rest to zero-load cycling, leg blood volume decreased 32±2% (mean±SEM), whereas relative end-diastolic blood volume increased 9.6±1.2%, and lung blood volume increased 18±2%, suggesting that the lungs may act as a blood volume buffer during periods of acutely increased venous return. With relative increasing exercise, leg blood volume stabilized, and then the blood volume in the abdominal organs decreased, further augmenting cardiopulmonary blood volume; leg blood volume and abdominal blood volume decreased by 23±2% and 19±2% from baseline, respectively, whereas thoracic blood volume increased 38±4%. In the abdomen, large decreases in blood volume were observed in the spleen (46±2%), kidney (24±4%), and liver (18±4%). In contrast, lung blood volume increased 50±4%, with the upper lung fields increasing more than the lower. Blood sampling revealed an increase in the hematocrit level by 4.3±0.4 units at peak exercise that paralleled the decrease in splenic blood volume (r²= -0.64, p<0.001), suggesting a role for the spleen in augmenting cardiovascular performance by the release of concentrated red blood cells into general circulation. We conclude that upright exercise results in marked blood volume shifts from the legs and abdominal organs to the heart and lungs in a dynamic process correlating closely with oxygen consumption.