Regulation of Red Blood Cell Volume with Exercise Training

Hypervolemia is a hallmark of endurance training (ET) and manifests by similar elevations in plasma (PV) and red blood cell volume (RBCV) so that hematocrit largely remains unaltered following weeks/months of training. While the mechanisms facilitating PV expansion with ET have been previously reviewed extensively this is not the case for RBCV. Endurance champions may have 40% more RBCV than controls and RBCV may increase up to 10% following months of regular exercise training in healthy individuals. Such adaptations are the main factor leading to concomitant changes in maximal oxygen uptake. The increase in RBCV is preceded by that of PV after few ET sessions, which in turn transiently decreases the hematocrit. The "critmeter" theory suggests that O₂ sensors located within the juxtamedullary apparatus regulate the hematocrit via modulation of renal erythropoietin (EPO) production according to arterial O₂ content-dependent changes in tissue O₂ pressure. Hence, the initial decrease in hematocrit can be considered as a primary mechanism facilitating RBCV expansion with ET. Furthermore, after a single endurance exercise bout blood volume-regulating hormones ANGII and VPN increase transiently. Both stimulate renal EPO production. Catecholamines and cortisol, stress hormones acutely increased by endurance exercise, may facilitate the release of red blood cells from the bone marrow, thus possibly contributing to ET-induced erythropoiesis. These and other endocrine effects could be enhanced by the hyperplasia of the hematopoietic bone marrow observed in endurance athletes. © 2019 American Physiological Society. Compr Physiol 9:149-164, 2019.