SAWKA, M. N., V. A. CONVERTINO, E. R. EICHNER, S. M. SCHNIEDER, and A. J. YOUNG. Blood volume: importance and adaptations to exercise training, environmental stresses, and trauma/sickness. Med. Sci. Sports Exerc., Vol. 32, No. 2, pp. 332–348, 2000. This paper reviews the influence of several perturbations (physical exercise, heat stress, terrestrial altitude, microgravity, and trauma/sickness) on adaptations of blood volume (BV), erythrocyte volume (EV), and plasma volume (PV). Exercise training can induce BV expansion: PV expansion usually occurs immediately, but EV expansion takes weeks. EV and PV expansion contribute to aerobic power improvements associated with exercise training. Repeated heat exposure induces PV expansion but does not alter EV. PV expansion does not improve thermoregulation, but EV expansion improves thermoregulation during exercise in the heat. Dehydration decreases PV (and increases plasma tonicity) which elevates heat strain and reduces exercise performance. High altitude exposure causes rapid (hours) plasma loss. During initial weeks at altitude, EV is unaffected, but a gradual expansion occurs with extended acclimatization. BV adjustments contribute, but are not key, to altitude acclimatization. Microgravity decreases PV and EV which contribute to orthostatic intolerance and decreased exercise capacity in astronauts. PV decreases may result from lower set points for total body water and central venous pressure, while EV decreases may result from increased erythrocyte destruction. Trauma, renal disease, and chronic diseases cause anemia from hemorrhage and immune activation which suppresses erythropoiesis. The re-establishment of EV is associated with healing, improved life quality, and exercise capabilities for these injured/sick persons.
Blood volume changes are often credited as being important adaptations to the perturbations imposed by physical exercise, environmental stressors, trauma, and certain sicknesses. The relative contributions of plasma volume and erythrocyte volume for adaptations to these perturbations have not been systematically reviewed, despite many recent advances. Blood volume represents the sum of erythrocyte volume and plasma volume. Erythrocyte volume and plasma volume can change independent of each other to alter blood volume. Physical exercise, environmental stresses, and trauma/illness can influence each of these vascular volumes. Erythrocyte volume expansion usually occurs slowly over many weeks to months, whereas plasma volume expansion can occur rapidly over several hours to days (131). A principle factor regulating erythrocyte production is the hormone erythropoietin (EPO) (101). Reduced tissue oxygen tension (renal and hepatic) is a primary factor stimulating EPO synthesis; however, other factors such as central venous pressure, growth hormone stimulation, and nutrition may contribute (131). Plasma volume is regulated by the extracellular fluid (ECF) volume, as well as by changes in total circulating protein and pre- to postcapillary resistance ratios (131).
The purpose of this review is to examine critically the influence of several perturbations (physical exercise, environmental (heat, terrestrial altitude, and microgravity) stress, and trauma/sickness on blood volume. In addition, the relative importance of changes in erythrocyte volume and plasma volume on minimizing the adverse effects of these perturbations will be examined.
U.S. Army Research Institute of Environmental Medicine, Natick, MA; U.S. Army Institute of Surgical Research, Fort Sam Houston, TX; University of Oklahoma, Oklahoma City, OK; and NASA Johnson Space Center, Houston, TX
Submitted for publication November 1998.
Accepted for publication June 1999.
Address for correspondence: Dr. Michael N. Sawka, Thermal & Mountain Medicine Division, U.S. Army Research Institute of Environmental Medicine, Natick, MA 01760-5007.