Erythrocyte indices during a competitive marathon.

Three adult male volunteers ran a competitive marathon (42.2 km) during which there was an average ambient air (dry bulb) temperature of 19.4 degree C, with an average ambient water vapor pressure of 9.5 Toor. Prior to and at 4.8-km (3-mi) intervals throughout the race, venous blood samples were drawn from an indwelling catheter in a peripheral arm vein. In each blood sample the total erythrocyte count (EC) was made, and whole blood hemoglobin concentration, hematocrit, and plasma osmolality were determined. From these measurements, the standard red blood cell (RBC) indices of mean corpuscular volume (MCV), mean corpuscular hemoglobin (MCH), and mean corpuscular hemoglobin concentration (MCHC) were calculated. The three subjects completed the race in 191, 240, and 310 min, with weight losses averaging 5.4% of prerace body weight. EC linearly decreased below the prerace control levels of 5.69 X 10(6) RBC/mm3 for the first 14.5 km of the race, so that EC was 4.7% below control levels by this point. Subsequently, EC rose back to control levels by 19.3 km, where it remained for the rest of the race. MCHC, MCH, and calculated total RBC volume did not significantly change during the race. Plasma osmolality related to plasma osmolality. These data indicate the dynamic nature of erythrocyte number and volume during long-duration submaximal exercise, and the extraordinary ability of the human body to maintain RBC indices within narrow limits when confronted with a severe stress such as the marathon.