What determines blood volume?
Blood volume scales primarily with body size — taller, heavier individuals generally have more blood. The Nadler formula uses both height and weight, making it more accurate than weight-only approaches. Endurance athletes can have blood volumes 20–40% above predicted values due to exercise-induced plasma expansion, which improves oxygen delivery.
Blood is approximately 55% plasma and 45% red blood cells in healthy adults (haematocrit ~45%), though this ratio varies with sex, fitness, altitude, and health status.