Men and women have different mean hemoglobin levels in health in venous blood – women have mean levels approximately 12% lower than men. A similar sex-related difference in hemoglobin levels in adult animals is found in many species of mammals, birds, and reptiles, indicating that it is an important physiological phenomenon. It is probably a direct effect of sex hormones, both estrogen, and androgens, on erythropoiesis.
However, since there is no difference in erythropoietin levels between the sexes, this effect most likely takes place at the kidney, rather than at the bone marrow. Estrogens dilate and androgens constrict the renal microvasculature: dilation and vasoconstriction in vessels below 300 microns in diameter respectively increase and decrease the hematocrit in blood in arterioles, capillaries, and venules, altering the oxygen delivery per unit red cell mass, and providing a mechanism for varying the red cell mass without compensatory changes in erythropoiesis.