Anaemia and marrow blood flow in the rat

An increase in bone marrow blood flow has been previously described in anaemic rabbits and dogs. We examined the effect of haemorrhage and haemolysis in female Sprague-Dawley rats, with the hypothesis that high blood flow was related to hyperplasia of bone marrow tissue and that the increase would affect bone as well. Blood flow was measured in tibia and femur by the microspheres trapping method. Chronic anaemia was accompanied at day 32 by a marked increase (factor of 1.7-1.9) in blood flow to bone marrow and to bone. On the other hand, no increase in blood flow was observed in chronic compensated anaemia, showing that anaemia per se and not stimulation of erythropoiesis was responsible for the increase. We then explored the effect on blood flow of two factors associated with anaemia. Erythropoietin 4 U/d for 4 d failed to induce any increase in bone and marrow blood flow. Systemic hypoxia (76 mmHg for 3 d) reduced the fraction of cardiac output conveyed to bone and marrow by 40-50%, but this decrease was more than compensated by an increase in cardiac output. We conclude that neither erythropoietin nor low tissue pO2 is the direct cause for the increase in bone and marrow blood flow observed in anaemia.