MECHANISMS BEHIND RAPID BLOOD VOLUME RESTORATION AFTER HEMORRHAGE IN BIRDS

The mechanisms responsible for the more efficient volume restoration after hemorrhage in birds as compared with mammals were studied in experiments on ducks. The results suggest that qualitatively similar mechanisms operate to mobilize tissue fluid in ducks as in cats (see Öberg 1964). The most important factor seems to be a strong reflex vasoconstriction in the skeletal muscles, increasing considerably the pre‐ to postcapillary resistance ratio with a fall in capillary pressure as a result. In this way net fluid absorption is reflexly initiated in the tissue which contains the largest tissue fluid reservoir in the body. At the same time the precapillary sphincters become secondarily relaxed, increasing the capillary surface area available for absorption. When in the course of a large blood loss arterial and venous pressures become reduced as well, this will further lower capillary pressure and do so in all tissues, thus enhancing fluid absorption. The results indicate that, from a quantitative point of view, considerable differences exist between ducks and cats. First, the reflex vasoconstriction and the subsequent capillary pressure fall are more pronounced in duck skeletal muscle. Second, the capillary surface area is approximately three times larger in duck muscle. These circumstances imply especially favourable conditions for rapid fluid absorption. It is probable that the greater resistance to blood loss and shock of most bird species as compared with mammals is directly related to their much greater capacity for fluid mobilization from the skeletal muscles. © 1968 Scandinavian Physiological Society