Mechanisms of blood vessel constriction during hemorrhage

The contributions of sympathetic nerves, circulating hormones, and passive vascular recoil to vasoconstriction and transcapillary fluid reabsorption in skin and skeletal muscle were examined in the forelegs of 55 pentobarbitalized dogs by using a combined gravimetric and segmental resistance technique. Hemorrhage elicited large increases in forelimb segmental vascular resistances and in the rate of extravascular fluid reabsorption. Only a small part of these responses could be attributed to the passive effects of reducing forelimb perfusion pressure.