Fluid transfer from skeletal muscle to blood during hemorrhage Importance of beta adrenergic vascular mechanisms

Vascular reactions in the cat lower leg in response to short-term (10 min) hemorrhagic hypotension (approximately 80 mmHg) were studied before and after regional blockade of the beta-adrenoceptors. In the muscle region with intact beta-adrenoceptors, hemorrhage raised vascular resistance by about 80% and caused a dilatation of the precapillary sphincters, the latter effect evidenced in terms of a 35% increase of the capillary filtration coefficient. Concomitantly, an absorption of extravascular fluid to the blood stream occurred, a process tending to compensate for the reduction of intravascular fluid volume. After regional beta-blockade there was quite a marked augmentation of the hemorrhage induced increase of vascular resistance whereas the inhibition of precapillary sphincter tone and the transcapillary fluid absorption were almost abolished. These observations indicate that bleeding is associated with a significant beta-adrenergic dilator influence in both the resistance vessels and precapillary sphincters of skeletal muscle and that the beta-dilator mechanism may be essential for the important, compensatory fluid gain from the extravascular to the intravascular space during hemorrhage. The observed beta-adrenergic mediation of the net transcapillary fluid absorption could be ascribed to resetting of the pre-/postcapillary resistance ratio, leading to decreased capillary hydrostatic pressure, and to the dilator influence in the precapillary sphincters, leading to an increased number of the patent capillaries available for the transcapillary fluid exchange.