Hormonal and neurogenic adrenergic control of the fluid transfer from skeletal muscle to blood during hemorrhage.

During hemorrhage net transcapillary absorption of interstitial fluid from skeletal muscle into the intravascular space compensates effectively for the blood loss. This absorption of fluid is mainly linked to decrease of the capillary hydrostatic pressure (Pc), as caused by reflex adrenergic re-adjustment of the ratio of pre- to postcapillary resistance (ra/rv). The present study demonstrates the existence of both a neurogenic and a humoral component in the adrenergic control of the fluid transfer for skeletal muscle to blood. In the early period of bleeding (less than 5 min) reflex activation of the vasomotor fibres contributed significantly to the fluid absorption. The subsequent, main part of the fluid gain from the extra- to the intravascular space was due to the action of the blood-borne catecholamines. Both the neurogenic and the hormonal control of the fluid absorption process was mainly linked ot beta-adrenergic inhibition of vascular smooth muscle tone. This control was effected via two mechanisms, viz. by a relatively larger beta-adrenergic dilation of post- than precapillary resistance vessels, leading to adjustment of ra/rv and thereby to decrease of Pc, and via beta-adrenergic dilation of "precapillary sphincters' leading to increased capillary surface area available for fluid exchange.