Postural vascular response vs. sympathetic vasoconstriction in human skin during orthostasis.

Sympathetic activation and local vascular smooth muscle reactions to vessel distension contribute to the increase in vascular resistance in the skin during orthostasis. The relative contribution of these two mechanisms to the changes of skin blood flow along the body axis on standing was investigated in healthy male subjects by laser-Doppler (LD) fluxmetry. Compared with recumbency, LD flux (LDF) in the standing subjects was reduced by -19.6 +/- 7.2% at the forehead and by -69.6 +/- 9.6% in the leg. In the absence of hydrostatic pressure changes, the LDF changes on standing averaged -29 +/- 13%, independent of skin region, reflecting the effect of vasoconstriction due to sympathetic activation. The postural vascular response, elicited by lowering the arm or the leg from heart level, was significantly attenuated in orthostasis compared with recumbency. The vessel reaction to local alteration of transmural pressure was studied in the skin of the forehead and lower leg by application of external pressure in supine subjects. No difference in vessel responsiveness to changes of transmural pressure was found between these skin sites. The findings suggest that the changes of skin perfusion in orthostasis result from a nonadditive interaction of height-dependent and -independent mechanisms.