Skin AVA and capillary dilatation and constriction induced by local skin heating

In conscious sheep, total femoral blood flow and flow through arteriovenous anastomoses (AVAs) and capillaries (CAP) in skin of the hindleg were measured employing electromagnetic and radioactive microsphere techniques. Core temperature (Tc) was manipulated using intravascular heat exchangers and hindleg skin temperature (Tsk) was manipulated by immersion in temperature controlled water. With Tc set 1 degree C above normal, AVA flow was highest at the lowest Tsk tested (34 degrees C); AVAs progressively constricted as Tsk was increased from 34 to 40-41 degrees C, then dilated again as Tsk reached the highest levels tested (42-44 degrees C). Skin CAP flow was not altered by Tsk of 34 to 42 degrees C but was increased at a Tsk of 44 degrees C. Therefore total skin blood flow followed essentially the same pattern as AVA flow; total femoral flow also followed this pattern. When Tc was set 0.5 degrees C below normal, AVA flow was low at all levels of Tsk. It is concluded that Tc plays a dominant role in control of skin blood flow, however, once Tc is at a level requiring increased heat loss, Tsk exerts an extremely potent influence on the nature and magnitude of changes in skin blood flow. The pattern of flow changes appears to reflect principally a negative feedback mechanism aimed at maintaining Tsk at approximately 40 degrees C; this may contrast with mechanisms associated with sweating and/or active vasodilatation in other species.