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

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Abstract
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 (T c) was manipulated using intravascular heat exchangers and hindleg skin temperature (T sk) was manipulated by immersion in temperature controlled water. WithT c set 1°C above normal, AVA flow was highest at the lowestT sk tested (34°C); AVAs progressively constricted asT sk was increased from 34 to 40–41°C, then dilated again asT sk reached the highest levels tested (42–44°C). Skin CAP flow was not altered byT sk of 34 to 42°C but was increased at aT sk of 44°C. Therefore total skin blood flow followed essentially the same pattern as AVA flow; total femoral flow also followed this pattern. WhenT c was set 0.5°C below normal, AVA flow was low at all levels ofT sk. It is concluded thatT c plays a dominant role in control of skin blood flow, however, onceT c is at a level requiring increased heat loss,T sk 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 maintainingT sk at approximately 40°C; this may contrast with mechanisms associated with sweating and/or active vasodilation in other species.