Regulation of subcutaneous blood flow during head‐up tilt (45°) in normals

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Abstract
Local and remote regulation of subcutaneous blood flow in the forearm and leg was studied during head-up tilt (45.degree.) in 6 young healthy male subjects. Relative blood flow was estimated by the local 133Xe washout technique. Lowering of a leg head to a 51% decrease in its subcutaneous blood flow due to a venoarteriolar reflex elicited by the increase in venous transmural pressure. During head-up tilt subcutaneous blood flow in the arm remaining at heart level decreased by 27%, in the leg blood flow decreased by 50%. Following proximal nervous blockade, head-up tilt did not induce vasoconstriction in forearm at heart level, but blood flow in distal leg decreased by 45%. There was no difference in the vasoconstrictor response in the leg to head-up tilt or lowering of the labeled area by 40 cm. Since head-up tilt caused neurogenically mediated vasoconstriction in subcutaneous tissue, subcutaneous blood flow in the extremities seems to be regulated by remote (baroreceptor), as well as local sympathetic reflex mechanisms (venoarteriolar reflex).