Contribution of local blood flow regulation mechanisms during head-up tilt in human subcutaneous tissue.

Local and remote regulation of subcutaneous blood flow in the forearm and leg was studied during head-up tilt (30 degrees, 45 degrees and 70 degrees) in 7 young healthy subjects. Relative blood flow was estimated by the local 133Xe washout technique. Incremental levels of head-up tilt elicited increasing vascular resistance on arm and leg, respectively. Positive pressure similar to a blood column of the same height was able to prevent a significant part of the vasoconstrictor response on the leg to head-up tilt. Thus if venous distension is prevented the local veno-arteriolar reflex is abolished, whereas arteriolar constriction due to centrally elicited reflexes remains unaffected. Subcutaneous blood flow in the extremities are regulated by remote (baroreceptor) as well as local sympathetic reflex mechanisms (veno-arteriolar reflex); but the relative influence of the local veno-arteriolar reflex on the increase in total peripheral resistance seems to decrease with increasing tilt angles.