Myogenic mechanisms in the control of systemic resistance and transcapillary fluid exchange in man.

Myogenic reactions to raised vascular transmural pressure were studied in the forearm of healthy male volunteers. The study was designed to allow adequate comparative analyses of myogenic stimulus-effector response characteristics in the resistance vessels and in the precapillary sphincters, with no or minimal interference with systemic or local venous-arteriolar reflexes or with metabolic vascular control. Both vascular sections proved surprisingly sensitive to transmural pressure as a stimulus, showing graded constrictor responses when transmural pressure was raised 5-20 mmHg. There was a clearcut difference, however, in that the precapillary sphincters reacted very intensely even when transmural pressure was only slightly raised (5 mmHg). This potent myogenic regulation of precapillary sphincter tone may be an important factor in the control of transcapillary fluid exchange, and thereby in the control of plasma and interstitial fluid volumes, for instance in dependent regions on standing.