Transmural pressures and tissue perfusion in man.

The relationship between the arterio-venous pressure gradient and blood flow - the Hagen-Poiseuille principle - was reinvestigated. A main theme was to compare transmural pressure changes induced by either changing pressure inside or outside the vessels. Blood flow was measured in subcutaneous and skeletal muscle tissue in the lower limb of man by using the 133Xenon washout technique. Intramural pressure changes were achieved by supine leg elevation or lowering and leg exercise in nearly erect position. Extramural pressure changes were induced by inflation of a leg cuff. The main finding was a cessation of blood flow, when local diastolic arterial pressure was reduced to zero, although a positive pressure head was still present. This phenomenon was believed to be caused by a collapse of the arterio-arteriolar bed with the pulse pressure energy extinguished by the Windkessel effect. Several other hemodynamic topics were examined and discussed such as pulsatile arterial pressure and flow, the Hagen-Poiseuille principle, autoregulation of blood flow, postcapillary vessel collapse, vasoconstrictor response, local sympathetic veno-arteriolar axon reflex, the central baroreceptor activity, and the musculo-venous pump in the calf. In conclusion, changes in transmural pressure, whether accomplished by intra- or extramural pressure changes, had the same impact on the arterial and venous sections of the vascular bed. One exception emerged. Blood flow resistance was increased by external compression in supine position due to postcapillary venous collapse even when a low compression force was applied.