Vascular and extravascular volume changes due to elevated venous pressure.

Abrupt elevation of venous pressure causes an initial rapid and a secondary slow increase in total tissue volume (plethysmography). Changes in total tissue volume and blood conductivity from the isovolumetric state were determined to assess what factors determined the two components of the tissue volume change. The initial component of the plethysmograph record would appear to be 90% vascular volume change and 10% extra-vascular volume change. Since the two techniques measured identical amounts of capillary filtration during the second component of the recording there would seem to be no slow component of vascular volume increase following venous pressure elevation. Evidence that elevation of venous pressure causes myogenic closure of precapillary vessels isolating a segment of the microvasculature is presented.