Microvascular plasma velocity and indicator dispersion with hemorrhage.

The effects of stepwise hemorrhages on plasma flow velocity and indicator (FITC-Dextran) dispersion in series and parallel coupled microvascular vessels has been studied. The mesentery of cats (0.6 kg) anesthetized with Dial-Urethane was exposed and studied with a microscope equipped for fluorescense microscopy. Indicator was injected as a bolus (0.1 ml in 0.5 seconds) in a small branch of the mesenteric artery. Indicator curves were recorded from a video tape recording of the passage of the indicator by a video sampler with intensity sensitive windows. The mean transit time (t), appearance time (t(a)), peak time (t(p)), curve duration (t(E)), plasma velocity and vessel diameter changes were determined. Arterioles 68 +/- 8 micron and 35 +/- 3 micron and venules 55 +/- 6 micron constricted gradually with hemorrhage. Arterioles 20 +/- 2 micron constricted with mild hemorrhage and remained constricted with further hemorrhage. Arteriolar and venular plasma velocities decreased abruptly with mild hemorrhage and then continued to decrease gradually with further hemorrhage. Capillary flow ceased after moderate hemorrhage and was not reestablished until at least an hour after reinfusion. Arteriole t increased with hemorrhage but t increased much more in capillaries and venules. t(E) changes paralled t changes. The ratio t(a)/t was lower for venules than arterioles and both decreased with hemorrhage but the venular ratio decreased significantly more. Venular t(p)-t(a) and t(E)-t(p) increased more than arteriolar values. Hemorrhage increased indicator dispersion with the greatest effect occurring between the arteriolar and venular vessels.