Analysis of a recent hypothesis of plasma flow in pericapillary spaces.

The hypothesis that the observed fluid annulus outside the wall of some capillaries is the reservoir of plasma responsible for ratios of organ hematocrit to large vessel hematocrit less than 1 is examined hydrodynamically. The extent to which the hematocrits can be reconciled on the basis of flow through a single capillary is examined. Results show that ratios between 0.5 and 1.0 can be accounted for even if plasma and cells travel the same distance through a common capillary without a plasma annulus. But with the annulus, ratios as low as 0.27 can be explained (ratios as low as 0.35 have been measured), and the corresponding capillary pressure gradients agree better with those cited elsewhere. The ratio of organ hematocrit to large vessel hematocrit for the heart, lungs, brain, gut, and carcass can be explained on a single capillary basis with or without pericapillary plasma, but that for the liver and kidney requires the plasma annulus. The analysis predicts a linear relationship between pressure gradient and flow in the capillary with or without the annulus. The hydrodynamic result provides a plasma transit time along the annulus that agrees well with that observed between the "bulk" disappearance and subsequent reappearance of labeled plasma in the microcirculation. ADDITIONAL