Capillary filtration-absorption balance reconsidered in light of dynamic extravascular factors.

The evidence for the functional importance of extravascular Starling pressures now seems overwhelming, and when these terms are taken into account it is difficult to uphold the traditional conception that upstream microvascular filtration is largely matched by a sustained downstream reabsorption. Transient absorption can occur, however, during spontaneous vasomotion cycles, during sympathetic-induced vasoconstriction and during hypovolaemic hypotension. Sustained absorption is possible in specialized tissues where the interstitium is 'flushed' by an independent stream (intestinal mucosa, renal cortex, lymph nodes). Both theory and experiment show, however, that absorption cannot be maintained across most low-pressure exchange segments due to the finite permeability of microvessels to plasma protein, which leads to a rise in pericapillary interstitial oncotic pressure with time around absorbing microvascular segments. Extravascular hydraulic resistance may be a further determinant of net fluid transfer rate in situations where capillary wall resistance is low.