Transcapillary fluid exchange during arteriolar vasomotion.

Rhythmic contraction of small arterioles and the resulting pulsatile flow in the capillaries they supply are phenomena that have frequently been observed in the microcirculation. When an arteriole contracts, the capillary hydrostatic pressure drops below the plasma osmotic pressure and fluid is reabsorbed from the tissue into the blood. Because the capillary flow rate is slowed during this time, the reabsorbed fluid dilutes the plasma protein enough to significantly reduce the driving force for reabsorption. It was concluded in an earlier analysis (Salathe, Venkataraman and Gross, 1982, henceforth referred to as I), that such dilution results in less tissue dehydration when blood flow is decreased through pulsatility than when it is decreased the same amount by steady-state contraction of feeding arterioles. The large and rapid changes in fluid movement across the capillary wall during arteriolar vasomotion alter the interstitial hydrostatic and osmotic pressures, and the analysis in I included the simultaneous interaction of all four Starling forces. In order to retain a tractable problem, it was necessary to make certain simplifying assumptions that restricted the results to a limited range of physiological conditions. It was found, however, that changes in the interstitial fluid pressures were small, and that their variation can be neglected without introducing significant error. We have therefore repeated the analysis without the simplifications introduced in I, but with the interstitial pressures assumed constant, and so the results reported here are applicable to the full range of physiological conditions.