Model of interstitial pressure as a result of cyclical changes in the capillary wall fluid transport

Reported interstitial pressures range from -8 to +6 mm Hg in different tiss ues and from < -20 mm Hg in burned tissue or more than +30 mm Hg in tumors. We have tried to link interstitial pressure to the here proposed cyclical changes in the fluid transport across the capillary wall. In the presented model interstitial pressure is considered as an average of pressures in numerous pericapillary spaces. A single pericapillary pressur e is a dynamic difference between the net outward (hydraulic pressure+inter stitial colloid osmotic pressure) and inward (plasma colloid oncotic pressu re) forces. Hence, dominating net outward forces would result in a positive pericapillary interstitial pressure, while stronger inward forces would pr oduce negative pressures in the pericapillary space. All interruptions of b lood flow leave some blood in capillaries with a normal oncotic pressure an d no hydrostatic pressure that might act as a strong absorber of interstiti al fluid until the blood flow is reestablished. Model assumptions for the systemic circulation capillaries include (a) prec apillary sphincters can almost entirely stop the capillary flow, (b) only a minority of sphincters are normally open in the tissue, and (c) hydrostati c pressures in unperfused capillaries are similar to the pressures at their venous ends. The key proposal is that capillaries with closed precapillary sphincters al ong their entire length have low hydrostatic pressure of 10 to 15 mm Hg. Th is pressure cannot force filtration, so these capillaries reabsorb intersti tial fluid from the pericapillary space along their entire length. In the o pen capillaries, hydrostatic pressure filtrates fluid to the pericapillary space along most of their length. Fluid enters, moves some 20 or 30 microme ters away and back to be reabsorbed at the same point. Closed periods are p eriods of intense fluid reabsorption, while the short open periods refill t he space with fresh fluid. It can be calculated that subcutaneous tissue in terstitial pressure values might develop if the closed periods are 1.14 to 2.66 times longer than the open periods. Positive interstitial pressures ob served in some organs might develop if open periods are longer than the clo sed periods. High interstitial colloid pressure in lungs makes both perfused and unperfu sed capillaries absorptive, resulting in more negative values of lung inter stitial pressure. The same model is used to explain interstitial pressure v alues in tumors, burned tissue and intestinal villi. (C) 2001 Harcourt Publ ishers Ltd.