The pulmonary interstitium in capillary exchange
When capillaries filter excessive fluid, tissue fluid pressure increases, tissue colloid osmotic pressure decreases, and lymph flow increases. The change in tissue forces and flows have been termed edema safety factors since they act to oppose alterations in pulmonary capillary pressure. It is well known that pulmonary capillary pressure can be acutely increased by approximately 20 mm Hg before fluid enters the alveoli, and that the changes in the tissue forces are responsible for this phenomenon. The decrease in interstitial colloid osmotic pressure appears to account for approximately 50% of the tissue's ability to oppose increases in pulmonary capillary pressure. The tissue colloids change because the capillaries filter a protein-poor fluid, and the exclusion of plasma proteins decreases with increasing interstitial volume. Tissue pressure, at least in the perivascular regions, increases with tissue hydration and provides another major tissue force opposing capillary filtration. The contribution of lymph flow to the overall edema safety factor is difficult to estimate at the present time. However, it is possible that the pressure drop associated with the flow of interstitial fluid between the alveolar septal interstitium and the larger perivascular spaces could serve as an edema safety factor, rather than the standard lymphatic flow pressure drop across the capillary membrane. To calculate the standard lymph flow contribution to the total edema safety factor, total lymph flow [LF] and the filtration coefficient of the capillaries [Kf,c] must be known, i.e., Capillary Wall Drop = LF/Kf,c. For normal lung lymph flows and Kf,c's, it would appear that this factor is small. However, if the total pressure drop for fluid movement through the entire lung tissue is used to estimate the lymphatic factor, then it may represent a major portion of the edema safety factor, i.e., Total Pressure Drop = LF/[Kf,cKf,t]/Kf,c + Kf,t] where Kf,t is the filtration coefficient of the tissues. Tissue forces at different site within the lung tissue are presently under intense investigation in serveral laboratories. The next years should provide the necessary information to discuss fluid accumulation in lung tissue in terms of local transcapillary forces rather than the average forces that are now the "present state of the art."