Extravascular forces in lung affecting fluid and protein exchange.

Abstract

Transvascular fluid flux follows the same general rules as does fluid flow elsewhere: flow = conductance X driving pressure. Starling (1, 2) clearly recognized that, in addition to hydrostatic pressure, protein osmotic pressure contributed to the driving pressure in fluid filtration because the pores in the microvascular endothelium are very tiny and permit the passage of water and salts, but hinder the passage of coagulable proteid. His concept is formalized in the well-known equation that bears his name: Qf = K(Pmv Ppmv) Ko-(IImv Ilpmv), where the net fluid filtration (Qf) is dependent on the membrane conductance for fluid (K) and protein (o-), and the 2 pairs of pressures, hydrostatic (P) and osmotic (II), within the microvascular lumen (mv) and in the perimicrovascular interstitial fluid (pmv). Reasonable numbers for the microvascular hydrostatic (Pmv) and protein osmotic (IImv) pressures, under various conditions, are available, as indicated in figure 1 (3, 4). The real limitation comes when we try to quantify the perimicrovascular forces. As figure 2 shows, it is easy enough to guess at suitable numbers to fit our preconceptions. But how to measure the forces in a convincing manner is a formidable problem!