A theoretical approach to analyze pressure equilibria in the interstitium.

Inherent problems concerning the interstitial fluid pressure (IFP) are reinvestigated on theoretical grounds. Analyzing the thermodynamic and mechanical equilibria in the interstitium, it is concluded that IFP includes a pressure term originating from the elastic forces and an osmotic pressure term. A quantitative relationship is established between the IFP and all of the parameters responsible for the changes in the recorded pressure. The theoretical results suggest that, under control conditions, 1) there are no permanently existing free fluid spaces, 2) the gel pressure is atmospheric, and 3) the fluid equilibration techniques measure an osmotic pressure difference between the gel phase and the fluid phase created artificially by any of the pressure measuring devices. The pressure response during acute volume changes is attributed to the changes in the osmotic pressure term, gel volume, and elasticity. Volume and elasticity changes are reflected in the recorded IFP as promptly developing and permanent effects; on the other hand, osmotic processes result in slowly developing and transient effects. The volume-pressure relationship is also analyzed.