INTERSTITIAL FLUID PRESSURE - CAPSULE, FREE FLUID, GEL FLUID, AND GEL ABSORPTION PRESSURE IN SUBCUTANEOUS TISSUE

Pressure-volume curves were measured for subcutaneous tissue samples from the dog. These were measured in isotonic physiological salt solutions and in 2 M sodium chloride. The latter solvent was used to eliminate (1) the Donnan osmotic pressure of the cations trapped in the interstitial gel and (2) the absorption pressure caused by electrostatic repulsion of gel polyelectrolytes. These studies indicate that about one-third to one-half of the total absorption pressure of normal subcutaneous tissue results from the Donnan osmotic pressure and electrostatic repulsion effects of interstitial polysaccharides while the remainder results from some other factor such as compressed elastic forces in the tissue; however, these fractions vary greatly with changes in total absorption pressure. These data can be used to interpret the forces responsible for the pressure measured with chronically implanted capsules. In general, the capsular pressure equals the total absorption force of the tissue. In terms of the two-phase model of the interstitium, capsular pressure also equals interstitial fluid pressure in the free fluid phase, and this in turn is equal to the sum of forces in the gel phase, i.e., gel hydrostatic pressure and gel osmotic pressure. © 1979.