Transcapillary Starling forces using membrane osmometry

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Abstract
Membrane osmometry was used to estimate the 4 transcapillary Starling pressures in subcutaneous tissue of rats, guinea pigs and dogs. Isolated subcutaneous tissue samples were placed on a large-pore or small-pore osmometer that measured the interstitial fluid pressure (Pif) and the difference between the interstitial fluid pressure and the interstitial protein osmotic pressure (Pif - .pi.if), respectively. The colloid osmotic pressure of the interstial fluid (.pi.if) was obtained from the differences in these 2 pressures. A plasma sample placed on the small-pore osmometer yielded the colloid osmotic pressure of the plasma proteins (.pi.c). Finally the capillary pressure (Pc) was calculated from the 3 other Starling forces. In the rat, guinea pig and dog, respectively, the estimated Starling forces were as follows: Pif, -2.2, -2.1 and -4.8 mm Hg; .pi.if, 7.3, 4.8 and 4.4 mm Hg; .pi.c, 21.3, 19.5 and 19.2 mm Hg; and Pc, 11.8, 12.6 and 10.0 mm Hg. A comparison with data obtained in other studies using different methods shows good agreement and strongly supports membrane osmometry as a method for measuring the Starling pressures in subcutaneous tissue.