Interstitial fluid, lymph and oedema formation.

Venous and interstitial fluid pressure, protein content of blood plasma, leg lymph and interstitial fluid were estimated in anaesthetized normal dogs and rabbits as well as in dogs with chronic inferior v. cava constriction. In the tissue fluid collected by the perforated capsule and by the wick methods protein concentrations were the same. In the rabbit hindlegs subcutaneous tissue fluid and lymph protein concentrations were 50 and 45%, respectively of plasma protein concentration. In the dog tissue fluid, protein was 42.5% but lymph protein content was only 17% of plasma protein. Tissue fluid hydraulic pressure (intracapsular) was in normal dogs--5.0 +/- 1.3 mm Hg and it increased in cava constricted dogs to +0.8 +/- 0.4 mm Hg. In caval constriction, protein concentration in the interstitial fluid decreased markedly. A linear relationship was found in dogs and rabbits between tissue fluid and lymph protein concentration (r = 0.68 and 0.73, respectively). The relationship between venous and tissue fluid (intracapsular) pressures was non-linear. In dog hindlimbs the calculated isogravimetric capillary pressure was 7.8 mm Hg. In venous congestion both the increase of interstitial fluid hydraulic pressure, and the decrease of oncotic pressure ace as "oedema safety factors". In the rabbit 20 h after the intravenous injection of labelled albumin there was no significant difference between the specific radioactive albumin contents of plasma, leg lymph and tissue fluid.