The influence of tissue hydrostatic pressure and protein concentration on fluid and protein uptake by diaphragmatic initial lymphatics; effect of calcium dobesilate.

Net uptake of fluid and protein by rat diaphragmatic initial lymphatics was observed, at various tissue hydrostatic pressures (-5 to 25 cm water)--using the peritoneal cavity as a huge tissue space. Between -5 and +5 cms, the amount of fluid uptake was much increased as the protein concentration in the tissue fluid rose. This is as predicted by the colloidal osmotic pressure hypothesis of initial lymphatic filling; it is completely contrary to the hydrostatic pressure gradient hypothesis. However at 15 and 25 cm, the protein concentration had no effect on the fluid uptake. The initial lymphatics were acting as simple conduits, rather than as force-pumps. Fluid flowed down a hydrostatic pressure gradient, from the tissues to the collecting lymphatics. Fluid uptake was not directly proportional to the hydrostatic pressure gradient; it increased disproportionately at higher pressures--probably because of the dilatation of the initial lymphatics or their open junctions. Calcium dobesilate had no effect on fluid or protein uptake at the lower pressures, but significantly increased both of these at 25 cm of water. Probably it caused the collecting lymphatics to pump more, removing excess lymph which otherwise accumulated in them. This reduced the intralymphatic pressures in the initial lymphatics so that more fluid entered them.