Regulation of water balance between blood and lymph in the frog, Rana pipiens

To determine the relative importance of the lymphatic system and transcapillary reabsorption in maintaining water distribution between blood and lymph in Rana pipiens, the hematocrit (HCT) was measured (12 frogs) when each frog was: 1) under tricaine methanesulfonate (MS222) anesthesia, which inactivates the lymph hearts, 2) conscious at rest (control), and 3) conscious after activity. Arterial pressure (AP), lymph-heart rate (LHR), and plasma and lymph protein concentrations and colloid osmotic pressures (COP) were also measured. The average HCT during control state was 26.0 +/- 5.2% (SD), AP was 21.9 +/- 5.2 mmHg, and LHR was 86 +/- 13 bpm. MS222 increased HCT (42.6 +/- 7.9%), and did not affect AP. Experiments, in which plasma volumes were determined by intra-arterial injection of Evans blue-labeled albumin (EB-BSA) (16 frogs), showed that the increased HCT reflected a reduced plasma volume. Activity did not affect HCT, but increased AP (33.1 +/- 6.4 mmHg, p < 0.01), and LHR (115 +/- 26 bpm, p < 0.01). Thus lymph hearts are largely responsible for maintaining blood volume, and changes in transcapillary filtration rate are rapidly mirrored by changes in lymph heart function. Plasma and lymph COP values after MS222 or activity were not significantly different from control. This observation is consistent with the protein and fluid fluxes resulting from Starling forces if an average reflection coefficient to protein, sigma, of 0.5 is assumed, which is consistent with our EB-BSA data.