Transcapillary Fluid Forces during Dehydration in Two Amphibians

Bufo marinus is more tolerant of dehydration than Rana catesbeiana. Dehydration led to increases in plasma sodium, protein, and hematocrit. The increases in plasma protein concentration were greater in both species than predicted from passive concentration during dehydration, indicating protein redistribution to the vascular space. Increases in hematocrit with dehydration were delayed in Bufo, indicating that plasma volume was maintained. The colloid osmotic pressure of plasma increased with protein concentration during dehydration, as predicted by data for mammals. Venous blood pressures did not vary with dehydration in either species. Interstitial fluid pressure became more negative with dehydration. The balance of transcapillary fluid forces predict ultrafiltration during all stages of dehydration in Rana and during dehydration below 24% in Bufo. The transcapillary forces do not predict the fluid redistribution to the plasma space, which is consequently attributed to lymphatic return. The changes in plasma protein concentration during dehydration are consistent with lymphatic return. Greater interstitial fluid volumes may be an important corollary of enhanced tolerance to dehydration.