Routes of Water and Small Solute Transport between Blood and Lymph in Frog

To determine the pathways for transvascular molecular transport in Rana pipiens we injected 0.25 mL 0.03% Evans blue (EB) in the peritoneal cavity or subvertebral lymph sac (SV) of 18 anesthetized frogs. Lymph hearts in nine frogs were immobilized with curare. Samples of plasma and subcutaneous lymph were obtained for up to 60 min to determine EB concentration. With lymph hearts functioning, EB was quickly detected in the blood and at lower concentrations in the lymph. These results are consistent with venous return of EB by the lymph hearts and subsequent interstitialpassage into the lymph sacs. With lymph hearts immobilized, EB injected into the SV was detected rapidly at high concentration in the lymph but was delayed in reaching the bloodstream. Evans blue injected into the peritoneal cavity gave similar results but over a longer time frame. We conclude that lymph passes from the peritoneal cavity via the mesothelial perforations into the SV from whence it is returned to the circulation. Another route for peritoneal drainage may be the nephrostomes, which link the peritoneal cavity to the renal venous circulation, but this route is inactivated when the lymph hearts are immobilized with curare.