The toad skin NaCl check-valve

Short-circuited toad skins exposed to chloride-containing Ringer's on the outer side exhibit a large inward chloride flux. Conversely, toad skins exposed to nominally chloride-free Ringer's externally and chloride Ringer's internally manifest a very low chloride outflux. This chloride-dependent rectification of chloride flux serves to conserve NaCl transported from low NaCl concentration outer solutions to which amphibians are normally exposed. Once chloride is transported inwardly it cannot return to the outer solution, with chloride rectification acting as a check-valve for transported NaCl.