The role of the gastrointestinal tract in salt and water balance

Fish living in marine and freshwater environments face contrasting challenges in terms of maintaining salt and water balance. The present chapter summarizes and reviews much of what is known about intestinal water absorption, which is essential for survival of fish in marine environments, and the impacts of intestinal processes on integrative organismal physiology. Transcellular Cl^- and Na⁺ absorption via parallel co-transport systems and anion exchange drive water absorption across the intestinal epithelium and leave behind high concentrations of Mg²⁺ and SO₄^2- in the intestinal lumen resulting in fluids isotonic to the blood plasma but of unique chemical composition. High rates of Cl^-/HCO₃^- exchange by the intestinal epithelium render the intestinal fluids alkaline and high in HCO₃^- and result in CaCO₃ formation in the intestinal lumen. Rectal excretion of these precipitates and rectal fluids rich in HCO₃^- comprise a substantial base loss which along with salt gain from water absorption are compensated for at the gill where both NaCl and acid are excreted. Suggested directions for future studies in the area of intestinal contribution to osmoregulation are provided.