A model to explain uphill water transport in the mammalian stomach

Under most conditions the osmotic gradient hypothesis can account for water movement in the stomach. However, in the resting in vivo dog stomach, with isotonic HCl bathing the mucosal surface, H + disappears and Na + appears at approximately the same rate and water moves from the blood to the mucosal bathing fluid, resulting in the production of a hypotonic fluid. In the present paper we have developed a conceptual model to explain these findings. According to the model the cells on the surface and those lining the pits are impermeable to both water and solutes, the site of the H +-Na + exchange is the tubules, and the tubules are permeable to water. In demonstrating how the model works we have first obtained expressions for the concentration profiles of the ions in the pits under conditions of no water flow and show that the fluid at the bottom of the pits would be hypertonic. We then allow the tubules to become permeable to water, with the result that water moves from blood to lumen while the number of osmotically active particles does not change (the loss of H + equals the gain of Na +). We obtain an approximate quantitative picture during water flow by making the assumption that HCl and NaCl diffuse independently.