The effect of hydrostatic and osmotic pressure on blood flow and undirectional fluxes of sodium and water in the canine ileum



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Increased hydrostatic and osmotic pressure effects on the unidirectional Na and H2O fluxes were investigated in the canine ileum in situ. Isotonic gut perfusates included saline and saline-mannitol (1/1 : v/v). Intravenous isotonic saline (1 ml/min/kg body weight) elevated capillary blood pressure, total intestinal blood flow and the unidirectional secretory fluxes of Na and H2O, while the absorptive fluxes and the absorptive site blood flow remained unchanged. Intravenous hyperoncotic dextran (77 000 daltons, 25 percent in isotonic saline) infused in volume-expanded dogs decreased the secretory fluxes despite increased blood pressure. Venous pressure correlated directly to the secretory fluxes; arterial and colloid osmotic (i.e., plasma total solids) pressures correlated inversely. Intestinal blood flow was shunted away from the absorptive site when hydrostatic pressure increased.