| Summary: | Osmotic pressure of the contents from various parts of the gut was related to the concentrations of Ca, Mg, NH 4+, Na, K, Cl, total carbonic acid, inorganic phosphate, sulphate and steam-volatile fatty acids in press-juice of gut contents and faeces of 16 cows.Abomasal contents were slightly hypotonic to the blood. The most important osmotically-active elements were Na and Cl.In the upper ileum the chyme was strongly hypertonic largely through organic non-electrolytes but also Na and Cl.Distally in the small intestine osmotic pressure gradually decreased and in the caecum they were almost isotonic with the blood, largely through Na and, to a lesser extent, K and NH 4+.Strong selective absorption of Na against a concentration gradient caused the hypotony of colonic contents and faeces. Re-absorption from the large intestine was important in the Na metabolism of the cow.
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