Urinary production in the healthy horse and in horses deprived of feed and water

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Abstract
SUMMARY: Total daily 24-hour urinary output was obtained from 11 healthy horses fed alfalfa hay with free access to salt during periods of high environmental temperatures. Daily urinary volume averaged 15.6 L, with mean specific gravity of 1.028, osmolality of 1,040 mOsm/kg, and urinary flow rate of 1.24 ml/kg/hr. Total 24-hour sample collections of urine were also obtained from horses held without access to feed or water for periods of 24, 48, and 72 hours during high environmental temperatures. Average urine production under these conditions was 6.3 L during the 1st day; 3.2 L, the 2nd day; and 3.0 L, the 3rd day. Although total daily urinary electrolyte excretion decreased during these periods, urinary electrolyte concentrations of the major ions often remained unchanged, or in the case of sodium, actually increased. There was a highly significant correlation between the urinary excretion of the principal cations, sodium and potassium, and the principal anion, chloride. The urinary excretion of water and electrolytes appeared to remain stable after 48 hours of feed and water deprivation.