The Fate of Ingested Water

Except in some of the lower orders, virtual constancy of the volume of body water is a prerequisite for the normal function and indeed the survival of the organism. In terrestrial animals, particularly, adaptive mechanisms provide for physiological regulation of the amounts of water which leave and enter the body. Although solid foods provide a considerable quantity of pre-formed water to the body, variation in water intake is largely dependent upon the volume of liquid imbibed. Physiological regulation of this volume is accomplished by the mechanisms of thirst and the drinking of water in quantities sufficient to satisfy thirst. Referring to observations in animals, presumably less influenced than man by emotional and sociological factors, Adolph' has written, "Regulation of body content of water, therefore, is accomplished not merely by the overflow of any excesses. Equally, it is adjusted by measured intakes. Drinking is just as reproducible as excretion, and water balance is approached with equal accuracy from both sides." Wolf"' has presented equations and diagrams describing the quantitative relationships between water requirement and water intake in the dog and man. Although thirst may be properly regarded as an entirely subjective phenomenon, it hardly seems pertinent to argue the validity of the inference that thirst exists in a dehydrated burro who, when offered water, promptly imbibes several gallons. It had long been recognized that "in man any large loss of fluid, as by sweating, diarrhoea, or haemorrhage, gives rise to an intense thirst that has its natural reaction in increased intake of water by mouth" (Starling1'8). However, Gilman's' observations demonstrated that a decrease in total body fluid was not required to stimulate drinking since dogs given hypertonic saline intravenously drank copiously even though body water was increased. A comparable rise in total solute concentration was produced by infusion of an equi-osmolar solution of urea, which unlike