WATER EXCHANGES OF DOG 35 



of ''reciprocal action of organs." It is now possible to say that a 

 path of intake (ingestive) is precisely correlated, rate for rate, 

 with a path of output (urinary). This continuous juggling of 

 swallowing against excreting, represents the dog's mode of restor- 

 ing and governing its water content. Were the rate of one to be 

 somehow disturbed in its relation to the rate of the other, different 

 recoveries and different balances would be found. 



It seems to me that the equilibration diagram yields a concrete 

 picture of the physiological activities by which water balance is 

 restored after disturbance of it. Among only total exchanges, 

 there are four ways in which the dog might restore its water con- 

 tent. In deficits its gain might increase, its loss might decrease ; 

 actually recovery depends almost entirely on the former. In ex- 

 cesses its loss might increase, its gain might decrease ; actually both 

 occur, but the former is more highly modified. Conceivable alterna- 

 tives, such as no modification of any exchange, would not accom- 

 plish recovery. An increase in loss or a decrease in gain when 

 deficit prevails, would oppose recovery. Of the four possible modi- 

 fications, the dog has three, and could maintain its water content 

 with only two ; that situation actually occurs in other species. 



Subtracting rates of loss from rates of gain, I obtain the rates 

 of net exchanges (fig. 16). For convenience all net rates are con- 

 sidered to be positive. The relations shown compare the rates of 

 recovery at various displacements from balance. For example, 

 at AW of ±2% of Bo the restoration is more rapid (in the first 

 hour) in negative increments than in positive ones. 



So long as processes of exchange are quantitatively fixed in 

 relation to water load, the water content inevitably slides back 

 toward its control value. This control value is the only one at 

 which ordinarily gain approximately equals loss. Hence it repre- 

 sents the only state in which constancy of water content persists ; 

 in fact, equality of gain and of loss characterizes this state of 

 balance with respect to water. All other rates indicated in the 

 equilibration diagram prevail only temporarily at loads that the 

 organism quickly abolishes. 



What can be said about this unique physiological state of bal- 

 ance with respect to water? Not only is the dog usually found in 

 this state, but in it gain and loss continue indefinitely, representing 

 the steady turnover of water. The magnitude of the turnover may 

 be thought of in relation to many factors ; it includes the ' ' require- 



