Physiological Regulation of Water Content 11 



both absolute (body size) and relative to body solids, varies 

 with the age of the rat (Fig. 8). What controls the absolute 

 content of water and of each solute? The answer to this ques- 

 tion is not available. Obviously all the items that enter the 

 determination of growth and its correlatives participate in 

 these controls. This is a problem that has barely been 

 visualized, and one whose analysis may occupy many physio- 

 logists in the future. 



In general, the ready corrections of water excesses and 

 deficits result from specific response systems for diuresis and 

 for water drinking. The systems vary between infant and 

 adult, not only quantitatively but possibly also in the medi- 

 ators and effectors used. Over a long lifetime, the regulation 

 depends also upon detectors of body size and proportions 

 whose characteristics and locations have not been determined. 



REFERENCES 



Adolph, E. F. (1943). Physiological Regulations. Lancaster: Cattell. 



Adolph, E. F. (1956). Amer. J. Physiol, 184, 18. 



Adolph, E. F. (1957). Quart. Rev. Biol., 32, 89. 



Adolph, E. F., Barker, J. P., and Hoy, P. A. (1954). Amer. J. Physiol., 



178, 538. 

 Ames, R. G. (1953). Pediatrics, Springfield, 12, 272. 

 Falk, G. (1955). Amer. J. Physiol, 181, 157. 

 Hamilton, B., and Dewar, M. M. (1938). Growth, 2, 13. 

 Heller, H. (1947). J. Physiol, 106, 245. 

 Heller, H. (1952). J. Endocrin., 8, 214. 



Hoy, p. a., and Adolph, E. F. (1956). Amer. J. Physiol, 187, 32. 

 Krecek, J., Kreckova, J. and Dlouha, H. (1956). Physiol Bohemo- 



slov., 5, suppl., p. 33. 

 McCance, R. a., and Wilkinson, E. (1947). J. Physiol, 106, 256. 



DISCUSSION 



Shock: We have obtained some data in our laboratory on the age 

 differences in the antidiuretic response to pitressin. Some of the results 

 of these experiments are in accord with the concept that in many in- 

 stances the senescent animal returns to a type of response and behaviour 

 that is seen during the course of development. In these experiments we 

 measured the concentrating ability of the kidney rather than total urine 

 flows. Total urine flows are not useful for age comparisons since in older 

 subjects the number of functioning units is reduced and hence there is a 



