294 PHYSIOLOGICAL REGULATIONS 



That is contrary to data presented above. And so for further 

 statements, including some that I have made. 



There is no evident hmit to the variety of ways in which living 

 units adjust their water contents. Varied though the compensa- 

 tions and behaviors contributing to water adjustment are, the out- 

 standing fact is that every unit that has been investigated gives 

 evidence of special activities, one of which is modification of water 

 exchanges, that result in maintaining water content more constant 

 than when those activities are experimentally prevented. The 

 apparent relations among those activities and modifications, both 

 qualitatively and quantitatively, is set forth in a manner that 

 coordinates the materials of the comparative physiology of water. 



§ 108. Agents and types op load 



A great many agents regularly modify water relations, whether 

 they are widely recognized as ''hydrators" and "dehydrators," or 

 not. The measurable water increments (AW and AV) observed 

 in animals under the influences of some few of these agents were 

 already studied. But other agents abound, and many of them 

 would scarcely be proclaimed as producers of AW or AV. Ex- 

 amples are : diuretics, diaphoretics, cathartics, blood substitutes, 

 secretagogs, hot atmospheres, and low oxygen tensions. 



Whatever designations such agents may have, a common 

 property is to influence water content or volume and water ex- 

 changes. Hence the modifications in the body and its parts at any 

 water load may be compared quantitatively in the same manner as 

 above. The effects of several agents may thus be characterized 

 with respect to any and all the modifications of volumes, concen- 

 trations, and metabolisms that can be measured. 



Something of this sort has been done for pituitary extracts, for 

 instance. With respect to water output after water administration, 

 the times to maximal rate of urinary output, or the times for elimi- 

 nating half the water increment (mid-excretion point, index of 

 diuresis, half -life) are compared (Burn, '31; Heller and Urban, 

 '35; Martin and Herrlich, '39). Equally useful in those assays 

 would be any of the parameters listed in § 105. 



Particularized tests of the organism's status with respect to 

 water may be useful in the comparison of all conceivable agents, 

 whether those who make the tests are interested in the agents or in 

 the organisms. Such would be: {!) Total water output, or total 



