WATER EXCHANGES OF DOG 37 



dissimilar from those of the sack might also be revealed. The only 

 direct tests available are not on dog but on man. No detectable 

 water is then taken through the skin; but output through it is 

 wholly inhibited, except when solutes are added to the bath (White- 

 house et al., '32). 



The dog's respiratory surfaces appear to lose water enough to 

 saturate very nearly all the air that is pumped in and out of them 

 (Hemingway, '38). No system appears to have been devised in 

 terrestrial animals whereby oxygen may be taken out of air with- 

 out giving up to the air at least an equal number of molecules of 

 water. 



Altogether the paths of water exchange have a group of proper- 

 ties that remain fixed in states of balance. They represent ana- 

 tomical, chemical, and functional arrangements suitable for the 

 continuance of water turnover and content. Two of the paths, the 

 kidneys and the alimentary tract, modify their activities in accord- 

 ance with water load. This fact is expressed quantitatively in the 

 equilibration diagram, which indicates in what degree each of the 

 exchanges enters into the recovery from each possible load. 



§ 10. Velocity quotient 



The equilibration diagram has the coordinates AW and SW/At. 

 The quantities AW and BW have the same dimensions in both vari- 

 ables, and represent the same substance (water). Care is exercised 

 to correlate only simultaneously occurring quantities. Dividing 

 rate by load, SW/At -^ AW equals 1/At, the velocity quotient. Its 

 significance is clearer if written SW/(AW X At) ; it is the volume 

 of water restored or removed, relative to the increment to be re- 

 stored or removed, per interval of time. If, for instance, an excess 

 of 2% of Bo is present in the dog's body (fig. 6) and three-fourths 

 of it (1.5% of Bo) is removed per hour of recovery, the 1/At equals 

 0.75/hour. The larger this quotient is, the faster is the recovery. 



From the data of figure 13 a train of values of 1/At is obtained 

 (fig. 17), all within the first 1.0 hour of recovery. Similarly figure 

 16 furnishes a series of other values (fig. 18). Two kinds of 

 quotients are thus available, depending on whether the rates of 

 total gain and loss are used (total, fig. 17) or the rates of net water 

 exchange (net, fig. 18). Whereas net exchanges give rather con- 

 stant values of 1/At at diverse loads (AW), total exchanges do not, 

 for near zero load the latter ratios approach infinity. 



