WATER BALANCE 



231 



§ 5.322 



medium; but water losses following exposure to air can quickly be 

 made good, on the animals' return to water, by imbibition through 

 the skin. This can be shown by ligaturing the cloaca of such de- 

 hydrated frogs to stop loss of urine; then increase in weight over 

 two or three hours from the time of their return to water will show 

 the amount of water uptake. Comparison with control frogs, 

 which had already been in water for some time, shows that the 

 rate of water uptake after dehydration may be three times greater 



12- 



2 3 



Hours in water 



Fig. 5-21 Rates at which normal and hypophysectomized frogs, 

 Rana pipiens, return to their initial weight after being dehydrated 

 and then placed in water at time 0. Hypophysectomy and the 

 consequent lack of frog ADH reduces, but does not inhibit, water 

 uptake through the skin (from Levinsky and Sawyer, 1953). 



than in the controls. In the latter, the loss of urine, if the cloaca 

 were not ligatured, would balance this uptake and the weight 

 would remain constant (Fig. 5-20«). Removal of the hypophysis 

 decreases, but does not inhibit, the rate of water uptake by 

 dehydrated frogs (Fig. 5-21), presumably because the osmotic 

 gradient is steeper than in the normally hydrated controls. In- 

 jection of a neurohypophysial extract (frog ADH) more than 

 restores the capacity for water uptake (Fig. 5-20^). Even isolated 

 skin treated with the same hormone shows an increase in water 

 uptake large enough to account for the changes seen in whole 

 frogs (Fig. 5-20^:). 



