§ 5.322 WATER BALANCE 233 



Experimentally the skin of a frog or toad behaves as if it had 

 ultra-microscopic pores that are enlarged by the neurohypophysial 

 hormone to allow an osmotic flow of water, instead of the slower 

 diffusion (Ussing, 1954). The rate of flow is then determined by the 

 osmotic gradient (Fig. 5-22). Even with 1/lOth Ringer as the 

 "outside medium" for isolated skin, the net influx of water is 

 several times greater than it could be by diffusion. In fresh water 

 the rate would be still greater. 



Ussing supported his *'pore theory" by putting thiourea on both 

 sides of the isolated skin; but the thiourea on one side had its 

 carbon, and on the other its sulphur, isotopically labelled, so that 

 the flux could be followed in both directions. When neurohypo- 

 physial hormone was present, the increased flux of thiourea was 

 much greater than the increase in flux of the water. It may be 

 assumed that the hormone dilates pores that are normally too small 

 for molecules of thiourea, though just large enough for water. 

 Moreover, "the influx of thiourea is more rapid than the outflux, 

 even when the concentration is the same on both sides", and this 

 reflects the fact that thiourea is swept through the pores in the 

 osmotic flow of water (Sav^er, 1957). 



Increase in permeability of the bladder 



Amphibia. Reabsorption of water from the bladder in Rana is 

 similar to that from the skin and kidney tubules and responds in 

 like manner to frog ADH injections and to dehydration, a state 

 which presumably stimulates the secretion of the frog's own 

 neurohypophysial hormone (Fig. 5-24). 



Mammalia. There is no evidence of water being reabsorbed 

 from the bladder in mammals. 



Increase in permeability of the kidney tubules 



Amphibia. When amphibians are exposed to dehydrating 

 conditions, conservation of water in the kidneys is stimulated by 

 the independent action of antidiuretin at two points, the glo- 

 merulus and the tubule. 



The glomerular filtration rate (G.F.R.) is determined by the 

 arterial blood pressure, the resulting filtrate being isosmotic with 

 the blood plasma and only differing from it in the lack of proteins 



