§ 5.322 WATER BALANCE 235 



In the tubules all the sugar is reabsorbed, but the urea, to which 

 the tubules are impermeable, is excreted. Monovalent salts, which 

 account for much of the osmotic pressure of the filtrate, arc less 

 actively reabsorbed in the tubules during antidiuresis than during 

 diuresis (§ 5.312 and Fig. 5-15^ p. 214). 



The action of ADH on the tubules is to increase their permea- 

 bility, presumably by increase in pore size, as in the skin. The 

 obligatory endosmotic flow of water then follows closely on the 

 active inward movement of salts, so that the final urine tends to be 

 isosmotic with the plasma. The reduction in clearance of "free 

 water" is shown in the lowest curve (Fig. 5-23) to be much 

 greater than could be accounted for by the reduction in glomerular 

 filtration rate alone, as shown by the creatinine clearance. It is 

 therefore assumed that the reduction in volume of urine (V, Fig. 

 5-23) is mainly due to reabsorption of water because of the increase 

 in tubule permeability, and not only to the reduced G.F. R. 



No Amphibia have been found to excrete urine more con- 

 centrated than that which is isosmotic with the plasma. There is, 

 therefore, no need to postulate for them any seat of active transport 

 of water from the kidneys to the tissues. 



There is considerable specific variation among Amphibia in 

 their sensitivity to their own ADH. Removal of the neurohypo- 

 physis, and therefore of the store of this hormone, has no effect 

 in Bufo bufo and Rana temporaria, as compared with intact controls. 

 Bufo arenarum, when kept out of water, behaves like R. catesbiana 

 (Fig. 5-23). Bufo martnus of South America may be even more 

 sensitive than these to frog ADH ; in hydrated and hypophysec- 

 tomized specimens only about 35 per cent of the filtered water was 

 reabsorbed by the tubules; but after injecting extracts containing 

 frog ADH this rose to an average of 65 per cent (Sawyer, 1956). 



The rate of secretion of antidiuretin from the neurohypophysis 

 in Amphibia may well be mediated through osmo-receptors in 

 contact with some of the main arteries, as it is in mammals 

 (Chester Jones, 1957a). 



Teleostei. In sea water the body fluids of teleosts are maintained 

 at a level considerably hypotonic to the medium by drinking sea 

 water and excreting the excess salts through the gills. Kidney 

 excretion is reduced to a minimum, largely by a loss of glomeruli, 



