§ 5.322 WATER BALANCE 237 



time in hypotonic sea water, where the tissues would tend to 

 become over-hydrated and there would be no call for the secretion 

 of an antidiuretic hormone. Presumably in normal sea water the 

 supply of the hormone is in balance with the demand and no 

 histological change is observable. 



Reptilia. Little seems to be known of water control in reptiles. 

 Chester Jones {\9S7a) writes: "Many reptiles are adapted to arid 

 conditions and even those whose habitat is in or near water have 

 no movement of water through the skin . . . nor is the reptilian 

 renal tubule specifically adapted for water reabsorption beyond the 

 plasma osmotic concentration. The alligator is very responsive to 

 the antidiuretic effect of . . . pitressin [mammalian ADH] rather 

 than the pitocin fraction, as in frogs. Antidiuresis is effected, at 

 least partly, by lowering the glomerular filtration rate, pitressin 

 [contracting] the smooth muscle of the afferent glomerular arteri- 

 ole." Metabolic waste products and especially the uric acid are 

 eliminated not only by glomerular filtration, but probably to a 

 much greater extent by tubular secretion. 



Ayes. Even less seems to be known of birds, but they are prob- 

 ably similar to reptiles. 



Mammalia. The hormone control of water balance in mammals 

 has been most fully investigated in rats, but seems to be basically 

 similar in other forms, including the dog. In one way the situation 

 is simpler than in the frog because the skin is practically waterproof, 

 and the supply of water is, therefore, derived entirely from the gut 

 contents by intestinal absorption, where no hormonal influence has 

 been detected. The loss of water from the sweat glands does not 

 appear as a rule to be under hormone control either (§ 4.14); nor 

 does the loss through the lungs. The main control of the water 

 balance occurs in the kidneys, but it is only partially dependent 

 on hormones, because mammals have, in addition to the systems 

 found in the frog, the "concentrating mechanism" already 

 mentioned (§ 5.321). 



If the ANTIDIURETIC HORMONE, ADH, were to have the same effect 

 in mammals as in the frog it would increase the permeability 

 of the distal kidney tubules and allow of water reabsorption there 

 until the urine became isosmotic with the plasma (Fig. 5-15^, 

 p. 214). It is clearly of importance for wholly terrestrial mammals 



