§ 5.321 WATER BALANCE 227 



which is for them a hypertonic medium (cf. G. Pickford and Atz 

 1957). 



The protective action claimed for the thyroid, in allowing fish to 

 migrate into waters of low salinity, has not been fully elucidated 

 (cf. § 5.311) ; but it may be similar to that of thyroxine, in causing 

 water (and salts) to pass from the tissues of mammals to their 

 blood, thereby increasing the possible rate of water diuresis by the 

 kidneys (Fontaine, 1956). 



The corpuscles of Stannius, derived from the pronephric 

 ducts of teleosts, may also have a diuretic function, as they tend 

 to hypertrophy when the tissues are loaded with water (Rasquin, 

 1956). 



Mammalia. When water is plentiful in the tissues, the mam- 

 malian kidney reacts like that of the frog and excretes a hypotonic 

 urine. The volume of urine is considerably less than that of the 

 glomerular filtrate, chiefly because reabsorption of salts by active 

 transport in the proximal tubules results in a correlated reabsorp- 

 tion of at least 4/5ths of the water by obligatory endosmosis, either 

 in the same part or in the thin loop of Henle. Nevertheless, the 

 diffusion rate for water limits the amount of endosmosis, so that, 

 as in the frog, "free water" remains in the tubule. Most of the 

 remaining sodium ions are actively reabsorbed, or exchanged for 

 other cations, and especially for H + , in the distal tubule, which is 

 made relatively impermeable by hydrocortisone. This causes 

 further water to become "free" and to pass on to the collecting 

 ducts. Thence it would be excreted, as it is in the frog, were it not 

 for a "concentrating mechanism", which is apparently independent 

 of hormone control. This is of most significance in reinforcing 

 antidiuresis, and it is referred to in more detail in that connexion 

 (§ 5.322). "Osmotic diuresis" is also unaffected by hormones ; it can 

 occur if the blood is loaded with extra urea, which then passes into 

 the glomerular filtrate and increases the tubular osmotic pressure, 

 so that more water than usual is excreted, instead of being re- 

 absorbed (Mudge, 1954). 



The action of hydrocortisone, in causing the relative imper- 

 meability of the distal tubules in normal diuresis, has been 

 examined indirectly in "water-loaded rats", i.e. rats which have 

 been given large doses of water by stomach tube. Such rats 



