206 metabolic hormones 



5.3 Balance of monovalent electrolytes and water 

 The control of salts and water in the blood and tissues is of 

 importance to all animals living in environments with which they 

 are not in osmotic and ionic equilibrium; but evidence of this 

 control being exerted by hormones is almost confined to the 

 vertebrates. Hormonal control of these factors among invertebrates 

 is as yet only known in relation to water in a few arthropods. 



There are, from the point of view of basic physiology, two 

 contrasting situations in which an animal must be able to control 

 its salt and water content if it is to survive. In the first, the blood 

 and tissues tend to lose salts and become diluted by imbibition of 

 water. This can arise when an aquatic animal moves into any 

 hypotonic medium; but it is particularly associated with the 

 migration from the sea to fresh water. Since the salt losses cannot, 

 as a rule, be made good merely from the food, most freshwater 

 animals not only reabsorb salts from their urine but also absorb 

 them from the environment against considerable osmotic gradients. 

 This involves the active transport of ions across certain cell 

 membranes (cf. Kitching, 1957). There are various hypotheses 

 concerning active transport, but they may all "be expected to 

 contain the following principles: 



(a) The cations [such as Na"*"] are not transported as the free 

 ions, but are bound in some complex, which operates as a 

 carrier ; 



(b) This carrier operates in a cyclical manner across a mem- 

 brane. Towards one side of the membrane it combines with 

 the ion and towards the other side it releases the ion, the 

 process being combined either with an exchange for another 

 cation or it is accompanied by an anion [such as Cl~]. 

 These associated exchanges may or may not be active in 

 turn; 



(c) The release of the ion must be associated with a simul- 

 taneous release or transfer of the necessary energy if osmotic 

 work is being done" (Conway, 1956). 



At the same time, water which enters the tissues by endosmosis 

 from a hypotonic medium must be baled out by some form of 



