Hypernatraemia and Hyponatraemia 37 



Before discussing the subject in more detail it may be 

 helpful to recall some of the factors which regulate the water 

 content of the body. 



Regulation of water 



Two mechanisms, closely linked, normally guard against 

 water depletion. One regulates the intake of water through 

 the sensation of thirst, the other the output of water through 

 the secretion of antidiuretic hormone. There are at least two 

 ways in which each may be invoked: the first, a rise in the 

 tonicity, the second, less well known, a fall in the volume of 

 the body fluids (Smith, 1957; Strauss, 1957). 



A rise in the sodium content of the extracellular fluid 

 (ECF) is well known to produce thirst and to stimulate the 

 release of antidiuretic hormone (ADH). The effective stimulus 

 is not simply the rise in ECF tonicity : if the ECF tonicity is 

 raised with a substance like urea, which diffuses freely across 

 the cell membrane and raises the tonicity of both extracel- 

 lular and cellular fluid equally, this does not stimulate thirst 

 and antidiuresis to the same extent (Gilman, 1937). When, 

 however, the extracellular tonicity is raised by a substance 

 which does not diffuse into the cells, water leaves the cells 

 until the tonicity of extracellular fluid and cells are again 

 equal. The cells shrink. It is assumed that certain cells in the 

 hypothalamus respond to shrinking and stimulate the sensa- 

 tion of thirst and the liberation of ADH. 



For the release of ADH there is much evidence that there 

 are localized receptors of this kind (Jewell and Verney, 1957; 

 Verney, 1957). Recently Andersson (1957) has also provided 

 additional evidence for a thirst centre. He found that when 

 he stimulated a certain area of the hypothalamus in goats, 

 they drank water as long as the stimulus went on, even to the 

 point of haemolysing their own red cells. With destructive 

 lesions in the same region, the goats would not drink water 

 when they obviously needed it. The thirst centre and the 

 receptors of the ADH mechanism are very close together, 

 but probably distinct. 



