Cellular Aspects of Body Electrolytes and Water 25 



This does not mean, however, that the maintenance of 

 differences of osmotic pressure between cells and their environ- 

 ment by the excretion of water does not occur; it is well 

 known that such fluids as urine and saliva have osmolarities 

 that are vastly different from that of the plasma; and the 

 elaboration of these fluids is best described by invoking an 

 active transport of w^ater — i.e. the functioning of a 'Svater 



Cations and Anions 305-7 Cations and Anions 303-3 



Cations and Anions 320 • 1 



pump". The cerebrospinal fluid would appear to represent 

 another example of a non-iso-osmotic fluid, and since it is in 

 such close relationship with the nervous tissue of the brain and 

 spinal cord, this lack of iso-osmolarity is of special interest, 

 suggesting as it does that these tissues, too, are not in osmotic 

 equilibrium with the blood. The results of a detailed analysis 

 of the ionic concentrations in plasma and cerebrospinal 

 fluid are shown in Table II: included are values for a similar 



