Glandular Secretion of Electrolytes 67 



the observed values from the asymptote could be that there 

 is a certain back-diffusion of water in the sequence of active 

 sodium reabsorption. As demonstrated below there is reason- 

 able qualitative evidence to suggest that water is, in fact, 

 reabsorbed from the precursors of sweat and parotid saliva.' 

 Fig. 3 illustrates that the concentration of urea in sweat, 

 tears, and parotid saliva remains proportional to the con- 



600 



(1) SWEAT-- 1,82(10,02) P-6(i3) 

 (ALL RATES) 



(3) PAROTID SALIVA=Q724(tQ002)P-3(i3) 

 (RATES>0,5ml/nin) 



100 200 300 400 500 



CONCENTRATION OF UREA IN PLASMA (mg/k)0ml) 



Fig. 3. The relation between the concentration of 

 urea m the plasma (P) and the concentration of 

 urea in sweat, parotid sahva and tears. From 

 the data of 1 : Schw^artz, Thaysen and Dole (1953) • 

 2 : Albrectsen and Thaysen (1955) ; and 3 : Thaysen 

 and Thorn (1954). 



centration of urea in the plasma within a wide range of varia- 

 tion in the latter. This finding indicates that urea is excreted 

 m these secretions by a process of simple diffusion and not via 

 a specific secretory mechanism which might become saturated 

 by increasing load. Potentially urea may, therefore, be used 

 as a tracer for the movement of water within the secreting 

 glands in a similar manner as in the glomerular nephron. 



