220 



Theories of Secretion 



saliva or to facilitate their entering. The importance of this obser- 

 vation is that in this case chloride is being affected independently of 

 bicarbonate and in association with a cation; this process cannot, 

 for instance, be proceeding at an anion exchange area in the distal 

 part of the salivary duct. 



Effects of adrenal corticoids. The effect of adrenal corticoids is to 

 raise the potassium and lower the sodium concentration in saliva. 



Fig. 10.14. Unloading experiment with C 14 urea in the dog submaxillary 



gland. 



During stimulation the rate of loss of labelled urea in the venous blood increased. This excess 

 venous loss rate corresponded to the amount of urea crossing the duct epithelium into the 

 periductal blood (Burgen and Seeman, 1958). 



This is most strikingly seen in the sheep parotid (Fig. 8.12), but 

 the same process has been noted in all the other glands although 

 to a lesser degree. No analysis of this effect has been carried out 

 but this appears to be a case where the primary change is on the 

 two major cations, without important changes in the anions. It is 

 possible that the corticoids control an exchange of sodium and 

 potassium in the duct system. 



REABSORPTION OF FLUID IN THE DISTAL DUCTS 



Several pieces of evidence point to the occurrence of fluid re- 

 absorption in the most distal region of the salivary ducts. At low 



