The Blood Supply of the Gland 225 



Towbin and Perkins (1958) found that as much as 88 per cent of 

 the iodide was extracted from blood flowing through the gland. 

 In this instance, very little of the blood cannot have perfused the 

 ducts. Where does the blood required by the acini come from? Is 

 this a separate supply or is it shared with the duct supply through 

 a portal system analogous to the glomerulo-tubular system of the 

 mammalian kidney? Information relevant to this may be obtained 

 from a consideration of the specific activity of potassium in the 

 venous blood from the submaxillary gland at rest after previous 

 intra-arterial loading (Burgen and Seeman, 1958). The specific 

 activity of the venous blood was usually about 60-80 per cent of 

 that in the gland. It is known that a large proportion of the gland 

 potassium must be contained in acinar cells because after intra- 

 venous infusion of K 42 the whole gland potassium reaches the 

 same specific activity as the plasma, and further that the kinetics 

 with which this occurs do not indicate the presence of more than 

 one major compartment. We must conclude, therefore, that at 

 least 60-80 per cent of the blood perfusing the gland comes in 

 contact with the acini. These results were of course obtained in 

 different glands but, nevertheless, ones that in other ways behave 

 in very similar fashion. This evidence then suggests that a con- 

 siderable degree of serial perfusion to the acinar and ducts occurs 

 in the resting gland. What of the stimulated gland? The iodide 

 clearance of the parotid gland rises about proportionately to the 

 rate of salivary secretion (Burgen and Seeman, 1957). In the sub- 

 maxillary gland the blood flow also bears a more or less constant 

 relationship to the rate of secretion (see Terroux, Sekelj and Bur- 

 gen, 1959). It therefore seems likely that the proportion of the 

 total gland blood flow that perfuses the ducts is not greatly changed 

 during activity. These facts, then, make it probable that at least 

 half the total blood flow through the gland passes through the 

 portal system both at rest and during activity. Which way does 

 the blood flow in such a system? Does it flow in the same direction 

 as the saliva (concurrent) or in the opposite direction (counter- 

 current)? Burgen and Seeman (1958) pointed out that if the direc- 

 tion of flow were concurrent, the minimum saliva specific activity 

 for a substance that exchanged freely across the duct epithelium 

 would be that of the venous blood. On the other hand, if the flow 

 is countercurrent, the limiting specific activity of the saliva is that 

 of the arterial blood which in an unloading type of experiment is 



P.S.G. — Q 



