ACTION OF CAFFEINE UPON THE KIDNEY 



19; 



twofold, i.e. either passive or active. The fall in blood pressure must 

 produce a diminution of the kidney volume unless it be over-compen- 

 sated by another volume change in the reverse direction. The fall of 

 volume that actually occurs is partly to be explained by the fall in 

 blood pressure, but not entirely, for the fall in blood pressure usually 

 precedes by a definite interval the fall in the kidney trace, and 

 secondly the blood pressure attains its original height long before the 

 kidney begins to expand. The main cause producing the diminution 

 of the kidney volume is therefore active, and due to constriction of its 



Fig. 158. — Effect of Caffeine upon the Kidney Volume ani> Blood Pressure. 

 Tracing reduced to Half Size. 



blood vessels. This constriction ultimately yields, and is followed by 

 a dilatation lasting a still longer time, and then the kidney returns to 

 its initial state. When the rate of secretion of the urine is recorded 

 at the same time as the kidney changes it is found that this rate 

 varies and accurately follows the changes in volume of the kidne\ 

 When the kidney vessels contract the rate of secretion drops, and in 

 the second stage, the period of relaxation, the secretion is accelerated, 

 and finally, as the kidney regains its original state, the rate of secre- 

 tion returns to that observed at the commencement of the experiment. 

 The fall in blood pressure produced in this experiment is due to the 



