240 



N. W. Shock 



functional capacities of the tubular cells to perform osmotic 

 work on the glomerular filtrate. 



The results of these observations lead to the concept that, 

 with increasing age, there is a gradual loss of nephrons in the 



1 — I r 



URINE COLLECTION PERIOD 



Fig. 10. Mean values of U/P inulin ratio for each of 

 three age groups before and after the intravenous ad- 

 ministration of pitressin. Urine collection periods 1-9 

 represent nine consecutive 12-minute periods. Pitressin 

 was administered immediately after the conclusion of 

 period 3. 



(From: Miller and Shock, 1953). 



are 



kidney. In addition to these structural losses there 

 functional changes. One of these is a gradual increase in the 

 vasoconstriction of the vascular bed of the kidney which 

 further reduces the flow of blood through it, even in the face 



