238 N. W. Shock 



expressed as percentages of the base line values, the rise in 

 renal blood flow for the young, middle, and old groups was 

 76, 86, and 91 per cent respectively. As shown by the upper 

 three curves, the filtration fraction diminished markedly in 

 all subjects, indicating a fall in effective filtration pressure, 

 which would result from a greater vasodilatation at the 

 efferent than at the afferent side of the glomerulus if there 

 were no change in blood pressure. Actually, the diastolic 

 blood pressure dropped slightly in the middle and old groups, 

 but remained constant throughout the reaction in the young 

 group. At the height of the reaction the differences in the 

 filtration fraction, observed under resting conditions, com- 

 pletely disappeared. The small absolute changes in renal 

 plasma flow in the older subjects, following pyrogen, are 

 consistent with the anatomical findings of a progressive 

 decrease in the number of glomeruli in the aged kidney 

 (Moore, 1931). On the other hand, the time of onset and the 

 percentage increase in renal plasma flow were similar in the 

 different age groups. Consequently, it must be concluded 

 that the responsiveness to pyrogen of the vascular elements 

 remaining in the aged kidney is not qualitatively different 

 from that in the young kidney. It is inferred from these 

 experiments that the renal arterioles in the aged kidney are 

 capable of dilating, and that in the resting state there is a 

 functional vasoconstriction of the afferent arterioles in the 

 aged which, under resting conditions, diverts blood from the 

 kidney to other parts of the circulation. 



To function effectively the kidney must respond to a 

 variety of stimuli. One of the most important signals for 

 altering the reabsorption of water by the renal tubule is the 

 antidiuretic hormone. Age differences in the inhibition of 

 water diuresis, following the intravenous administration of 

 small amounts of pitressin, have been observed (Miller and 

 Shock, 1953). In these experiments a maximum water 

 diuresis was established by the oral administration of 500 ml. 

 water at 6.00 a.m., followed by 250 ml. water at half-hour in- 

 tervals until completion of the test. To ensure maximum urine 



