Age Changes in Renal Function 239 



flows, oral fluid intake was supplemented by the intravenous 

 administration of 5 per cent dextrose in distifled water, in 

 which appropriate quantities of inulin and sodium amino- 

 hippurate had been added at the rate of 8 ml./min. by a 

 constant infusion pump. Twenty-nine adult males, ranging 

 in age from 26 to 86 years, served as subjects. The total 

 sample was arbitrarily divided into three age groups: young 

 (no. = 9, age range from 26-45), middle (no. = 10, age range 

 from 46-65), and old (no. = 10, age range from 66-86). After 

 three control collection periods, 0-05 milliunits pitressin/kg. 

 body weight was administered intravenously. Subsequently, 

 six consecutive urine collections, each of 12 minutes duration, 

 were made. During the control periods, the average urine 

 flow for the young subjects was approximately 14 ml./min.; 

 middle-aged, 11 ml./min. and old subjects, 10 ml./min. The 

 urine/plasma (U/P) inulin ratio was calculated as an index 

 of water reabsorption. The results of this experiment are 

 shown in Fig. 10, where the U/P inulin ratio was plotted 

 against the urine collection period. During the control 

 periods, the U/P inulin ratios were approximately 10 for all 

 three age groups. Following the administration of pitressin, 

 prompt antidiuresis was noted in all three groups. Peak 

 antidiuresis and peak concentration of inulin were observed in 

 all three age groups during this period which was 12-24 

 minutes after pitressin. As indicated in Fig. 10, there was a 

 marked age difTerence in the antidiuretic response to this 

 standard stimulus. The young subjects showed the maxi- 

 mum response and the old subjects showed the minimum. 

 In Fig. 11, the relationship between the maximum observed 

 tubular response to the standardized dose of pitressin and age 

 is shown. Correlation coefficient was — 0-73, and the regres- 

 sion of the concentration on age was described as U/P inulin = 

 162 —1-6 X age (in years). Although the administered 

 pitressin resulted in a rise of blood pressure, it averaged only 

 10 mm. at two minutes after injection, and fell to control 

 levels within five minutes. These experiments indicate that, 

 in the older individual, there is an impairment in the 



