244 



N. W. Shock 



after 12 hours of water deprivation falls from an average of 

 1-032 at age 20 to 1-024 at age 80-90. Although the absolute 

 magnitude of the decrement is small, it is statistically signi- 

 ficant (Lewis and Alving, 1938) and indicates impairment of 

 the concentrating ability of the kidney, which is no doubt a 

 reflection of the reduction in Tm as reported from our studies. 

 With regard to volume regulation, our observations on a 



24 



'h 



T r 



.. • - • 

 • • •• • 



. •• 



s • • , 



• • • 



• • • 



^h^ 



J L 



20 30 40 50 60 70 80 90 100 



Age Years 



Fig. 14. Relationship between extracellular fluid space (thio- 



cyanate space) and age in males. 

 (From: Shock, 1956). 



series of 152 males failed to demonstrate any systematic 

 changes in either plasma volume (Cohn and Shock, 1949) or 

 in total extracellular fluid volume (Shock, Watkin and Yiengst, 

 1954) as estimated by thiocyanate determinations (Figs. 13 

 and 14). 



Although the aged kidney has a capacity for maintaining 

 acid-base equilibrium of the plasma under resting conditions, 

 when an extra load is imposed upon it age differences appear. 

 Thus, for example, we have found that a single dose of 



