Studies of Human Organ System Function 89 



the average decrease per year in renal perfusion turns out to 

 represent more than half of the total average reduction in car- 

 diac output, although the kidneys in a young adult receive 

 less than one-fourth of the total flow. The increase in renal 

 resistance with age is, therefore, more striking than is the 

 increase, on the average, of the non-renal portion of the 

 greater circulation. Consequently, the changes in renal 

 resistance contribute in good measure to the overall dispro- 

 portionality which we have deduced between decreased per- 

 fusion and diminished oxygen uptake. Not only each circula- 

 tory segment but each metabolite may contribute to the overall 

 picture in a different fashion. The older individual therefore 

 not only has a reduced total circulation, but, logistically, has 

 altered the distribution of blood to various organs. It is 

 tempting to speculate teleologically that he reduces circula- 

 tion in regions where it can be more readily reduced, but this 

 finds him less prepared to cope with urgent or sustained needs. 

 This concept is supported by our estimates that, despite an 

 increased fixed renal resistance, there may be no reduction in 

 renal vasoconstriction in the older subject (Landowne and 

 Shock, 1951). 



From the observations on cardiac output and blood pressure, 

 we have also drawn deductions indicating that there is a 

 decrease in the work of the left ventricle, and in the rate at 

 which this work, on the average, is performed; i.e., in the 

 "power" (Landowne, Brandfonbrener and Shock, 1955). 

 The decrease in cardiac output and work need not be con- 

 sidered as evidence that the older subjects had subclinical 

 heart disease, since, in compensated heart disease, normal or 

 even high values may be obtained. It appears more likely 

 that the decrease in cardiac output is related, in part at least, 

 to alterations in the circulatory requirements of older people 

 at rest. As a byproduct of a loss of circulatory pathways, 

 the demands upon the pump are reduced. Cardiac size, how- 

 ever, does not decrease — thus, the heart is larger in relation- 

 ship to body size and to cardiac work. This relative increase 

 in heart size, the decrease in power, and an increase in the 



