Studies of Human Organ System Function 77 



limitations of arterial function compromise the function 

 of the tissues served, disease may be in a well established 

 stage. 



In the area of cardiovascular physiology then, the con- 

 temporary pattern of early and increasing age incidence of 

 atherosclerosis makes the possibility of selecting "pure" 

 normal subjects remote, if by "normal" we mean free of this 

 disease. Moreover, for the time being, the dependence of man 

 on his vascular system places the burden of proof upon any 

 hypothesis which does not consider the role of the circulation 

 in relation to age changes in any tissue. To a greater or less 

 degree, the agewise changes which have been described in 

 renal function (Shock, 1952), glucose tolerance (Silverstone 

 et al., 1956), nerve conduction velocity (Norris, Shock and 

 Wagman, 1952), etc., might be secondary to vascular altera- 

 tion. Before proposing that these agewise effects represent 

 primary manifestations of ageing, the role of the circulation is 

 to be considered. 



It would be much simpler to select a representative sample 

 of a population, and, in this manner, give a picture of the 

 "norm". This would be descriptively acceptable. Severely 

 limiting the success of this approach are those areas where the 

 degree of abnormality will hardly remain constant over any 

 age span, and much of our variance becomes ascribable to the 

 increasing age incidence of pathology. A recent study of 

 pulmonary ventilatory function (Norris et al., 1956) may be 

 used to emphasize the double quandary, and here disease of 

 more than one system may be present to affect pulmonary 

 bellows function. Fig. 3 indicates the incidence of clinically 

 diagnosed conditions. The incidence of acute and severe 

 chronic pulmonary disease is approximated, for such subjects 

 were not included in this study; we have actually practised 

 selection in all, or almost all, of our studies. Agewise increase 

 is noted in several relevant diagrammed disorders, and multiple 

 disorders commonly coexist. If the severely ill subjects had 

 been included in this study, we should have had no age 

 study, but a study of disease. On the other hand, if all cases 



