Studies of Human Organ System Function 87 



index by • 8 per cent per year, and to a slight reduction in 

 calculated surface area. The regression of cardiac index on age 

 is due partly to an average decrease in heart rate averaging 

 • 3 per cent per year, while the remainder, or • 5 per cent per 

 year, represents a reduction in stroke index, the blood pumped 

 per beat per unit of body size (Fig. 7). 



50 60 



Age years 



Fig. 7. The relation between cardiac stroke index and age for 

 67 "normal" men. 



Blood pressure was measured in these subjects by brachial 

 intra-arterial needle and the mean blood pressure was found 

 to increase by only 0-1 per cent per year. Therefore, the 

 decrease in cardiac output was in association with an increase 

 in pressure flow ratio, the vascular resistance, which averaged 

 1-1 ±0-14 per cent per year. The change in output and 

 resistance are greater than the rates calculated (Landowne, 



