The gradual decline in Pao 2 that occurs with age parallels the decrease in elastic recoil and the increase in physiologic dead space. These changes may lead to the collapse of peripheral airways, which decreases ventilation to distal gas exchange units but with much less effect on perfusion. Also, lower cardiac output in the elderly results in increased tissue oxygen uptake, decreased mixed venous oxygenation, and, consequently, decreased Pao 2 . Control of Breathing Heart rate and ventilatory responses to hypoxia and hypercapnia diminish with age because of diminished responsiveness of peripheral and central chemoreceptor function and integration of central nervous system pathways with age. Age also decreases neural output to respiratory muscles and lowers chest wall and lung mechanical efficiency.
Effects are even greater in persons who are deconditioned.
Work capacity increases during childhood, peaks in the late teens, plateaus until the mid-20s, then gradually declines. The increase in work capacity that occurs during childhood results from the growth of muscle, heart, and lungs, and the decline results in part from gradual reductions in maximal heart rate and muscle mass that occur with age. Although pulmonary function measurements such as FEV 1 decline with age, reduced ventilation seldom limits exercise in healthy persons. The age-related decrease in aerobic exercise capacity is due only in part to pulmonary changes. With age, the rate of mucociliary transport declines, although the effect of decreased clearance on clinical infection has not been proved.