The aging respiratory system--pulmonary structure, function and neural control.

Pulmonary structure and function change significantly between young adulthood and old age. Elastic elements of the lung degenerate, parenchymal tissue is lost, alveolar ducts and bronchioles dilate, chest wall compliance decreases, intercostal muscle mass and force are reduced and gas exchange surface lessens. Disturbances of innate immunity predispose the elderly to pulmonary inflammation. These changes affect pulmonary function tests and gas exchange, but adaptive changes in breathing frequency and tidal volume serve to maintain adequate ventilation. Aging depresses cough reflexes and ventilatory responsiveness to hypoxia and hypercapnia. Sleep-associated apnea and periodic breathing occur more frequently in the elderly, implying that neural feed back and feed-forward control (loop gain) are impaired. Low loop gain may contribute to sleep apnea but not to periodic breathing. A likely cause of age related pulmonary tissue degeneration and a future therapeutic target is defective protein folding in the endoplasmic reticulum. Nervous system adaptations that accompany structural and functional changes in the elderly are poorly understood.