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The chapter focuses on age-related changes in pathophysiological mechanisms in apneic and normal subjects, and on the potential relevance of the findings to obstructive sleep apnea (OSA). Aging has been reported to be a major factor affecting the risk of OSA. In the extremes of aging, some data suggest a survivor effect, such that apnea prevalence may decrease among elderly patients. The upper airway (UAW) requires stiffness of the soft tissue walls around it and activity of the dilator muscles to maintain patency. Any reduction in UAW cross-sectional area, change in its length, muscle activity, or a combination of these variables, may lead to vulnerability of the UAW to collapse. The term loop gain is used to refer to the intrinsic stability or instability in the ventilatory control system. Once the patient with apnea falls asleep and the cycle of repetitive airway obstruction begins, recurrent hypoxemia and hypercapnia develop.