Homeostatic fragility in the elderly.

Age-related alterations in adaptation capacity to physiologic changes in physical workload, posture, and water and electrolyte metabolism are reviewed in this article. Other than the increased prevalence of disease in the elderly, normal aging is associated with changes in hormonal secretion patterns, tissue sensitivity to hormones, control of breathing, sleep, and renal function that make this population particularly vulnerable. Orthostatic hypotension, sleep apnea, hypernatremia, hyponatremia, and hyperkalemia, as well as other conditions, are discussed in light of these changes.