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Learning Objectives
Describe general features of physiological aging shared across tissues and organ systems including loss of complexity, increased heterogeneity, loss of resilience, homeostenosis, diminished physiological reserves, diminished end-organ responsiveness, loss of negative feedback, and allostatic load.
Understand the concepts of homeostasis and homeostenosis.
Describe the impact of aging on homeostatic mechanisms which help maintain a normal physiologic body temperature and fluid/salt balance in the face of lowered or increased ambient temperature, as well depletion or overload of fluids and sodium.
Describe the clinical features—including epidemiology, symptoms, signs, results of diagnostic tests and treatment—of hypothermia, hyperthermia, and water and sodium excess or depletion.
Describe the impact of aging on the ability to maintain a normal blood pressure in the face of orthostasis, meal ingestion, hypovolemia, and volume overload.
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Key Clinical Points
In addition to tissue- and organ-specific changes, shared features of physiological aging include loss of complexity, increased heterogeneity, loss of resilience, homeostenosis, diminished physiological reserves, diminished end-organ responsiveness, loss of negative feedback, and allostatic load.
Homeostasis reflects the aggregate effect of varied mechanisms that maintain normal physiologic constancy in the face of different extrinsic challenges. Aging is associated with impaired homeostasis, or homeostenosis, in the form of diminished capacity to respond to varied challenges.
Aging is associated with a failure of several different homeostatic mechanisms that enhance the risk of hypothermia in the face of decreased ambient temperature.
Aging is associated with a failure of homeostatic mechanisms that enhance the risk of hyperthermia and heatstroke in the face of increased ambient temperature.
The clinical presentation of hypothermia and hyperthermia may be subtle in older adults, requiring a high index of suspicion and careful supportive management in order to avoid the high rate of mortality associated with these conditions in late life.
Aging is also associated with homeostatic deficits when confronted with the assumption of the upright posture, eating, hypovolemia or a fluid challenge, increased or decreased sodium level, increased or decreased glucose level, bladder filling or bladder outlet obstruction, major burns or trauma, bed rest, or exercise.
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“Besides more or less obvious physical changes in old age, physiological investigation may reveal increasing limitation of the effectiveness of homeostatic devices which keep the bodily conditions stable.”
Walter Bradford Cannon (1871–1945)
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All organ systems undergo physiological changes with aging. However, the rate and nature of such changes varies both among organ systems and across individuals. This book includes individual chapters that address physiological changes associated with aging within the context of specific organ systems or tissues (see Part V: Organ Systems and Diseases).
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Nevertheless, approaches to the older patient, which do not consider the entire person and ignore considerations cutting across different organ systems or disciplinary perspectives often fail to produce clinically meaningful improvements in the health of older adults, especially in the context of multiple morbidity (see Chapter 41). This chapter discusses ...