Chapter 45: Aging and Homeostatic Regulation

This chapter addresses the following Geriatric Fellowship Curriculum Milestone: #9

Learning Objectives

• Understand the concepts of homeostasis and homeostenosis.

• Describe the impact of aging on homeostatic mechanisms which help maintain a normal physiologic body temperature in the face of lowered or increased ambient temperature.

• Describe the clinical features—including epidemiology, symptoms, signs, results of diagnostic tests and treatment—of both hypothermia and hyperthermia.

• Describe the impact of aging on the ability to maintain a normal blood pressure in the face of orthostasis, meal ingestion, and hypovolemia.

Key Clinical Points

1. 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.

2. Aging is associated with a failure of several different homeostatic mechanisms that enhance the risk of hypothermia in the face of decreased ambient temperature.

3. Aging is associated with a failure of homeostatic mechanisms which enhance the risk of hyperthermia and heatstroke in the face of increased ambient temperature.

4. The clinical presentation of hypothermia and hyperthermia may be subtle in the 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.

5. 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.

6. Features of homeostatic dysregulation in old age include diminished physiologic reserve, loss of complexity, enhanced variability, normal or enhanced basal activity, excess response to stressors in terms of the sympathetic nervous system, diminished end-organ responsiveness, and loss of negative feedback.

7. The accumulation of repeated exposure of such challenges resulting in an increased biological burden—termed allostatic load has been shown to be predictive of mortality and frailty including future declines in cognitive and physical function.