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Characteristic of the normal aging process is a decline in physiologic reserve in many body regulatory systems, including those involved in the maintenance of fluid balance. The confluence of normal aging changes, common diseases, and the administration of many classes of drugs can readily lead to clinically evident disturbances of fluid balance, such as water retention or loss and to hyponatremia or hypernatremia with resultant symptomatic consequences. In some individuals, an impaired ability to conserve water may underlie the development of nocturnal urinary frequency as well as urinary incontinence.

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The normal regulation of water and electrolyte balance involves the interplay of many homeostatic systems that operate to maintain the composition of fluid and electrolyte compartments within a narrow range. Because of alterations in the normal aging process, these homeostatic systems maybe compromised. The key regulatory components of fluid balance include (1) thirst perception, which governs fluid intake, (2) the kidney, which is governed by hemodynamic forces, and (3) hormonal influences of arginine vasopressin (AVP) or antidiuretic hormone (ADH), atrial natriuretic hormone (ANH), and the renin–angiotensin–aldosterone system, which control renal water and electrolyte excretion. Clinicians who are involved in the care of the elderly recognize that disturbances of water and electrolyte balance are common in this age group, especially when older persons are challenged by disease, drugs, or extrinsic factors such as access to fluids or control of diet composition.

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Table 88-1 Aging Effects on Water and Sodium Regulatory Systems
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Body Composition

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Aging effects on body composition have the potential to contribute to derangements in fluid balance. Normal aging is accompanied by a decrease in lean body mass, an increase in fat mass, and a decrease in total body water. Thus, total body water declines from the approximate values of 60% of body weight in young men and 52% in young women to 54% and 46%, respectively, in individuals older than age 65 years, primarily through a decrease in the intracellular fluid compartment. The decrease in total body water may place the elderly patient at increased risk for dehydration and/or hyponatremia when challenged by fluid loss or decreased fluid intake and at increased risk for fluid overload and hyponatremia when exposed to excessive oral or parenteral fluid intake.

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Thirst and Fluid Intake

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