RENAL FUNCTION

Although age-related anatomic and physiologic changes occur, the aging kidney remains capable of maintaining fluid and electrolyte balance remarkably well. Aging changes result in a decreased reserve capacity, which may alter the kidney's ability to maintain homeostasis in the face of ill­nesses or stressors. Overall, there is a general decline in kidney mass with aging, predominantly in the renal cor­tex. The number of functional glomeruli decreases by 30% to 50%, with an increased percentage of sclerotic or ab­normal glomeruli.³⁵

Numerous cross-sectional and longitudinal studies have documented a steady, age-related decline in total re­nal blood flow of approximately 10% per decade after 20 years of age, so that the renal blood flow of an 80-year-old person averages approximately 300 mL/minute, com­pared with 600 mL/minute in a younger adult. The major decline in blood flow occurs in the cortical area of the kid­ney, causing a progressive, age-related decrease in the glo-merular filtration rate (GFR). Serum creatinine, a byproduct of muscle metabolism, often is used as a measure of GFR. The decline in GFR that occurs with aging is not accompa­nied by an equivalent increase in serum creatinine levels because the production of creatinine is reduced as muscle mass declines with age.³⁶ Serum creatinine levels often are used as an index of kidney function when prescribing and calculating drug doses for medications that are eliminated through the kidneys; this has important implications for older adults. If not carefully addressed, improper drug dos­ing can lead to an excess accumulation of circulating drugs and result in toxicity. A formula that adjusts for age-related changes in serum creatinine for individuals 40 through 80 years of age is available (see Chapter 36).

Renal tubular function declines with advancing age, and the ability to concentrate and dilute urine in response to fluid and electrolyte impairments is diminished. The aging kidney's ability to conserve sodium in response to sodium depletion is impaired and can result in hypo-natremia. A decreased ability to concentrate urine, an age-related decrease in responsiveness to antidiuretic hormone, and an impaired thirst mechanism may account for the older adult's greater predisposition to dehydration during periods of stress and illness. Older adults also are more prone to hyperkalemia and hypokalemia when stressed than are younger individuals. An elevated serum potas­sium may result from a decreased GFR, lower renin and al-dosterone levels, and changes in tubular function. Low potassium levels, on the other hand, are more commonly caused by gastrointestinal disorders or diuretic use. Neither is the result of aging.³⁶