Читать книгу Pathy's Principles and Practice of Geriatric Medicine - Группа авторов - Страница 69

Kidney size and the thickness of the renal cortex decrease slightly with age, while the structure of the renal medulla is not affected much. Microscopically, the nephron number is reduced and interstitial, and glomerular fibrosis occurs.⁵⁹ While some of the glomeruli become sclerotic and shrink, the remaining glomeruli become hypertrophic in an attempt to compensate for an age‐related reduction in the glomerular filtration rate (GFR). GFR declines by 0.8–1 ml/min per 1.73 m² each year in approximately one‐third of older adults after age 40.⁶⁰ Why this reduction is not seen in all older adults has not been explained, and the existence of older adults with normal GFR demonstrates that renal function decline is not an inevitable destiny. GFR is mostly calculated by using equations related to the serum creatinine level. However, an age‐related decrease in muscle mass (as the primary source of creatinine) may result in false estimations of GFR that use the creatinine level. Measuring cystatin C is recommended in such circumstances.^61‐62

Renal blood flow decreases with age, too. Perfusion of the renal medulla remains nearly intact compared with the cortex. Atherosclerotic processes and glomerulosclerosis contribute to the reduction of renal blood flow. Glomerular blood flow is regulated by angiotensin 2. The vasoconstrictive response to angiotensin 2 in the afferent arteriole is accelerated, while the compliance of renal arterioles decreases in older adults.⁶³

Tubular functions are slightly impaired in older adults, and water, sodium, and potassium resorption and excretion decrease. The aldosterone response of Henle is also reduced. Thus, older adults need longer to reset their electrolyte balance after it becomes unbalanced.⁶⁴