How does age affect kidney function?
Functional changes in the kidney occur in parallel to changes in structure.
• Decreased effective renal plasma flow (ERPF): ERPF decreases approximately 10% per decade with age in relation to progressive vascular sclerosis and loss of nephron number. Both changes in the number of functioning glomeruli and altered intrarenal signal and response to vasodilatory and vasoconstrictive mediators may affect renal plasma flow in the elderly.
• Decrease in glomerular filtration rate (GFR): An estimated drop of 0.8 to 1.0 mL/min per 1.73 m 2 /year in GFR is noted with progressive age depending on methodology used to measure clearance. Decreases in GFR with age may also be affected by race, gender, genetic variation, and underlying comorbidities including hypertension, diabetes, and cardiovascular disease.
• Decreased ability to conserve filtered sodium: An increase in solute load per nephron in the face of decreased nephron number and increased medullary flow, and lower levels of plasma renin and aldosterone with age, likely contribute to individuals 60 years and older taking nearly twice the number of hours (31 vs. 18 hours) compared to those 30 years and younger to reach appropriate distal tubular sodium reabsorption when sodium restriction is imposed.
• Decreased natriuretic ability: Individuals older than 40 years also handle a salt load less efficiently, as seen by taking a longer time to excrete 2 L of saline than those younger than 40 years. Although levels of the natriuretic hormone atrial natriuretic peptide appropriately increase, an incremental increase in urine sodium excretion is not evident in older compared to younger subjects, suggesting a possible decreased tubular sensitivity to natriuretic stimuli.
• Abnormal tubular concentrating and diluting capacity: The older individual may not be able to reach maximal urinary concentration despite 12 hours of overnight water deprivation. Studies in aged animals indicate a decrease in tubular transporters, Na-K-2Cl, and ENaC beta and gamma subunits, urea transporters UT-A1, UT-B1, and intrarenal resistance to arginine vasopressin may be reasons for decreased urinary concentration. Similarly, maximally dilute urine is also not found with increasing age given that appropriate solute extraction, suppression of arginine vasopressin, and distal delivery of the filtered load is necessary.
• Decreased net acid excretion: A diminished capacity for net acid excretion is found in older adults as both renal mass and GFR decrease are particularly noted when there is increased acid generation or acid load.
• Changes in potassium handling: Although total body potassium is lower given a decrease in muscle mass in older individuals, lower plasma renin and aldosterone levels and decreased aldosterone response to potassium load in the elderly predispose to decreased tubular excretion of potassium. In the face of a sudden potassium load, older individuals may have a decreased ability to shift potassium into cells because Na-K-ATPase activity is decreased with increasing age.
• Decreased kidney phosphate reabsorption: With phosphate restriction, older kidneys display evidence for decreased tubular phosphate absorption.
• Tubular calcium excretion: Remains unchanged in the kidney with increasing age.