Aging and Normal Physiologic Changes
Reserve, not disease
Aging produces predictable loss of physiologic reserve across organ systems, but it does not by itself produce symptoms. The candidate must distinguish age-associated change from pathology: presbycusis and presbyopia are aging; a new focal deficit is not. Renal plasma flow and glomerular filtration fall roughly 8 mL/min/1.73 m² per decade after age 40, so a serum creatinine of 1.0 mg/dL in a thin 88-year-old woman may represent a creatinine clearance near 40 mL/min. Lean mass declines while fat mass rises, increasing the volume of distribution for lipophilic drugs and decreasing it for hydrophilic drugs such as digoxin and alcohol. Total body water falls, concentrating water-soluble agents. Hepatic first-pass metabolism and hepatic blood flow decline, raising oral bioavailability of drugs with high first-pass extraction.
Baroreflex sensitivity and arterial compliance decrease, producing orthostatic hypotension and a blunted heart-rate response to volume loss. Thirst and renal concentrating ability decline, predisposing to dehydration. Thermoregulation is impaired, so infection may present as hypothermia. Immunosenescence increases susceptibility to infection while blunting fever and leukocytosis, so a frail elder with pneumonia may have a normal temperature and white count. Mucociliary clearance and cough strength decline. Bone mineral density and muscle mass fall, and skin thins with reduced subcutaneous fat.
Traps
Questions ask you to label a finding as "normal aging" when it warrants a workup (weight loss, incontinence, confusion, falls, depression), or to accept a "normal" creatinine as normal renal function. Another common trap is dosing a renally cleared drug (e.g., gabapentin, DOACs, antibiotics) at full dose.
Worked rule
Estimate creatinine clearance with Cockcroft-Gault: (140 − age) × weight kg ÷ (72 × SCr), multiplied by 0.85 for women. For an 85-year-old woman weighing 60 kg with SCr 1.0: (55 × 60) ÷ 72 = 45.8, then × 0.85 ≈ 39 mL/min. Dose accordingly.