Expansion
Nephron loss with falling GFR, reduced concentrating ability and blunted responses
Expansion
Structural
- Loss of about 10 per cent of nephrons per decade after 40
- Glomerulosclerosis, tubular atrophy and interstitial fibrosis
- Reduced renal mass, mainly cortical
Functional
- GFR falls by roughly 1 ml/min/1.73m2 per year after 40
- Reduced concentrating and diluting ability
- Reduced ability to conserve sodium and to excrete a potassium or acid load
- Blunted renin, aldosterone and vitamin D activation
- Reduced thirst sensation
The critical clinical point is that serum creatinine may remain normal, because declining muscle mass reduces production in parallel with the falling GFR. Estimating equations or cystatin C are therefore needed, and drug doses must be adjusted even when creatinine looks reassuring.
The combination of blunted thirst, impaired concentrating ability and impaired sodium conservation makes older patients prone to dehydration and hypernatraemia in illness or hot weather, and equally to fluid overload and hyponatraemia when given intravenous fluids.