Expansion
Low GFR and limited concentrating ability, maturing over the first two years
Expansion
- Nephrogenesis completes by about 34 to 36 weeks; no new nephrons form after birth, so prematurity permanently reduces nephron number and increases later risk of hypertension and chronic kidney disease
- GFR at term is only about 20 to 30 ml/min/1.73m2, doubling in the first two weeks and reaching adult values by about 2 years
- Concentrating ability is limited to about 600 to 700 mOsm/kg, half the adult maximum, because the medullary gradient is shallow and urea recycling is reduced by the anabolic state
- Diluting ability is relatively well preserved
- Obligatory sodium loss is high in the preterm infant, from tubular immaturity, so hyponatraemia is common
- Reduced ability to excrete an acid load, giving a physiologically lower bicarbonate
Consequences: neonates dehydrate quickly, cannot excrete a large water or solute load, are prone to both hyponatraemia and hypernatraemia, and require careful drug dosing since renally cleared drugs accumulate.