Postnatal Renal Maturation

The kidney functions to maintain a constant volume and composition of the extracellular fluid volume. The glomerulus forms an ultrafiltrate of plasma which enters the proximal tubule where most of the fluid, organic solutes, and electrolytes are reabsorbed in an isotonic fashion. The more distal nephron segments are responsible for the final regulation of electrolyte and solute absorption and secretion as well as urinary concentration and dilution. Glomerulogenesis continues until 34-36 weeks gestation. The term neonate thus has a comparable nephron endowment as an adult. The term neonate has a much lower glomerular filtration rate than an adult even when factored for body surface area. There is postnatal maturation of the glomerulus and a parallel increase in tubular resorptive capacity. Transport occurs across tubules and also across the paracellular pathway. The very premature infant is far less equipped for life outside the mother’s womb. There is glomerular tubular imbalance for infants less than 34 weeks as evidenced by glucosuria and very premature infants filter far more sodium than they can recover resulting in renal salt wasting. Postnatal maturation is much more complex than originally thought. There is not only a maturational increase in transporter abundance but there are changes in isoforms of key renal transporters during development as well as change in the paracellular pathway. This chapter discusses the postnatal changes in glomerular filtration rate and tubular transport of solutes and water that occur in the term and preterm neonate.