Abstract
The kidney plays an important role to maintain water and electrolyte balance. This article focuses on the maturational changes in kidney function (especially, glomerular filtration rate, fractional excretion of sodium, and urine concentration) that occur during postnatal renal development. In the human newborn, GFR is quite low at birth, even when corrected for body surface area. GFR continues to increase after birth and reaches adult levels by 2 years old. The fractional excretion of sodium (FENa) is relatively high in premature newborns of less than 32 weeks’ gestation. However, in newborns of more than 34 weeks’ gestation, FENa is less than 1% soon after birth. We have to pay enough attention to premature babies who often have negative sodium balance during the first 2 to 3 weeks of life. In the fetal period, the urine-diluting ability is mature. On the other hand, the urine-concentrating mechanism makes remarkable progress during the neonatal period. The recent studies of rat kidneys found that the neonatal thin ascending limb of Henle’s loop has a thick morphological appearance and the urine-concentrating mechanism dependent on urea is immature in neonates. Furthermore, the basic ontogeny and phylogeny of the urine-concentrating mechanism will be discussed in this article.
