Concurrent with the morphogenesis of the kidney discussed in Chapter 460, functional development of the fetal kidney also progresses with increasing gestational age. Prenatally, the placenta regulates renal clearance and fluid and electrolyte homeostasis, while the primary function of the kidney is the production of urine in order to maintain the volume of the amniotic fluid. Renal development begins at 5 weeks of gestation, followed by the appearance of rudimentary nephrons, which begin producing urine at 10 weeks. The critical role of the kidney in maintaining the amniotic fluid is highlighted in situations when abnormalities of renal development result in oligo- or anhydramnios leading to significant postnatal morbidity and mortality, particularly pulmonary hypoplasia. In the later stages of gestation, urinary output, renal tubular function, and glomerular filtration rate (GFR) continue to increase with gestational age. By 32 to 34 weeks of gestation, nephrogenesis is complete, following which no new nephron units are formed.
After birth, the neonatal kidney must undergo several changes to adapt to the extrauterine environment. Further increases in GFR and tubular handling of fluid and electrolytes occur over the first weeks of life and continue until reaching adult levels at 1 to 2 years of age.
This chapter reviews the changes in renal function during the prenatal and postnatal periods in order to provide a framework for understanding both normal and abnormal development of renal function.
DEVELOPMENT OF FETAL KIDNEY FUNCTION
RENAL BLOOD FLOW AND GLOMERULAR FILTRATION RATE
GFR refers to the volume of filtrate formed by the transudation of plasma across the glomerular filtration barrier per unit of time. In adults, a normal GFR is approximately 120 mL/min or 170 L/d. For a typical adult with a total plasma volume of 4 L, this translates into filtration of the entire plasma volume across the glomerular tuft 40 times per day. GFR is influenced by several factors, one of which is renal blood flow (RBF). This interrelation between RBF and GFR is evident as changes in both occur during fetal and postnatal development.
Development of Renal Blood Flow
As noted, prenatally, the placenta assumes many of the functions that the kidney will perform following birth. Reflecting this, the percentage of cardiac output (CO) received by the placenta is significantly higher compared to the kidney. In fetal lambs, the kidney receives approximately 2.5% of CO compared to 40% for the placenta. In humans, recent studies employing Doppler ultrasound to estimate fetal RBF confirm animal data showing that RBF is 3% to 4% of total fetal CO and increases to 12% to 16% in the first year of life. The mechanism responsible for the age-related increase in RBF at birth is largely attributed to a postnatal drop in renal vascular resistance (RVR) in relation to the increase in CO. In the neonatal period, increased CO and ...