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In the perinatal period, kidney function, as it relates to fluid
and electrolyte homeostasis in the premature and full-term neonate,
is predominantly influenced by (1) anatomical kidney development
at the time of birth, (2) physiologic adaptations that occur during
the transition from intrauterine to extrauterine environment, and
(3) diminished capacity of the newborn kidney to respond to increased
fluid, electrolyte, or acid load. Their combined effects on renal
function in the newborn infant are in indirect proportion to gestational
age such that premature infants are most vulnerable to effects of
anatomical immaturity, changes in renal blood flow, and limited
functionality of hormonal regulatory mechanisms. Consequently, a clear
understanding of changes in glomerular and tubular function and
renal hemodynamics in the perinatal period is crucial for appropriate management
of fluid and electrolyte problems in the sick neonate. In this chapter,
general and specific aspects of renal function as they relate to
the antenatal and postnatal periods are reviewed to provide a framework for
evaluating renal function in the healthy and sick newborn infant. Diagnostic
and therapeutic aspects regarding normal and abnormal renal function
are discussed elsewhere.
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Morphologic aspects of human kidney development are discussed
in detail in Chapter 464. Here, attention
is given to spatiotemporal relationships between anatomical and functional
development of renal structures. Table 465-1 provides a summary of relationships
between anatomical and functional kidney development.
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Human kidney development begins at the fifth week of gestation
(Fig. 465-1).1,2 The
first functioning nephrons are formed by week 9 and excrete urine
by week 12. By 32 to 34 weeks, nephrogenesis is completed, following
which no new nephron units are formed.3,4 In humans who
suffer fetal or perinatal renal injury, the developing kidney is
incapable of compensating for irreversible nephron loss by either
accelerating the rate of nephron formation ex utero in infants born
prematurely, or by de novo generation of nephrons once nephrogenesis
is completed.3,5
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