RENAL SYSTEM
Structural and Functional Development of the Excretory
System
The embryologic development of the renal system begins in the
middle of the third week of gestation with development of the pronephric tubule,
which undergoes further development and regression through the stages of pronephric
duct, mesonephros, and ultimately, the metanephric kidney. By 10 weeks' gestation,
a functioning kidney and collection system exist, and fetal urine is discharged into
the bladder. At 32 to 36 weeks' gestation, nephron induction is complete, and the
full complement of nephrons per kidney is present.[217]
Table 76-14
presents the
normal values for pediatric renal function.
Functional Development of the Kidney
Because the placenta is the major excretory organ of the fetus,
renal growth is not governed by functional requirements. Renal growth during the
third trimester increases linearly with body weight and body surface area.[218]
The glomerular filtration rate (GFR) increases rapidly
TABLE 76-14 -- Normal values for pediatric renal function
|
Age |
Value |
|
Creatinine (mg/dL) |
1 yr |
0.41 ± 0.1 |
|
10 yr |
0.61 ± 0.22 |
|
18 yr |
0.91 ± 0.17 |
|
Glomerular filtration rate (mL/min/1.72 m2
) |
2–8 days |
39 (range, 17–60) |
|
6–12 mo |
103 (range, 49–157) |
|
2–12 yr |
127 (range, 89–165) |
|
Urine concentration (mOsm/L) |
1 mo |
600–1100 |
|
2–16 yr |
1089 (range, 870–1309) |
|
Modified from Goldsmith DI: Clinical and laboratory
evaluation of renal function. In Edelman CM Jr (ed):
Pediatric Kidney Disease. Boston, Little, Brown, 1978, p 213. |
from 28 to 35 weeks' gestation—the period of greatest nephron growth and functional
maturation. At term birth, GFR is 10 mL/min/m2
and increases to 20 mL/min/m2
by 2 weeks of age. Although the GFR is lower in a premature infant, the rate of
increase is the same as in a term infant.[219]
Tubular functions are not fully mature in a full-term newborn at birth. The newborn
kidney is sensitive to antidiuretic hormone (ADH), or vasopressin, but urine osmolarity
can vary from only 50 up to 780 mOsm/L.[220]
The
tubular function of premature infants is less mature; they can concentrate their
urine only to 600 to 700 mOsm/L.
The renal threshold for bicarbonate in the newborn is about 20
mEq/L (≅4 to 6 mEq/L less than that of an adult), which means that standard acid-base
nomograms do not apply to infants because a serum bicarbonate concentration of 20
mEq/L is normal and does not indicate metabolic acidosis. The renal tubular glucose
threshold is at adult levels in a term infant, but it is reduced to approximately
150 mg/dL in a premature infant. A full-term infant handles sodium well, with 1%
or less fractional excretion by the third day of life. Fractional excretion of sodium
can be as high as 5% in a premature infant.[221]
Renin, angiotensin, and aldosterone levels are high in the newborn and decrease
over the first few weeks of life. Table
76-14
presents the normal values for pediatric renal function.[222]
