The maximum urinary concentrating capacity in a preterm newborn (∼400 mOsm/L) is less than that in afull-term newborn (600 to 800 mOsm/L), which is less thanthat in older children and adults (∼1200 mOsm/L). Neonates can dilute urine similar to adults (75 to 90 mOsm/L),but, because the glomerular filtration rate (GFR) is lower,the capacity to excrete a water load is less in infants. GFRreaches adult levels by 1 to 2 years of age. Tubular reabsorption of sodium, potassium, bicarbonate, and phosphateand excretion of hydrogen are all reduced in infants relative to adults. These functions mature independently andat different ages, so a neonate rapidly develops the abilityto reabsorb sodium efficiently, but it takes 2 years for bicarbonate reabsorption to mature. Erythropoietin is secreted by interstitial cells in the renal medulla in response to lowoxygen delivery and helps regulate bone marrow red bloodcell production.
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Renal disorders can be classified as primary or secondary (due to systemic illnesses) (see Table 161-2). Renal diseases may present with obvious signs, such as hematuria oredema, or with subtle signs detected on screening examinations (abdominal or flank mass, HTN, proteinuria). Fever, irritability, and vomiting may be presenting symptoms inneonates and infants with urinary tract infections (UTIs),whereas frequency and dysuria suggest UTI in older children.Chronic kidney disease often is associated with poor growthand feeding but may be first detected on screening examinations (HTN, hematuria). An abnormal urine stream mayindicate posterior urethral valves, other bladder disorders, orobstructive lesions.
The GFR is measured most accurately by infusion of a substance that is freely filtered by the glomerulus but is not metabolized, reabsorbed, or secreted in or by the tubules. The GFR is calculated as follows: where [U] is urine concentration, [P] is serum concentration of a substance (mg/dL) used to measure clearance, and V is urine flow rate (mL/min).
By convention, GFR is corrected to a body surface area of 1.73 m2 to allow comparison between different sized individuals. In a full-term newborn, an uncorrected GFR of 4 to 5 mL/min corrects to approximately 40 mL/min/1.73 m2. GFR increases rapidly during the first 2 years of life when it achieves adult values (100 to 120 mL/min/1.73 m2). After that, GFR and body size increase proportionately, and GFR remains stable.
Plasma creatinine reflects muscle mass, increases with age, and is used to approximate GFR. Creatinine is also secreted by PT, resulting in a less accurate measurement of GFR with immature kidneys or with decreased renal function. Blood
Figure 161-1 Major transport functions of each nephron segment, including representative osmolalities in vasa recta, interstitium, and tubule at different levels within the kidney. ADH, Antidiuretic hormone; Glu, glucose; OA, organic acid. (From Andreoli TE, Carpenter CCJ, Plum F, et al, editors: Cecil Essentials of Medicine, Philadelphia, 1986, WB Saunders.) urea nitrogen is affected by renal function but is greatly altered by hydration, nutrition, catabolism, and tissue breakdown. The correlation between creatinine and GFR can be used to estimate GFR. The revised Schwartz formula is the following: 0.413×Ht