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John D. Mahan and Hiren P. Patel 22
The kidneys preserve homeostasis through multiple mechanisms: maintain fluid and electrolyte balance, excrete metabolic waste products through glomerular filtration and tubular secretion, generate energy (gluconeogenesis), and produce important endocrine hormones (renin, vitamin D metabolites, erythropoietin). Renal disorders, by disturbing homeostasis, can affect growth and development and result in a variety of clinical manifestations (Table 161-1). Renal disorders in children may represent intrinsic renal diseases or derive from systemic conditions (Table 161-2).
Fetal urine production contributes to amniotic fluid volume, lung maturation, and somatic development. Congenital renal disorders may be associated with reduced (oligohydramnios) or increased amniotic fluid volume (polyhydramnios). Pulmonary hypoplasia and fetal maldevelopment of the face and extremities may result from insufficient amniotic fluid (Potter syndrome) (see Chapters 58 and 60).
Risk factors for renal disease (e.g., perinatal hypoxia-ischemia, acute tubular necrosis, renal vein thrombosis) can be detected by a careful history. A detailed family history may identify hereditary renal conditions. Poor growth and/or feeding, abnormal fluid intake and/or output may indicate underlying renal dysfunction. Other common manifestations of renal and/or urinary tract disorders are listed in Table 161-1.
Renal diseases in children may be initially asymptomatic and detected during routine visits. Abnormal growth, hypertension (HTN), dehydration, or edema may suggest occult renal disease (see Chapter 33). Abnormal facial features may suggest syndromes associated with renal disorders (fetal alcohol syndrome, Down syndrome). Preauricular tags, deformities of the external ear, or ophthalmologic abnormalities (keratoconus, aniridia, iridocyclitis, cataracts) can be associated with congenital renal defects. Abdominal examination may reveal renal masses. Bladder distention may suggest a urine-concentrating defect or urinary tract obstruction.
Normal renal function depends on intact glomerular filtration and tubular function (proximal tubule [PT], loop of Henle, and distal tubule [DT]) that results in urine formation (Fig. 161-1). The net glomerular pressure favors movement of fluid out of the capillaries into Bowman’s space. Intraglomerular pressure is regulated by afferent and, particularly, efferent arteriolar tone. Renin is generated by cells in the juxtaglomerular apparatus, located at the base of each glomerulus in proximity to the afferent/efferent arterioles and DT of that nephron and released in response to glomerular flow and perfusion.
The PT conducts isosmotic reabsorption (see Fig. 161-1)to reabsorb two thirds of the filtered volume, sodium, and chloride. Glucose, amino acids, potassium, and phosphate are almost completely reabsorbed. Approximately 75% of filtered bicarbonate is reabsorbed in the PT. When filtered bicarbonate exceeds the PT threshold, bicarbonate is spilled into the urine. The PT also secretes compounds such as organic acids, penicillins, and other drugs. The most potent vitamin D analog—1,25(OH)2-cholecalciferol (calcitriol)—is produced by PT cells in response to parathyroid hormone and intracellular calcium/phosphorus concentrations.