The loop of Henle is the site of reabsorption of 25% of sodium chloride filtered in the glomerulus (see Fig. 161-1).Active chloride transport drives the countercurrent multiplier and composes the medullary interstitial hypertonic gradient required for urinary concentration.
The DT is composed of the distal convoluted tubule (DCT) and collecting ducts (CD). The DCT is water impermeable and contributes to urine dilution by active sodium chloride absorption. The CD is the primary site of antidiuretic hormone (vasopressin) response, which leads to urine concentration. Sodium-potassium and sodium-hydrogen exchange in the CD is regulated by aldosterone. Active hydrogen ion secretion, which is responsible for the final acidification of the urine, also occurs in the CD.
Renal ammonia production and intraluminal generation of ammonium (NH4+) facilitate urine hydrogen ion excretion. Urinary NH4+ is not measured easily but can be inferred by calculation of the urinary anion gap. The gap between measured anions and cations largely consists of NH4+. Problems with renal acid excretion or ammonia production decrease the
Table 161-1 Common Manifestations of Renal Disease by Age FINDING SIGNIFICANCE NEONATE Flank mass Multicystic dysplasia, urinary tract obstruction (hydronephrosis), polycystic disease, tumor
Hematuria Acute tubular/cortical necrosis, urinary tract malformation, trauma, renal vein thrombosis
Anuria and oliguria Renal agenesis, obstruction, acute tubular necrosis, vascular thrombosis CHILD/ADOLESCENT
Cola red-colored urine Hemoglobinuria (hemolysis); myoglobinuria (rhabdomyolysis); pigmenturia (porphyria, urate, beets, drugs); hematuria (infection, glomerulonephritis, Henoch-Schönlein purpura, hypercalciuria, stones)
Gross hematuria Infection, glomerulonephritis, trauma, benign hematuria, nephrolithiasis, tumor
Edema Nephrotic syndrome, glomerulonephritis, acute/chronic renal failure, cardiac/liver disease
Hypertension Acute glomerulonephritis, acute/chronic renal failure, obstruction, cysts, dysplasia, coarctation of the aorta, renal artery stenosis
Polyuria Diabetes mellitus, central and nephrogenic diabetes insipidus, obstruction, dysplasia, hypokalemia, hypercalcemia, psychogenic polydipsia, sickle disease/trait, polyuric renal failure, diuretic abuse
Oliguria Dehydration, acute tubular necrosis, acute glomerulonephritis, interstitial nephritis, hemolytic uremic syndrome
Dysfunctional voiding/urgency Neurogenic bladder, urinary tract infection, vaginitis, hypercalciuria, foreign body
Polycystic kidney disease Cystinosis, oxalosis
Renal/urinary tract tumor Galactosemia, hereditary fructose intolerance
Extrarenal malignancy (leukemia, lymphoma) urinary anion gap. A urinary gap of approximately 1 mEq/kg body weight is reasonable in normal children.