Page 256 - Medicine and Surgery

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252 Chapter 6: Genitourinary system

NCCT channel
+
+
Na (60%) Na Cl – Ca 2+

Glomerulus
Proximal tubule Distal tubule
K +
ROMK channel
+
Na (25%)
K +
Aminoacids 2CI – +
Glucose NKCC2 channel K
Bowman's capsule
PO 4 3– ROMK channel
HCO – Na +
3
ENaC channel
Collecting duct
Loop of Henlé

Figure 6.9 Renal tubular function.

nephron affected (see Fig. 6.9). See also Renal Tubu- depletion, polyuria and immunodeficiency secondary
lar Acidosis (see below). The single defects are discussed to immunoglobulin loss. Phosphate loss leads to vita-
here. minDresistantricketsinchildhood,andosteomalacia
Proximal tubule: The proximal tubule is the site of max- in adults.
imal reabsorption of glucose and sodium. Water and Phosphate transport defects: There are several types,
anions such as aminoacids follow sodium. Osmotic di- usually X-linked, although occasional sporadic inher-
uretics and carbonic anhydrase inhibitors act at this site. ited or acquired cases do occur. They cause inappro-
Disordersoftheproximaltubulemayleadtooneormore priate loss of phosphate from the tubules and result in
of the following syndromes: hypophosphataemia and vitamin D resistant rickets
Renal glycosuria is an autosomal recessive inherited (VDDR). Treatment is with oral phosphate supple-

condition characterised by glycosuria with normal ments with vitamin D or 1,25 dihydroxyvitamin D
blood glucose. There is impaired reabsorption of glu- (calcitriol).
cose in the proximal tubules, with no clinical sequelae. Thick ascending loop of Henle: Sodium is pumped
Glycosuria is a normal response during pregnancy. out of the lumen in the ascending loop, water is drawn
Aminoaciduria may affect only one amino acid or sev- out of the descending loop by osmosis. This creates a

eral. The most important single defect is cystinuria, an concentration gradient within the medulla of the kid-
autosomal recessive condition which predisposes to ney, which draws water out of the collecting duct and
urinary stone formation (see page 270). Treatment is hence concentrates the urine. Loop diuretics such as
with high fluid intake and alkali ingestion, because the furosemide act from within the lumen of the ascending
cystine is more soluble in alkaline conditions. Drugs loop binding to the chloride site of the NKCC2 channel.
such as penicillamine may be used to form complexes This interferes with the pump reducing the concentra-
with the cystine. In Fanconi syndrome (which may be tion gradient resulting in more dilute urine.
an inherited juvenile form or acquired in adults) there Bartter type I syndrome is an autosomal recessive
is defective reabsorption of most aminoacids, glucose, defect in the gene encoding the NKCC2 pump. It
phosphate and bicarbonate. There may be potassium results in high urinary sodium loss, dehydration,