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1,25(OH) 2 D-mediated active absorption contributes to the rest. The kidney is

the prime organ involved in phosphate homeostasis, and it is accomplished by

modifying renal phosphate excretion. Eighty-five to 90 % of the filtered phos-

phate is reabsorbed and the rest is excreted in urine. Out of this, approximately

85 % is reabsorbed in PCT, while the rest in distal tubules. Phosphate reabsorp-

tion is mediated through active transport via sodium–phosphorus co- transporters

present at these sites (Na-Pi 2a and 2c). PTH inhibits the expression of Na-Pi 2a

in PCT, while FGF-23 inhibits the expression of Na-Pi 2a and 2c in the

PCT. PTH also inhibits the reabsorption of phosphate in distal convoluted

tubule (Fig. 5.6).

PCT

75%

10%

5%

10%

TALH

CCD

MCD

DCT

Phosphate Homeostasis

PCT - Proximal convoluted tubule

DCT - Distal convoluted tubule

TALH - Thick ascending limb of

loop of Henle

CCD - Cortical collecting duct

MCD - Medullary collecting duct

Fig. 5.6 Sites of phosphate reabsorption from the kidney

5 Rickets–Osteomalacia