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8. How is serum calcium maintained in the normal range?

Calcium absorption from the intestine, resorption from the bone, and reab-

sorption from the kidney are tightly regulated to maintain serum calcium

within the normal range. Only 20–30 % of the ingested calcium is reabsorbed

from the upper intestine (duodenum and upper jejunum). Of the total calcium

absorbed, passive absorption accounts for only 8–23 %, while 1,25(OH) 2 D-

mediated active absorption contributes to the rest. PTH-mediated bone remod-

eling results in exchange of 200–500 mg of calcium between extracellular

fluid and bone per day. In the kidney, 98 % of the filtered calcium is reabsorbed

and the rest is excreted in urine. Majority of the filtered calcium is passively

reabsorbed in proximal convoluted tubules (60–70 %) and thick ascending

limb of loop of Henle (TALH, 20 %), whereas PTH-mediated active reabsorp-

tion in distal convoluted tubule and collecting ducts contribute to the rest

(8–10 %). Calcium reabsorption in TALH is regulated by CaSR, independent

of PTH and 1,25(OH) 2 D. FGF-23 also contributes to calcium homeostasis

through the regulation of 1α-hydroxylase activity in the proximal convoluted

tubule (Fig. 5.5).

ECF

[Ca++]

[Ca++]

ECF

parathyroid

glands

1,25(OH) 2 D PTH

Ca++ absorption Ca++ reabsorption

phosphate excretion

bone resorption

small

intestine

kidney

bone

FGF 23

+

−

Ca++ efflux to ECF

Fig. 5.4 Regulation of calcium homeostasis

5 Rickets–Osteomalacia