IV. Disorders of Plasma Calcium
ConcentrationGenerally, the kidney, the gastrointestinal tract and the skeleton
play a key role in body calcium and phosphate homeostasis.The contribution of the kidney in calcium and phosphate
metabolism includes:
1- Synthesis of 1,25 dihydroxycholecalciferol
Inactive vitamin D (cholecalciferol) is activated in the liver by
hydroxylation to 25, hydroxycholecalciferol, the second step of its
activation is in the kidney to be 1, 25, dihydroxycholecalciferol. The
active vitamin D promotes the gut calcium absorption and the normal
calcification of bone.
2- Renal excretion of calcium
85-90% of the filtered Ca2+ is reabsorbed by the PCT while the rest is
reabsorbed by the DCT, under the influence of PTH, only <2% of
filtered calcium is excreted in the urine (equals about 5.5 mmol/day).
3- Renal excretion of phosphate
Urinary excretion of phosphate varies from 5-40 mmol/day. 80-95% of
the filtered load is absorbed in PCT (as Ca2+, glucose, aminoacids and
low molecular weight proteins). Phosphate is the major buffer for H+
excretion.Hypercalcaemia
Is a total plasma Ca2+ concentration more than 2.6 mmol/litre (10.5
mg/dl)
Causes of hypercalcaemia