Outline the regulation of plasma calcium concentration. Outline the mechanism of action of biphosphonates for the management of hypercalcaemia.

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College Answer

The main points expected for a pass were:
· The components of plasma calcium are diffusible Ca ( free and complexed ) and nondiffusible
Ca ( protein bound ). Only the plasma free Ca is physiologically active and
regulated by homeostatic mechanisms. Plasma free Ca is also affected by plasma pH and
albumin concentration.
· The distribution of Ca in the body and the fact that ECF Ca is less than 0.1% of total body
Ca. ECF and hence plasma Ca is the result of a balance between dietary intake,
gastrointestinal absorption and excretion, renal excretion and exchange with bone Ca.
· Tight hormonal regulation of GIT absorption, bone exchange and renal excretion mainly
by parathyroid hormone and calcitriol.

Also expected were details of the actions of PTH on bone and the kidney and the actions of
calcitriol on the gut and bone. No candidates described the feedback control mechanisms
involving PTH and calcitriol. Additional marks were given for mention of other hormones
that have a lesser effect on plasma Ca concentration.

The second part of the question on the mechanism of action of biphosphonates was poorly
answered.