For each action or function related to calcium and phosphate metabolism A–E, select the most appropriate option from the following list.
1. Increased blood calcium.
2. Decreased blood calcium.
3. Increased urinary phosphate excretion.
4. Decreased urinary phosphate excretion.
5. Increased alimentary absorption of calcium.
6. Decreased alimentary absorption of calcium.
7. Hydroxylation of cholecalciferol.
- An action of parathormone which increases the likelihood of renal calculi by its effect on renal tubular function.
- A hepatic function which is necessary for normal absorption of calcium from the gut.
- An outcome of the action of parathormone on osteoclasts.
- A renal function which is necessary for the absorption of calcium from the gut.
- A consequence of vitamin D deficiency in childhood which can lead to softening and deformity of bones (rickets) due to inadequate mineral content.
Medicine
A. Option 3 Increased urinary phosphate excretion. Parathormone liberates both calcium and phosphate ions from bone and in order not to exceed the solubility product for these ions it is necessary to excrete the excess phosphate. Parathormone favours this by inhibiting reabsorption of filtered phosphate. It thereby tends to raise the solubility product for these ions in urine, favouring development of renal calculi.
B. Option 7 Hydroxylation of cholecalciferol. Cholecalciferol is ingested or synthesized in the skin under the influence of sunlight. To become an active hormone promoting absorption of calcium from the gut it must be converted into 1:25 dihydroxycholecalciferol. The first hydroxylation takes place in the liver.
C. Option 1 Increased blood calcium. Parathormone stimulates osteoclasts to erode bone, thereby releasing calcium and phosphate. This raises the blood calcium level; the phosphate is excreted as discussed above.
D. Option 7 Hydroxylation of cholecalciferol. The second hydroxylation necessary for activating vitamin D (cholecalciferol) takes place in the kidney under the influence of parathormone. Both hydroxylations (liver and kidney) are necessary before vitamin D can regulate total body calcium (mainly in bones) by stimulating its active absorption in the upper small intestine.
E. Option 6 Decreased alimentary absorption of calcium. When vitamin D is deficient (dietary plus lack of adequate sunlight) the substrate for hydroxylation and activation is not available and absorption of calcium is deficient so that there is inadequate calcium for normal bone mineralization.
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