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Answer
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Article
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| What functions/mechanisms is calcium involved in within the body:
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- Enzymatic reactions and membrane stability
- Second messenger signalling systems
- Nerve conduction and neuromuscular transmission
- Muscle contraction (smooth and skeletal)
- Blood coagulation
- Milk Production
- Bone Functioning
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Answer article
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| Which Three hormones affect calcium homeostasis, and what organs synthesize them?
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- Parathyroid Hormone - from the parathyroid glands.
- Calcitriol aka Active Vitamin D3 - activated in the kidney
- Calcitonin - from the C Cells of the thyroid gland.
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Answer article
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| Give the 4 Actions of PTH.
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- Removal of Calcium from the bone by activating the Fast Bone Phase
- Removal of Calcium from the bone by activating the Slow Bone Phase
- Increases calcium absorption from the gut indirectly, by activating Vitamin D3.
- Decreases calcium and magnesium excretion in the kidneys at the expense of phosphorus
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Answer article
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| What are the sources of Vitamin D?
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- Dietary - Vitamin D3 is from animal food sources, Vitamin D2 is from plant food sources.
- Skin - 7-dehydrocholesterol is converted to vitamin D3 by Ultraviolet radiation at wavelength 300nm in the skin.
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Answer article
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| Describe the synthesis of Active Vitamin D3 (Calcitriol)
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- Vitamin D is transported in the blood bound to Vitamin D Binding Globulin
- In the liver it's converted to 25-hydroxycholecalciferol - 25(OH)Vitamin D3.
- Within the kidney, 25-hydroxycholecalciferol is further 'hydroxylated' using the enzyme '1-a-Hydroxylase'. to create 1,25-dihydroxycholecalciferol = ACTIVE VITAMIN D3.
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Answer Article
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| What are the actions of Calcitriol?
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- Increases calcium absorption from the intestine - via active transport mechanisms. It increases the synthesis of Calbindin (Calcium binding protein) which transports calcium from the lumen to the Vitamin D activated Calcium ATPase pumps on the basolateral membrane of the enterocytes (secondary active transport).
- Increases phosphorous absorption from the intestine - Phosphorous is found in grains, and is absorbed in the small intestine via active transport which responds to calcitriol.
- Decreases Calcium and Phosphorous excretion via the kidney - Calcitriol acts on the renal tubular epithelial cells to increase calcium and phosphorous reabsorption from the nephron. This action is WEAK compared to the PTH action (which acts to reabsorb calcium but lose phosphorous from the nephron).
- Normal bone functioning (osteoclast and osteoblast functions) - Calcitriol is needed for normal bone absorption and deposition. Without Vitamin D3, bone is not resorbed in response to PTH.
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Answer Article
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| What is the function of Calcitonin and how does it work?
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- Calcitonin acts to decrease calcium levels in the plasma.
- Calcitonin is stimulated by hypercalcemia, and has the opposite effects of PTH on the bone:
- Fast Phase - puts calcium to bone fluid by inhibiting osteoclasts' absorptive abilities
- Slow Phase - puts calcium into bone by reducing formation of new osteoclasts.
There are also slight (insignificant) effects on the kidney and GIT.
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Answer Article
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| How does the body raise blood calcium levels:
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- By increasing PTH concentration
- By increasing Active Vitamin D3
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Answer article
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| How does the body lower blood calcium levels:
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- By decreasing PTH
- By reducing Vitamin D activation
- By increasing Calcitonin synthesis and secretion.
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Answer article
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