Calcium regulation

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When calcium is low:

  1. The parathyroid gland secretes parathyroid hormone
  2. Parathyroid hormone increases osteoclast activity and more calcium and phosphate is released into the blood stream from the bone. 
  3. Parathyroid hormone also increases secretion of phosphates by the kidneys, and also increases calcium reabsorption. 
  4. This causes an instant increase in serum calcium levels. 
  5. Parathyroid hormone also works on the kidney to increase the rate of vitamin D activation. Activated vitamin D (calcitriol) is needed for calcium absorption. 
  6. Increased calcitriol levels increase the absorption of calcium from the terminal ileum of the small intestines. 
  7. This leads to an increase in serum calcium and restores the total amount of calcium in the body in the long term.
  8. calcitriol also increases bone sensitivity to parathyroid hormone further increasing amount of calcium in blood. 
The parathyroid glands are sensitive to changes in serum calcium, therefore when calcium levels are raised sufficiently, parathyroid hormone secretion stops. The parathyroid gland is the main regulator of calcium levels. 

Vitamin D 
Vitamin D is essential in the absorption of calcium and therefore in the maintenance of calcium levels in the long term. Vitamin D needs to be activated to become calcitriol before it will act on the intestines. Deficiency in vitamin D or malfunction of the activation system will therefore lead to bone problems. 

  • Vitamin D may originate from the diet or be activated by UV in the skin. 
  • Cholecalciferol from the diet or skin is then hydroxylated by the liver
  • It is then hydroxylated again by the kidneys. This produces calcitriol. 

Calcitonin, secreted by C cells of the thyroid are capable of decreasing plasma calcium levels. Calcitonin works by inhibiting the movement of calcium from bone fluids to the blood and inhibits osteoclast activity leading to an overall increase in bone resorption. Calcitonin is also controlled by plasma calcium levels. However it is not essential in the regulation of calcium levels. 


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