Calcium Metabolism

• Elevated phosphate lowers free Calcium in the blood and inhibits the formation of Vitamin D.
• Calcium exists in the blood in 3 forms:

  • Free-ionised - diffusible, biologically active
  • Bound to anions - diffusible, biologically inactive
  • Bound to proteins - not diffusible, biologically inactive
• There are three molecules which regulate the amount of calcium in blood and ensure it is maintained within the normal range. These are calcitriol (vitamin D), parathyroid hormone and calcitonin.
• Vitamin D is a long-term regulator of serum calcium, with a half life of around 6 hours. Its main function is to increase the intestinal absorption of calcium.
• Calcitriol synthesis:

  1. In the skin, 7-dehydrocholesterol is converted into cholecalciferol under the influence of UV radiation.
  2. In the liver, cholecalciferol is converted into calcidiol by 25-hydroxylase. This is relatively inactive.
  3. In the kidney, 1-alpha-hydroxylase converts calcidiol into calcitriol.
• Calcitriol travels in the bloodstream and acts on intestinal epithelial cells to synthesize calbindin-D, which facilitate the transport of calcium from the brush border to the basolateral membrane.
• When the amount of calcium in blood rises above the normal range, parafollicular cells in the thyroid gland secrete calcitonin. Calcitonin decreases calcium levels by inhibiting osteoclasts, thus reducing bone resorption.
• Calcium in the blood exists in 3 forms:

  • Free-ionized
  • Bound to anions
  • Bound to proteins
• There are 3 molecules that regulate calcium concentration in the blood:

  • Vitamin D
  • PTH
  • Calcitonin