Calcium homeostasis

Contained within the skeleton

Parathyroid hormone (PTH), vitamin D and calcitonin.

Both growth hormone and thyroxine also play a small role in calcium metabolism.

• - ncrease calcium levels and decrease phosphate levels
• - Increases bone resorption
• - Immediate action on osteoblasts to increase ca2+ in extracellular fluid
• - Osteoblasts produce a protein signaling molecule that activate osteoclasts which cause bone resorption
• - Increases renal tubular reabsorption of calcium
• - Increases synthesis of 1,25(OH)2D (active form of vitamin D) in the kidney which increases bowel absorption of Ca2+
• - Decreases renal phosphate reabsorption

1,25-dihydroxycholecalciferol

• - Increases plasma calcium and plasma phosphate
• - Increases renal tubular reabsorption and gut absorption of calcium
• - Increases osteoclastic activity at high levels and osteoblasts at low levels
• - Increases renal phosphate reabsorption

• - Secreted by C cells of thyroid
• - Inhibits intestinal calcium absorption
• - Inhibits osteoclast activity
• - Inhibits renal tubular absorption of calcium