Calcium Homeostasis | Parathyroid Hormone and Vitamin D

Main Ideas

• Calcium levels must be tightly regulated. Calcium is essential for heart and skeletal muscle contraction and normal nerve function. Low blood calcium can interfere with these functions.
• Parathyroid hormone (PTH) responds to low blood calcium. The parathyroid glands, located behind the thyroid, release PTH when calcium levels fall.
• Vitamin D is activated in stages. Sunlight acts on a cholesterol derivative in the skin to produce vitamin D₃, and dietary sources also provide vitamin D. The liver converts vitamin D into an intermediate form, which the kidneys then convert into calcitriol, its active form.
• PTH and calcitriol work together to raise blood calcium. Their effects include:
  • Increasing bone breakdown, releasing calcium into the bloodstream.
  • Reducing calcium loss through the kidneys by increasing calcium retention.
  • Increasing calcium absorption from the intestines into the blood.
• Negative feedback restores balance. As blood calcium rises, the low-calcium stimulus decreases, PTH release is inhibited, and the corrective response winds down. This regulatory process is called calcium homeostasis.

Vitamin D Terminology Note

The subtitles identify egg-derived vitamin D as ergocalciferol (D₂). This identification may reflect a transcription or speaker error.

Speakers and Sources

• Speaker: Dr. Mike
• Vitamin D sources mentioned: Sunlight and fish for vitamin D₃; eggs are also mentioned in the subtitles as a source of vitamin D.