Blood Calcium levels when it’s too low pth and calcitriol lecture chpt 6

Sep 15, 2024

Lecture Notes: Regulation of Blood Calcium Levels

Introduction

  • Discussion on the body's response when calcium blood levels fall too low.
  • The body's response is to bring calcium levels back to normal through negative feedback.
  • Normal blood calcium levels: 9 to 11 milligrams per deciliter.

Detection of Low Calcium Levels

  • The body constantly monitors blood calcium levels through a feedback cycle.
  • When levels fall too low, the controlled condition (blood calcium level) is disrupted.
  • Sensory Receptor: Parathyroid gland cells detect low calcium levels (different from when levels are too high, where the thyroid gland is involved).

Role of the Parathyroid Gland

  • Location: Parathyroid glands are four small glands on the back of the thyroid.
  • Function: Not dependent on the thyroid gland.
  • Detection Mechanism:
    • Parathyroid gland cells produce cyclic AMP (CAMP) when they detect low blood calcium.

Control Center and Signal

  • Control Center: Parathyroid hormone gene in parathyroid gland cells.
  • Signal: Increased cyclic AMP turns on the parathyroid hormone (PTH) gene.
    • PTH is produced as a response.
    • PTH acts to increase blood calcium levels (PTH "picks it up").

Effectors and Actions

  • Bone Cells:
    • Osteoclasts: Increase activity to resorb bone and release calcium into the bloodstream.
    • Osteoblasts: Decrease activity to prevent calcium storage in bones.
  • Kidneys:
    • Retain calcium to prevent loss through urine.
    • Produce calcitriol (active vitamin D) to aid calcium absorption in the GI tract.
  • GI Tract: Calcitriol increases calcium absorption from the diet.

Feedback Loop

  • As calcium levels increase back to normal, the negative feedback loop completes.
  • Key steps in the cycle:
    1. Detection: Low calcium detected by parathyroid cells.
    2. Signal: Cyclic AMP produced in high amounts.
    3. Control Center Activation: PTH gene activates, leading to PTH release.
    4. Effectors:
      • Osteoclasts and osteoblasts in bones.
      • Kidneys for calcium retention and calcitriol production.
      • GI tract for increased calcium absorption.

Conclusion

  • The regulation of blood calcium is an example of a negative feedback mechanism.
  • PTH plays a crucial role in increasing calcium levels by targeting bones, kidneys, and the GI tract.
  • Remember the mnemonic: "PTH picks it up," and calcitonin tones it down.