Blood Calcium Level Regulation and Homeostasis

Normal Blood Calcium Levels

• Normal Range: 8.6 to 10.3 mg/dL
• Importance: Maintaining levels within this range is crucial for physiological functions.
• Homeostasis: The body constantly adjusts to keep calcium within these limits.

Homeostasis and Feedback Cycles

• Components of Feedback Cycle:
  1. Sensory Receptors: Monitor controlled conditions, e.g., calcium blood serum levels.
  2. Stimulus Detection: Sensory receptors detect changes in calcium levels.
  3. Input: Chemical (endocrine) or electrical (nervous system) signals sent to control center.
  4. Control Center: Evaluates input and sends output.
  5. Output: Signals sent to effectors to correct imbalance.

High Blood Calcium Levels

• Stimulus: High blood calcium levels disrupt homeostasis.
  • Detected by Sensory Receptors: Located on the thyroid gland.

Thyroid Gland and C Cells

• Thyroid Gland Location: On the neck, over the trachea.
• C Cells (Parafollicular Cells):
  • Detect changes in calcium levels.
  • Release autocrine signal known as the "boomerang effect".
  • Signal is an autocrine signal that acts on C cells themselves.

Role of Calcitonin

• Calcitonin (CT): Released by C cells when activated.
  • Type: An endocrine output.
  • Function: Lowers blood calcium levels by:
    • Inhibiting osteoclasts (prevent bone breakdown).
    • Stimulating kidneys to excrete calcium in urine.
    • Indirectly activating osteoblasts to build bone.

Effectors

• Osteoclasts: Break down bone and release calcium.
  • Response to CT: Inhibition to prevent further calcium release into the blood.
• Kidneys: Excrete calcium through urine.
  • Response to CT: Increase calcium excretion.
• Osteoblasts: Build bone and use calcium.
  • Indirect Activation: Encouraged to build bone, utilizing excess calcium.

Negative Feedback Cycle

• Outcome: Decreases blood calcium levels back to normal.
• Cycle: Once levels normalize, signaling for calcitonin release stops.
• Summary: Calcitonin "tones down" calcium levels by:
  • Inhibiting osteoclasts.
  • Excreting calcium via kidneys.
  • Activating osteoblasts to utilize calcium in bone formation.

Review Questions

• Calcitonin decreases activity of which cell?
  • Answer: Osteoclasts.
• Calcitonin indirectly increases activity of which cell?
  • Answer: Osteoblasts.
• Calcitonin signals kidney to do what?
  • Answer: Excrete calcium.

Key Concept

• Understanding the balance between osteoclast and osteoblast activity is crucial in managing calcium levels and maintaining homeostasis.