Effects on Respiration: Central and Peripheral Chemoreceptors

Central Chemoreceptors

• Location: Central chemoreceptors are located in the cerebrospinal fluid (CSF), where neurons are present.
• Function: Monitor CO2 levels indirectly by sensing changes in pH due to carbonic acid formation.

Mechanism

• High CO2 Levels:

  • Increased partial pressure of CO2 in blood.
  • CO2 crosses blood-brain barrier, combines with water to form carbonic acid (H2CO3).
  • Carbonic acid disassociates into protons (H+) and bicarbonate (HCO3-), lowering pH and stimulating chemoreceptors.
  • Stimulates dorsal respiratory group (DRG), pneumotaxic center, and ventral respiratory group (VRG) leading to increased ventilation (increase in respiration rate and depth).
  • Hyperventilation adjusts CO2 levels in blood.

• Low CO2 Levels:

  • Reduced stimulation of chemoreceptors, leading to less frequent respiratory impulses.
  • Decreases in respiration rate and depth to increase CO2 levels.
  • Hypoventilation leads to CO2 retention to normalize pH.

Peripheral Chemoreceptors

• Location: Located at carotid bodies and aortic bodies.
• Function: Respond to changes in CO2, O2, and pH levels, with a significant response below 60 mmHg of O2.

Mechanism

• Stimuli:

  • Low oxygen levels (hypoxia) inhibit potassium channels leading to dopamine release.
  • High CO2 indirectly increases protons by forming carbonic acid, inhibiting potassium channels.
  • Metabolic acids (e.g., lactic acid, ketone bodies) lower pH and inhibit potassium channels.

• Response:

  • Dopamine release stimulates cranial nerves IX (glossopharyngeal) and X (vagus), increasing DRG activity, leading to increased ventilation.
  • Increase in ventilation exchanges more CO2 and O2, normalizing blood levels.
  • If opposite conditions are met (high O2, low CO2, high pH), potassium channels remain open, decreasing stimulation and respiration.

Summary of Respiratory Regulation

• High CO2/low pH: Increased ventilation to reduce CO2 levels and normalize pH.
• Low CO2/high pH: Decreased ventilation to retain CO2 and correct pH balance.
• Low O2 (below 60 mmHg): Primary trigger for increased ventilation in peripheral chemoreceptors.
• Both central and peripheral chemoreceptors adjust respiratory rates to maintain homeostasis.