Chemicals That Directly Affect Respiration

Which of the following chemical directly affects the respiration?

Correct Answer: Both CO₂ & H⁺

CO₂ and H⁺ (hydrogen ions / protons) are the primary chemical stimuli that directly and powerfully regulate respiration. O₂ plays a secondary role.

Central Chemoreceptors (Medulla)

• Located on the ventral surface of the medulla oblongata.
• Sensitive to H⁺ concentration in the cerebrospinal fluid (CSF).
• CO₂ freely crosses the blood-brain barrier → reacts with CSF water → H₂CO₃ → H⁺ + HCO₃⁻ → increased H⁺ stimulates central chemoreceptors → increased ventilation.
• CO₂ works primarily indirectly via H⁺ at central chemoreceptors.

Peripheral Chemoreceptors (Carotid and Aortic Bodies)

• Respond to: ↓ PaO₂ (primary), ↑ PaCO₂, ↑ H⁺
• O₂ works through peripheral chemoreceptors — but only when PaO₂ falls significantly (below 60 mmHg), hence O₂ is a less potent immediate stimulus than CO₂/H⁺.

Practical Example

• When a cow runs (exercise) → more CO₂ produced → CO₂ crosses BBB → H⁺ rises in CSF → central chemoreceptors stimulated → respiratory rate increases to eliminate CO₂.