Central chemoreceptors in the ventral medulla respond primarily to changes in CSF pH rather than directly to arterial PaCO₂. Why does CO₂ changes affect CSF pH faster than arterial HCO₃⁻ changes?
- A CO₂ is actively transported across the blood-brain barrier via carbonic anhydrase XIV
- B HCO₃⁻ crosses the blood-brain barrier faster than CO₂, causing rapid pH changes
- C The blood-brain barrier is freely permeable to CO₂ but has limited permeability to HCO₃⁻ and H⁺ ✓
- D Choroid plexus actively secretes H⁺ in response to arterial PaCO₂ changes
Correct answer: C. The blood-brain barrier is freely permeable to CO₂ but has limited permeability to HCO₃⁻ and H⁺
Explanation
CO₂ is highly lipid-soluble and diffuses freely across the blood-brain barrier in seconds, rapidly equilibrating between blood and CSF. Once in the CSF (which has low buffering capacity compared to blood), CO₂ hydrates to H₂CO₃ and dissociates to H⁺ + HCO₃⁻, causing rapid pH changes that stimulate central chemoreceptors. In contrast, HCO₃⁻ and H⁺ cross the blood-brain barrier slowly (hours), which is why metabolic acid-base changes have slower effects on ventilatory drive than equivalent PaCO₂ changes.
Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.
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