central chemoreceptor

Central chemoreceptors are the brain's own carbon dioxide alarms. They sit on the surface of the medulla and constantly check the acidity of the fluid bathing the brain, raising the urge to breathe whenever that fluid turns even slightly acidic.

Strictly, they respond to changes in pH rather than carbon dioxide directly. But carbon dioxide crosses easily from blood into the cerebrospinal fluid, where it reacts with water to form acid and lowers the local pH. The chemoreceptors detect this drop and signal the respiratory center to breathe faster and deeper, which blows off carbon dioxide and corrects the acidity. This loop is the dominant minute-to-minute regulator of normal breathing.

Because they sense brain pH rather than oxygen, central chemoreceptors respond more slowly to acute changes than the peripheral ones, and they can be blunted by chronic carbon dioxide retention. In long-standing lung disease the cerebrospinal fluid pH partly normalizes, weakening this carbon dioxide response and shifting reliance toward oxygen-driven breathing.