The _____ regulates natural ventilation by detecting the increase of CO2 level.

• A. Medulla
• B. Cerebellum
• C. Pons
• D. Cortex

Answer: (A)

Automatic breathing is governed by the medulla oblongata, which houses the respiratory centers that set the rate and depth of ventilation. Central chemoreceptors in the medulla monitor the hydrogen ion concentration in cerebrospinal fluid, which reflects CO2 levels in the blood. When CO2 rises, CSF H+ increases and stimulates these chemoreceptors, prompting the medullary respiratory centers to increase ventilation to blow off CO2 and restore pH balance. The cerebellum handles movement coordination, the cortex can influence breath voluntarily but not control the automatic CO2-driven drive, and the pons modulates breathing rhythm rather than being the primary detector of CO2. Peripheral chemoreceptors in the carotid bodies contribute, especially at low oxygen, but the key detector for automatic CO2-regulated ventilation is the medulla.