Questions: The central chemoreceptors respond to , not to hydrogen ions; carbon dioxide carbon dioxide; hydrogen ions

The answer is the second one: carbon dioxide; hydrogen ions.

Explanation: Central chemoreceptors, located in the medulla oblongata of the brain, primarily respond to changes in the partial pressure of carbon dioxide (pCO₂) in the blood. They do not directly respond to hydrogen ions (H⁺) in the blood because the blood-brain barrier is impermeable to H⁺. However, CO₂ can cross the blood-brain barrier and, once inside the cerebrospinal fluid, it reacts with water to form carbonic acid, which then dissociates into hydrogen ions and bicarbonate. The increase in hydrogen ions in the cerebrospinal fluid is what the central chemoreceptors actually detect, leading to a response to regulate breathing.

Summary: Central chemoreceptors respond to carbon dioxide, not to hydrogen ions.