Central chemoreceptors located in the medulla provide feedback to increase the rate and depth of breathing when they sense:

• A. increased levels of oxygen in the blood and a decrease in the pH of the cerebrospinal fluid.
• B. slight decreases in carbon dioxide and an increase in the pH of the cerebrospinal fluid.
• C. decreased levels of oxygen in the blood and an increase in the pH of the cerebrospinal fluid.
• D. slight increases in carbon dioxide or a decrease in the pH of the cerebrospinal fluid.

Answer: (D)

Central chemoreceptors in the medulla monitor the acidity of cerebrospinal fluid, which reflects carbon dioxide levels in the blood. When CO2 rises, it diffuses into the CSF and converts to carbonic acid, dissociating into hydrogen ions and bicarbonate. The increased hydrogen ion concentration lowers CSF pH, and these receptors sense that change and send signals to the respiratory centers to increase both the rate and depth of breathing to blow off CO2 and raise pH. That’s why slight increases in CO2 or a decrease in CSF pH provoke a stronger ventilatory drive.

Oxygen levels play a smaller direct role for central chemoreceptors; the primary oxygen-sensitive control comes from peripheral chemoreceptors (like carotid bodies) when oxygen is low. So the scenario that best fits is a small rise in CO2 or a drop in CSF pH, which directly stimulates these medullary sensors to boost breathing.