Chemoreceptors are stimulated by which change in blood gases?

• A. Increased PaCO2
• B. Increased PaO2
• C. Decreased pH
• D. None of the above

Answer: (A)

Chemoreceptors regulate breathing primarily in response to rising carbon dioxide levels in the blood. CO2 freely crosses into the brain; there it becomes carbonic acid, releasing hydrogen ions and lowering the pH of the cerebrospinal fluid. This pH drop stimulates central chemoreceptors in the medulla, increasing the rate and depth of ventilation to blow off CO2 and restore pH toward normal. Peripheral chemoreceptors in the carotid bodies also respond to low oxygen and to pH/CO2 changes, but their strongest stimulus is a rise in PaCO2. An increase in PaO2 does not stimulate ventilation. A decrease in pH is related to CO2 buildup and can contribute to stimulation, but the primary driver tested here is the increase in PaCO2.