The human body's arterial blood pH is tightly maintained around 7.4 by buffering agents that bind hydrogen ions to stop any change in pH. If an acid-base imbalance overcomes the buffer system, the body changes the ventilation rate, or the rate at which gas enters or leaves the lungs. Changing the ventilation rate changes the concentration of CO2 in the blood, which alters the pH of the blood. In this case, the muscles that control the ventilation rate are acting as the: