What is the most important factor controlling ventilation under normal conditions?

The most important factor controlling ventilation under normal conditions is the partial pressure of carbon dioxide (PCO2) in the blood. This is primarily due to the body's need to maintain a homeostatic balance of carbon dioxide levels, which directly influences blood pH and, consequently, respiratory drive.

When PCO2 levels rise, the body's chemoreceptors are stimulated, leading to an increase in the rate and depth of breathing to expel more carbon dioxide. Conversely, if PCO2 levels drop, ventilation decreases to retain CO2. This mechanism ensures that acid-base balance is maintained within the body, as CO2 is inherently linked to changes in blood pH. Thus, while factors like oxygen concentration, blood pH, and chest wall compliance can influence breathing, it is the concentration of carbon dioxide that serves as the primary regulator of ventilation under normal physiological conditions.