Which factor affects alveolar ventilation by changing dead space?

• A. Cardiac output
• B. Pulmonary capillary blood flow
• C. Dead space amount
• D. Oxygen saturation

Answer: (C)

Alveolar ventilation is determined by how much of each breath actually reaches the gas-exchanging units, not just how much air you move overall. The key relationship is alveolar ventilation = (tidal volume minus dead space) times respiratory rate. Dead space is the portion of each breath that fills the conducting airways or nonperfused alveoli and does not participate in gas exchange. If the dead space amount increases, a larger portion of each breath goes to nonexchange air, so the air reaching the alveoli decreases and alveolar ventilation drops—unless you raise tidal volume or breathing rate to compensate. This is why dead space amount is the factor that directly affects alveolar ventilation.

The other options influence either perfusion (cardiac output and pulmonary capillary blood flow) or the outcome of gas exchange (oxygen saturation), not the volume of air available for gas exchange per breath.