When ventilation (V) is increased while perfusion (Q) is held constant, the V/Q ratio rises. More fresh air reaches the alveolus, so alveolar PO2 rises toward the inspired value and alveolar PCO2 falls. Arterial blood leaving that alveolus carries more oxygen and less carbon dioxide. When V is decreased while Q is held constant, the ratio falls, alveolar PO2 drops, alveolar PCO2 rises, and arterial blood becomes less oxygenated and more carbonated. The opposite pattern occurs when Q is changed: increasing Q while holding V constant lowers the ratio and worsens gas exchange, while decreasing Q raises the ratio and improves it. These shifts follow from the alveolar gas equation, which relates alveolar PO2 to the ratio of CO2 production to oxygen consumption and to the inspired PO2. The simulation lets you move V and Q independently and observe the resulting changes in alveolar and arterial partial pressures.
Learn MotionVentilation–Perfusion Matching in the Lung
The V/Q Ratio as the Currency of Gas Exchange
What Happens When You Change V or Q Alone
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Use the two sliders to change ventilation and perfusion one at a time. Start at the normal setting, where V is about four point two and Q is about five, giving a ratio near zero point eight. Now raise V alone. The ratio climbs, alveolar oxygen rises, and carbon dioxide falls. The blood leaving this alveolus is better oxygenated. Return to normal and instead raise Q alone. The ratio drops, alveolar oxygen falls, and carbon dioxide rises. The blood is less oxygenated. The key observation is that the direction of the change in gas exchange depends on which variable you moved and in which direction, because the ratio is what the alveolus responds to. Watch the arterial values track the alveolar values as you move the sliders.
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