Ventilation-perfusion ratios and V/Q mismatch

Alveolar ventilation (V) is the amount of air that reaches alveoli in the lungs, measured in liters/minute (L/min); and perfusion (Q) is the pulmonary blood flow, or cardiac output, that reaches the arteries, and specifically the capillaries, surrounding the alveoli, also measured in L/min.

When the lungs are upright and at rest, ventilation is about 4 L/min and perfusion is about 5 L/min, giving a ratio of 0.8.

Now, the lungs can be divided into three distinct zones.

Zone 1 is the top of the lungs, or the apexes; zone 2 is the middle of the lungs; and zone 3 is the bottom, or bases, of the lungs.

In an upright position, gravity dramatically affects both ventilation and perfusion across all three zones, and overall the V/Q ratio progressively decreases from zone 1 to zone 2 and finally to zone 3.

In zone 1, the flow of air and blood is the lowest with ventilation of around 0.25 L/min, and perfusion of around 0.07 L/min; generating a V/Q ratio of 3.6.

In zone 2, ventilation is equal to perfusion; generating a V/Q ratio of about 1.

In zone 3, the flow of air and blood is the highest with ventilation of around 0.8 L/min, and perfusion of around 1.3 L/min; generating a V/Q ratio of 0.6.

So the V/Q ratio varies depending on which part of the lung is involved, but the overall ratio is an average of the three zones and works out to be 0.8.

Now, the ratio of V to Q influences how efficiently gases, specifically O2 and CO2 , are exchanged in the lungs.

In healthy lungs with a V/Q ratio of 0.8, the alveolar partial pressure of O2 (PAO2), is about 100 mmHg or millimeters of mercury; and the alveolar partial pressure of CO2 (PACO2) is about 40 mmHg.

Meanwhile, the arterial partial pressure of O2 (PaO2) is around 95 mmHg - slightly lower than what’s on the alveolar side; and the arterial partial pressure of CO2 (PaCO2) is about 40 mmHg - the same as what’s on the alveolar side.

But these partial pressures are also an average over the the three lung zones.

In zone 1 the arterial partial pressure of O2 (PaO2) is 130 mmHg and arterial partial pressure of CO2 (PaCO2) is 28 mmHg.

In Zone 2 the arterial partial pressure of O2 (PaO2) is about 108 mmHg and arterial partial pressure of CO2 (PaCO2) is about 39 mmHg.

And in zone 3, the arterial partial pressure of O2 (PaO2) is 88 mmHg and arterial partial pressure of CO2 (PaCO2) is 42 mmHg.

So, a drop of blood in zone 1 gets more oxygen diffused into it than a drop of blood in zone 3, but because zone 3 has about 19 times more blood flowing through per minute, it ends up accounting for more of the overall gas exchange.