alveolar ventilation
Alveolar ventilation is the part of your breathing that actually counts — the fresh air per minute that reaches the alveoli, where oxygen enters the blood and carbon dioxide leaves it. It is the portion of breathing that does real work, the rest being overhead.
Each breath drags air down the windpipe and bronchi before it reaches the gas-exchanging alveoli. That conducting plumbing — the dead space, about 150 millilitres in an adult — must be refilled every breath before any fresh air gets through, so it is subtracted from each tidal volume.
The formula makes this concrete: alveolar ventilation equals (tidal volume minus dead space) times respiratory rate. For 500 mL breaths, 150 mL of which is dead space, at 12 breaths a minute, that is about 4.2 litres per minute reaching the alveoli, less than the 6 litres of total minute ventilation.
This is why it is alveolar ventilation, not minute ventilation, that sets the arterial CO2 level. It also explains the danger of rapid shallow breathing: dropping tidal volume toward the dead-space volume can make alveolar ventilation collapse even when the chest is heaving and minute ventilation looks adequate.
A panicking patient taking 200 mL breaths 40 times a minute has an 8 L/min minute ventilation but, after dead space, may achieve only about 2 L/min of alveolar ventilation — and so retains CO2 despite the frantic effort.
Rapid shallow breaths starve alveolar ventilation.
Arterial CO2 is inversely proportional to alveolar ventilation: halve the alveolar ventilation and CO2 roughly doubles. This tight, reciprocal link is the cornerstone of how breathing controls acid–base balance.