Model of inspiration with respect to the volume/time cycle

In a patient breathing 20 breaths per minute with an I:E ratio of 1:2 and assuming no dwell time between breaths, the last 150 mL of inspired gas which fills the dead space requires about one-half the inspiratory time while the first part of the breath, representing about 0.5 seconds in this schematic, is the actual gas that reaches the alveoli and participates in gas exchange. Note that the very first gas in the airways is air that was exhaled from the alveoli. It is oxygen poor and CO₂ rich.