Airplane O2 bags

Article here

If the bag isn’t inflating at all during the pause between inhalation and exhalation you’d still be wasting quite a bit of the O2 - it is possible to get around this issue (google “Lack circuit” if you’re interested) but maybe they don’t bother.

To add to Cecil’s explanation though, the reason there is more CO2 in the later part of the expiration is that the first part contains the air that was just sitting in the upper airways (trachea, bronchi, bronchioles) which don’t have the apparatus to absorb oxygen and get rid of CO2. So this fills the bag, you blow out the stuff that’s been sitting in the alveoli (which is where gas exchange actually takes place) then you inhale the contents of the bag (not sure whether this is actually neat oxygen or just air) before topping up with the contents of the tube.