Plants take in carbonic-acid gas through their leaves, and send the oxygen back into the air ready for us to use again.
~29.~ We have already learned that the oxygen taken in by the lungs is carried to the various parts of the body by the little blood corpuscles.
The effect of strong liquors is to injure these corpuscles so that they cannot carry so much oxygen as they ought to do.

For this reason, the blood of a drunkard is darker in color than that of a temperate person, and contains more carbonic-acid gas.

The drunkard's lungs may supply all the air he needs, but his blood has been so damaged that he cannot use it.

Excessive smoking has a similar effect.
SUMMARY.
1.

Our bodies need air, just as a candle or a fire does.
2.