As I recall from my human physiology class, it is called tidal volume. The game is to be able to have more volume in the lungs than the trachea. And that is pretty easy to do. Giraffes do have pretty big chests. And the next area to think about is the rate of ventilation. One is able to test this for themselves. How long can nominal breaths be skipped before needing a full breath to recover? You can go for a pretty long time taking every other. So that indicates that a single breath may have more O2 than needed as well as the capacity to take CO2. And so partial mixing of the last and next breath works. It is surprisingly complex.

As far as pressure, outside and inside the giraffe are the same pressure. And so it is a matter of muscles moving gas in a manner similar to a billows. But the diaphragm does the work via a pressure differential by expanding and contracting the chest cavity causing the pluera that contains the lungs to pull the lungs open.

What is cool is if you breathe sulfur hexafluoride, it is heavier than air. And so it doesn't mix. And it is difficult if not possible to ventilate. And so drowning! Unless you hang yourself upside down.

That is the extent of my recollection of that chapter. Good Q! Hope someone more up to date can correct me.