Your Lungs Have Never Been Empty

Breathe out until nothing is left. You never get there. After the hardest exhale you can make, about 1 to 1.2 litres of air is still in your chest.

It is not weak muscles. As you push, the smallest airways - the ones with no cartilage holding them open - pinch shut before you finish, like the neck of a balloon, and seal the rest of the air behind them. That leftover air is your residual volume, and it is doing a job: it holds your air sacs open and keeps gas exchange going between one breath and the next.

Your lungs were not empty before you were born either. They were full of liquid, about a hundred times harder to move than air. Your first breaths drove that liquid deeper into the lungs, through the airway walls and into the surrounding tissue - over the first few breaths, not in one. The first of them was the hardest breath of your life. Since then the space inside your lungs has never once been empty. The air in it changes with every breath; the space never clears.

Sources:
https://www.ncbi.nlm.nih.gov/books/NBK493170/
https://en.wikipedia.org/wiki/Lung_volumes
https://en.wikipedia.org/wiki/Closing_capacity
https://pmc.ncbi.nlm.nih.gov/articles/PMC4818244/
https://pubmed.ncbi.nlm.nih.gov/24035400/