The oxygen molecules bumped and jiggled their way through the tracheae via a process called diffusion. This is the same process that causes a drop of food coloring to tint a glass of unstirred water and that lets you smell a spritz of perfume from across the room. Molecules of oxygen, food-coloring, and perfume are in constant motion, jostling with other molecules. In this way, they slowly mix with each other.

Diffusion happens all the time — but it’s slow. The further a molecule has to travel via diffusion, the longer it takes to get there.