Diffusion of oxygen against a concentration gradient. It's basically the same process that happens when you sprinkle salt on a slug and it dies: the salt lowers the water concentration outside of the slug, and water flows out of the slug to balance the water concentrations in and out of the slug.
Partial pressure of oxygen outside of the lungs (pressure produced only by oxygen molecules, nothing else) is much lower than the partial pressure of oxygen inside the lungs. Oxygen flows out of the lungs to equalize the partial pressures. CO2 flows into the lungs to replace the displaced oxygen.
And, you die, just like the slug.