Video narration
Where does the oxygen in your breath actually enter your blood? Follow one breath. As you inhale, air passes down your airways, branching through your lungs until it reaches tiny air sacs. That's where the real exchange happens. The lungs do more than fill with air: these sacs swap gases with nearby blood. Oxygen moves from the air into the blood, while carbon dioxide moves from the blood into the sacs. Your blood carries oxygen away to the body and brings carbon dioxide back. Your diaphragm, a broad muscle beneath the lungs, tightens and moves down as you breathe in, helping draw air inside. When it relaxes, air flows back out, carrying carbon dioxide with it. So that breath isn't simply air going in and out. Your breath reaches tiny air sacs: oxygen enters your blood there, while carbon dioxide leaves it to be breathed out.