Your lungs are two spongy organs inside your chest that let you breathe. Every time you inhale, air rushes into your lungs through your nose or mouth, travels down your windpipe, and fills millions of tiny air sacs called alveoli. These air sacs grab oxygen from the air and pass it into your bloodstream, while pulling out carbon dioxide (a waste gas your body needs to get rid of) and sending it back out when you exhale.

You have two lungs, one on each side of your heart. Your right lung is slightly larger and has three sections, while your left lung has two sections and a little dent to make room for your heart. Together, they work constantly without you thinking about it, expanding and contracting about 20,000 times a day.

When someone has healthy lungs, they can run, swim, sing, or play without getting winded too quickly. Athletes often train to increase their lung capacity, meaning how much air their lungs can hold. You might notice your lungs working harder when you sprint across the playground: they're rushing to get more oxygen to your muscles. When you catch your breath after running, that's your lungs returning to their normal, steady rhythm.