Alveoli are tiny little air sacs in your lungs that help you breathe. You can think of them like small balloons that fill up with air. When you take a breath in, the alveoli fill up with fresh air that has oxygen in it, which your body needs to feel good and have energy.
After your body uses the oxygen, it sends back a gas called carbon dioxide. The alveoli help get rid of this carbon dioxide when you breathe out. So, you can think of alveoli as special helpers that make sure your body always has the air it needs and gets rid of what it doesn't need!