As you inhale and fill your lungs with air, nitrogen, oxygen, and carbon dioxide enter your mouth, move through your trachea, your windpipe, into the two branches of your bronchi, and then into your bronchioles, which keep dividing and getting smaller, like the roots of a tree.
Ultimately, the air winds up in your alveoli, which look like little bundles of grapes, and that’s where the real work happens. The alveoli take the carbon dioxide out of your blood and put oxygen in, then send it throughout the body via two arteries that connect to the heart.