Breathing starts when you inhale air into your nose or mouth.

It travels down the back of your throat and into your windpipe, which is divided into air passages called bronchial tubes.

As the bronchial tubes pass through your lungs, they divide into smaller air passages called bronchioles. The bronchioles end in tiny balloon-like air sacs called alveoli. Your body has about 600 million alveoli.

The alveoli are surrounded by a mesh of tiny blood vessels called capillaries. Here, oxygen from inhaled air passes into your blood. After absorbing oxygen, blood goes to your heart. Your heart then pumps it through your body to the cells of your tissues and organs.

As the cells use the oxygen, they make carbon dioxide, which goes into your blood. Your blood then carries the carbon dioxide back to your lungs, where it’s removed from your body when you exhale.