Two spongy lungs
You have two lungs, one on each side of your chest. They're pink, soft and spongy, a bit like a sponge full of tiny holes.
Your ribs make a cage around them to keep them safe.
Your lungs aren't the same size. The left lung is a little smaller, to make room for your heart. The right lung has three sections, called lobes, and the left lung has two.
An upside-down tree
Air comes down your windpipe, which splits into two big tubes, one for each lung. Inside the lungs, those tubes split again and again into smaller and smaller branches.
Turn the picture over and it looks like a tree, with the windpipe as the trunk and the tubes as branches. Doctors even call it the bronchial tree.
At the tips of the tiniest branches are bunches of little air sacs called alveoli (al-VEE-oh-lye). They look a bit like bunches of grapes.
You have hundreds of millions of alveoli. All together they make a huge surface for air to touch, dozens of times bigger than all the skin on your body.
Oxygen in, carbon dioxide out
The air around you has oxygen (OK-sih-jen) in it. Every part of your body needs oxygen to make energy.
Each air sac is wrapped in tiny blood vessels called capillaries. Oxygen passes from the air sac into your blood, and your heart pumps that blood all over your body.
At the same time, your body makes a waste gas called carbon dioxide. Your blood carries it back to the lungs, it moves into the air sacs, and out it goes when you breathe out.
Plants do the opposite! They take in carbon dioxide and give off oxygen. People and plants make a pretty good team.
The diaphragm does the work
Your lungs can't pull in air by themselves. The work is done by the diaphragm (DYE-uh-fram), a dome-shaped muscle under your lungs.
When you breathe in, the diaphragm tightens and flattens, moving down. That makes more room in your chest, and air rushes into your lungs. When it relaxes, it moves back up and air flows out.
Muscles between your ribs help too, lifting your ribs up and out.
Try it: put your hands on your belly and take a big, slow breath. Feel your belly push out? That's your diaphragm moving down.
Hiccups happen when your diaphragm suddenly jerks. Each jerk makes you gulp air, and a flap in your throat snaps shut with a hic!
Breathing fast and slow
You breathe all day and all night without thinking. Your brain stem checks how much carbon dioxide is in your blood and tells your diaphragm when to breathe.
A resting grown-up takes about 12 to 20 breaths a minute. Kids usually breathe a little faster, and babies faster still.
When you run or play hard, your muscles need more oxygen. So you breathe faster and deeper, and your heart beats faster too.
Try it: count your breaths for one minute while you sit still. Then do 20 jumping jacks and count again. What changed?