The respiratory system, along with most bodily systems, is complicated. But it's important for children to know how their bodies work, so here's how you can explain it to a child.
Every person and animal needs to breathe to stay alive. Your respiratory system is the group of organs (body parts) that helps you breathe. You have two lungs in your chest. They are full of air, like balloons. You wouldn't want them to pop, so twelve bones called ribs make a cage around your lungs to keep them safe.
When you breathe, air comes into your body through your mouth and nose. The air goes down your windpipe (a tube in your throat) and then into two tubes or bronchi. One bronchus goes into each lung. The tubes get smaller and smaller like branches on a tree until they get to tiny air sacs. These air sacs, called alveoli, take oxygen (a special kind of gas that your body needs in order to get energy) from the air you breathe to all the different parts of your body.
Under your lungs is a muscle called the diaphragm. This muscle helps your lungs get bigger to fill up with fresh air and get smaller as they breathe out (old, stale air). When your body uses up the oxygen in the air you breathe in, your blood becomes full of dangerous waste gas, like carbon dioxide, that needs to leave your body. Air full of these wastes goes out the way it came in, from the blood to the alveoli, through the bronchii, up your windpipe and out your mouth and nose.
To have a long and healthy life, make sure you keep your lungs healthy. That means getting lots of exercise and never smoking cigarettes, which are bad for your lungs and can make you very sick.