How we breathe

Taking air in and pushing it out is called breathing. It is hard to live a week without food, or a day or two without water — but without breathing we usually cannot survive more than a few minutes.

The body has a special system for breathing, the respiratory system, and air follows a fixed path through it.

It begins at the two nostrils. Air goes into two small tubes behind them, the nasal passages. Tiny hairs inside your nose, along with mucus, trap dust and dirt. That is why we should breathe through the nose, not the mouth: the nose cleans the air first.

From the nose, air goes down the windpipe to the lungs. The windpipe splits into two branches, one into each lung. Inside the lungs these branch again and again into finer and finer tubes, ending in tiny balloon-like sacs called alveoli. The lungs are protected by the rib cage.

Activity 9.2 makes a model. Take a bottle with its bottom cut off. Through the lid, fix a Y-shaped tube with a balloon on each fork. Stretch a thin rubber sheet across the open bottom. Pull the sheet down: the balloons fill up. Let it go up: they go flat. The balloons are the lungs, and the rubber sheet is the diaphragm.

Your body works the same way. When you breathe in, your ribs move up and out, and the diaphragm — a dome-shaped muscle under the lungs — moves down and flattens. There is more space in the chest, so air rushes into the lungs.

When you breathe out, the ribs move down and in, and the diaphragm moves back up into its dome. The space shrinks and air is pushed out. Put your hands on your ribs and breathe deeply to feel it.