Breathing and gas exchange
Biology · Key Stage 3 · The School
Breathing is a pressure trick
Lungs have no muscle of their own. The DIAPHRAGM flattens and the ribs swing up and out, which increases the volume of the chest; a bigger volume means lower pressure inside than outside, and air rushes in to equalise it. Breathing out is mostly the reverse happening passively as those muscles relax.
Where the exchange actually happens
At the end of the airways sit the ALVEOLI — hundreds of millions of tiny sacs. They are one cell thick, wrapped in capillaries and permanently moist, which is exactly what diffusion needs. Oxygen moves from high concentration in the air to low concentration in the blood; carbon dioxide moves the other way. Together they give a surface area roughly the size of a tennis court, folded into your chest.
In versus out
Inhaled air is about 21% oxygen and 0.04% carbon dioxide. Exhaled air is about 16% oxygen and 4% carbon dioxide — so you use only a fraction of the oxygen you take in, which is precisely why mouth-to-mouth resuscitation works at all. Exhaled air is also warmer and carries much more water vapour.
Breathe normal room air through limewater: it stays clear. Now breathe OUT through limewater: it turns milky within a few breaths, showing carbon dioxide. The figures: inhaled air is about 21% oxygen and 0.04% carbon dioxide; exhaled is about 16% oxygen and 4% carbon dioxide. Note that plenty of oxygen comes back out — which is precisely why rescue breathing works.