How the lungs exchange gases — Health, 14–17 years
Breathing is more than moving air in and out. Deep inside the lungs, oxygen crosses into the blood and carbon dioxide crosses out, because thin surfaces and tiny blood vessels make that exchange possible.
A huge, thin exchange surface
Air travels through branching tubes to millions of tiny air sacs called alveoli. Each sac has a very thin wall and lies next to tiny blood vessels. Oxygen moves from the air into the blood, while carbon dioxide moves from the blood into the sac to be breathed out.
Why the exchange must be fast
Your cells use oxygen to release energy from food, and they make carbon dioxide as a waste product. A body that is moving needs this exchange continuously, not once in a while. Many air sacs create a large surface, while thin walls keep the distance short.
A runner’s next breath
A runner’s leg cells need more energy, so they use oxygen faster and produce more carbon dioxide. Sensors detect the rise in carbon dioxide, and breathing becomes deeper and quicker. More fresh air reaches the alveoli, where oxygen enters blood and carbon dioxide leaves it.
Air is not the same as oxygen
It is reasonable to say that we breathe in oxygen, because oxygen is the gas our cells need. But air is a mixture: it contains mostly nitrogen, plus oxygen, carbon dioxide and other gases. The lungs transfer some oxygen into blood; they do not turn all inhaled air into oxygen.
Why smoke and dust matter
The exchange surface works best when its thin walls stay clear and healthy. Smoke and fine dust can irritate airways, damage lung tissue or make mucus harder to move. This is one reason air quality matters, especially during exercise when the body needs more oxygen.
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