Air reaches the alveoli
Air enters through the nose or mouth and passes down the trachea. Two bronchi lead into the lungs, branching into smaller airways that end in many tiny alveoli.
Lesson 2.9 · AQA Combined Science
Lungs and gas exchange
Follow each gas
Oxygen diffuses from alveolar air into the blood. Carbon dioxide diffuses from blood into the alveoli to be breathed out. Each gas moves down its own concentration gradient.
Built for rapid diffusion
| Adaptation | Why it helps |
|---|---|
| Many alveoli | Large total surface area |
| Alveolus and capillary walls each one cell thick | Short diffusion distance |
| Moist lining | Gases dissolve before diffusing |
| Good blood supply | Brings carbon dioxide and carries oxygen away |
| Ventilation | Refreshes alveolar air, maintaining gradients |
Breathing is not respiration
Ventilation moves air into and out of lungs. Gas exchange is diffusion between air and blood. Respiration is the chemical process in cells that releases energy. During exercise, breathing and blood flow increase to supply more oxygen and remove more carbon dioxide.
Remember
Oxygen: air → blood. Carbon dioxide: blood → air. Large area, thin walls and maintained gradients.
20 quick questions
Try it
Build confidence with pictures, clues and instant feedback.
Question 1 of 20Spot · Match · Explain
Check your understanding
Look at the diagram.
Case closed!
One point for each answer you get right first time.
No timer. Take your time and use a clue whenever you need one.