The respiratory system
Pathway of air
Air is drawn through the nose / mouth, down the trachea, into the bronchi, then the bronchioles, and finally into the alveoli (tiny air sacs) in the lungs.
Gas exchange in the alveoli
Alveoli are adapted for fast diffusion:
- Large surface area — millions of alveoli give an enormous total area.
- Thin walls — one cell thick, so a short diffusion path.
- Moist surface — gases dissolve before crossing.
- Rich blood supply — capillaries surround each alveolus, maintaining a steep concentration gradient.
Oxygen diffuses from alveolar air (high O_2) into the blood. Carbon dioxide diffuses from the blood (high CO_2) into the alveolar air, ready to be exhaled.
Ventilation (breathing)
| Phase | Diaphragm | Intercostals | Ribcage | Volume | Pressure | Air |
|---|---|---|---|---|---|---|
| Inhale | Contracts and flattens | Contract | Up and out | Increases | Decreases | In |
| Exhale | Relaxes and domes | Relax | Down and in | Decreases | Increases | Out |
Effect of exercise
Exercise increases the demand for oxygen and the production of CO_2 by respiring muscle.
- Breathing rate increases (more breaths per minute).
- Tidal volume increases (deeper breaths).
- Heart rate also rises to deliver O_2 faster.
If oxygen supply cannot keep up, muscles respire anaerobically, producing lactic acid and an oxygen debt.
WJEC exam tip
When listing alveolar adaptations, link each feature to the function: "thin walls (one cell thick) so the diffusion distance is short" — both halves are needed to score the mark.
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