Part 1 of 4Structures of the Lungs
The lungs are organs in the thorax and are the site of gas exchange.
Trachea
- The windpipe carries air from the nose and mouth down towards the lungs.
Bronchi
- The trachea divides into two bronchi, one leading to each lung.
Bronchioles
- The bronchi branch into smaller tubes called bronchioles, which carry air to the alveoli.
Alveoli
- Millions of tiny air sacs at the ends of the bronchioles are the site where gases are exchanged.
Protection and movement
- The lungs lie in the thorax, protected by the ribs, and ventilation is brought about by the diaphragm and intercostal muscles.
Part 2 of 4Adaptations of the Alveoli
Adaptations of the Alveoli
Many alveoli
- There are very many alveoli, which together provide a large surface area for gas exchange.
Thin walls
- The alveolar wall is very thin, so the diffusion distance is short and gas exchange is rapid.
Well ventilated
- Breathing keeps oxygen high and carbon dioxide low in the alveoli, which maintains steep concentration gradients.
Many capillaries
- A dense network of capillaries brings blood low in oxygen and high in carbon dioxide and keeps the gradients steep.
Moist lining
- A layer of moisture lining each alveolus lets gases dissolve, which speeds up diffusion.
| Feature | Adaptation |
|---|---|
| Many alveoli | Large surface area |
| Thin wall | Short diffusion distance |
| Ventilation | Steep concentration gradient |
| Capillary network | Steep gradient maintained |
Part 3 of 4Gas Exchange Sequence
1
Air is breathed in through the trachea, bronchi and bronchioles until it reaches the alveoli.
2
Oxygen dissolves in the moisture lining the alveolus and diffuses across the thin wall into the blood in the capillaries.
3
Carbon dioxide diffuses in the opposite direction, from the blood into the alveolar air space.
4
Breathing out removes the carbon dioxide, and fresh air breathed in replaces the oxygen that has been absorbed.
Part 4 of 4Exam Checklist & Tips
2.7.3 – Key Points for the Exam
- Trachea The windpipe that carries air to the lungs.
- Bronchus One of two tubes branching from the trachea, leading to each lung.
- Bronchiole A small branch of a bronchus leading to alveoli.
- Ventilation The movement of air into and out of the lungs.
- Gas exchange The diffusion of oxygen into the blood and carbon dioxide out of it.
Common Mistakes
- Saying that gas exchange takes place in the bronchi or trachea; it occurs at the alveoli.
- Describing alveoli as muscular; they are thin-walled air sacs.
- Stating that oxygen enters the blood by active transport, when it diffuses down a concentration gradient.
- Forgetting to mention both oxygen and carbon dioxide and the direction in which each moves.