The lungs (AQA GCSE Biology Combined Science): Revision Notes
The lungs
The lungs are part of the respiratory system. They are specially designed to help gases move in and out of your body.
Structure of the lungs
Air pathway
Air travels through your body in a specific route:
- Mouth or nose → Trachea (windpipe) → Bronchi → Bronchioles → Alveoli
- The trachea splits into two bronchi, one for each lung
- These bronchi branch into smaller tubes called bronchioles
- The bronchioles end in tiny air sacs called alveoli
The respiratory system works like an upside-down tree - starting with one main trunk (trachea) that branches into smaller and smaller tubes until it reaches the leaves (alveoli) where the actual work happens.
Alveoli - the key structures
Alveoli are millions of tiny air sacs where gas exchange happens. They have special features:
- Huge surface area - millions of alveoli create a large area for gas exchange (about )
- Very thin walls - only one cell thick to keep distances short
- Rich blood supply - each alveolus is surrounded by tiny blood vessels called capillaries
- Moist surface - helps gases dissolve and move across
Gas exchange
Gas exchange is the essential function that keeps your body alive by delivering oxygen to cells and removing carbon dioxide waste.
Your body cells need oxygen and must get rid of carbon dioxide:
- Oxygen moves in: From alveolar air → into blood in capillaries → carried to body cells
- Carbon dioxide moves out: From body cells → into blood → from capillaries into alveolar air
The process in alveoli
- Oxygen moves from the air in alveoli into the blood
- Carbon dioxide moves from the blood into the air in alveoli
- This happens because gases naturally move from areas where there's more of them to areas where there's less
Inhaled and exhaled air
The air you breathe in is different from the air you breathe out:
| Gas | Inhaled air | Exhaled air |
|---|---|---|
| Oxygen | 21% | 16% |
| Carbon dioxide | 0.04% | 5% |
Key differences
- Less oxygen in exhaled air because your body uses it for cellular respiration
- More carbon dioxide in exhaled air because your body produces it as waste
- More water vapour in exhaled air - you can see this when you breathe out on a cold day
The dramatic increase in carbon dioxide from 0.04% to 5% shows just how much waste your body produces from cellular respiration!
Perfect Adaptations for Gas Exchange
Alveoli are perfectly designed for their job:
- Spherical shape gives the largest possible surface area in a small space
- Thin walls mean gases don't have far to travel
- Lots of capillaries ensure good blood flow for carrying gases away
Key Points to Remember:
- Air travels: mouth/nose → trachea → bronchi → bronchioles → alveoli
- Alveoli are tiny air sacs where gas exchange happens
- Oxygen moves from alveoli into blood, carbon dioxide moves from blood into alveoli
- Exhaled air has less oxygen (16%) and more carbon dioxide (5%) than inhaled air
- Alveoli are specially adapted with thin walls, huge surface area, and rich blood supply