The Cardiovascular & Respiratory System (AQA GCSE Biology): Revision Notes
The lungs
The lungs are vital organs in the respiratory system. They are specially designed for gaseous exchange - swapping oxygen and carbon dioxide between the air and blood.
The human respiratory system is incredibly efficient, processing about 20,000 breaths per day and moving approximately 10,000 litres of air through your lungs daily.
Structure of the lungs
The pathway air takes
Air travels through your body in this order:
- Mouth or nose → Trachea (windpipe) → Bronchus (main branch) → Bronchioles (smaller branches) → Alveoli (tiny air sacs)
Think of it like a tree - the trachea is the trunk, bronchi are main branches, bronchioles are smaller branches, and alveoli are like leaves.
This branching structure is called the bronchial tree and it ensures air reaches every part of your lungs efficiently. The branching continues up to 23 generations, creating an enormous internal surface area.
What are alveoli?
Alveoli are tiny air sacs at the end of the bronchioles. You have millions of them in your lungs! Each alveolus is surrounded by a network of capillaries (tiny blood vessels).
Key features of alveoli:
- They are one cell thick
- They have a spherical (round) shape
- They create a huge surface area for gas exchange
These three features work together to make gas exchange as efficient as possible. Without these adaptations, your body wouldn't be able to get enough oxygen to survive.
Gas exchange
This is how your lungs swap gases with your blood.
What cells need
Oxygen: Your cells need oxygen for respiration. Oxygen travels in red blood cells and diffuses from capillaries into body cells.
Getting rid of waste: Carbon dioxide is a waste product from respiration. It diffuses from cells into blood plasma in capillaries.
How gas exchange works in alveoli
- Oxygen moves from alveolar air into the blood
- Carbon dioxide moves from blood into alveolar air
- This happens by diffusion - gases move from where there's more to where there's less
Diffusion is a passive process - it doesn't require energy from your body. Gases naturally move down their concentration gradients, making this process automatic and continuous.
Inhaled and exhaled air composition
The air you breathe in is different from the air you breathe out:
Gas Composition Comparison:
Oxygen:
- Inhaled air: 21%
- Exhaled air: 16%
Carbon dioxide:
- Inhaled air: 0.04%
- Exhaled air: 5%
This shows that your body extracts about 5% of the oxygen from each breath and adds about 5% carbon dioxide as waste.
Why exhaled air contains water vapour
The surface of alveoli is moist. This means exhaled air contains more water vapour than inhaled air. You can see this water vapour when you breathe out on a cold day!
Why alveoli are perfectly designed for gas exchange
Large surface area
Millions of tiny alveoli create a massive surface area (about 50-75 m²). This is much more efficient than having fewer, larger air sacs.
One cell thick walls
The walls of alveoli and capillaries are extremely thin. This means gases only have a short distance to travel when diffusing.
Good blood supply
Each alveolus is surrounded by capillaries. This maintains concentration gradients and keeps diffusion happening quickly.
Moist surface
The moist surface helps gases dissolve, making diffusion more efficient.
These four adaptations work together perfectly. Scientists estimate that if your alveoli were spread out flat, they would cover about half the area of a tennis court - all packed inside your chest!
Summary
Key Points to Remember:
- The lungs are made up of millions of tiny alveoli - these create a huge surface area for gas exchange
- Gas exchange happens by diffusion - oxygen goes into blood, carbon dioxide comes out
- Exhaled air has less oxygen and more carbon dioxide than inhaled air
- Alveoli are perfectly adapted - they're one cell thick, have huge surface area, and excellent blood supply
- The pathway is simple - mouth/nose → trachea → bronchus → bronchioles → alveoli