The Lungs and Gas Exchange

The lungs are the organs responsible for gas exchange in humans — taking in oxygen from the air and removing carbon dioxide from the blood. For AQA GCSE Biology, you need to understand the structure of the lungs, the adaptations of the alveoli for efficient gas exchange, and the mechanism of breathing (ventilation). This lesson covers all of these topics in detail.

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Structure of the Gas Exchange System

The gas exchange system is made up of several structures that work together to move air into and out of the lungs:

graph TD
    A[Nose/Mouth] --> B[Trachea — windpipe, lined with cartilage rings]
    B --> C[Bronchi — two tubes, one to each lung]
    C --> D[Bronchioles — smaller branching tubes]
    D --> E[Alveoli — tiny air sacs where gas exchange occurs]
    F[Diaphragm — sheet of muscle below lungs]
    G[Intercostal muscles — muscles between ribs]
    F --> H[Breathing mechanism]
    G --> H

Structure	Description	Function
Trachea (windpipe)	A tube held open by C-shaped rings of cartilage	Carries air from the throat to the bronchi; cartilage prevents it from collapsing
Bronchi (singular: bronchus)	Two tubes that branch off from the trachea, one going to each lung	Carry air into each lung
Bronchioles	Smaller tubes that branch off from the bronchi	Carry air to the alveoli
Alveoli (singular: alveolus)	Tiny, hollow air sacs at the end of the bronchioles; there are approximately 300 million in each lung	The site of gas exchange between the air and the blood
Diaphragm	A large, dome-shaped sheet of muscle beneath the lungs	Contracts and relaxes to change the volume of the thorax during breathing
Intercostal muscles	Muscles located between the ribs	Contract and relax to move the ribcage up and down during breathing
Pleural membranes	Thin membranes surrounding the lungs, with pleural fluid between them	Reduce friction as the lungs expand and contract; maintain a pressure difference

Exam Tip: The trachea is held open by C-shaped rings of cartilage (not complete rings). This allows it to stay open for airflow while still being flexible enough for the oesophagus (which lies behind it) to expand when food passes through.

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The Alveoli and Gas Exchange

The alveoli are the functional units of the lungs — this is where gas exchange actually takes place. Oxygen diffuses from the air in the alveoli into the blood, while carbon dioxide diffuses from the blood into the alveoli.

Adaptations of the Alveoli for Efficient Gas Exchange

Adaptation	How It Increases the Rate of Gas Exchange
Enormous surface area	There are approximately 300 million alveoli in each lung, giving a total surface area of about 70 square metres (roughly the size of a tennis court) — this maximises the area available for diffusion
Very thin walls	The alveolar walls are only one cell thick, and the adjacent capillary walls are also one cell thick, giving a total diffusion distance of only two cells — this makes diffusion extremely rapid
Rich blood supply	A dense network of capillaries surrounds each alveolus, continuously bringing deoxygenated blood and carrying away oxygenated blood — this maintains a steep concentration gradient
Moist lining	The inner surface of each alveolus is coated with a thin layer of moisture — gases dissolve in this moisture before diffusing across the membrane
Good ventilation	Breathing constantly refreshes the air in the alveoli, maintaining a high concentration of oxygen and a low concentration of carbon dioxide in the alveolar air

graph LR
    A[Alveolar Air — High O2, Low CO2] -->|O2 diffuses across thin walls| B[Blood in Capillary]
    B -->|CO2 diffuses across thin walls| A
    B --> C[Oxygenated blood to pulmonary vein]
    D[Deoxygenated blood from pulmonary artery] --> B

Gas Exchange by Diffusion

Gas exchange occurs by diffusion — the net movement of particles from an area of higher concentration to an area of lower concentration.

Gas	Direction of Diffusion	Reason
Oxygen	From alveolar air INTO the blood	Oxygen concentration is higher in the alveolar air than in the deoxygenated blood arriving in the capillaries
Carbon dioxide	From the blood INTO the alveolar air	Carbon dioxide concentration is higher in the blood than in the alveolar air

The concentration gradient is maintained because:

• Fresh air is constantly brought into the alveoli by breathing (ventilation)
• Blood is constantly flowing through the capillaries, bringing more CO2 and taking away O2

Exam Tip: When explaining gas exchange, always mention: (1) diffusion, (2) concentration gradient, (3) the adaptations that maintain the gradient (thin walls, large surface area, good blood supply, ventilation). Covering all four points will secure full marks.

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The Mechanism of Breathing (Ventilation)

Breathing is the process of moving air into and out of the lungs. It involves two movements — inhalation (breathing in) and exhalation (breathing out).

Inhalation (Breathing In)

Step	What Happens
1	The intercostal muscles contract, pulling the ribcage up and out
2	The diaphragm contracts and flattens (moves downward)
3	The volume of the thorax (chest cavity) increases
4	The pressure inside the thorax decreases (below atmospheric pressure)
5	Air is drawn into the lungs through the trachea

Exhalation (Breathing Out)

Step	What Happens
1	The intercostal muscles relax, and the ribcage moves down and in
2	The diaphragm relaxes and curves upward (returns to dome shape)
3	The volume of the thorax decreases
4	The pressure inside the thorax increases (above atmospheric pressure)
5	Air is forced out of the lungs

Summary of Breathing Mechanics

Feature	Inhalation	Exhalation
Intercostal muscles	Contract	Relax
Diaphragm	Contracts (flattens)	Relaxes (dome shape)
Ribcage	Moves up and out	Moves down and in
Thorax volume	Increases	Decreases
Thorax pressure	Decreases	Increases
Air movement	Into the lungs	Out of the lungs

Exam Tip: The most common mistake students make is saying the diaphragm "moves down" during inhalation without saying it contracts and flattens. Always include the term "contracts" for inhalation and "relaxes" for exhalation. Pressure changes drive air movement — volume up means pressure down means air in.

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Measuring Lung Function

Doctors can measure lung function using a spirometer or a peak flow meter: