Lungs at Higher Level: The same statement as SL. At HL you also meet the two kinds of pneumocyte in the alveolar wall and how haemoglobin loads the oxygen that crosses it.
Practise this as you read
- Outline the structural features of lungs and the role of surfactant.
- Explain how lung structure and breathing together bring about gas exchange.
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The big idea: The mammalian lung is filled with millions of tiny air sacs called alveoli (singular: alveolus). Each alveolus is where gas exchange happens.
What happens there
- Oxygen diffuses from the air into the blood
- Carbon dioxide diffuses from the blood into the air to be breathed out
How alveoli are **adapted** to make this diffusion fast
- They give a huge surface area
- Walls just one cell thick
- Kept moist
- Wrapped in a rich supply of blood capillaries
Gas exchange at an alveolus: oxygen diffuses from the air sac across the thin, moist wall into the blood capillary, while carbon dioxide diffuses the other way.
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- Alveolus (plural: alveoli)
- A tiny air sac in the lung where gas exchange between air and blood takes place.
- Gas exchange
- The diffusion of oxygen into the blood and carbon dioxide out of the blood across the alveolar wall.
- Diffusion distance
- How far a gas must travel to cross a surface. The alveolar wall is one cell thick, so the distance is very short.
- Capillary
- A tiny blood vessel with very thin walls; capillaries form a dense network around each alveolus.
- Pneumocyte
- A cell that makes up the wall of an alveolus. There are two kinds: type I and type II.
Why so many tiny sacs?: Splitting the lung into millions of tiny alveoli packs an enormous surface area into a small chest.
A single large bag would have far less surface — and surface area is exactly what controls how fast gases can diffuse across.
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If you zoom in on the wall of one alveolus, you find three different cell types, each with its own job.
Two of them build the wall (type I and type II pneumocytes) and one keeps it clean (a phagocyte).
Type I pneumocyte — the thin lining: Type I pneumocytes are extremely thin, flat cells that form most of the alveolar wall.
Being so thin gives a short diffusion distance, so oxygen and carbon dioxide cross the wall quickly. Type I cells are the actual gas-exchange surface.
Type II pneumocyte — makes surfactant: Type II pneumocytes are smaller, rounder cells that secrete surfactant — a fluid that coats the moist inner lining of the alveolus.
Surfactant lowers the surface tension of that watery lining. Without it, the surface tension would pull the wet walls together and make tiny alveoli collapse or stick shut, especially as you breathe out.
Phagocyte — the cleaner: A phagocyte (alveolar macrophage) patrols the inside of the alveolus and engulfs and digests any pathogens, dust or debris that get breathed in.
This protects the delicate, thin gas-exchange surface from infection and blockage.
| Cell in the alveolus | What it does | Why it matters |
|---|---|---|
| Type I pneumocyte | Forms the very thin, flat lining of the alveolus | A wall one cell thick gives a short diffusion distance for fast gas exchange |
| Type II pneumocyte | Secretes surfactant onto the moist lining | Surfactant lowers surface tension so alveoli do not collapse and are easier to inflate |
| Phagocyte (macrophage) | Engulfs and digests pathogens and dust that reach the alveolus | Keeps the delicate gas-exchange surface clean and protected from infection |
Why surfactant is the star of the topic: Surfactant reduces surface tension in the moist lining. This does two things.
What surfactant does
1
Stops collapse
It stops alveoli collapsing (or sticking shut) when air leaves on exhalation.
2
Easier breathing
It makes the lungs easier to inflate on the next breath.
The real-world link: A newborn baby that has not yet made enough surfactant struggles to keep its alveoli open.
Type I pneumocyte
- Thin, flat cells
- Form most of the alveolar wall
- Give a short diffusion distance
- Are the gas-exchange surface itself
Type II pneumocyte
- Rounder, smaller cells
- Secrete surfactant
- Surfactant lowers surface tension
- Stops alveoli collapsing; eases inflation
An easy way to remember: Type I = thin = exchange (1 cell thick). Type II = surfactant (think 'II' = 'two jobs': stops collapse AND eases inflation). The phagocyte is the cleaner.
The big idea: A branched network of bronchioles carries air to millions of alveoli, each wrapped in capillaries and lined with surfactant.
The points to remember
- A branched network of bronchioles carries air to every part of the lung.
- Millions of alveoli give a very high surface area for gas exchange.
- Extensive capillary beds cover the alveoli, so blood is close to every part of the surface.
- Alveolar walls are one cell thick: a short diffusion distance.
- Surfactant lines the alveoli: it lowers surface tension, so alveoli do not stick together or collapse, and lets oxygen dissolve.
Remember it as: Branches, sacs, vessels, surfactant: the four features of a lung.
Real example: emphysema: In emphysema, often caused by smoking, the walls between alveoli break down and merge them into larger air spaces. The surface area for gas exchange falls, so patients are breathless.
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How this is tested: Alveolar cells are tested on Paper 1 and Paper 2. Know each cell's job and what happens if it is lost.
| Paper | What you are asked | The answer |
|---|---|---|
| Paper 1 multiple choice | Given the three alveolar cell types: match each to its function | Type I = thin lining for exchange, type II = make surfactant, phagocyte = engulf pathogens |
| Paper 1 data/reasoning | An alveolar cell is labelled on a micrograph: predict the consequence of destroying it | Destroy the type II cells and surfactant production stops, so alveoli collapse |
| Paper 2 short Outline (about 2 marks) | The role of surfactant | Lowers surface tension and stops alveoli collapsing |
Exam-style question — outline the role of surfactant
Outline the role of surfactant in the mammalian lung. [2]
How to score both marks
- Name what it does to surface tension. Surfactant lowers (reduces) the surface tension of the moist film lining the inside of each alveolus.
- Give the consequence. This stops the alveoli collapsing (or sticking shut) as air leaves on exhalation, and makes the lungs easier to inflate on the next breath. (Mark 1: reduces surface tension. Mark 2: prevents alveolar collapse / eases inflation.)
Final answer
Surfactant lowers the surface tension of the moist alveolar lining, which stops the alveoli collapsing on exhalation and makes the lungs easier to inflate.
Why this scores full marks: The two marks are a cause and an effect: lowers surface tension (cause) → alveoli do not collapse / lungs inflate more easily (effect).
Writing only 'it helps breathing' is too vague — you must name surface tension and the collapse it prevents.
| Cell in the alveolus | What it does | Why it matters |
|---|---|---|
| Type I pneumocyte | Forms the very thin, flat lining of the alveolus | A wall one cell thick gives a short diffusion distance for fast gas exchange |
| Type II pneumocyte | Secretes surfactant onto the moist lining | Surfactant lowers surface tension so alveoli do not collapse and are easier to inflate |
| Phagocyte (macrophage) | Engulfs and digests pathogens and dust that reach the alveolus | Keeps the delicate gas-exchange surface clean and protected from infection |
A 2-mark Outline, in the Paper 2 style:
IB-style questionOutline[2 marks]
Outline the role of surfactant in the mammalian lung.
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