Alveoli and the mammalian lung

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: oxygen diffuses from the air into the blood, and carbon dioxide diffuses from the blood into the air to be breathed out.

Alveoli are adapted to make this diffusion fast — they give a huge surface area, have walls just one cell thick, are kept moist, and are wrapped in a rich supply of blood capillaries.

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.

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.

Why surfactant is the star of the topic: Surfactant reduces surface tension in the moist lining. This does two things:

1. It stops alveoli collapsing (or sticking shut) when air leaves on exhalation.

2. It makes the lungs easier to inflate on the next breath.

This is why a newborn baby that has not yet made enough surfactant struggles to keep its alveoli open — the classic real-world link the IB uses.

A memory hook: Type I = thin = exchange (1 cell thick). Type II = surfactant (think 'II' = 'two jobs': stops collapse AND eases inflation). The phagocyte is the cleaner.

How this is tested: On Paper 1 a multiple-choice question often gives the three alveolar cell types and asks you to match each to its function — type I = thin lining for exchange, type II = make surfactant, phagocyte = engulf pathogens.

A common Paper 1 data/reasoning item labels an alveolar cell on a micrograph and asks you to predict the consequence of destroying it — destroy the type II cells and surfactant production stops, so alveoli collapse.

On Paper 2 a short Outline question (about 2 marks) asks for the role of surfactant — lowers surface tension and stops alveoli collapsing.

✓ 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.