Your lungs pack an enormous exchange surface into a chest cavity the size of a shoebox. The trick is branching: one tube becomes two, then four, then millions, ending in tiny air sacs wrapped in blood vessels. Every part of that structure has a job, and the exam wants you to name them.
📚 What you need to know
Air path: nose and mouth → trachea → two bronchi → bronchioles → alveoli.
The trachea is held open by rings of cartilage, which support its shape while still letting it flex.
The bronchi also have cartilage plus smooth muscle. Trachea and bronchi are lined with ciliated epithelium that sweeps out mucus and trapped particles.
Bronchioles have smooth muscle but no cartilage, so they can dilate or constrict to control airflow.
Many small alveoli give a huge surface area, spread evenly by the branched network.
Each alveolar cluster has an extensive capillary bed: pulmonary artery in, pulmonary vein out, maintaining the concentration gradient.
Surfactant lowers surface tension so alveoli do not collapse and stick together on expiration.
The route air takes
The branching also matters for distribution. Because bronchioles spread out across each lung, the alveoli end up evenly spaced rather than bunched in one corner.
Structure
Key features
Why it is built that way
Trachea
Rings of cartilage; ciliated epithelium with mucus
The rings hold it open and stop it collapsing, while still letting it move and flex with the body. Cilia sweep mucus and trapped particles back up out of the airway
Bronchi
Cartilage plus a layer of smooth muscle; ciliated epithelium
Cartilage keeps them open; smooth muscle contracts or relaxes to change the diameter of the airway. One bronchus leads to each lung
Bronchioles
Smooth muscle, no cartilage
They dilate when more air is needed and constrict when, for example, an allergen is present. This regulates airflow into the lungs
Alveoli
Millions of tiny sacs, one cell thick, wrapped in capillaries, coated in surfactant
Huge total surface area, an extremely short diffusion distance, and a steep gradient maintained by blood flow
Do not confuse the alveolar wall with a cell wall. The alveolar wall is a single layer of epithelial cells. A cell wall is the rigid cellulose structure around a plant cell. Using the wrong term here is a common and avoidable error.
Inside an alveolus
Both barriers are a single layer of flattened cells, so the total diffusion distance is a fraction of a micrometre. Thicken it — as happens in some lung diseases — and gas exchange falls sharply.
The four adaptations to quote
Many small alveoli. A very large total surface area for diffusion.
A branched network of bronchioles. Alveoli are evenly distributed throughout each lung rather than clustered.
An extensive capillary bed. More surface for diffusion, and blood entering from a branch of the pulmonary artery and leaving via a branch of the pulmonary vein keeps the gradient steep.
Surfactant. Secreted by cells of the alveolar wall, it lowers surface tension so alveoli do not collapse and stick together during expiration.
🤔 Why surfactant matters more than it sounds
An alveolus is a tiny wet bubble. Water molecules at a curved wet surface pull towards each other, and in something that small the pull is strong enough to squash the sac shut every time you breathe out. Reopening a collapsed alveolus takes far more effort than keeping an open one inflated. Surfactant reduces that inward pull, which is why premature babies, whose lungs have not yet made enough of it, struggle so badly to breathe.
Worked examples
WORKED EXAMPLE
Explain why the trachea contains rings of cartilage but bronchioles do not. [3]
Point 1: what cartilage does in the tracheaThe rings support the trachea and keep it open, preventing it collapsing as air pressure changes.Point 2: why rings rather than a tubeSeparate rings still allow the trachea to move and flex with the body.Point 3: why bronchioles differBronchioles have smooth muscle instead, so their diameter can be changed to regulate airflow — cartilage would prevent this
WORKED EXAMPLE
Explain three ways in which mammalian lungs are adapted for efficient gas exchange. [3]
Adaptation 1Many small alveoli provide a very large total surface area for diffusion.Adaptation 2Alveolar and capillary walls are each one cell thick, giving a very short diffusion distance.Adaptation 3An extensive capillary bed with continuous blood flow maintains a steep concentration gradient for both gasesSurfactant and the branched bronchiole network are also creditable.
WORKED EXAMPLE
A disease causes the alveolar walls to become thicker and fibrous. Predict the effect on gas exchange and explain your answer. [3]
Step 1: state the predictionThe rate of gas exchange would fallStep 2: explain using diffusionA thicker wall means a longer diffusion distance, so oxygen and carbon dioxide cross more slowly.Step 3: the consequence for the personLess oxygen enters the blood, so the person would become breathless, especially during exercise when demand is highest.
💡 Exam tip
Learn the airway in order and spell it correctly: trachea, bronchi (singular bronchus), bronchioles, alveoli (singular alveolus).
Attach a reason to every structure: cartilage to hold open, cilia to sweep out mucus, smooth muscle to change diameter.
For alveolar adaptations, cover all three diffusion factors: area, distance, gradient.
Name the vessels: deoxygenated blood arrives from the pulmonary artery, oxygenated blood leaves via the pulmonary vein.
Surfactant is worth one clean sentence: it lowers surface tension so alveoli do not collapse.
Say alveolar wall or epithelium, never “cell wall”.
⚠ Common mix-up
Confusing alveolar wall with cell wall. One is a layer of epithelial cells; the other is plant cellulose.
Putting cartilage in the bronchioles. They have smooth muscle only.
Mixing up the pulmonary vessels. The pulmonary artery carries deoxygenated blood, which feels backwards but is correct.
Saying alveoli are large. They are tiny; it is the number of them that gives the surface area.
Bronchi and bronchioles used interchangeably. Bronchi are wide and cartilage-supported; bronchioles are narrow and muscular.
Thinking surfactant helps gases dissolve. Its job is reducing surface tension.
Up next: How Ventilation Works — the muscles and pressure changes that actually move the air in and out.
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