The structure of the thorax

Structure and functions in living organisms · Gas exchange in humans · note 1 of 5

Spec 2.46Download all Gas exchange in humans notes (PDF)Download all notes (PDF)8 pages

The structure of the thoraxSpec 2.46

In short

The thorax is the chest, which contains the lungs and heart and is protected by the ribs. Air passes through the trachea, bronchi and bronchioles to the alveoli, where gas exchange happens. The intercostal muscles between the ribs and the diaphragm below the lungs move the chest, and fluid between the pleural membranes reduces friction.

The thorax is the chest. It contains the lungs and the heart, and it is protected by the ribs. Air enters through the nose or mouth and passes along the airways to the alveoli.

Structures of the thorax
Structure	Description and function
Ribs	Bones that form a cage and protect the lungs and heart. They can move up and out, and down and in.
Intercostal muscles	Muscles between the ribs. They move the rib cage.
Diaphragm	A sheet of muscle under the lungs that separates the thorax from the abdomen.
Trachea	The windpipe. It carries air to the lungs. Rings of cartilage keep it open.
Bronchi (one is a bronchus)	Two tubes that branch from the trachea, one to each lung.
Bronchioles	Small tubes that branch from the bronchi and carry air to the alveoli.
Alveoli	Tiny air sacs at the ends of the bronchioles, where gas exchange happens.
Pleural membranes	Two thin membranes around the lungs and lining the chest wall, with a thin layer of fluid between them. The fluid reduces friction when the lungs move.

The path of air when you breathe in is: nose or mouth, trachea, bronchi, bronchioles, alveoli.

Exam tip:

Learn the order of the airways. Marks are often lost by writing bronchi and bronchioles the wrong way round: the bronchi are the larger tubes.

Front section of the thorax showing the trachea with cartilage rings, a bronchus to each lung branching into bronchioles that end in alveoli, the two lungs, the ribs with intercostal muscles between them, the diaphragm under the lungs, and an enlarged inset of the pleural membranes with pleural fluid between them. — Air passes down the **trachea**, **bronchi** and **bronchioles** to the **alveoli**. The ribs, intercostal muscles and diaphragm surround the lungs.

Written and checked against the Edexcel IGCSE Science Double Award (4SD0) specification · Updated October 2026

Frequently asked questions

How are alveoli adapted for gas exchange?

Alveoli are adapted for gas exchange by diffusion. Millions of alveoli give a very large surface area, and their walls and the capillary walls are each one cell thick, giving a short diffusion distance. A rich capillary network and ventilation maintain a steep concentration gradient, and the moist lining lets gases dissolve.

What happens when you breathe in?

When you breathe in, the intercostal muscles contract and move the ribs up and out, and the diaphragm contracts and flattens. This increases the volume of the thorax, so the pressure inside falls below atmospheric pressure. Air moves from the higher pressure outside into the lungs. The lungs themselves have no muscle.

How does smoking affect the lungs?

Smoking damages the lungs because tar destroys the cilia, so mucus and trapped microorganisms are not removed, causing infections and a smoker's cough. Smoke irritates the airways, causing chronic bronchitis. In emphysema the alveolus walls break down, reducing the surface area for gas exchange. Carcinogens in tar can cause lung cancer.

All 5 questions on Gas exchange in humans