Structure of the thorax

The thorax is the upper part of your body — the chest. It contains the lungs and the heart, and it is sealed off from the abdomen (the lower part with the gut) by a muscular sheet called the diaphragm.

Think of the thorax as a protective, airtight box:

• The walls of the box are the ribs (bone) with intercostal muscles between them.
• The floor of the box is the diaphragm.
• Inside sit the two lungs, each wrapped in pleural membranes.

This box can change its volume by moving the ribs and the diaphragm — and that is exactly how we draw air in and out (ventilation). For spec 2.46 you only need to name the parts and give the function of each — the actual breathing movements come in the next subtopic.

Exam tip. Examiners want the parts in the correct place. Get clear on three things: what protects (ribs), what moves (intercostal muscles + diaphragm), and what carries/exchanges air (the airways and alveoli).

These three structures form the moving walls and floor of the thorax.

Ribs

• A cage of curved bones around the chest.
• Function: protect the lungs and heart, and provide a framework the muscles can pull on.

Intercostal muscles

• Muscles found between the ribs ("inter" = between, "costal" = rib).
• Function: contract and relax to move the ribs up/out and down/in, changing the volume of the thorax during breathing.

Diaphragm

• A dome-shaped sheet of muscle stretched across the bottom of the thorax, below the lungs.
• Function: when it contracts it flattens (moving down), increasing the volume of the thorax; when it relaxes it domes upward, decreasing the volume.

Exam tip. A very common slip is calling the diaphragm a bone or an organ. It is a muscle. And the intercostal muscles are between the ribs, not on top of them.

Air taken in through the nose and mouth passes down a series of tubes that branch like an upside-down tree.

Trachea (windpipe)

• A single large tube carrying air from the throat down into the chest.
• Held open by C-shaped rings of cartilage so it cannot collapse when you breathe in.
• Function: carries air towards the lungs.

Bronchi (one = bronchus)

• The trachea divides into two bronchi, one going to each lung.
• Also strengthened with cartilage.
• Function: carry air from the trachea into each lung.

Bronchioles

• Inside each lung, the bronchi branch repeatedly into smaller and smaller tubes called bronchioles.
• Function: carry air from the bronchi to the alveoli.

The order is: trachea → bronchi → bronchioles → alveoli.

At the very end of each bronchiole are clusters of alveoli (one = alveolus) — tiny, balloon-like air sacs.

• There are millions of them in each lung, which together give a very large surface area for gas exchange.
• Their walls are very thin (one cell thick) and moist, and they have a rich blood supply (capillaries).
• Function: this is where oxygen passes into the blood and carbon dioxide passes out of the blood — the actual gas exchange.

You do not need the full detail of how gas exchange works here (that comes later) — for spec 2.46 you just need to know that the alveoli are the site of gas exchange and that they provide a large surface area.

Exam tip. "Alveoli" is plural. If the question says "name the structure", the answer is "an alveolus" (singular). Examiners notice the difference.

Each lung is wrapped in two thin sheets called the pleural membranes (the "pleura"):

• One membrane lines the inside of the chest wall; the other covers the surface of the lung.
• Between them is a thin film of pleural fluid.
• Function: the fluid lubricates the surfaces so the lungs slide smoothly against the chest wall as you breathe, without friction. It also helps the lungs stay attached to the chest wall so they move with it.

So when the rib cage moves out, the lungs are pulled out with it — the slippery pleural fluid means this happens without the surfaces catching or tearing.

Exam tip. A typical mark is awarded for "reduces friction / lets the lungs move smoothly". Don't just write "protects the lungs" — that's vague and usually not credited.