Thorax and Ventilation Diagram: Breathing Mechanism
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Reviewer participation may vary by guide. Checked against Cambridge IGCSE Biology 0610 (2026–2028); last reviewed August 2026.
Key diagram
The thorax contains the lungs, ribs, intercostal muscles and diaphragm. During inhalation, the diaphragm contracts and flattens, the intercostal muscles contract raising the ribs, the thorax volume increases, pressure decreases and air is drawn in.
The thorax diagram is tested alongside the breathing mechanism: students must label the structures and explain the pressure changes during inhalation and exhalation. The key is connecting muscle contraction to volume change to pressure change.
Drawing the thorax
Draw a bell-shaped outline for the thorax. Inside, draw two lung-shaped regions. Between the lungs, draw the trachea dividing into two bronchi, which branch into smaller bronchioles ending in clusters of alveoli. Around the lungs, draw curved lines for the ribs with intercostal muscles between them. At the base, draw a curved line for the diaphragm.
Label the pleural membranes around the lungs and the pleural fluid between them, which reduces friction during breathing.
Inhalation (breathing in)
The diaphragm contracts and moves downward, becoming flatter. The external intercostal muscles contract, pulling the ribs upward and outward. These two actions increase the volume of the thorax. As the volume increases, the pressure inside the thorax decreases (becomes lower than atmospheric pressure). Air is pushed into the lungs by the higher atmospheric pressure outside.
Exhalation (breathing out)
The diaphragm relaxes and moves upward, becoming dome-shaped again. The internal intercostal muscles contract (or the external intercostal muscles relax), moving the ribs downward and inward. The volume of the thorax decreases. The pressure inside increases (becomes higher than atmospheric pressure). Air is forced out of the lungs.
Normal exhalation is largely passive: the elastic recoil of the lungs helps push air out.
Label checklist
Check these labels are present and correctly positioned before finishing your drawing.
✓Trachea
✓Bronchi (left and right)
✓Bronchioles
✓Alveoli
✓Ribs
✓Intercostal muscles (external and internal)
✓Diaphragm
✓Pleural membranes
✓Pleural fluid
Try a diagram question (4 marks)
Describe the changes that occur in the thorax during inhalation. [4]
Show the answer and marking guidance
The diaphragm contracts and flattens (moves downward).
The external intercostal muscles contract, moving the ribs upward and outward.
The volume of the thorax increases.
The pressure inside the thorax decreases, so air is drawn into the lungs.
Original Cambridge-style practice written for this site.
Common diagram mistakes
Check these against your own drawing before moving on.
Saying the diaphragm "moves down" without stating it contracts.
Confusing inhalation and exhalation sequences.
Writing that "the lungs expand and suck air in": the lungs are passive; pressure changes drive air movement.
Forgetting that volume increase causes pressure decrease.