Knowledge of the lungs is restricted to the trachea, bronchi, alveoli and…

The idea

The lungs are the organs of gas exchange, and the required route is trachea → bronchi → alveoli, with a capillary network surrounding each alveolus. The trachea is the windpipe that carries air from the mouth and nose towards the lungs; it divides into two bronchi, one entering each lung. The bronchi branch into smaller airways that end at alveoli, tiny air sacs where gas exchange occurs. Each alveolus is surrounded by a dense capillary network, so oxygen diffuses from the alveolar air into the blood, and carbon dioxide diffuses from the blood into the alveolar air. A useful method is to trace one oxygen molecule from the trachea to a red blood cell, naming each structure in order and stating that the capillary network brings blood close to the alveolar surface.

What earns the marks

• Names the trachea as the airway that carries air from the mouth and nose towards the lungs.
• Names the bronchi as the two tubes that branch from the trachea and enter the lungs.
• Names the alveoli as the air sacs where gas exchange takes place.
• Names the capillary network surrounding the alveoli as the blood supply that receives oxygen and releases carbon dioxide.
• Describes the direction of diffusion of oxygen and carbon dioxide across the alveolar surface.
• Sequences the structures correctly as trachea, bronchi, alveoli and surrounding capillaries.

Common mistakes and how to put them right

• Writing that the bronchi lead directly to the capillary network.

  Put it right: The bronchi branch into smaller airways that end at alveoli, and the capillaries surround the alveoli.

• Confusing the trachea with the bronchi.

  Put it right: The trachea is a single tube before it divides into two bronchi.

• Stating that gas exchange happens in the bronchi.

  Put it right: Gas exchange occurs across the alveolar surface into the surrounding capillary network.

Exam tips

• Practise drawing a simple lung diagram with the trachea, two bronchi, a cluster of alveoli and surrounding capillaries, then label the direction of oxygen and carbon dioxide movement.
• Use the phrase 'trachea, bronchi, alveoli, capillary network' as a checklist when answering questions about the route of air or the site of gas exchange.
• If asked to explain gas exchange, link each structural feature to a function, such as a thin alveolar surface for a short diffusion distance.