Human defence systems: trachea and bronchi

The idea

The trachea and bronchi are airways that conduct air between the external environment and the lungs. Their walls contain C-shaped cartilage rings that hold the airway open, preventing collapse when pressure changes during breathing. The inner surface is lined with ciliated epithelial cells and mucus-secreting goblet cells. Mucus traps particles such as dust, pollen and microorganisms. Cilia beat in a coordinated way to move mucus upwards towards the throat, where it is swallowed. This is a non-specific defence because it acts against a wide range of potential pathogens before they reach the alveoli. In the context of human defence systems, these structures form part of the first line of defence, reducing the chance of infection.

For example: Inhaled smoke can paralyse cilia, allowing mucus and trapped pathogens to accumulate, which increases the risk of respiratory infections.

What earns the marks

• The trachea is the windpipe that carries air from the larynx towards the lungs; it divides into two bronchi.
• The bronchi are the two large airways that enter the left and right lungs and then branch into smaller bronchioles.
• Rings of cartilage in the trachea and bronchi provide support and keep the airways open, preventing collapse during inhalation and exhalation.
• The lining of the trachea and bronchi contains ciliated epithelial cells and mucus-secreting cells that trap particles and microorganisms.
• Cilia beat to move mucus, with trapped pathogens and particles, upwards towards the throat to be swallowed or coughed out.

Common mistakes and how to put them right

• Confusing the trachea with the oesophagus.

  Put it right: The trachea is supported by cartilage rings and lies in front of the oesophagus.

• Thinking that cilia trap pathogens.

  Put it right: State that mucus is sticky and traps particles, and cilia beat to move the mucus.

• Believing that the bronchi are tiny air sacs.

  Put it right: Describe the bronchi as branching airways that lead to bronchioles and then alveoli.

Exam tips

• When describing defence, always link the structure to its function: cartilage keeps airways open, mucus traps pathogens, cilia move mucus.
• Use the correct terms: trachea, bronchi, bronchioles, alveoli, ciliated epithelial cells, goblet cells, mucus.
• If asked to explain how smoking increases infection risk, refer to paralysis or damage of cilia and the build-up of mucus.