Recognising Structures (Cambridge (CIE) A Level Biology): Revision Note

Recognising structures in the gas exchange system

Trachea

• A tracheal cross-section shows the large lumen, which air has to travel through

• The innermost cells of the trachea are the ciliated epithelia with projections called cilia

• The cells of the ciliated epithelium are shown in the light micrograph below

  • The cells are tightly packed and interspersed with goblet cells, which are shown in light grey

• The density of the cilia are shown in the electron micrograph below

  • The cilia are essential for sweeping bacteria and dust-filled mucus away from the lungs and up the trachea into the back of the mouth

  • This mucus is then swallowed, with any pathogens hopefully destroyed by the acidic conditions in the stomach

Bronchi

• Bronchi are distinctive from the trachea because their lumen is narrower (around 8-9mm instead of 18mm

• However, like the trachea, they are lined by ciliated epithelium

Bronchioles

• Bronchioles are approximately 1mm or less in diameter

• Smooth muscle and cuboidal epithelium are found in their walls

Alveoli

• Alveoli have a sponge-like appearance under a microscope due to their air spaces.

• They are surrounded by an extensive capillary network and are lined by squamous epithelium

Walls of the trachea and bronchus

Trachea

• The tracheal lumen is around 15 to 20 mm and is lined by ciliated epithelia

• The tracheal wall is surrounded by strong and flexible cartilage which flexes during breathing

• The smooth muscle of the trachea constricts and allows air to be expelled with more force

 Bronchus

• Like the trachea, the bronchus is lined with ciliated epithelium

• The cartilage hoops provide structural support to the bronchi and the gland ducts secrete mucus in the bronchial tubes