Recognising Structures (Cambridge International AS Biology)

Recognising Structures in the Gas Exchange System

Spec point — 9.1.4 · recognise trachea, bronchi, bronchioles and alveoli in microscope slides, photomicrographs and electron micrographs and make plan diagrams of transverse sections of the walls of the trachea and bronchus

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

Trachea cross-section as shown under a light microscope

• 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

A photomicrograph image showing tissue of the trachea

• 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

An electron micrograph of the tissues that can be found inside the trachea

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

A photomicrograph image showing tissue of the bronchi in cross section

Bronchioles

• Bronchioles are approximately 1mm or less in diameter
• Smooth muscle and cuboidal epithelium are found in their walls

A photomicrograph showing bronchiole tissues in the gas exchange system

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

The cells surrounding the alveolus air sac as viewed by a light microscope

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

Labelled diagram of a cross section of the trachea

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

Parts of the cross section of a bronchus. This would be the same in both bronchi