Recognising Structures (CIE A Level Biology)

Trachea

Trachea cross-section

• 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 – light micrograph

• The cells of the ciliated epithelium are shown here – the cells are tightly packed and interspersed with goblet cells, which are shown in white

Trachea – electron micrograph

• The density of the cilia are shown in this electron micrograph
• 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 cross-section

• Bronchi are distinctive from the trachea because their lumen is narrower; 8.7mm instead of 18mm
• However, like the trachea, they are lined by ciliated epithelium

Bronchioles

Bronchiole – photomicrograph

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

Alveoli

Alveoli

Emphysema 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 & bronchus

Walls of the 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

Walls of the 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