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

Published by Patrick Mutisya · 14 days ago

Cambridge A‑Level Biology 9700 – The Gas Exchange System

The Gas Exchange System

Learning Objective

By the end of this lesson you should be able to:

  • Recognise the trachea, bronchi, bronchioles and alveoli in microscope slides, photomicrographs and electron micrographs.

  • Draw accurate plan diagrams of transverse sections of the walls of the trachea and a bronchus, labelling all major layers.

Key Anatomical Structures

The respiratory tract can be divided into a series of progressively smaller tubes. The main structures are:

  1. Trachea – the principal airway that extends from the larynx to the primary bronchi.

  2. Bronchi – the right and left primary bronchi that enter the lungs and then divide into secondary and tertiary bronchi.

  3. Bronchioles – small airways without cartilage, ending in terminal bronchioles.

  4. Alveoli – sac‑like air‑filled structures where gas exchange occurs.

Microscopic Identification

When viewing slides or micrographs, look for the following diagnostic features:

StructureKey Microscopic FeaturesTypical Magnification
Trachea (transverse section)

  • C‑shaped hyaline cartilage rings (open posteriorly)

  • Columnar pseudostratified ciliated epithelium

  • Submucosal seromucous glands

  • Loose connective tissue (adventitia)

40× – 100×
Bronchus (transverse section)

  • Irregular cartilage plates (not complete rings)

  • Similar ciliated epithelium to trachea

  • Thicker smooth‑muscle layer in the wall

  • More abundant elastic fibres in adventitia

100× – 200×
Bronchioles (transverse section)

  • Absence of cartilage

  • Simple cuboidal epithelium (ciliated in larger bronchioles)

  • Prominent smooth‑muscle layer

  • Thin basement membrane

200× – 400×
Alveolus (cross‑section)

  • Thin squamous epithelium (type I pneumocytes)

  • Scant type II pneumocytes with lamellar bodies

  • Capillary network closely apposed

  • Elastic fibres forming a supportive scaffold

400× – 1000×

Plan Diagrams – Transverse Sections

When drawing plan (top‑down) diagrams, follow these steps:

  1. Start with a circle representing the lumen.

  2. Draw the epithelial layer and label it (pseudostratified ciliated columnar for trachea/bronchus; simple cuboidal for bronchioles).

  3. Indicate the basement membrane as a thin line.

  4. Show the submucosal layer:

    • For trachea – C‑shaped cartilage rings, labelled “hyaline cartilage”.

    • For bronchus – irregular cartilage plates, labelled “cartilage plates”.

  5. Add the smooth‑muscle layer (thicker in bronchi and bronchioles).

  6. Finish with the outermost connective‑tissue layer (adventitia) and, where appropriate, elastic fibres.

Suggested diagram: Plan view of a transverse section of the trachea showing C‑shaped cartilage, ciliated epithelium and seromucous glands.

Suggested diagram: Plan view of a transverse section of a bronchus illustrating irregular cartilage plates, thicker smooth muscle and surrounding elastic connective tissue.

Comparison of Trachea and Bronchus Walls

FeatureTracheaBronchus
CartilageC‑shaped complete rings (open posteriorly)Irregular, incomplete plates
EpitheliumPseudostratified ciliated columnarPseudostratified ciliated columnar (similar)
Submucosal glandsNumerous seromucous glandsFewer glands; more smooth muscle
Smooth muscle layerThin layerProminent, thicker layer
Elastic tissueLimitedAbundant elastic fibres in adventitia
Diameter≈ 2 cm (adult)Varies; secondary bronchi ≈ 1 cm

Common Pitfalls in Identification

  • Confusing cartilage plates with surrounding smooth muscle – remember cartilage appears eosinophilic and has a clear, regular shape.

  • Missing the posterior membranous part of the tracheal ring – it is composed of smooth muscle and connective tissue, not cartilage.

  • Assuming all small airways have cilia – distal bronchioles lose cilia and become simple cuboidal.

  • Over‑looking type II pneumocytes in alveolar sections – they are smaller, cuboidal cells containing lamellar bodies.

Suggested Exam‑Style Questions

  1. Label a transverse section of the trachea, indicating cartilage, epithelium, submucosal glands and smooth muscle.

  2. Compare the structural adaptations of the bronchus that allow it to regulate airflow.

  3. Explain why alveolar walls are extremely thin and how this relates to the diffusion of \$O2\$ and \$CO2\$.

Summary

The respiratory tract shows a clear progression from rigid, cartilage‑supported tubes (trachea, bronchi) to highly compliant, thin‑walled structures (bronchioles, alveoli) that maximise surface area for gas exchange. Mastery of microscopic identification and the ability to produce accurate plan diagrams are essential skills for A‑Level examinations.