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

The Gas‑Exchange System (Cambridge IGCSE/A‑Level 9700 – Topic 9)

Learning Objectives

• Recognise the trachea, bronchi, bronchioles and alveoli in microscope slides, photomicrographs and electron micrographs.
• Identify all histological layers required by the syllabus (epithelium, basement membrane, cartilage/plates, smooth‑muscle, elastic fibres, adventitia, sub‑mucosal seromucous glands where appropriate).
• Explain the functional significance of each layer for airway patency, airflow regulation and gas exchange.
• Draw accurate plan (top‑down) diagrams of transverse sections of the trachea and a bronchus, labelling every major layer.
• Link structural features to physiological concepts such as airway resistance, compliance, surface‑area‑to‑volume ratio and the role of surfactant.

Quick Anatomical Overview

1. Trachea – main conduit from the larynx to the primary bronchi; reinforced by C‑shaped hyaline cartilage.
2. Bronchi – primary, secondary and tertiary branches; cartilage becomes irregular plates.
3. Bronchioles – small airways without cartilage; end in terminal bronchioles.
4. Alveoli – thin‑walled sacs where O₂ and CO₂ diffuse between air and blood.

Sub‑Mucosal Seromucous Glands – Required Syllabus Label

These glands are present only in the trachea (and in the proximal main bronchi). They are a mandatory label in any plan diagram of the trachea.

Microscopic Identification – “What to Look For”

Layer‑by‑Layer Function Box (AO2)

Plan (Top‑Down) Diagrams – Transverse Sections

Use the checklist below to ensure every required layer is drawn and labelled. The bronchus checklist now separates “Irregular cartilage plates” and “Elastic fibres (adventitia)” as distinct items.

Step‑by‑step drawing guide

1. Draw a central circle for the lumen.
2. Sketch the epithelial layer; add tiny cilia if the magnification permits.
3. Insert a thin line for the basement membrane.
4. Place the cartilage:

   • Trachea – three or four C‑shaped rings evenly spaced.
   • Bronchus – irregular, incomplete plates; label each as “Irregular cartilage plate”.

5. Indicate sub‑mucosal seromucous glands (trachea only) as small ovals and label them.
6. Draw the smooth‑muscle band – thin for trachea, thick for bronchus.
7. Show elastic fibres as short, wavy lines in the outermost layer; label them “Elastic fibres (adventitia)”.
8. Encircle everything with the adventitial connective tissue.
9. Label each component clearly; a legend can be placed beneath the sketch if space is limited.

Sample labelled sketch (appendix)

For visual reference, see the Appendix – Sample labelled plan diagrams which provides a simple line‑drawing of a transverse tracheal section and a bronchial section, each with all required labels.

Comparison of Trachea and Bronchus Walls

Surface‑Area‑to‑Volume Ratio – Whole‑System Perspective

The alveolar sacs, each a tiny sphere, give the lung a combined internal surface area of about 70 m² while occupying only ~6 L of volume. This exceptionally high surface‑area‑to‑volume (SA:V) ratio is the principal reason why the respiratory system can meet the metabolic demands of the whole body. In exam answers you should link the thin walls of type I pneumocytes, the dense capillary network and the presence of surfactant directly to this high SA:V ratio, emphasizing that any reduction (e.g., alveolar collapse, fibrosis) markedly impairs gas‑exchange efficiency.

Clinical Connections (Real‑World Relevance)

• Tracheomalacia – Weakening of cartilage rings leads to airway collapse, especially during coughing.
• Asthma – Hyper‑responsive smooth muscle in bronchi/bronchioles causes excessive bronchoconstriction, raising airway resistance and producing wheeze.
• Chronic obstructive pulmonary disease (COPD) – emphysema – Destruction of elastic fibres reduces recoil, causing airway collapse and reduced expiratory flow.
• Neonatal respiratory distress syndrome – Insufficient type II pneumocyte activity → low surfactant → high surface tension → alveolar collapse.

Common Pitfalls & How to Avoid Them

• Cartilage vs. smooth muscle – Cartilage stains eosinophilic, appears as clear, regular shapes; smooth muscle is basophilic with a granular texture.
• Posterior membranous part of the trachea – Composed of smooth muscle and connective tissue, not cartilage; look for the absence of the C‑shape.
• Cilia in small airways – Distal bronchioles lose cilia; only larger bronchioles retain them.
• Type II pneumocytes – Small cuboidal cells with dark‑staining lamellar bodies; easy to miss if you focus only on the thin squamous cells.
• Labeling the basement membrane and adventitia – These layers are explicitly tested; always include them in diagrams and micrograph annotations.

Suggested Exam‑Style Questions

1. Label a transverse section of the trachea, indicating cartilage, epithelium, basement membrane, sub‑mucosal seromucous glands, smooth muscle, elastic fibres and adventitia.
2. Compare the structural adaptations of the bronchus that enable it to regulate airflow and maintain flexibility.
3. Explain why alveolar walls are extremely thin and how type I and type II pneumocytes contribute to efficient gas exchange.
4. Discuss how changes in smooth‑muscle tone affect airway resistance and relate this to a clinical condition such as asthma.
5. Describe the role of surfactant in maintaining alveolar stability and the consequences of its deficiency.

Summary

The respiratory tract shows a clear structural gradient: rigid, cartilage‑supported tubes (trachea, bronchi) transition to highly compliant, thin‑walled bronchioles and alveoli that maximise surface area for diffusion. Mastery of microscopic identification, the ability to label every histological layer (including sub‑mucosal glands), and an understanding of how each feature supports ventilation and gas exchange are essential for success in the Cambridge IGCSE/A‑Level examinations.