3.2.3 Looking at the Gas Exchange under the Microscope

Preparing a Microscope Slide:

1. Sample Preparation:

• Use prepared lung tissue slides or dissect a lung to obtain a thin tissue sample.
• Place the sample on a microscope slide.

2. Staining:

• Apply a stain such as haematoxylin and eosin (H&E) to highlight different cell types:
• Haematoxylin stains nuclei (purple/blue).
• Eosin stains cytoplasm and connective tissues (pink).
• Carefully mount the sample with a coverslip to avoid air bubbles.

3. Microscope Setup:

• Place the slide on the stage and use the lowest magnification objective to locate the sample.
• Adjust focus and increase magnification to observe details.

Structures to Observe:

1. Trachea:

• Ciliated Epithelium:
• Cells with cilia that move mucus and trapped particles.
• Goblet Cells:
• Secrete mucus to trap dust and microorganisms.
• Cartilage Rings:
• Seen as incomplete C-shaped structures, providing support.

2. Bronchi and Bronchioles:

• Smooth Muscle:
• Present in the walls, allowing the airways to constrict or dilate.
• Elastic Fibres:
• Provide flexibility, allowing bronchioles to expand and recoil.

3. Alveoli:

• Thin Squamous Epithelium:
• One cell thick to reduce diffusion distance for gases.
• Capillaries:
• Surround the alveoli, forming a dense network to maintain a steep concentration gradient.
• Elastic Fibres:
• Allow the alveoli to stretch during inhalation and recoil during exhalation.

Observations Under the Microscope:

1. Trachea:

• Lining shows cilia and goblet cells.
• Thick walls with cartilage and layers of connective tissue.

2. Bronchioles:

• Narrower than bronchi, with visible smooth muscle and fewer cartilage structures.

3. Alveoli:

• Appear as honeycomb-like structures.
• Thin walls and close association with capillaries are visible.

Adaptations Seen at the Cellular Level:

1. Thin Walls:

• Squamous epithelium in alveoli reduces diffusion distance for oxygen and carbon dioxide.

2. Large Surface Area:

• Numerous alveoli ensure maximum gas exchange efficiency.

3. Cilia and Goblet Cells:

• Protect the lungs by trapping and moving particles out of the respiratory tract.

4. Elastic Fibres:

• Found in alveoli and bronchioles, enabling expansion and recoil.

Practical Tips:

• Use appropriate staining to distinguish structures clearly.
• Start with low magnification to locate key structures and switch to higher magnifications for detailed observation.
• Compare the observed structures with textbook diagrams for better understanding.

Key Learning Points:

1. Trachea:

• Lined with ciliated epithelium and goblet cells, supported by cartilage rings.

2. Bronchioles:

• Contain smooth muscle and elastic fibres for flexibility.

3. Alveoli:

• Specialised for gas exchange with thin walls, large surface area, and dense capillary networks.