3.2.5 The Alveolar Epithelium

Structure of the Alveolar Epithelium:

1. Alveoli:

• Small, air-filled sacs located at the ends of bronchioles.
• Approximately 300 million alveoli in the lungs, providing a large surface area (~70 $m²$) for gas exchange.

2. Epithelium:

• The walls of alveoli are made of squamous epithelial cells, which are one cell thick, reducing the diffusion distance for gases.

3. Capillaries:

• Each alveolus is surrounded by a dense network of capillaries, also lined with a single layer of endothelial cells.
• The close association between alveoli and capillaries ensures a short diffusion pathway.

4. Elastic Fibres:

• Present in the walls of alveoli.
• Allow the alveoli to stretch during inhalation and recoil during exhalation, helping to expel air effectively.

Adaptations for Efficient Gas Exchange:

1. Large Surface Area:

• The large number of alveoli provides a vast surface for gas exchange.

2. Thin Barriers:

• Both the alveolar and capillary walls are one cell thick, minimising the diffusion distance.

3. Steep Concentration Gradient:

• Maintained by:
• Ventilation: Ensures oxygen-rich air is brought into the lungs and carbon dioxide is removed.
• Blood Flow: Continuously transports oxygen away from and carbon dioxide to the alveoli.

4. Moist Surface:

• The alveoli are lined with a thin layer of water to dissolve gases, facilitating their diffusion.

5. Elastic Recoil:

• Elastic fibres in the alveolar walls help maintain airflow and prevent collapse.

Role in Gas Exchange:

1. Oxygen Diffusion:

• Oxygen diffuses from the alveoli (high concentration) into the blood in capillaries (low concentration).

2. Carbon Dioxide Diffusion:

• Carbon dioxide diffuses from the blood in capillaries (high concentration) into the alveoli (low concentration), where it is exhaled.

Diseases Affecting the Alveolar Epithelium:

1. Emphysema:

• Destruction of alveolar walls reduces surface area, impairing gas exchange.

2. Pulmonary Fibrosis:

• Scarring thickens the alveolar walls, increasing the diffusion distance and reducing efficiency.

3. Pneumonia:

• Fluid accumulation in alveoli increases the diffusion distance, making gas exchange less effective.

Key Features of the Alveolar Epithelium:

Feature	Adaptation	Function
Thin walls	Squamous epithelial cells, one cell thick.	Reduces diffusion distance.
Large surface area	Many alveoli (~300 million).	Maximises gas exchange.
Close capillary network	Dense capillary network surrounding alveoli.	Maintains steep concentration gradient.
Elastic fibres	Allow alveoli to stretch and recoil.	Aids ventilation and prevents collapse.
Moist lining	Thin film of water inside alveoli.	Facilitates diffusion of gases.