3.2.1 The Human Gas Exchange System

Structure of the Human Gas Exchange System:

1. Lungs:

• Pair of lobed structures with a large surface area, allowing for efficient gas exchange.
• Enclosed in the thoracic cavity and protected by the rib cage.
• Lubricating fluid reduces friction between the lungs and rib cage during breathing.

2. Rib Cage:

• Surrounds the lungs and provides protection.
• External intercostal muscles contract to lift the rib cage during inhalation.
• Internal intercostal muscles contract to lower the rib cage during exhalation.

3. Diaphragm:

• A sheet of muscle separating the thoracic cavity from the abdominal cavity.
• Contracts and flattens during inhalation, increasing lung volume and reducing pressure.

Pathway of Air:

1. Nasal Cavity:

• Air is filtered, warmed, and moistened before entering the lungs.

2. Trachea:

• A wide tube supported by C-shaped rings of cartilage, preventing collapse during inhalation.
• Ciliated epithelium and goblet cells line the trachea, trapping and moving mucus with debris away from the lungs.

3. Bronchi:

• The trachea divides into two bronchi, each leading to a lung.
• Similar structure to the trachea but narrower.

4. Bronchioles:

• Smaller airways branching from the bronchi.
• Walls contain smooth muscle and elastic fibres; the larger bronchioles have some cartilage.
• End in alveolar clusters, where gas exchange occurs.

Gas Exchange in Alveoli:

Alveoli are tiny air sacs adapted for efficient gas exchange:

5. Thin Walls:

• One cell thick (squamous epithelium), reducing the diffusion pathway.

6. Capillary Network:

• Surrounding capillaries are also one cell thick, further reducing the diffusion distance.

7. Large Surface Area:

• Approximately 300 million alveoli collectively provide a surface area of ~70 m².

8. Steep Concentration Gradient:

• Maintained by a constant blood flow in capillaries and ventilation of the lungs.

9. Elastic Fibres:

• Allow alveoli to stretch and recoil during breathing, helping to expel air effectively.

Role of Cartilage, Smooth Muscle, and Elastic Fibres:

1. Cartilage:

• Supports the trachea and bronchi, keeping airways open.
• The C-shaped rings in the trachea allow the oesophagus to expand during swallowing.

2. Smooth Muscle:

• Found in the bronchioles.
• Can contract to restrict airflow, controlling air movement in certain conditions.

3. Elastic Fibres:

• Present in the alveoli and bronchioles.
• Allow for expansion and recoil during inhalation and exhalation.

Key Features of the Gas Exchange System:

1. Ventilation:

• Inhalation increases lung volume and decreases pressure, drawing air in.
• Exhalation reduces lung volume and increases pressure, pushing air out.

2. Efficient Diffusion:

• Thin barriers (alveoli and capillaries).
• Large surface area provided by numerous alveoli.
• Steep concentration gradient maintained by ventilation and blood flow.

Summary of Adaptations:

Feature	Adaptation	Function
Large Surface Area	Numerous alveoli (~300 million).	Increases the area for gas exchange.
Thin Barriers	Alveolar walls and capillaries are one cell thick.	Reduces the diffusion distance for gases.
Steep Gradient	Maintained by blood flow and ventilation.	Ensures efficient diffusion of gases.
Cartilage	C-shaped rings in trachea.	Keeps airways open during breathing.
Elastic Fibres	In alveoli and bronchioles.	Allow for stretching and recoil.