3.2.6 Ventilation & Gas Exchange

Inspiration (Inhalation):

1. Muscle Actions:

• External intercostal muscles contract, lifting the rib cage upwards and outwards.
• Internal intercostal muscles relax.
• The diaphragm contracts and flattens.

2. Volume and Pressure Changes:

• The volume of the thorax increases.
• This causes the pressure inside the thorax to decrease, creating a pressure gradient.

3. Air Movement:

• Air is drawn into the lungs as the pressure inside the lungs is lower than the atmospheric pressure.

Expiration (Exhalation):

1. Muscle Actions:

• Internal intercostal muscles contract, pulling the rib cage downwards and inwards.
• External intercostal muscles relax.
• The diaphragm relaxes, returning to its domed shape.

2. Volume and Pressure Changes:

• The volume of the thorax decreases.
• This increases the pressure inside the thorax, forcing air out of the lungs.

3. Air Movement:

• Air is expelled as the pressure inside the lungs is higher than the atmospheric pressure.

Key Differences Between Inspiration and Expiration:

Feature	Inspiration	Expiration
Rib Movement	Ribs move up and out.	Ribs move down and in.
Diaphragm	Contracts and flattens.	Relaxes and domes upwards.
Volume of Thorax	Increases.	Decreases.
Pressure in Thorax	Decreases (below atmospheric pressure).	Increases (above atmospheric pressure).
Air Movement	Air flows into the lungs.	Air flows out of the lungs.

Gas Exchange in the Alveoli:

Gas exchange occurs by diffusion across the thin alveolar and capillary walls:

4. Oxygen:

• Diffuses from the alveoli (high oxygen concentration) into the blood (low oxygen concentration).

5. Carbon Dioxide:

• Diffuses from the blood (high carbon dioxide concentration) into the alveoli (low carbon dioxide concentration), to be exhaled.

Ventilation and Gas Exchange Efficiency:

1. Steep Concentration Gradient:

• Ventilation replenishes oxygen in the alveoli and removes carbon dioxide, maintaining a steep diffusion gradient.

2. Large Surface Area:

• Provided by millions of alveoli for maximum gas exchange.

3. Thin Exchange Surface:

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

4. Good Blood Supply:

• Capillaries surrounding the alveoli transport oxygen away and bring carbon dioxide, maintaining the gradient.