Mechanism of Ventilation in Mammals

Ventilation — the mechanical process of moving air in and out of the lungs — is essential for maintaining the steep concentration gradients on which gas exchange depends. Without ventilation, oxygen in the alveolar air would be rapidly depleted by the blood and carbon dioxide would accumulate, halting diffusion. Mammals use negative pressure breathing: the thoracic cavity is actively expanded, pressure inside the lungs falls below atmospheric pressure, and air flows down the pressure gradient into the lungs. This lesson examines the anatomy of the ventilation apparatus, the roles of the diaphragm and intercostal muscles, and the subtle differences between quiet and forced breathing, matching OCR A-Level Biology A specification 3.1.1 (g).

Key Definitions:

• Inspiration (inhalation) — the active phase of breathing in which air enters the lungs.
• Expiration (exhalation) — typically passive at rest; air leaves the lungs.
• Boyle's law — at constant temperature, pressure is inversely proportional to volume ($p \propto 1/V$p∝1/V).

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The Thoracic Cavity

The thoracic cavity is sealed from the abdomen below by the diaphragm, a sheet of skeletal muscle shaped like a dome. Its walls are formed by the ribcage, to which the external and internal intercostal muscles attach. The lungs themselves are enclosed in a double-walled membrane called the pleura, containing a thin film of pleural fluid that lubricates movement and couples the lung surface to the chest wall.

• Parietal pleura — outer layer, attached to the ribcage.
• Visceral pleura — inner layer, attached to the lung surface.
• Pleural cavity — the narrow, fluid-filled space between them.

Because the pleural fluid is incompressible, expansion of the thoracic wall automatically pulls the lung surface outwards with it — this is the basis of negative-pressure breathing.

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Inspiration (Active)

Inspiration is an active, energy-requiring process driven by muscle contraction.

1. External intercostal muscles contract — they run downwards and forwards between the ribs. When they shorten, they pull the ribs upwards and outwards, rather like a bucket handle being lifted.
2. Diaphragm contracts — it flattens from its dome shape downwards.
3. These combined movements increase the volume of the thoracic cavity.
4. By Boyle's law, an increase in volume causes a decrease in pressure inside the thorax (and therefore inside the lungs), to just below atmospheric pressure.
5. Air flows down the pressure gradient from the atmosphere into the lungs, inflating them.

flowchart LR
    A[External intercostals contract] --> C[Thoracic volume increases]
    B[Diaphragm contracts and flattens] --> C
    C --> D[Intra-thoracic pressure falls]
    D --> E[Air flows into lungs down pressure gradient]

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Expiration (Passive at Rest)

At rest, expiration is largely a passive process, driven by elastic recoil.

1. External intercostal muscles relax; the ribs return downwards and inwards under gravity.
2. Diaphragm relaxes and returns to its dome shape as the elastic tissue in the lungs recoils and abdominal pressure pushes it upwards.
3. The elastic fibres in the lung tissue (abundant in alveolar walls and bronchiole walls) recoil, reducing lung volume.
4. Thoracic volume decreases, so by Boyle's law pressure inside the lungs rises above atmospheric pressure.
5. Air is pushed out of the lungs to the atmosphere.

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Forced Expiration (Active)