Mechanics of Breathing

The Thoracic Wall, Pleura, and Lungs

The space between the outer surface of the lungs and inner thoracic wall is known as the pleural space. It contains a thin film of pleural fluid, the surface tension of which holds the visceral and parietal layers of the pleura closely apposed.

This surface tension creates a sealed interface between the lungs and the thoracic wall. Because the two pleural layers cannot separate with one layer adherent to the thoracic wall and the other adherent to the lung surface, any movement of the thoracic wall is directly transmitted to the lungs.

Thus, when the respiratory muscles contract or relax, the thoracic wall expands and recoils respectively. This movement of the thoracic wall is transmitted via the sealed pleural interface, causing the lungs to expand and recoil in turn.

OpenStax College, CC BY 3.0 https://creativecommons.org/licenses/by/3.0, via Wikimedia Commons

Figure 1
The relationship between the lungs, the pleurae (visceral and parietal), the pleural space and the thoracic wall.

Intrapulmonary Pressure and Air Flow

When the thoracic cavity expands or contracts, the lungs change in volume accordingly. This alters the pressure inside the lungs, known as intra-pulmonary pressure (equivalent to intra-alveolar pressure).

Boyle’s law states that the volume of gas is inversely proportional to its pressure, at constant temperature. Therefore:

• As thoracic volume increases, intra-pulmonary pressure falls and air moves into the lungs

• As thoracic volume decreases, intra-pulmonary pressure rises and air moves out of the lungs

Adapted from OpenStax College, CC BY 3.0 <https://creativecommons.org/licenses/by/3.0>, via Wikimedia Commons

Figure 2
Demonstration of Boyle’s law: when volume increases, pressure decreases and vice versa.

Intrapleural Pressure

Intrapleural pressure refers to the pressure within the pleural space. It is normally negative relative to atmospheric pressure, because the elastic recoil of the lungs and chest wall act in opposing directions – the lungs tend to recoil inwards while the chest wall tends to spring outwards.

During inspiration, expansion of the thoracic cavity causes intrapleural pressure to become more negative. This increases the transpulmonary pressure (TPP) – the difference between intra-pulmonary and intrapleural pressure.

TPP is the distending force that keeps the lungs expanded. As TPP increases during inspiration, the lungs expand. During passive expiration, intrapleural pressure becomes less negative as the thoracic cavity recoils. Therefore, TPP falls and the lungs return to their resting volume, known as the functional residual capacity (FRC).

Inspiration

During the inspiratory phase of ventilation, air enters the lungs, initiated by the contraction of inspiratory muscles:

• Diaphragm – flattens downwards, increasing the superior-inferior (vertical) diameter of the thoracic cavity
• External intercostal muscles – elevates the ribs and sternum, increasing the anteroposterior (horizontal) diameter of the thoracic cavity

As the inspiratory muscles contract, the chest wall movement is transmitted to the lungs via the pleural seal which holds the structures together and the rise in TPP which drives the lungs to expand, consistent with the pleural mechanics described above.

In line with Boyle’s law, as lung volume increases, intra-pulmonary pressure falls below atmospheric pressure. Air is then driven into the lungs down a pressure gradient.

Created in BioRender

Figure 3
Changes in diaphragm position (black arrows) and external intercostal muscle activity (blue arrows) during inspiration (left) and passive expiration (right).

Passive Expiration

During the expiratory phase of ventilation, air is expelled from the lungs. In quiet breathing, expiration is considered passive as it occurs primarily through relaxation of the inspiratory muscles:

• Diaphragm – returns to its resting dome shape, reducing the superior-inferior diameter of the thoracic cavity
• External intercostal muscles – depresses the ribs and sternum, reducing the anteroposterior diameter of the thoracic cavity

As the inspiratory muscles relax, thoracic volume decreases. The elastic recoil of the lung tissue allows it to passively return to its resting size, without any active muscular effort.

As lung volume decreases, intra-pulmonary pressure rises above atmospheric pressure, driving air out of the lungs down a pressure gradient.

At the end of quiet expiration, intra-pulmonary pressure equalises with atmospheric pressure and airflow (temporarily) ceases. The lungs are now at their FRC.

Forced Breathing

Forced breathing is an active form of ventilation that recruits accessory muscles to rapidly increase airflow. It commonly occurs during exercise or respiratory distress.

Active Inspiration

During active (forced) inspiration, accessory muscles assist the diaphragm and external intercostals to increase the volume of the thoracic cavity. These include:

• Scalenes – elevates the upper ribs
• Sternocleidomastoid – elevates the sternum
• Pectoralis major and minor – lifts the rib cage (when the upper limb is fixed)
• Serratus anterior – elevates the ribs (when the scapulae are fixed)
• Latissimus dorsi – elevates the lower ribs

Active Expiration

During active (forced) expiration, thoracic and abdominal muscles are recruited to decrease the volume of the thoracic cavity:

• Anterolateral abdominal wall – increases intra-abdominal pressure, pushing the diaphragm upwards into the thoracic cavity
• Internal and innermost intercostals – depresses the ribs