Part of the TeachMe Series

Mechanics of Breathing

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Original Author(s): Kamashi Pandirajan
Last updated: 24th November 2019
Revisions: 19

Original Author(s): Kamashi Pandirajan
Last updated: 24th November 2019
Revisions: 19

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The processes of inspiration and expiration are vital for providing oxygen to tissues and removing carbon dioxide from the body. Inspiration occurs via active contraction of muscles such as the diaphragm – whereas expiration tends to be passive.

In this article, we shall look at the physiology of ventilation – the process of inspiration and expiration and rest, during forced breathing and their clinical correlations.

The Lungs and Breathing

The space between the outer surface of the lungs and inner thoracic wall is known as the pleural space. This is usually filled with pleural fluid, forming a seal which holds the lungs against the thoracic wall. This seal ensures that when the thoracic cavity expands or reduces, the lungs undergo expansion or reduction in size accordingly.

During breathing, the contraction and relaxation of muscles acts to change the volume of the thoracic cavity – thereby altering the volume of the lungs, and changing the pressure inside the lungs.

Boyle’s law states that the volume of gas is inversely proportional to pressure (when temperature is constant). Therefore:

  • When the volume of the thoracic cavity increases – the volume of the lungs increases and the pressure within the lungs decreases.
  • When the volume of the thoracic cavity decreases – the volume of the lungs decreases and the pressure within the lungs increases.

Fig 1 – Demonstration of Boyle’s law: an increase in volume results in a decrease in pressure.

Process of Inspiration

Inspiration is the phase of ventilation in which air enters the lungs. It is initiated by contraction of the inspiratory muscles:

  • Diaphragm – flattens, extending the superior/inferior dimension of the thoracic cavity.
  • External intercostal muscles – elevates the ribs and sternum, extending the anterior/posterior dimension of the thoracic cavity.

The action of the inspiratory muscles results in an increase in the volume of the thoracic cavity. As the lungs are held against the inner thoracic wall by the pleural seal, they also undergo an increase in volume.

As per Boyle’s law, an increase in lung volume results in a decrease in the pressure within the lungs. The pressure outside the lungs is now higher – and so air rushes into the lungs, moving down the pressure gradient.

Fig 2 – Diagram showing the process of inspiration and expiration at rest.

Process of Passive Expiration

Expiration is the phase of ventilation in which air is expelled from the lungs. It is initiated by relaxation of the inspiratory muscles:

  • Diaphragm – returns to resting position, reducing the superior/inferior dimension of the thoracic cavity.
  • External intercostal muscles – relaxes to depress the ribs and sternum, reducing the anterior/posterior dimension of the thoracic cavity.

The relaxation of the inspiratory muscles results in a decrease in the volume of the thoracic cavity. The elastic recoil of the previously expanded lung tissue allows them to return to their original size.

As per Boyle’s law, a decrease in lung volume results in an increase in the pressure within the lungs. The pressure inside the lungs is now higher than outside – and so air moves out of the lungs, down the pressure gradient.

Forced Breathing

Forced breathing is an active mode of breathing which utilises additional muscles to rapidly expand and contract the thoracic cavity volume. It most commonly occurs during exercise.

Active Inspiration

Active inspiration involves the contraction of the accessory muscles of breathing (in addition to the diaphragm and external intercostals). All these muscles act to increase the volume of the thoracic cavity:

  • Scalenes – elevates the upper ribs.
  • Sternocleidomastoid – elevates the sternum.
  • Pectoralis major and minor – pulls ribs outwards.
  • Serratus anterior – elevates the ribs (when the scapulae are fixed).
  • Latissimus dorsi – elevates the lower ribs.

Active Expiration

Active expiration utilises the contraction of several thoracic and abdominal muscles. These muscles act the decrease the volume of the thoracic cavity:

  • Anterolateral abdominal wall – increases the intra-abdominal pressure, pushing the diaphragm upwards into the thoracic cavity.
  • Internal intercostal – depresses the ribs.
  • Innermost intercostal – depresses the ribs.

Fig 3 – The muscles of the anterolateral wall are utilised in forced expiration.

Clinical Relevance: Diaphragmatic Paralysis

The phrenic nerve provides motor innervation to the diaphragm. If the nerve becomes damaged, paralysis of the diaphragm can result. Causes of phrenic nerve palsy include:

  • Mechanical trauma – ligation or damage to the nerve during surgery.
  • Compression – due to a tumour within the chest cavity.
  • Neuropathies – such diabetic neuropathy.

Paralysis of the diaphragm produces a paradoxical movement. The affected side of the diaphragm moves upwards during inspiration, and downwards during expiration. A unilateral diaphragmatic paralysis is usually asymptomatic and is most often an incidental finding on x-ray. If both sides are paralysed, the patient may experience poor exercise tolerance, orthopnoea and fatigue. Lung function tests will show a restrictive deficit.

Management of diaphragmatic paralysis is two-fold. Firstly, the underlying cause must be identified and treated (if possible). The second part of treatment deals with symptomatic relief. This is usually via non-invasive ventilation, such as a CPAP (continuous positive airway pressure).

Fig 3 – Chest X-Ray showing paralysis of the right hemidiaphragm.