Mechanics of Breathing

This article provides an overview of the mechanics of breathing, including the stages of ventilation, relevant anatomy and associated pathology.


What is Breathing?

Breathing is the process by which oxygen and CO2 are transported to and from the lungs (also known as ventilation).


The Mechanics of Breathing 

In order to understand the mechanics of breathing it is important to be aware of the physics and anatomy that underpin this process.

Physics

Boyle’s law helps us to understand the mechanics of breathing.

Boyle’s law

At constant temperature, the volume of a gas is inversely proportional to pressure.

As the volume of a container of a gas decreases, the pressure increases. This is demonstrated by the image below (Figure 1).

Boyle's law
Figure 1. Boyle’s law 1

 

Relevant anatomy

The key anatomical structures involved in ventilation include:

  • Lungs
  • Diaphragm
  • Rib cage
  • External intercostal muscles

 

Inspiration and expiration
Figure 2. Demonstration of the anatomical structures involved with ventilation. 2

 


Inspiration

Inspiration is the process of transporting air into the lungs.

During inspiration:

  • the diaphragm contracts, allowing the lung bases to expand
  • the external intercostal muscles contract to move the rib cage and sternum upwards and outwards

These actions significantly increase the volume of the thoracic cavity. This increase in intra-thoracic volume causes a reduction in intra-alveolar pressure, which in turn allows air to move into the lungs along the negative pressure gradient.


Passive Expiration

Expiration is the process of transporting air out of the lungs.

During expiration:

  • the diaphragm relaxes and flattens, reducing the volume of the lung bases
  • elastic recoil causes the chest wall to move downwards and inwards

These actions significantly decrease the volume of the thoracic cavity. This decrease in intra-thoracic volume causes an increase in intra-alveolar pressure, forcing air to move out of the lungs and into the atmosphere along the negative pressure gradient.


Active Breathing

Active breathing involves the recruitment of accessory muscles to assist with ventilation, allowing the thoracic cavity to expand and contract more rapidly. Active breathing most commonly occurs during exertion (e.g. exercise), however active breathing also occurs in disease states (e.g. acute asthma attack).

The accessory muscles recruited during active respiration include:

  • Sternocleidomastoid – assists with the elevation of the sternum
  • Pectoralis major and minor – assist with the expansion of the ribs laterally
  • Serratus anterior – assists with the elevation of the ribcage
  • Latissimus dorsi – assists with the elevation of the lower ribcage
  • Scalenes – assists with the elevation of the upper ribcage
Muscles of the abdominal wall
Figure 3. Muscles of the anterior-lateral abdominal wall 3

Relevant Pathology

Bowel obstruction

Why do some surgical patients who are presenting with bowel obstruction (Figure 4) find it difficult to breathe? Let’s go back to our principles and apply Boyle’s law.

Remember pressure is inversely related to volume. In this case, large bowel obstruction prevents the diaphragm from being able to flatten effectively, reducing intrathoracic volume generated via inspiration and in turn reducing the volume of gas that can be inspired. As a result, patients can feel short of breath due to the inability to inspire air effectively. Decompression of the bowel via a nasogastric tube can significantly improve the mechanics of breathing in this situation.

Figure 4. An abdominal x-ray demonstrating a distended large bowel 4

 

Neuromuscular disease 

Degenerative neuromuscular diseases such as motor neurone disease can result in inadequate ventilation, due to weakness of the respiratory muscles such as the intercostal muscles and diaphragm.

The weakness of these muscles:

  • reduces the intrathoracic volume able to be generated during inspiration and therefore reduces the pressure gradient for air to be drawn in from the atmosphere (this can result in inadequate levels of oxygen, known as hypoxia)
  • reduces the ability to decrease the intrathoracic volume during expiration, reducing the pressure gradient for air to be expelled from the lungs into the atmosphere (this can result in a build-up of CO2 known as hypercapnia)

As a result, these patients often need ventilatory support.


Key points

  • Ventilation is the process by which oxygen and CO2 are transported to and from the lungs.
  • The physics of breathing are based on Boyle’s law.
  • The key anatomical structures involved in ventilation include the lungs, diaphragm, rib cage and external intercostal muscles.
  • Active breathing involves the recruitment of accessory muscles to assist with ventilation, allowing the thoracic cavity to expand and contract more rapidly.

References

  1. Mechanism of Breathing by OpenStax College [CC BY 3.0]. Available from: [LINK]
  2. Muscles of the anterior-lateral abdominal wall diagram by OpenStax College [CC BY 3.0]. Available from: [LINK]
  3. Author: James Heilman, MD (Own work) – Large bowel obstruction – via Wikimedia Commons – Licence: CC BY-SA 3.0 

 

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