Breathing is the process by which oxygen and CO2 are transported to and from the lungs (also known as ventilation).
Relevant physics and anatomy
In order to understand the mechanics of breathing it is important to be aware of the physics and anatomy that underpin this process.
Boyle’s law helps us to understand the mechanics of breathing.
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).
The key anatomicalstructures involved in ventilation include:
External intercostal muscles
Inspiration is the process of transportingairinto the lungs.
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 thoraciccavity. This increase in intra-thoracic volume causes a reduction in intra-alveolar pressure, which in turn allows air to move into the lungs along a negative pressure gradient.
Expiration is the process of transporting air out of the lungs.
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 a negative pressure gradient.
Activebreathing involves the recruitment of accessorymuscles to assist with ventilation, allowing the thoracic cavity to expand and contractmorerapidly. Active breathing most commonly occurs during exertion (e.g. exercise), however active breathing also occurs in diseasestates (e.g. acute asthma attack).
The accessorymuscles recruited during activerespiration 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.
Clinical relevance: Bowel obstruction
Why do some surgical patients who are presenting with bowelobstruction (Figure 4) find it difficulttobreathe? 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 their inability to inspire effectively. Decompression of the bowel via a nasogastric tube can significantly improve the mechanics of breathing in this situation.
Clinical relevance: Neuromuscular disease
Degenerative neuromusculardiseases such as motor neurone disease can result in inadequateventilation, due to weakness of the respiratory muscles such as the intercostal muscles and diaphragm.
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 oxygenation, 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, patients with advanced neuromuscular disease often require ventilatorysupport.
Ventilation is the process by which oxygen and CO2 are transported to and from the lungs.
The physics of breathing are based on Boyle’slaw.
The keyanatomicalstructures involved in ventilation include the lungs, diaphragm, ribcage and externalintercostalmuscles.
Activebreathing involves the recruitment of accessorymuscles to assist with ventilation, allowing the thoracic cavity to expand and contractmorerapidly.
OpenStax College. Boyle’s Law. Licence: [CC BY 3.0]. Available from: [LINK].
OpenStax College. Mechanism of Breathing. Licence: [CC BY 3.0]. Available from: [LINK].
OpenStax College. Muscles of the anterior-lateral abdominal wall diagram. Licence: [CC BY 3.0]. Available from: [LINK].
James Heilman, MD. Large bowel obstruction. Licence: [CC BY-SA 3.0]. Available from: [LINK].