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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).
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 anatomical structures involved in ventilation include:
- Rib cage
- External intercostal muscles
Inspiration is the process of transporting air into 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 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 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.
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.
Clinical relevance: 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 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 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.
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 ventilatory support.
- 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.
- 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].