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Mechanics of Breathing – Respiratory Physiology

The mechanics of breathing describes the physical and muscular processes that enable inhalation and exhalation. It is the foundation of pulmonary ventilation.

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Things worth knowing about "Mechanics of Breathing"

The mechanics of breathing describes the physical and muscular processes that enable inhalation and exhalation. It is the foundation of pulmonary ventilation.

What Are the Mechanics of Breathing?

The mechanics of breathing refers to the physical and muscular processes that drive inspiration (inhalation) and expiration (exhalation). These processes form the physiological basis of external respiration, ensuring adequate oxygen supply to the body and the removal of carbon dioxide.

Basic Principle

Breathing mechanics are governed by the physical principle that air flows from areas of higher pressure to areas of lower pressure. By contracting and relaxing specific muscles, the volume of the chest cavity changes, creating pressure differences between the outside air and the tiny air sacs in the lungs, known as alveoli. These pressure differences drive airflow in and out of the lungs.

Inspiration (Inhalation)

Inhalation is an active process requiring muscular effort. The primary muscle of breathing is the diaphragm, a dome-shaped muscle that separates the chest from the abdominal cavity. When the diaphragm contracts, it flattens and moves downward, increasing the volume of the thoracic cavity. At the same time, the external intercostal muscles lift the ribcage outward and upward.

Lung volume increases.
Alveolar pressure drops below atmospheric pressure.
Air is drawn into the lungs through the nose or mouth, down the trachea and bronchi.

During intense activity or labored breathing, accessory breathing muscles such as the sternocleidomastoid and scalene muscles provide additional support.

Expiration (Exhalation)

Quiet exhalation is largely a passive process, driven by the natural elastic recoil of the lungs and chest wall returning to their resting state after being stretched.

The diaphragm and intercostal muscles relax.
Thoracic volume decreases.
Alveolar pressure rises above atmospheric pressure.
Air is expelled from the lungs.

During forced expiration -- such as coughing, sneezing, or vigorous exercise -- the internal intercostal muscles and abdominal muscles actively contract to accelerate airflow out of the lungs.

Pressure Relationships and Compliance

Two key pressure values are essential to understanding breathing mechanics:

Alveolar pressure (P_alv): The pressure inside the alveoli, which fluctuates rhythmically around atmospheric pressure (used as a reference of 0 mmHg) during the breathing cycle.
Pleural pressure (P_pl): The pressure within the pleural space -- the thin gap between the lung surface and the chest wall. It is always slightly negative, keeping the lungs partially expanded and preventing collapse.

Compliance measures the distensibility (stretchability) of the lungs and chest wall. High compliance means the lungs expand easily. Compliance is reduced in conditions such as pulmonary fibrosis and increased in emphysema.

Airway Resistance

Alongside compliance, airway resistance plays a critical role in breathing mechanics. It refers to the opposition that the airways present to the flow of air. Increased resistance, as seen in conditions like asthma or chronic obstructive pulmonary disease (COPD), makes breathing more difficult -- particularly exhalation.

Clinical Relevance

Disruptions to normal breathing mechanics can significantly impair lung function. Key clinical conditions include:

Restrictive ventilatory disorders: Reduced lung expansion due to conditions such as pulmonary fibrosis or pleural effusion.
Obstructive ventilatory disorders: Narrowing of the airways, as seen in asthma or COPD.
Pneumothorax: Air entering the pleural space, disrupting the negative pressure balance and causing lung collapse.

Assessing breathing mechanics is a core component of pulmonary function testing, including spirometry and body plethysmography.

References

West, J. B.: Respiratory Physiology -- The Essentials. 9th Edition. Lippincott Williams & Wilkins, Philadelphia, 2012.
Guyton, A. C., Hall, J. E.: Textbook of Medical Physiology. 13th Edition. Elsevier, Philadelphia, 2016.
World Health Organization (WHO): Chronic respiratory diseases. Available at: https://www.who.int/health-topics/chronic-respiratory-diseases

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