The Mechanics of Breathing: Ventilation and Pressure Changes
Ventilation, the movement of air into and out of the lungs, relies on precise pressure gradients created by the diaphragm, intercostal muscles, lung elasticity, and pleural membranes. These mechanical processes ensure that oxygen continuously enters the alveoli and carbon dioxide is removed. When ventilation fails, gas exchange collapses rapidly, making the understanding of respiratory mechanics foundational to all clinical respiratory assessment.
What You Need to Know
Breathing occurs because of pressure differences between the atmosphere, the lungs, and the pleural cavity. During inspiration, the diaphragm contracts and flattens while the external intercostal muscles lift and expand the rib cage. This increases the volume of the thoracic cavity, which lowers intrapulmonary pressure below atmospheric pressure, allowing air to flow into the lungs. At the same time, the lungs expand as they are pulled outward by the chest wall through the pleural membranes.
During expiration, the diaphragm and external intercostal muscles relax and the elastic tissues of the lungs recoil, decreasing thoracic volume. This raises intrapulmonary pressure above atmospheric pressure and drives air out of the lungs. Under resting conditions, expiration is passive and does not require active muscle contraction, although forced breathing can recruit additional muscles to accelerate airflow.
Breathing depends on three linked pressure relationships:
atmospheric pressure outside the body
intrapulmonary pressure within the airways and alveoli
intrapleural pressure within the pleural cavity
The lungs remain inflated because intrapleural pressure is always slightly negative relative to atmospheric pressure. This negative pressure creates a suction effect that keeps the visceral pleura attached to the parietal pleura and therefore to the chest wall. The thin film of pleural fluid between these layers allows smooth, frictionless movement during breathing while also transmitting the mechanical forces that expand and deflate the lungs.
If this pressure relationship is disrupted, such as when air enters the pleural space during a pneumothorax, the lung can collapse because it is no longer held open by negative intrapleural pressure. This highlights why the integrity of the pleural cavity is essential for effective ventilation.
Beyond the Basics
Lung Compliance and Pressure–Volume Relationships
Lung compliance describes the relationship between changes in transpulmonary pressure and changes in lung volume. Highly compliant lungs expand easily with minimal pressure, whereas poorly compliant lungs require greater pressure to achieve the same volume change. This pressure–volume relationship is central to the mechanics of ventilation.
In conditions such as emphysema, destruction of alveolar walls reduces elastic recoil, resulting in increased compliance. While the lungs inflate readily, expiration becomes inefficient, leading to air trapping and increased work of breathing. In contrast, disorders such as pulmonary fibrosis, pulmonary oedema, and acute respiratory distress syndrome reduce compliance by increasing lung stiffness. In these conditions, greater negative intrapleural pressure must be generated during inspiration, substantially increasing the mechanical workload on the respiratory muscles.
Airway Resistance and Airflow Dynamics
Airway resistance reflects the opposition to airflow within the respiratory tract and is strongly influenced by airway radius. Because resistance is inversely proportional to the fourth power of airway radius, even minor narrowing of bronchioles produces a dramatic increase in resistance. This relationship explains why conditions affecting small airways have profound effects on ventilation.
Under normal conditions, airflow through larger airways is turbulent, while flow through distal bronchioles becomes increasingly laminar as total cross-sectional area increases. When airway narrowing occurs, higher pressure gradients are required to maintain airflow, increasing the work of breathing. In obstructive conditions such as asthma, smooth muscle contraction, mucosal oedema, and mucus accumulation combine to elevate resistance, significantly impairing ventilation despite relatively small anatomical changes.
Surface Tension and Alveolar Stability
Surface tension within the alveoli acts to resist expansion and promote collapse, particularly at low lung volumes. Pulmonary surfactant plays a crucial mechanical role by reducing surface tension, thereby decreasing the pressure required to inflate alveoli and stabilising alveoli of different sizes.
Without adequate surfactant, alveoli collapse at end-expiration and require disproportionately high inspiratory pressures to reopen with each breath. This phenomenon increases the work of breathing and reduces ventilated surface area. The mechanical consequences of surfactant deficiency highlight its importance in maintaining efficient ventilation and minimising pressure fluctuations during the respiratory cycle.
Ventilation–Perfusion Relationships and Gas Exchange Efficiency
Ventilation alone does not guarantee effective gas exchange; adequate perfusion must be present at ventilated alveoli. The ventilation–perfusion (V/Q) relationship describes the matching of airflow to pulmonary blood flow and is essential for optimal oxygen uptake and carbon dioxide elimination.
When ventilation exceeds perfusion, as occurs distal to a pulmonary embolism, alveoli are ventilated but not perfused, creating regions of physiological dead space. Conversely, when perfusion exceeds ventilation, such as in pneumonia or atelectasis, blood passes through poorly ventilated alveoli, resulting in shunt-like physiology. In both scenarios, oxygenation is impaired despite intact respiratory mechanics, underscoring the integrated relationship between ventilation, pressure changes, and pulmonary circulation.
Integration of Mechanical Factors in Ventilation
Effective ventilation depends on the coordinated interaction between lung compliance, airway resistance, surface tension, and perfusion. Alterations in any one of these factors disrupt normal pressure gradients and increase the mechanical workload required to move air in and out of the lungs.
Understanding these mechanical principles provides the foundation for interpreting clinical signs such as increased work of breathing, hypoxaemia, and respiratory fatigue. More importantly, it explains why seemingly small changes in lung structure or airway calibre can lead to significant ventilatory compromise.
Clinical Connections
Disorders that disrupt normal pressure relationships or lung mechanics quickly compromise ventilation. In pneumothorax, air enters the pleural space and abolishes negative intrapleural pressure, allowing the lung to recoil inward and collapse. This reduces the surface area available for gas exchange and causes sudden breathlessness, chest pain, and asymmetrical chest movement. In asthma, airway narrowing from smooth muscle contraction and mucosal swelling greatly increases resistance, making it harder to move air into and out of the lungs. In COPD, destruction of elastic lung tissue leads to air trapping, hyperinflation, and difficulty fully exhaling, which increases the work of breathing and contributes to fatigue and hypoxia.
Mechanical and neuromuscular factors can impair ventilation even when the lungs themselves are structurally normal. Weakness, pain, or altered chest wall mechanics reduce the ability to generate the pressure changes required for effective breathing.
Common causes of impaired ventilation include:
respiratory muscle fatigue or neuromuscular disease
obesity limiting chest wall expansion
rib fractures or chest trauma
postoperative pain or splinting
spinal cord or brainstem injury
Assessing ventilation is achieved by observing the effort and pattern of breathing rather than relying solely on oxygen saturation. Increased respiratory rate, use of accessory muscles in the neck or chest, nasal flaring, or paradoxical chest movements indicate rising work of breathing. Reduced chest expansion, shallow breaths, or declining alertness may signal impending respiratory failure. Early recognition allows timely escalation to oxygen therapy, bronchodilators, non-invasive ventilation, or airway support before critical deterioration occurs.
Concept Check
Why does air move into the lungs during inspiration?
How does surfactant influence alveolar stability?
What conditions reduce lung compliance?
Why do small airway diameter changes dramatically affect airflow resistance?
What is V/Q mismatch, and why does it impair oxygenation?