Lung Volumes and Capacities
Lung volumes and capacities describe how much air the lungs can hold during different phases of the respiratory cycle. These measurements not only reflect the physical size of the lungs but also the flexibility of the chest wall, the strength of respiratory muscles, and the integrity of lung tissue. Although breathing feels effortless and automatic, it represents a finely coordinated balance between pressure changes, muscle contraction, and elastic properties of the lungs and thoracic cage.
What You Need to Know
Each breath consists of a tidal volume- the amount of air moved during a normal, quiet breath. Beyond this lies the inspiratory reserve volume, the additional air that can be drawn into the lungs with a deep inhalation. Similarly, the expiratory reserve volume is the extra air that can be expelled during a forceful exhalation. Even after the deepest possible exhalation, a residual volume of air remains in the lungs, preventing airway collapse and maintaining continuous gas exchange between breaths. These individual volumes combine to form larger functional capacities that describe how the lungs operate as a whole.
The most clinically relevant lung capacities include:
Vital capacity: the maximum amount of air that can be exhaled after a full inspiration
Functional residual capacity: the volume of air left in the lungs after a normal exhalation
Total lung capacity: the total amount of air the lungs can hold after maximal inspiration
These capacities reflect both lung size and mechanical properties such as compliance and airway resistance, which is why they change in diseases like fibrosis, emphysema, or neuromuscular weakness.
At rest, tidal breathing uses only a small fraction of the lungs’ full capacity. Most of the available volume remains in reserve, allowing the respiratory system to respond quickly to increased metabolic demand, exercise, or illness. These reserve volumes also help keep alveoli open between breaths, improve oxygen reserves during brief apnoea, and provide the ability to generate deep breaths needed for coughing, clearing secretions, and preventing atelectasis.
Beyond the Basics
Lung Volumes as Indicators of Respiratory Mechanics
Lung volumes reflect the interaction between airway patency, lung elasticity, and chest wall mechanics. They provide important insight into how easily the lungs can be inflated and deflated, as well as how effectively air can be exchanged during breathing. Alterations in lung volumes therefore often signal underlying mechanical or structural abnormalities within the respiratory system.
In restrictive lung diseases, such as pulmonary fibrosis or conditions that limit chest wall movement like scoliosis, total lung capacity is reduced. Increased lung stiffness or restricted thoracic expansion makes lung inflation energetically costly. Patients compensate by adopting a pattern of rapid, shallow breathing, which minimises the work required for each breath but reduces tidal volume and limits effective alveolar ventilation.
Obstructive Disease and Air Trapping
Obstructive lung diseases alter lung volumes through a fundamentally different mechanism. In conditions such as asthma and emphysema, airflow limitation is most pronounced during expiration. Narrowed airways or loss of elastic recoil prevent complete emptying of the lungs, resulting in air trapping.
As residual volume increases, functional lung volumes shift upward. Over time, trapped air leads to hyperinflation, increasing total lung capacity despite compromised ventilation. Although the lungs may appear larger, much of this volume is poorly ventilated and inefficient for gas exchange. Hyperinflation also places the respiratory muscles at a mechanical disadvantage, increasing the work of breathing and contributing to dyspnoea.
Functional Residual Capacity and Oxygen Reserve
Functional residual capacity (FRC) is the volume of air remaining in the lungs at the end of a normal passive exhalation. This volume plays a critical physiological role by acting as an oxygen reservoir that buffers against fluctuations in oxygen levels between breaths.
A reduction in FRC decreases this reserve and makes patients more susceptible to hypoxaemia, particularly during apnoea or periods of increased oxygen demand. Several common physiological and clinical factors reduce FRC, including anaesthesia, obesity, pregnancy, and supine positioning. These conditions alter chest wall mechanics or diaphragmatic position, limiting lung expansion at end-expiration.
Diseases that promote alveolar collapse, such as atelectasis or pulmonary oedema, further reduce FRC by eliminating ventilated lung units. As FRC falls, greater inspiratory effort is required to reopen collapsed alveoli, increasing the work of breathing and further impairing gas exchange.
Spirometry and Interpretation of Lung Volumes
Spirometry is the most widely used clinical tool for assessing lung function and provides indirect insight into lung volumes and mechanics. Although spirometry cannot directly measure residual volume or total lung capacity, it reveals characteristic patterns associated with obstructive and restrictive disease.
In obstructive disease, spirometry demonstrates prolonged expiration and reduced expiratory flow rates, reflecting airflow limitation and air trapping. In restrictive disease, volumes are uniformly reduced, but airflow relative to lung size may remain normal. Recognising how lung volumes change in response to mechanical constraints allows spirometry results to be interpreted in a meaningful physiological context rather than as isolated numerical values.
Integration of Lung Volumes and Breathing Efficiency
Normal lung volumes depend on a balance between lung elasticity, airway resistance, chest wall compliance, and respiratory muscle function. Disruption in any of these components alters the distribution of lung volumes and increases the energetic cost of breathing.
Understanding how lung volumes shift in health and disease provides a foundation for interpreting symptoms such as dyspnoea, predicting vulnerability to hypoxaemia, and guiding clinical interventions aimed at improving ventilation and gas exchange.
Clinical Connections
Lung volumes play a crucial role in the presentation and management of respiratory diseases. In asthma, narrowing of the airways traps air, increasing residual volume and giving the lungs a hyperinflated appearance. Patients may report difficulty exhaling and feeling “full of air,” even though oxygenation may be adequate early in an exacerbation. In contrast, patients with restrictive diseases complain of an inability to take a deep breath because their total lung capacity is reduced. These individuals rely heavily on respiratory rate rather than depth of breathing, leading to rapid, shallow breaths that are inefficient for gas exchange.
Key clinical patterns to recognise:
Obstructive disease → air trapping, increased residual volume, hyperinflation
Restrictive disease → reduced total lung capacity, shallow breathing
Low functional residual capacity → rapid desaturation when oxygen supply is interrupted
Increased lung volumes → sensation of incomplete exhalation
Mechanical ventilation is also strongly influenced by lung volumes. Patients with low functional residual capacity may desaturate quickly when disconnected from the ventilator because their oxygen reserves deplete faster. Positive end-expiratory pressure (PEEP) is often used to maintain alveolar recruitment and increase functional residual capacity, improving oxygenation.
Age-related changes further alter lung volumes and respiratory reserve. Residual volume increases due to loss of elastic recoil, while vital capacity declines, even in otherwise healthy individuals. These changes reduce respiratory reserve and contribute to increased vulnerability during illness.
Concept Check
Why does restrictive lung disease reduce total lung capacity?
How does air trapping in obstructive disease affect residual volume?
Why is functional residual capacity so important during anaesthesia?
How does hyperinflation alter the mechanics of breathing?
What makes tidal volume only a small fraction of total lung capacity?