Lung volume is a quantitative measurement that assesses how much air the lungs can hold and move during the breathing cycle. These measurements are fundamental tools in respiratory physiology, providing a snapshot of the mechanical efficiency of the pulmonary system. Quantifying these volumes allows clinicians and researchers to understand the limits of gas exchange and the reserve capacity of the lungs. The movement of air is a function of the respiratory muscles, the flexibility of the chest wall, and the elastic properties of the lung tissue.
The Four Primary Lung Volumes
The overall volume of air the lungs can contain is subdivided into four fundamental, non-overlapping components. These primary volumes are the basic building blocks from which all other lung measurements are derived.
The Tidal Volume (\(\text{V}_\text{T}\) or \(\text{TV}\)) represents the amount of air that moves into or out of the lungs during a single, normal, quiet breath. This volume typically averages around 500 milliliters in a healthy adult and increases significantly during physical activity.
The Inspiratory Reserve Volume (\(\text{IRV}\)) is the extra amount of air that can be forcibly inhaled after a normal tidal inspiration. This volume reflects the maximum reserve capacity available for a deep breath, often around 3,000 milliliters. It is used when the body requires a greater intake of oxygen, such as during exercise.
The Expiratory Reserve Volume (\(\text{ERV}\)) is the additional volume of air that can be forcefully exhaled after a normal tidal expiration. This reserve volume typically measures about 1,200 milliliters. The \(\text{ERV}\) relies on the contraction of accessory respiratory muscles, such as the abdominal muscles, to push air out beyond the resting state.
The Residual Volume (\(\text{RV}\)) is the air that remains in the lungs even after a maximal, forced exhalation. This volume cannot be expelled and typically measures around 1,200 milliliters in adults. The presence of \(\text{RV}\) is functionally important because it prevents the small air sacs (alveoli) from completely collapsing. It also ensures that gas exchange can continue between breaths.
Calculated Lung Capacities
In pulmonary function testing, the primary volumes are combined to create four calculated lung capacities. These capacities provide a broader assessment of respiratory function by reflecting the total air available at different points in the respiratory cycle.
The Total Lung Capacity (\(\text{TLC}\)) is the sum of all four primary volumes (\(\text{RV}\), \(\text{ERV}\), \(\text{V}_\text{T}\), and \(\text{IRV}\)), representing the maximum volume of air the lungs can hold after a maximal inspiration. For an average adult male, \(\text{TLC}\) is approximately 6,000 milliliters.
The Vital Capacity (\(\text{VC}\)) is the maximum volume of air a person can exhale after taking the deepest possible breath, calculated as the sum of \(\text{IRV}\), \(\text{V}_\text{T}\), and \(\text{ERV}\). \(\text{VC}\) is a commonly measured indicator of respiratory muscle strength and lung compliance.
The Functional Residual Capacity (\(\text{FRC}\)) is the volume of air remaining in the lungs at the end of a normal, quiet expiration, calculated by summing the \(\text{ERV}\) and \(\text{RV}\). This volume is considered the resting volume of the lungs. It is determined by the balance between the lung’s tendency to collapse and the chest wall’s tendency to expand.
The Inspiratory Capacity (\(\text{IC}\)) is the maximum amount of air that can be inhaled after a normal tidal expiration, calculated as the sum of \(\text{V}_\text{T}\) and \(\text{IRV}\). \(\text{IC}\) reflects the reserve available for inspiration above the resting breathing level.
Methods for Measuring Lung Function
The most common technique for assessing lung volumes and capacities is spirometry, which measures the volume of air moved over time. A spirometer tracks the volume of air inhaled and exhaled, generating a graph called a spirogram.
Spirometry can directly measure \(\text{V}_\text{T}\), \(\text{IRV}\), \(\text{ERV}\), \(\text{VC}\), and \(\text{IC}\) because these measurements involve the movement of air in and out of the lungs. However, spirometry cannot directly measure the Residual Volume (\(\text{RV}\)) since this air remains trapped within the lungs after maximum expiration.
Because \(\text{RV}\) cannot be exhaled, capacities that include it (\(\text{TLC}\) and \(\text{FRC}\)) must be measured using indirect methods. One method is the helium dilution technique. A subject breathes a known concentration of helium until it is evenly distributed throughout the lungs. The final concentration of the diluted helium allows for the calculation of the volume of air in which it mixed, which is the \(\text{FRC}\).
Another technique is body plethysmography, which involves the subject sitting in an airtight chamber, or “body box.” By applying Boyle’s Law and measuring the changes in pressure and volume within the chamber and the subject’s mouth, the total volume of air inside the lungs (\(\text{TLC}\)) can be accurately determined.
Interpreting Abnormal Lung Volume Results
The analysis of lung volume measurements is an informative method for differentiating between the primary categories of respiratory disease. Changes in expected values provide clear patterns that point toward a specific type of impairment.
One distinct pattern is the restrictive ventilatory defect, characterized by a proportional reduction in all lung volumes, resulting in a significantly reduced \(\text{TLC}\) and \(\text{VC}\). This pattern suggests the total amount of air the lungs can hold is limited. Restrictive diseases often involve stiffness in the lung tissue (e.g., pulmonary fibrosis) or an external limitation on chest wall expansion (e.g., severe obesity).
The second major pattern is the obstructive ventilatory defect, which involves air trapping due to difficulty with exhalation. This is characterized by an increase in \(\text{RV}\) and \(\text{FRC}\), as air is retained in the lungs after expiration. The obstruction is caused by narrowed airways, common in conditions like asthma, emphysema, and Chronic Obstructive Pulmonary Disease (\(\text{COPD}\)).
In obstructive disease, the increased \(\text{RV}\) leads to an elevated \(\text{TLC}\) as the lungs become hyperinflated over time. A key diagnostic indicator of obstruction is a reduced ratio of the forced expiratory volume in one second (\(\text{FEV}_1\)) to the Forced Vital Capacity (\(\text{FVC}\)). Analyzing these static volumes alongside dynamic flow measurements allows for precise classification of the underlying respiratory problem.