The air exchanged during quiet, normal respiration is only a small fraction of what the lungs can hold. Lung volumes quantify the amount of air the lungs can exchange, providing a window into respiratory health and function. These measurements are divided into four main volumes, and the two “reserve volumes” represent the capacity to move air beyond a standard, relaxed breath. Quantifying these reserves helps clinicians determine the flexibility and overall maximum capacity of the respiratory system, especially when the body’s needs increase.
Defining Inspiratory and Expiratory Reserve Volumes
The two distinct reserve volumes are the Inspiratory Reserve Volume (IRV) and the Expiratory Reserve Volume (ERV). The IRV is the maximum amount of air that can be inhaled forcefully following a normal, relaxed inspiration (tidal volume). This volume is usually the largest of the four main lung volumes, averaging around 2.5 to 3 liters in a healthy adult male.
The IRV reflects the lungs’ ability to increase oxygen intake significantly during moments of elevated physical demand, such as intense exercise. To draw in this extra air, the diaphragm and external intercostal muscles must contract with greater force than during quiet inspiration. A large IRV indicates a healthy, compliant lung and chest wall system capable of substantial expansion.
The Expiratory Reserve Volume (ERV) is the maximum amount of air that can be forcefully exhaled after a normal, relaxed breath out. This volume is generally smaller than the IRV, averaging about 1.1 to 1.5 liters in a healthy adult. The ERV is measured from the end of a normal, passive exhalation down to the point of maximum forceful exhalation.
Forceful exhalation requires the active contraction of accessory respiratory muscles, primarily the internal intercostal and abdominal muscles, to push the diaphragm upward. This volume represents the air that is not exchanged during quiet breathing but remains available for forceful expulsion.
How Reserve Volumes Create Lung Capacities
Reserve volumes combine with other volumes to form lung capacities, which provide a more comprehensive picture of respiratory function. Capacities are the sum of two or more primary lung volumes. The most physiologically significant capacity involving the ERV is the Functional Residual Capacity (FRC), which is the volume of air remaining in the lungs after a normal, relaxed exhalation.
The FRC is calculated as the sum of the Expiratory Reserve Volume (ERV) and the Residual Volume (RV). This capacity is the resting point of the respiratory system where the inward elastic recoil of the lungs is balanced by the outward elastic recoil of the chest wall. The FRC is a reservoir of air that ensures continuous gas exchange between breaths, preventing the blood’s oxygen content from fluctuating too widely.
The most comprehensive capacity involving both reserve volumes is the Vital Capacity (VC), the maximum volume of air a person can exchange in one complete breath cycle. It is the sum of the Inspiratory Reserve Volume (IRV), the Expiratory Reserve Volume (ERV), and the Tidal Volume (TV). The VC represents the total usable volume of the lungs, measuring the range of motion from maximum inspiration to maximum expiration.
The average VC in a healthy adult can range from 4.5 to 5 liters, and it is a powerful indicator of overall respiratory muscle strength and lung elasticity. Another capacity, the Inspiratory Capacity (IC), combines the IRV with the Tidal Volume (TV) and measures the maximum air that can be inhaled from the end of a normal exhalation. Analyzing these capacities helps diagnose whether a patient’s breathing limitation is due to issues with lung expansion or airflow obstruction.
Measurement and Clinical Significance
The measurement of reserve volumes is a fundamental part of a complete pulmonary function test, most commonly performed using a spirometer. Spirometry involves the patient breathing into a mouthpiece connected to the device, which records the volume of air moved over time. Simple spirometry can directly measure the Inspiratory Reserve Volume, the Expiratory Reserve Volume, and the Tidal Volume.
Changes in these reserve volumes are significant in diagnosing and managing respiratory diseases. A reduced ERV and, consequently, a reduced FRC, is a common finding in restrictive lung diseases, such as pulmonary fibrosis or severe obesity. These conditions limit the lung’s ability to expand and the chest wall’s ability to move, directly compressing the lung volume.
In contrast, obstructive lung diseases like Chronic Obstructive Pulmonary Disease (COPD) or emphysema can cause an increase in FRC, a condition known as hyperinflation. The destruction of lung tissue in emphysema causes the lungs to lose their elastic recoil, trapping air inside. This air trapping elevates the FRC and RV, often at the expense of the IRV and ERV, which can become smaller because the lungs are already operating at a higher resting volume.
While spirometry is effective for measuring IRV and ERV, it cannot measure the Residual Volume (RV) or capacities that include it, such as FRC. For a full assessment, specialized techniques like body plethysmography or gas dilution methods, such as helium dilution, are required. These measurements provide the complete static lung volumes, which are necessary for distinguishing between restrictive and obstructive patterns of respiratory impairment.