22.3
Lung Capacity
The air in the lungs is measured in volumes and capacities. Lung volume measures reflect the amount of air taken in, released, or left over after a lu…
Lung size can vary depending on experience and body size. The amount of air that can enter or leave the lungs is measured as four volumes, single functions, like inhalation, and four capacities, comparative amounts of multiple volumes.
The amount of air normally inhaled and exhaled is called the tidal volume, whereas the air that can be further breathed in or out after normal inhalation or exhalation is referred to as the inspiratory and expiratory reserve volumes.
The forth volume is the amount of air that remains in the lungs after the expiratory reserve volume, known as the residual reserve volume, the small amount of air that doesn't leave the lungs keeping the lungs inflated.
This amount combined with the other volumes is the total lung capacity, the entire amount of air that the lungs can hold, approximately six liters.
Within a single respiratory cycle, the maximal amount of air that can be moved in or our is the vital capacity, essentially a summation of the tidal and both reserve volumes.
The remaining capacities are the inspiratory capacity, how much can be inhaled after normal exhalation and the functional residual capacity, how much air remains after breathing out. Ultimately these measurements are used to diagnose lung diseases, like asthma where more air remains in the lungs taking longer for exhalation or pulmonary fibrosis where a volume is rapidly emptied resulting in being short of breath.
View the full transcript and gain access to JoVE Core videos
Q1: What is the difference between lung volumes and lung capacities?
Lung volumes measure air during single respiratory functions like inhalation or exhalation, while lung capacities represent sums of two or more volume measures. There are four lung volumes: tidal volume, inspiratory reserve volume, expiratory reserve volume, and residual volume. Four capacities combine these volumes to show total air movement potential during breathing cycles.
Q2: How much air does tidal volume represent in normal breathing?
Tidal volume is the amount of air normally inhaled and exhaled during a regular breath without forced effort. This is the baseline air movement during quiet, resting breathing. It represents the air exchanged during each normal respiratory cycle before any reserve volumes are engaged.
Q3: What is residual volume and why does it matter?
Residual volume is the small amount of air remaining in the lungs after forced exhalation, keeping the lungs inflated and preventing collapse. This air cannot be voluntarily expelled. Combined with other volumes, residual volume contributes to total lung capacity, approximately six liters in adult males and 4.2 liters in adult females.
Q4: What does vital capacity measure during breathing?
Vital capacity is the maximum volume of air that can be exhaled after maximal inhalation, representing the sum of tidal volume, inspiratory reserve volume, and expiratory reserve volume. It measures the maximal amount of air that can be moved in or out during a single respiratory cycle, excluding residual volume.
Q5: How do inspiratory and expiratory reserve volumes differ?
Inspiratory reserve volume is the additional air that can be inhaled after normal inhalation, while expiratory reserve volume is the extra air that can be exhaled after normal exhalation. Together with tidal volume, these reserves determine how much extra breathing capacity exists beyond routine respiration and are essential for activities requiring increased oxygen intake.
Q6: How are lung capacity measurements used to diagnose respiratory diseases?
Lung capacity measurements help identify diseases by revealing abnormal air retention or rapid emptying patterns. In asthma, more air remains in the lungs, prolonging exhalation. In pulmonary fibrosis, lung volume empties rapidly, causing shortness of breath. These measurements provide objective data for diagnosing and monitoring respiratory conditions.
Q7: What is functional residual capacity and how is it calculated?
Functional residual capacity is the volume of air remaining in the lungs after normal exhalation, calculated as the sum of expiratory reserve volume and residual volume. This measurement shows how much air stays in the lungs during quiet breathing and is important for understanding gas exchange and transport during the respiratory cycle.