22.3:
Lung Capacity
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.
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 lung function, like a single inhalation. Lung capacity measures are sums of two or more lung volume measures.
There are four defined lung volume measures: tidal volume, inspiratory reserve volume, expiratory reserve volume, and residual volume.
Tidal volume is the amount of air inhaled and exhaled in a normal breath. Inspiratory (inhalation-related) reserve volume is the additional amount of air that can be inhaled after a regular inspiration. Expiratory (exhalation-related) reserve volume is the amount of air that can be exhaled after normal exhalation. Residual volume is the amount of air remaining in the lungs after forced exhalation (i.e., blowing).
There are four defined lung capacity measures: inspiratory capacity, functional residual capacity, vital capacity, and total lung capacity.
Inspiratory capacity is the volume of air that can be inhaled after normal exhalation. It is the sum of the two volume measures involving inhalation: tidal volume and inspiratory reserve volume. Functional residual capacity is the volume of air remaining in the lungs after normal exhalation and is equal to the sum of the expiratory reserve and residual volumes. Vital capacity is the maximum volume of air that can be exhaled after a maximal inhalation. It is the sum of the tidal, expiratory reserve, and inspiratory reserve volumes.
Total lung capacity is the total volume of air the lungs can hold. It is the sum of all four lung volume measures. Total lung capacity in humans is 6 liters in an average adult male and 4.2 liters in an average adult female! However, lungs do generally not operate at this maximal capacity.
Suggested Reading
Ji, Jie, Su-qing Wang, Yu-jian Liu, and Qi-qiang He. “Physical Activity and Lung Function Growth in a Cohort of Chinese School Children: A Prospective Study.” PLoS ONE 8, no. 6 (June 19, 2013). [Source]
Melén, Erik, and Stefano Guerra. “Recent Advances in Understanding Lung Function Development.” F1000Research 6 (May 19, 2017). [Source]