Hint: In order to understand the physical condition of our lungs we need to measure their respiratory volumes and respiratory capacities. These are interlinked, but are not exactly the same thing.
Complete Answer: - Lung capacity is the amount of air that is contained in the lungs after an individual has put in the maximum effort for inspiration. On average, for a healthy adult, this should be around 6 litres. Also known as total lung capacity, the volume increases from childhood to adulthood, reaching a peak around age 25. - Of course, this value differs depending on the age, gender, ethnicity, physical health, athleticism, and so on. For instance, males tend to have a greater lung capacity than females. Athletes or people who exercise regularly and are in good physical health will also have an increased total lung capacity. Smokers would have a lower figure. - Total Lung Capacity is a function of the inspirational, functional residual, and vital capacities combined. VC = IC+ - If lung capacity takes inspiration as the main factor, lung volume is the total amount of air retained after forced exhalation. Lung volume again deals with various figures. These volumes are tidal, expiratory reserve, inspiratory reserve, and residual. The average residual lung volume is about 1.2 litres. This is what is left behind after a forced exhalation.
Note: For a normal, living lung, no matter how forced the expiration, the lungs will never be completely empty, there is always some reserve amount left. Dynamics between the chest walls and lungs ensure that there is a negative pressure which keeps the smaller airways always open. Puncturing the chest cavity causes the lungs to collapse totally.