Total lung capacity refers to the maximum amount of air a person’s lungs can hold, about four to six liters (4000 to 6000 cm3) for the average human. Only a third of this capacity is used during normal activity, but this fraction increases during strenuous activity when the body requires more oxygen. On average, males have a 20-25% higher capacity than females, and tall individuals have a greater capacity than short people.
The human lungs enable respiration. When air is inhaled, the lungs extract oxygen and transfer it to the blood where the oxygen is then carried to cells. The blood supplies cells with oxygen and collects carbon dioxide, a waste resulting from cell functions. The carbon dioxide is then carried back to the lungs where it is exhaled.
Those who live in higher altitudes also have a higher lung capacity than those who live at sea level. This is because air is less dense at high altitudes, and thus the concentration of oxygen is lower. Lungs, therefore, have to inhale a greater volume of air to extract the same amount of oxygen than they would have to at sea level.
Total lung capacity (TLC) is measured by adding together Inspiratory Reserve Volume (IRV), Tidal Volume (Vt), Expiratory Reserve Volume (ERV), and Residual Volume (RV) to come up with the formula, TLC=IRV + Vt + ERV + RV. Tidal Volume is the amount of air normally inhaled or exhaled. Inspiratory Reserve Volume is the amount of additional air that could be inhaled in order to completely fill up the lungs. Expiratory Reserve Volume is the amount of additional air that can be expelled after a normal exhale. These values are measured with a spirometer. There is a certain amount of air that cannot be exhaled under any circumstances. This is the Residual Volume and is measured by other pulmonary function tests.
It is important to keep the lungs healthy because all cells need oxygen to function. Cigarette smoke reduces lung capacity by causing lung disease. One of these diseases is emphysema which destroys the alveoli--the air sacs within the lungs where the exchange of oxygen and carbon dioxide takes place. As a result, the lungs lose elasticity and are unable to stretch properly, thus reducing the amount of air that is able to be inhaled. Asthma, a disease that causes inflammation of the lungs, and chronic obstructive pulmonary disease (COPD), the combination of chronic bronchitis and emphysema, also decrease total lung capacity.