Final answer:
The amount of air left in the lungs after a normal exhalation is called the residual volume. It is vital for preventing the collapse of the lungs, and ensuring continuous gas exchange, and it cannot be measured directly.
Step-by-step explanation:
After a normal exhalation, the amount of air left in the lungs is the residual volume (RV). Residual volume is the air remaining in the lungs after a forceful exhalation and plays a crucial role in keeping the lungs from collapsing. It ensures that lung tissues do not stick together and allows for continuous exchange of respiratory gases (O₂ and CO₂) even between breaths. This volume helps prevent large fluctuations in respiratory gases and cannot be directly measured as the lungs cannot be completely emptied; it can only be calculated.
The total lung capacity, which is the maximum amount of air the lungs can hold, includes the residual volume along with the tidal volume (TV), inspiratory reserve volume (IRV), and expiratory reserve volume (ERV). Tidal volume is the amount of air inhaled or exhaled during a normal breath. The inspiratory reserve volume is the additional air that can be inhaled after a normal inhalation, while the expiratory reserve volume is the additional air that can be exhaled after a normal exhalation. Together, these volumes determine how much air can be moved in or out of the lungs in a single respiratory cycle.