Final answer:
The term for the volume of air left in the lungs after a maximal expiration is known as the residual volume (RV). It is a measure that cannot be directly observed but is crucial for maintaining lung health and function.
Step-by-step explanation:
The volume of air remaining in the lungs after a maximal expiration is called the residual volume (RV).
Human lungs are never completely empty of air, because after a maximal exhalation, there is still air left which is known as residual volume. This is crucial for the lungs to function properly, as complete emptying could cause the lungs' tissues to stick together, making reinflation difficult. The residual volume also helps in preventing drastic fluctuations in the levels of respiratory gases like oxygen (O₂) and carbon dioxide (CO₂). Unlike other lung volumes such as tidal volume, inspiratory reserve volume, or expiratory reserve volume, the residual volume cannot be measured directly. Instead, it is calculated, as our lungs do not possess the capability to exhale all the air completely.