What does residual volume refer to in medical terms?

Residual volume specifically refers to the amount of air that remains in the lungs after a person has exhaled fully. This air cannot be expelled from the lungs, and it is crucial for maintaining the constant exchange of gases, preventing alveolar collapse, and ensuring that oxygen supply is always available for gas exchange during the next inhalation.

Understanding residual volume is vital in assessing respiratory function and can aid in diagnosing conditions such as chronic obstructive pulmonary disease (COPD) or restrictive lung diseases. The concept is distinct from the other options, which involve fluids in the stomach, blood flow in circulation, and urine in the bladder, as those pertain to different physiological systems.