Expiratory reserve volume (ERV) is the amount of air that can be forcibly exhaled after a normal exhalation. It represents the extra air that remains in the lungs after a typical breath, allowing for deeper breathing when needed. ERV is an important measure in assessing lung function and respiratory health. This volume is typically measured during a spirometry test, which evaluates how well the lungs are working. Understanding ERV helps healthcare professionals identify conditions like asthma or chronic obstructive pulmonary disease (COPD) that may affect breathing efficiency and overall lung capacity.