Final answer:
The lung volume when respiration muscles are relaxed and alveolar pressure is zero is known as Functional Residual Capacity (FRC), which includes air from Expiratory Reserve Volume and Residual Volume.
Step-by-step explanation:
When all muscles of respiration are relaxed and alveolar pressure is zero, the lung volume is equal to the Functional Residual Capacity (FRC).
The Functional Residual Capacity is the volume of air that remains in the lungs after a normal, passive exhalation. This volume includes both the Expiratory Reserve Volume (ERV) and the Residual Volume (RV). The ERV is the additional air that can be exhaled after a standard expiration, and the RV is the air remaining in the lungs after a maximal exhalation to prevent lung collapse. Together, they ensure that the lungs are never completely deflated, which maintains the alveolar architecture and facilitates efficient gas exchange.