<prompt> When the respiratory muscles are relaxed, the lungs are in which of the following state?Functional residual capacity FRCExpiratory reserve volume ERVResidual volume RVInspiratory reserve volume IRV  <response>: The functional residual capacity FRC occurs at the lung volume at which the outward recoil of the chest wall is equal to the inward recoil of the lungs, which occurs when no respiratory muscles are actively contracting, that is, at the end of a tidal expiration.The FRC is therefore also referred to as the resting expiratory level, or simply, the resting level. 