<prompt> In which of the following conditions the respiratory muscles are relaxedResidual volumeFunctional residual capacityExpiratory reserve volumeInspiratory reserve volume  <response>: Functional residual capacity 652- Ganong 22th 621- K.S. 4th 444- A.K.Jain 3rd 593- Ganong 23rdThe lung volume at which airway pressure is zero mm Hg is called relaxation volume, whch is equal to FRC.Therefore, relaxation volume is the point where the recoil of the chest and the recoil of the lungs balanceResidual volume - The amount of air remaining in the lung even after forced expirationi It helps to aerate the blood in between breathing and during expirationii It maintains the contour of the lungs 