<prompt> Nitrogen washout method is used for estimating:Dead space volumeFunctional residual capacityTidal volumeDiffusion capacity  <response>: B i.e. Functional residual Capacity- Residual volume is the amount of air left inside the lungs at the end of forced expiration. It cannot be measured by simple spirometerQ because the air in the residual volume of lungs can not be expired into spirometer. Because spirometer cannot measure RV, it cant be used to measure lung capacities requiring RV such as functional residual capacity RFC ERV RV and total lung capacity TLC VC RV. Spirometery can not be used in a direct way to measure - Functional residual capacity FRC, residual volume RV and total lung capacity TLCQ. Helium dilution method is used to measure them.Spirometry can measure tidal volume TV, inspiratory reserve volume IRV, inspiratory capacity IC, expiratory reserve volume ERV and vital capacity VC. 