In a data collected from nitrogen wash-out method in a patient, volume of nitrogen was found to be 800 mL in his residual volume (RV). How much is the RV in this patient?: (A) 400 mL, (B) 800 mL, (C) 1000 mL, (D) 1600 mL
Answer is C. In all the respiratory volumes, 80% fraction is of nitrogen. Thus, if total nitrogen volume in the RV is 800 mL, then RV is 1000 mL. (80% of 1000 mL is 800 mL.)