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Residual volume (RV) is

Athe air left in the lungs after maximal expiration
Bthe air inhaled during a normal breath
Cthe extra air inhalable beyond a normal breath
Dthe air that crosses into blood per minute
Answer & Solution
Correct answer: A. the air left in the lungs after maximal expiration
RV is the volume of air still left in the lungs after the most forceful expiration possible, about 1200 mL in adults. It can't be measured directly with a spirometer (needs helium dilution or similar). Tidal volume is normal breath air; IRV is the extra inhalable air.
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