Least amount of CO2 is in:

Ans. A. Anatomical dead space -- end inspiration phasea. During inspiration atmospheric air enters into respiratory passage (that contain O2 , 20.84%, and CO2 , 0.04%) that air in the respiratory passages before entering the alveoli, becomes humified (i.e. at the end of inspiratory phase, anatomical dead space contains humified air that contains O2 , 19.67% and CO2 , 0.04 %)b. During expiration: Expired air is a combination of dead space air and alveolar air.i. First portion of expired air, is the dead space air, and is typically humified air.ii. Then progressively more and more alveolar air becomes mixed dead space air until all the dead spaceiii. air has finally been washed out and nothing but alveolar air is expired at the end of expiration.iv. Thereforev. Dead space (Anatomical) At the end of inspiration contains hummed air i.e. O2 19.69% (or 149.3mmHg), CO2 - 0.04% (or 0.3 mmHg) and N2 -74.09% (or 563 mm Hg)vi. Alveoli alveolar air composition remains relatively constant either during expiration or inspiration i.e. Alveolar air contain O2 13.6% (or 104 mm Hg), CO2 5.3% (or 40 mm Hg)vii. Dead space at the end of expiration: contains alveolar air Le. O2 13.6%, and CO2 5.3%.Therefore, Ans is Dead space at the end of inspiration contains least amount of CO2 .