In below capnograph the red pa represent:-

First phase (A-B) represents the initial stage of expiration. Gas occupying anatomic deed space, devoid of Co2 represent this phase. Second phase (B-C)- represent a sharp upstroke when Co2 containing alveolar gas presents at sampling site. Third phase (C-D) - represents alveolar phase Fouh phase (D-E) - inspiration begins and fresh gas is entrained and touches baseline. Phase C-D may take an upward slope. Point D is the highest Co2 value and is called the end-tidal Co2 (ET Co2). ET Co2 is the best reflection of the alveolar Co2 (PA Co2). Normal value of ET Co2 - 35-45 mm of Hg.