Carbon dioxide is considered as a waste product of Krebs Cycle.Before Krebs cycle begins pyruvic acid which has 3 carbon splits into two parts. Two carbon atom combines with co enzyme A and forms a compound called acetyl-co enzyme A.The third carbon atom left combines with oxygen to form carbon dioxide which is a by product of this cycle.The carbon dioxide formed here diffuses into the bloodstream and then carried to the lungs for exchange of gas.