Carbon dioxide gas is produced as a waste product during respiration in the cells of the body tissues. This carbon dioxide diffuses into the blood. Blood carries the carbon dioxide back to the lungs where it diffuses into the alveoli. When we breathe out air, the diaphragm and muscles attached to the ribs relax due to which our chest cavity contract and become smaller. This contraction movement of the chest pushes the carbon dioxide from the alveoli of lungs in to tracheas then into the nostrils and then out of the body into air.