I will choose a frog.The CO2 molecule is produced in aerobic respiration as a product from decarboxylation(R-COOH--->CO2+R-H) in Kreb's cycle. The CO2 molcule will leave the mitochondria. CO2 from the will diffuse from the muscle cell to the capilaries. From the capilaries the molecule will go into a vein. It will brought to the right atrium by the inferior vena cava. During atrial systole the molecule will be pumped in the ventricle(frogs only have 1 ventricle). During ventricular systole the molecule will be pumped into the pulmonary artery into the lungs where it'll diffuse out of the capilaries into the lungs and will be expired.