Answer:
Both the respiratory and circulatory systems works in coordination in order to purify the blood.The heart continuously pumps blood and the lungs at alveolar sacs the CO2 and O2 get exchanged.Continuously through the diffusion.
From body parts the blood reaches at heart through Vena cava To right atrium,then from Right ventricle through pulmonary artery it reaches to lungs and get purified.Form there it again comes back to heart through pulmonary vein and again through Aorta it again reaches to the body parts.
Hope it helps....