Snails, unlike humans, have an open circulatory system with a two chambered heart. The heart pumps the blood into the aorta and then it gets pumped into smaller arteries. From these smaller arteries, the blood empties into the body cavity. The body cavity is called the haemocoel. The blood has a blue color because of the haemocyanin. The blood passes from the haemocoel into vessels surrounding the lung where O2 is released from the blood. The blood then re-enters the heart via the pulmonary vein.