Snails have an open circulatory system with a simple two-chambered heart. The blood is usually pumped into the aorta and then into smaller arteries. However, the blood does not always travel through blood vessels. Instead, the blood passes through sinuses, which lead to vessels that pass through gills where oxygen and carbon dioxide are exchanged. The blood then goes back to heart. The blood of snails is usually bluish due to hemocyanin in the blood.