Possessing an open cardiovascular system as well due to its status as an invertebrate, the "main" parts of the maine lobster's Cardiovascular system are made up of a single-chambered heart held in place by a blood sinus called the pericardium. Arteries run toward the abdomen of the lobster, supplying blood to the various parts of the lobster, which include the gut, ventral nerves, and abdominal muscles. From the arteries, the blood enters blood cavities/sinuses that are present within the tissue of the lobster. These blood cavities proceed to bathe the various organs in oxygen rich blood. No veins are present to return the blood to the heart. Instead, blood comes back to the heart through connected cavities within the lobster known as venous sinuses which open back into the pericardium.