Lobsters have a cardiovascular system, containing one single-chambered heart. Their hearts are a bit different from ours, because their hearts are a sac of striated muscle with several openings called ostia. Blood gathers in the pericardium, which surrounds the heart, until it enters the heart through the ostia, and into other parts of the body for different needs. This is the main function of the ostia. Lobsters have closed circulatory systems, so there are no veins to bring blood back to the heart. Blood returns to the heart through a serious of interconnected spaces that empty back into the pericardium where the blood gathers. After a certain amount of collection, the cycle is restarted.