A spiny lobster's circulatory system is open. The spiny lobsters have a heart that is a single-chambered sac that contains muscle and several small openings called ostia. The heart is located above the stomach on the upper surface of the animal. Also, the heart is suspended in a blood sinus called pericardium. The blood, or hemolymph, usually passes from the sinuses, through the ostia, and into the heart. When the hemolymph is pushed out of the heart, major arteries carry the hemolymph to sensory organs, systems, and ventral nerves, guts, and abdominal muscles. Blood returns to the heart through venous sinuses. The heartbeat of an adult lobster has an average heartbeat of 50-136 beats per minute and is controlled by the cardiac ganglia, which provides a pacemaker mechanism.