In a shark's circulatory system, the heart pumps blood first to the gills where it is oxygenated, then to the organs and muscles, and finally back to the heart. To remain oxygenated, the blood must flow continually, and this is accomplished through the opening and closing of a shark's mouth as well as the muscle movements of its body.
The heart of a shark is S-shaped and has two chambers, the atrium and the ventricle. It is located in an area near the head. The blood flows through ventral aorta, or arteries, to the gills and other organs and then back to the heart through the veins. Sharks have low blood pressure, which is why some species must move continuously to keep the blood flowing.
Some sharks are cold-blooded, or poikilothermic, which means that their internal temperatures match the outside environment. Others, such as mackerel, shortfin makos, common threshers and great white sharks, are homeothermic, which means that their body temperatures are considerably higher than the water around them. To achieve this, they have special red muscle that generates heat which travels through the veins back to the heart. Being warm-bodied greatly increases the shark's speed and strength. To fuel this heat, a warm-bodied shark must consume far more food than a cold-bodied shark.