The circulatory system of reptiles is closed, as it is in humans. Reptiles have a double circulatory system, but the heart is not always completely separated into two separate pumps. In reptiles, the ventricular septum is incomplete and the pulmonary artery is equipped with a sphincter muscle. This allows for a second possible blood flow path. Sometimes the sphincter muscle may cause the blood to flow from the capillaries to the heart, then right back to the capillaries again. This type system is useful to cold-blooded organisms like reptiles in helping regulate their body temperature.