How is the circulatory system of reptiles characterized?

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.