In the course of evolution vertebrae heart got modified alot.in Pisces heart is two chambered made up of a auricle and a ventricle.it is a single circuit heart..due to this pure blood will not be supplied.
In amphibians heart got modified to 3 chambered with two auricles and one ventricle. It is a double circuit heart and impure blood only supplied to body structures.
In reptiles except crocodile(4) heart is incompletely divided 4 chambered heart. Here also double circuit heart is seen. Due to incomplete division of ventricles, impure blood only supplied to body structures.
In aves, mammals, heart is fully developed with two auricles and two ventricles with double circuit heart. Here proper separation of pure and impure blood happens. So each and every cell is supplied with pure blood.they are warm blooded organisms, because enough energy will be produced by complete oxidation of glucose.