In mammals and birds, the heart is divided into four chambers: two atria and two ventricles.
The oxygenated blood is separated from the deoxygenated blood, which improves the efficiency of double circulation and is probably required for the warm-blooded lifestyle of mammals and birds.
Reptiles have a three-chambered heart – two atria and one partially divided ventricle.
There is a mixing of oxygenated and deoxygenated blood because the ventricle is not split completely.
Pisces are aquatic animals, they belong to phylum Chordata, their heart is two-chambered, one auricle and one ventricle and there is the mixing of pure and impure blood takes place.