Question

why do fishes have two chambered heart whereas humans have four chambered heart?

Answer 1

because they need less o2 than human beings.......

Answer 2

system with blood vessel present on the alveoli which dissolves only the gasseous and pure oxygen. By the help of blood vessels the blood goes from the lungs to the heart then to the body organs and then again to the heart and then thereby following to the lungs for purification.

While in fish there are gills present instead of lungs which only binds with the dissolved oxygen in water that directly reaches to the body organ and then to the heart which then goes out through gills.