Transport system in higher animals

1. The circulatory system in higher animals has undergone several changes during evolution.
2. The heart has become totally muscular.
3. It pumps the blood into well-defined blood vessels
4. The two chambered heart in fishes evolved into a three chambered heart in amphibians and finally into a four chambered heart in birds and mammals.
5. Deoxygenated blood is collected from the body parts and sent to the respiratory system for oxygenation.
6. In animals, like fish the blood is directly sent from the gills to all the body parts.
7. In higher animals oxygenated blood is sent back to the heart and then from there to body parts.
8. Single circuit circulation occurs in fish. In amphibians, reptiles, birds and mammals double circuit circulation takes place.