What is meant by double circulation? What is its significance ?

Double circulation : It is the pattern of blood circulation in which blood passes through the heart twice in a complete circuit. It includes two completely separated circulatory pathways namely pulmonary and systemic circulations. It is seen in the vertebrates with four chambered heart.
Significance :
* The systemic circulation provides nutrients, O₂ and other essential substances to the tissues and collects CO₂ and other harmful substances, for elimination.
* The pulmonary circulation eliminates CO₂ into alveolar air and collects O₂ from the alveolar air.
* Mixing of the oxygenated and deoxygenated types of blood is prevented.