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

Double circulation : It means that the blood passes through the heart twice,  through two separate pathways for completing one cycle. In double blood circulation the blood has two circulations pathways, namely, pulmonary and systemic are present.

1. Pulmonary circulation : It is the circulation in which impure blood from right. ventricle of heart goes to lungs and after purification (oxygenation) is poured back into the heart (left auricle).
Mechanism - In this the blood is pumped by the right ventricle enters the pulmonary artery, whereas the left ventricle pumps blood into the aorta. The deoxygenated blood pumped into the pulmonary artery is passed on to the lungs from where the oxygenated blood is carried by the pulmonary veins into the left atrium.

Right auricle Fig. Double circulation of blood in mammals

2. Systemic circulation : The systemic circulation starts with the pumping of oxygenated blood by the left ventricle to the aorta which is carried to all the body tissues and the deoxygenated blood from there is collected by the veins and returned to the right atrium.
Mechanism -  The oxygenated blood entering the aorta is carried by a network of arteries, arterioles and capillaries to the tissues from where the deoxygenated blood is collected by a system of venules, veins and vena cava and emptied into the right atrium.
Significance of Double circulation
It  allows a highly efficient supply of oxygen to the body cells which is necessary for producing a lot of energy. This energy is useful in case of human beings that have high energy need because they constantly required energy to maintain their body temperature.