Describe double circulation in human beings. Why is it necessary?

The blood passes through the heart twice through separate pathways for completing one cycle. This type of circulation is called double circulation.
The double circulation of the blood includes:
(i) Systematic circulation (ii) Pulmonary circulations.
(i) Systematic circulation: it supplies oxygenated blood from the left auricle to left ventricle, which is pumped to the aorta to distribute blood to various body parts. The deoxygenated blood is collected from the various body organs by the veins to pour ven cava and finally into the right atrium(auricle).Right atrium transfer this blood into the right ventricle.
(ii) Pulmonary circulation: The deoxygenated blood is pushed by the right ventricle into the lungs for oxygenation through the pulmonary artery. The oxygenated blood is brought back to the left atrium of the human heart through the pulmonary vein
Double circulation is necessary in human beings in order to avoid mixing of oxygenated and deoxygenated blood which makes the circulatory system more efficient and helps in maintaining a constant body temperature.