Human beings have 'double circulation' . …

In human beings, the blood goes through the heart twice during each cycle i.e. the blood passes through the human heart two times to supply once to the whole body. So, it is called Double Circulation of blood.
The double circulation of blood includes:
(i) Systematic Circulation
(ii) Pulmonary Circulation
Systematic Circulation: It supplies oxygenated blood from left auricle to left ventricle, thereby pumped to various body parts. The deoxygenated blood is collected from the various body organs by the veins to pour into vena cava and finally into the right atrium (auricle). Right atrium transfers the blood into the right ventricle.

Pulmonary Circulation: The deoxygenated blood is pumped by the right ventricle into the lungs for oxygenation. The oxygenated blood is brought back to left atrium (auricle) of the human heart. From left atrium, the oxygenated blood is pushed into the left ventricle. The left ventricle pumps oxygenated blood into aorta for systematic circulation.

It is necessary in human being to separate oxygenated and de-oxygenated blood because this makes their circulatory system is more efficient and helps in maintaining constant body temperature.