Human circulatory system is double circulating. …

The double circulation of blood includes:
(i) Systematic Circulation
(ii) Pulmonary Circulation

Systematic Circulation

• Systemic circulation transfers oxygenated blood from left ventricles to capillaries in the tissue.
• The oxygen-rich blood is passed to the aorta for distribution into various body sections.
• The veins and venules later absorb the deoxygenated blood which is rich in carbon dioxide from different parts of the body.
• The deoxygenated blood is transferred back to the superior vena cava, then on to the right atrium.
• The right atrium transports blood to the right ventricle for pulmonary circulation, after receiving the deoxygenated blood.

Pulmonary Circulation

• The distribution of blood starts in the pulmonary circulation from the right atrium to the left atrium.
• The pulmonary artery receives the blood from the right ventricle and carries to the lungs for oxygenation.
• When the oxygenated blood is pumped back to the left atrium via the pulmonary vein, that is brought to the left ventricles, after the purification process.

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