Blood has to pass through the human heart twice to reach once in the body, hence it is called double circulation. There are two circulations under this :

(i) Systemic circulation

(ii) Pulmonary circulation

(i) Systemic circulation : Systemic circulation transfers oxygenated blood from the left ventricles to capillaries in the tissue. The oxygen-rich blood is passed to the aorta for distribution into various body parts. The veins and vessels later absorb the deoxygenated blood, 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.

(ii) Pulmonary circulation : The pulmonary artery receives the blood from the right ventricle and carries it 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. The left ventricles pump the oxygenated blood to the aorta for systemic circulation.

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The need for double circulation : The right and left sides of the human heart do not allow oxygenated and deoxygenated blood to mix. Due to the separation of oxygenated and deoxygenated blood, oxygen reaches the body effectively. It keeps on giving energy to regulate the temperature of the body.