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Blood Circulation by heart occurs in 2 closed-circuits, i.e. double circulation, which operates in series. Output of one becomes the input of the other.  One circulation is of deoxygenated (oxygen-deficient/oxygen poor) blood and other is of oxygenated (oxygen-rich) blood.

Pulmonary circulation: Deoxygenated blood is pumped by right side (right atrium and right ventricle) of the heart called as Pulmonary circulation. Right atrium receives deoxygenated dark red blood (output of systemic circulation) from large systemic veins. Right ventricle pumps this deoxygenated blood into pulmonary artery which carries it to the lungs (pulmon). In lungs, exchange of gases occur (CO₂ lossed and O₂ gained by blood). This fresh bright red colour oxygenated blood flows into pulmonary veins and received by left atrium.

Double circulation

Systemic Circulation: Oxygenated blood is handled by the left side (left atria and left ventricle) of the heart called as systemic circulation.  Left atrium receives oxygenated blood (output of pulmonary circulation) from lungs by pulmonary veins. Left ventricle pumps this blood into aorta which further divides into systemic arteries, arterioles and systemic capillaries. In systemic capillaries, blood losses O₂  and gains CO₂  from tissues. Then this deoxygenated blood is taken by systemic venules, veins and vena cava to flow back to right atrium.

• Systemic circulation provides nutrients, O₂ and other essential substances to the tissues and takes CO₂ and other harmful substances away for elimination.
• Right side of heart pumps deoxygenated blood into pulmonary circulation to the alveoli of lungs, while left side pumps oxygenated blood into systemic circulation to all tissues of the body except the alveoli of lungs.

• Blue indicating pulmonary circulation and red is for systemic circulation.

NOTE–  1) Hepatic portal system: A unique vascular connection exists between the digestive tract and liver called as hepatic portal system/hepatic portal circulation. In this, the hepatic portal vein carries venous blood from intestine and spleen to the liver before it is delivered to the systemic circulation. Liver modifies, stores and eliminates some substances present in the blood before they pass into general circulation.

2) Coronary circulation: Heart has its own network of blood vessels to maintain sufficient supply of nutrients in all the layers of heart wall and chambers. This network is called as coronary circulation consisting of coronary arteries and coronary veins. This network is present in myocardium.