The human blood circulation is divided into two kinds: body circulation and pulmonary circulation. In the body circulation, small arteries supply blood with the capillary network of tissues and organs for full blood oxygen exchange, and the arterial blood becomes venous blood, and then converges into larger veins through some tiny veins and small veins. The veins of the general circulation merge into the superior vena cava and inferior vena cava respectively, and these veins merge the venous blood into the right atrium, then through the right atrium to the right ventricle, which pumps it through the pulmonary artery and the small pulmonary artery to the lung tissues, and then further oxygenation and exchange occurs into arterial blood. Therefore, the blood flowing inside the above mentioned vessels is venous blood with low oxygen content, and the right heart system is mainly venous blood.