Most hearts are located on the left side of the chest, but a small number of people have a heart located on the right side of the chest, more often in children with complex precordial disease. The heart is the organ that pumps blood and is divided into four chambers, called the atria above and the ventricles below, so there is a left atrium, a right atrium, and a left ventricle and a right ventricle. There are septa between the atria and septa between the ventricles, called the interatrial septum and the interventricular septum, which divide the heart into two distinct left and right halves. For most normal hearts, the structure is shown in the diagram above. The superior and inferior vena cava are the central venous vessels that connect to the right atrium, and the right and left pulmonary veins, four in total, connect to the left atrium; the pulmonary artery originates from the right ventricle and the aorta from the left ventricle. There are atrioventricular valves between the atria and the ventricles, the right one is called tricuspid valve and the left one is mitral valve, and there are arterial valves between the ventricles and the aorta, the pulmonary valve and the aortic valve respectively. These four valves are all one-way valves that ensure that blood flows in the direction of the arrow in the diagram without backflow. The never-ending beating of the heart is a process in which the ventricles are constantly contracting and diastolic. When the ventricles contract, they pump blood into the aorta, while when the ventricles are diastolic, blood from the atria can enter the ventricles. Blood circulation Venous blood from all over the body passes through the small veins – the large veins – and finally venous blood from the upper body passes through the superior vena cava, while venous blood from the lower body passes through the inferior vena cava, all of which drains back into the right atrium, where blood passes through the tricuspid valve into the right ventricle. The right ventricle contracts and then pumps the blood into the pulmonary artery. The blood passes through the main pulmonary artery and then to the right and left pulmonary arteries and enters the lungs, where gas exchange takes place. The oxygen inhaled by the body enters the blood, while the carbon dioxide in the blood is expelled from the body by whistling. After this process, the blood changes from venous blood to bright red arterial blood. The blood then returns to the left atrium through the right and left pulmonary veins and then enters the left ventricle through the mitral valve. The left ventricle contracts, pumping blood into the aorta, which supplies blood to tissues and organs throughout the body. After the metabolism of tissues and organs, the blood turns from arterial blood to venous blood again, and then returns to the right atrium through the small vein – large vein, and finally through the superior and inferior vena cava, completing a blood circulation. Through the cycle of blood circulation, human life can be maintained.