Structural diagram of human heart - blood circulation. Systemic circulation starts in the left ventricle. After the blood is pumped out from the left ventricle, it flows through the aorta and several derived arterial branches to send the blood to the corresponding organs. The heart is located in the thoracic cavity, between the two lungs above the diaphragm, and about two-thirds is on the left side of the midline. The human heart is divided into four chambers: left atrium, left ventricle, right atrium and right ventricle. The two atriums are connected with the two ventricles respectively, and the two ventricles are connected with the two major arteries.