The heart and the lungs are the primary organs of the circulatory system. The lungs are responsible for adding oxygen to blood. This oxygen-rich blood is then pumped to the heart, which then circulates it through the arteries to all the tissues throughout the body. Veins then return the blood for the addition of new oxygen and recirculation.
Blood circulates through two bodily systems: the systemic and the pulmonary circulatory systems. The systemic circulatory system is the system that transports the blood from the heart to the other organs and tissues in the body. The human heart is made of four separate chambers. The two top chambers allow blood to enter the heart, while the two lower chambers allow blood to leave the heart. The heart receives signals from the body telling it how much blood to pump.
The pulmonary circulatory system consists of a short loop that circulates the blood from the lungs to the heart. Without this connection, the heart would not be able to pump out blood containing the oxygen needed by the other parts of the body. Oxygenation of the blood occurs in the lungs, with oxygen attaching itself to the hemoglobin in the red blood cells.