The main biological function of the heart is to pump blood throughout the body. The heart is a muscular organ that's divided into four chambers. The flow of blood through these chambers is controlled by valves.
The pumping phase of the heart is the systole phase. It begins when the atria, which are the upper chambers of the heart, contract. They push blood through the mitral and tricuspid valves into the ventricles, which are the lower chambers in the heart. The ventricles contract and push blood through valves into the aorta and the pulmonary artery. The heart relaxes in between beats in what's called the diastole phase.