The heart forms the core of the circulatory system in the body which performs the function of circulating blood and providing oxygen and nutrients to cells. The circulatory system sends blood to the lungs through the pulmonary circulation and throughout the body through the systemic circulation. The heart is separated into four sections or chambers. The two upper chambers are known as the right atria and left atria and these perform the function of receiving the blood. The blood is pumped throughout the body by the lower chambers or the left and right ventricles. The blood enters the lower chambers through valves known as the tricuspid and mitral valve for right and left sides respectively. The heart is entirely made up of muscle and the ventricles by contracting perform the bulk of the hearts function and send blood soaring to different parts of the body. This contraction is what is heard through a stethoscope or felt while taking someone's pulse as a heartbeat.