How Your Heart Works Your heart is a muscle that is responsible for pumping blood throughout the body. The heart is made up of four chambers. The upper chambers are the called atria and act as the receiving chambers. The lower chambers are called ventricles; these are the pumping chambers. There are four valves within the heart, which help control the direction of blood flow. Blood low in oxygen returns from the body and enters the right atrium. The blood travels through the tricuspid valve into the right ventricle. The right ventricle pumps the blood through the pulmonary valve and into the pulmonary artery. The blood enters the main pulmonary artery, which then branches into the right and left lung where the blood absorbs oxygen. The blood, now rich in oxygen, returns to the heart and enters the left atrium. It crosses the mitral valve into the left ventricle. The left ventricle pumps the blood through the aortic valve into the aorta and out to the body. Blood then returns to the superior and inferior vena cava and drains into the right atrium and the cycle continues.