The heart is like a “pump” The human heart is a hollow, muscular organ that continuously pumps blood throughout the body. The heart can be divided into four parts, or chambers, two on the “left” side and two on the “right” side. The upper chambers on each side are called the atria, which receive blood from all parts of the body back to the heart, and the lower chambers on each side are called the ventricles, which pump blood out of the heart. These are responsible for pumping blood out of the heart. The four chambers work together to contract and pump blood, giving the circulation the power to move the blood carrying oxygen and nutrients throughout the body. The heart’s electrical conduction system The heart’s rhythmic diastolic activity is completely dependent on its electrical conduction system, which transmits electrical impulses to every corner of the heart. The sinus node is a special group of cells located in the right atrium, which is the origin of normal electrical impulses throughout the heart. The sinoatrial node is like the heart’s “natural pacemaker” and determines the rhythm of the heartbeat. The impulses from the sinoatrial node travel along a fixed pathway throughout the atria, triggering contraction of the atria and squeezing blood into the ventricles. The impulse travels out of the atria and then reaches the atrioventricular node, which is located between the atria and the ventricles. Under normal conditions, the AV node is the only electrical pathway between the atria and the ventricles. The atrioventricular node acts as a “staging area” where each electrical impulse slows down and stops briefly before reaching the ventricles to give the ventricles enough time to fill with blood. After the “stop” at the AV node, the impulse continues its descent to the ventricle through a bundle of specially differentiated muscle fibers. In the ventricle, these bundles are dispersed into numerous small fibers, and a “grid” of these small fibers transmits the impulse throughout the ventricle, causing the ventricular muscle to contract and pump blood out of the heart.