The heart is made up of four main chambers, two upper chambers (left atrium and right atrium), and two lower chambers (right ventricle and left ventricle). Heartbeats are not random, but rather very systematic and controlled by a system called the heart’s electrical system. This system controls the heart rate through a region called the SA node, which sends electrical signals that cause the heart to contract. If a defect happens in this system the heart does not beat normally and a pacemaker (a small device that sends electrical signals to the heart muscle to regulate its rate) might be needed.