Heart beat realistic colour animation. The heart is a muscular organ found in all vertebrates that is responsible for pumping blood throughout the blood vessels by repeated, rhythmic contractions. Blood is circulated around the body through blood vessels by the pumping action of the heart. Heart beat animation videos for download from Polymime Animation Studio. The heart beats as a result of the coordinated contraction of its four chambers. The heart has two upper chambers called the atria and two lower chambers called the ventricles. The atria contract first, pumping blood into the ventricles. Then, the ventricles contract, pumping blood out of the heart and into the body. The heart's rhythm is controlled by a group of specialized cells in the right atrium called the sinoatrial (SA) node, and another group of cells in the interventricular septum called the atrioventricular (AV) node. The SA node generates electrical impulses that initiate the contraction of the atria, and the AV node regulates the timing of the contraction of the ventricles. The heart's rhythm is also influenced by the autonomic nervous system, which can speed up or slow down the heart rate in response to changes in the body's needs. The sympathetic nervous system increases heart rate and the parasympathetic nervous system slows it down. The heart's ability to contract and pump blood effectively is dependent on the function of the heart muscle cells (myocytes) and the electrical conduction system. Any problem with these systems can lead to heart rhythm disturbances (arrhythmias) and heart failure.