The heart is the main organ of the cardiovascular system. Its role is to provide the body with the blood and oxygen its needs to function properly. It is diveded into four chamber or cavities. The upper chambers, callled atria, collect blood returning to the heart. The right atrium accumulateds blood from the body that is poor in oxygen and the left atrium receives blood replnished with oxygen from the lungs. The atria empty blood inot the lower chambers, rescectively right and left ventricles. the blood in the right ventricle is passed into the lungs to be reoxygenate and the blood in the leftt ventricle i
s pumed out the reste of the body.

During a normal heart beat, an electrical impulse originates in the right atrium in the sinoatrial node (SAN), the intrinsic pacemaker, and spreads throughout both the right and left atria and down the interatrial septum to the atrioventricular node (AVN).  
The impulse slows down briefly at the AVN and then travels down a common pathway, the His bundle, splitting off into the left and right bundle branches located in both ventricles. This elecgtrical activation causes the muscular contraction, which allows the ejection of the blood through the cardiovascular system. The whole precess is repeated at a reate of 60 to 100 beats per minute.

When the heart does not beat at a steady rate, the episodes are called arrhythmias. If the heartbeat is too slow, it is called a bradyarrhythmia or bradycardia. If it is too fast, it is called a tachyarrhythmia or tachycardia. However, depending on the part of the heart affected, the consequences for the person involved are quite different. The most serious cardiac rhythm disturbance is called ventricular fibrillation. Here, the ventricles quiver,which is due to a chaotic, uncoordinated electrical activity and the heart is prevented from pumping, leading inevitably to cardiac arrest and sudden death.