Heart is a muscular organ whose prime function is to pump the oxygen and nutrient rich blood to different part of body through  vessels called “arteries” (except pulmonary artery which carries impure blood). The impure blood return back to the heart by means  of vessels called “veins” (except pulmonary vein which carries pure blood) The pure and impure blood inside the adult heart are separated by a wall (septum) which divides the heart in two separate parts. Each part is further subdivided into upper and lower  chambers. The upper ones receives the blood and lower ones pumps the blood out. The left upper chamber (left atrium) receives  pure blood from lungs and passes it to left lower chamber (left ventricle), through a valvular opening (mitral valve), which pumps the  oxygen rich blood to body organs. The right upper chamber (right atrium) receives impure blood through veins and passes it to right  lower chamber (right ventricle), through the valvular opening (tricuspid valve), which pumps it to lungs for oxygenation.

In order to reach different organs, blood has to be pumped at a high pressure. For this heart muscle work very  hard especially of the left ventricles and need a good supply of oxygen, hence blood. This blood supply to heart muscles is accomplished by vessels called coronary arteries. The coronary arteries and its branches not only supplies blood to the heart  muscles but also to the electrical impulse conducting system. The left coronary artery is larger and divided into two arteries – the anterior descending artery and circumflex artery. It supplies blood to most of the left atrium, left ventricle and inter ventricular  wall. The right coronary artery is smaller and supplies the blood to the right atrium and most of the right ventricle. This means the blood supply to the muscles of heart is through three arteries namely left anterior descending, left circumflex and right  coronary artery. The blood flows in these coronary arteries only between the beats when the heart muscles relax (i.e. during disstolic phase).