Your heart, which is about the size of your clenched fist, is made up of four chambers: two on the left and two on the right. The two upper chambers receive blood into the heart and are called atriums, while the two lower chambers pump blood to the rest of the body and are called ventricles. The left ventricle sends blood out to all parts of the body, except the lungs; the right ventricle supplies only the lungs with blood.
The left and right sides of the heart are separated by a wall of muscle, called the septum. The valves allow blood to flow in only one direction. When the atria fill with blood from the veins returning to the heart, the valves have higher pressure above than below, and the atria contract, letting blood flow into the ventricles. The filled ventricles then contract and force the blood out through the mitral and tricuspid valves.
The aortic and pulmonary valves open and snap shut, letting blood into the aorta and pulmonary artery. As these valves snap closed, they prevent blood from returning to the ventricle. The sound it makes when this happens is what we call a heartbeat. It supplies every organ in the body with life-sustaining oxygenated blood, except the heart itself. The coronary arteries are the heart’s private circulatory system, fed directly from the aorta.
The two main coronary arteries, right and left, branch and divide into smaller and smaller tributaries. Heart muscle would soon die if a main coronary artery became blocked and could not supply enough blood, but the smaller branches are capable of connecting with those of other arteries, providing what is called collateral circulation. The coronary arteries start from the aorta and spread to many smaller branches that supply the heart muscles.
When plaque blocks these arteries, blood does not reach the muscles. Without oxygenated blood, muscles cannot contract to pump blood.