The heart is a hollow, muscular organ, located in the center of your chest. Roughly the size of a human fist, the heart is the center of your body's circulatory system, and it performs 2 important functions: supplying oxygenated blood to your body and helping rid the body of waste products, such as carbon dioxide. With every beat your heart intakes, it pumps blood to and from your lungs, which in turn clear and reoxygenate your blood.
Your heart has a built-in pacemaker (sinoatrial node) that regulates your heartbeat and consequently, the pumping of blood. Your pacemaker works by generating regular electrical signals that spread through your heart muscle (myocardium), triggering your heart to pump blood out.
Pulmonary circulation - the circulation within the heart and lungs
When the heart muscle beats in response to electrical signals, it forces blood through the four chambers of the heart, namely the left and right atria and the left and right ventricles. The right atrium (upper chamber) and ventricle (lower chamber) receive oxygen-depleted blood from your body and send it to your lungs through your pulmonary artery. Your lungs get rid of the carbon dioxide as you exhale and restore your used blood's oxygen level as you inhale. The now oxygen-rich blood flows through your pulmonary veins into your left atrium and ventricle. When these chambers are full, your heart pumps the blood through the aortic valve into the aorta, the largest artery in your body. This oxygenated blood is then pumped away from your heart to supply all of your body, except your lungs.
In addition to regular electrical signals, your heart must have a constant supply of oxygenated blood in order to keep pumping. So, you have coronary arteries, which are large blood vessels in your heart muscle that supply your heart with all the oxygenated blood it requires.
The 2 main coronary arteries, the left main coronary artery and the right coronary artery, come off your aorta, just as it leaves your heart. These arteries run through your heart muscle, nourishing it with a vital supply of oxygen-rich blood. The left main artery has 2 major branches, called the left anterior descending and the circumflex.
The left anterior descending branch goes down the front of your heart and is a main source of blood to the left ventricle, your heart's main pumping chamber. The second major branch, the circumflex, goes around the side and back of your heart, supplying that area of your left ventricle. These arteries are like branches of a tree and have secondary, smaller branches that then divide into even smaller arteries and arterioles. If these arteries become narrow, blocked, inflamed, infected or injured, the blood supply to the heart muscle is reduced, which can lead to injury of the heart muscle and heart disease.