Why doesn't the heart act like an opportunist and just take its oxygen straight from the blood that's constantly passing through its chambers? There are two important reasons. First, not all of the blood in the heart has oxygen. Remember, your heart is divided into four chambers: the left atrium and left ventricle, as well as the right atrium and right ventricle. The left side is receiving "good," oxygen-rich blood from your lungs, sending it through the left atrium down into the left ventricle, where the heart's contraction forces it through a valve into the aorta.
The left side of the heart would be OK if it took its oxygen right there in the chambers, but the right side would be out of luck. The right atrium handles "bad," oxygen-poor blood from all over the body and ships it down into the right ventricle. From there, the heart's contraction forces this bad blood out into the pulmonary artery, which forwards it to the lungs. This blood gains oxygen again, but it still returns to the side of the heart dealing in good blood -- the left side. The right side never sees oxygenated blood pass through it, so it needs its own supply.
The second reason for this tricky route is that the rest of the body wants fully oxygenated red blood cells, not just castoffs from the heart. If the heart replenished its own oxygen supply directly from the river of blood flowing through it, the blood leaving the heart would have less oxygen to deliver to the body. Instead, the muscle exercises some restraint and feeds itself through arteries that tap into the aorta, just like every other part of the body.
Once the aorta leaves the heart, blood can branch off almost immediately into the left coronary artery and the right coronary artery. The left artery splits into two large branches: the left anterior descending and the left circumflex.Their names describe their routes along the surface of the heart -- circumflex just means the artery winds around the heart instead of hanging straight down. The right coronary artery supplies the right side of the heart, and branches off into the posterior descending artery.
Interestingly, we don't all have the same network of arteries. Some people have a third main coronary artery that juts off the aorta, and others just have one main coronary artery. In addition, sometimes it's the right coronary artery, instead of the left, that branches into two more arteries.
Regardless of these differences, the result is the same: The arteries branch off into smaller arteries, and all the arteries and their branches wrap around the heart, like a crown (hence, the "coronary"). This way, all the heart's tissue receives a fresh blood supply, giving it the fuel it needs to pump day after day.
Next, we'll look at what happens when those coronary arteries don't deliver all the fuel that the heart needs. It's not good.