The heart is not an opportunist. It does not take its oxygen straight from the blood that is passing constantly through its chambers.
There are a couple of reasons it doesn't. First of all, not all of the blood in the heart has oxygen.
Don't forget, the heart is divided into four chambers: the left atrium and the left ventricle and the right atrium and the right ventricle.
The left side of the heart is receiving oxygen-rich blood from your lungs and sends it through the left atrium, down into the left ventricle. That's where the heart's contraction forces it through a valve in the heart's aorta.
The left side of the heart would be just fine if it took its oxygen right there in the chambers, but the right side would not be so fortunate. The right atrium handles the oxygen-poor blood from throughout the body and ships it down into the right ventricle.The heart's contraction then forces this bad blood into the pulmonary artery, which takes it to the lungs where it gains oxygen again. Then it returns to the left side of the heart. The right side of the heart never sees oxygenated blood, so it requires its own supply.
Once the aorta leaves the heart, the blood can branch off into the left and right coronary arteries. The left artery splits into two large branches which are called the left anterior descending and the left circumflex. The names of these branches describe their routes along the heart?s surface. Circumflex just means the artery winds around the heart instead of hanging straight down. The right coronary artery supplies blood to the right side of the heart. Then it branches off into the posterior descending artery.
Some people have a third main coronary artery and others have just one main coronary artery.Also, sometimes right coronary artery, instead of the left coronary artery, branches off into two more arteries.
Despite these differences, the result is the same. The arteries branch off into smaller arteries, and the arteries and their branches wrap around the heart, like a crown. Thus, the heart's tissue receives fresh blood, giving it the fuel it needs to work.