Like the rest of the body, the heart muscle (myocardium) needs oxygen-rich blood to survive. Blood is supplied to the heart by its own vascular system, called coronary circulation.
As the aorta ascends from the left ventricle, the two main coronary blood vessels (the right and left coronary arteries) branch away from it and encircle the heart. These main coronary arteries then branch off into smaller arteries to supply oxygen-rich blood to the entire heart muscle.
The right coronary artery supplies blood to the right side of the heart. The right side of the heart is smaller because it pumps blood only to the lungs.
The left coronary artery branches into the left anterior descending artery and the circumflex artery to supply blood to the left side of the heart. The left side of the heart is larger and more muscular because it pumps blood to the rest of the body.
Texas Heart Institute www.texasheartinstitute.com/HIC/Anatomy/coroanat.cfm