The coronary system is comprised of arteries, arterioles, capillaries, and cardiac veins and venules. The coronary arteries originate with right and left main coronary arteries which exit the ascending aorta just above the aortic valve. These two branches subdivide and course over the surface of the heart (epicardium) as they traverse away from the aorta. These arteries divide into progressively smaller branches that then progress inward to penetrate the epicardium and supply blood to the transmural myocardium. Coronary arteries eventually branch into arterioles; arterioles then branch into innumerable capillaries that deliver oxygenated blood to all of the heart's cells. Blood continues through the capillaries to begin the return back into the cardiac chambers through venules, and then coalesce into the coronary veins. The coronary veins collect deoxygenated blood and return it to the right atrium through the coronary sinus, where it joins the systemic deoxygenated blood entering from the superior and inferior vena cavae.