Location:
The coronary system is comprised of arteries, arterioles, capillaries, venules and veins. From the innumerable cardiac capillaries, blood flows back to the cardiac chambers through venules, which in turn coalesce into the cardiac veins. Most cardiac veins collect and return blood to the right atrium through the coronary sinus; there may or not be a Thebesian valve covering the ostium of the coronary sinus. The major venous vessels of the human heart are: coronary sinus, the anterior interventricular veins, left marginal veins, posterior veins of the left ventricle, and the posterior interventricular veins (see also the Coronary System Tutorial).

Function:
The cardiac veins returns deoxygenated blood (containing metabolic waste products) from the myocardium to the right atrium. This blood then flows back to the lungs for reoxygenation and removal of carbon dioxide.

Importance in cardiovascular diseases:
Cardiac veins contain valves preventing back flow; a Thebesian valve may or may not cover the ostium of the coronary sinus. Typically cardiac veins are free of atherosclerotic plaques.

Importance in device delivery:
Left heart pacing can be achieved by placing leads into the cardiac veins through the coronary sinus, which is located within the right atrium. While cannulating the coronary sinus ostium can be challenging due to the presence of the Thebesian valve, venous valves within the coronary venous system may hinder advancement of guide wires, catheters or pacing leads. Stem cell or other biologic therapies can be delivered to the heart via the coronary vasculature.