Smallest cardiac veins (Thebesian veins)

Location and function

The smallest cardiac veins are located within the subendocardial part of the myocardium of all four chambers of the heart. They comprise the smaller cardiac venous system, which represents the direct connection between the chambers of the heart and the larger cardiac vessels that drain the epicardial surface of the myocardium.

The smallest cardiac veins open into lumina of the corresponding cardiac chamber. They can do so directly or indirectly, according to which they are classified as:

• Venoluminal veins, which empty directly into the chambers of the heart
• Venosinusoidal veins, which indirectly drain into the cardiac chambers via the subendocardial sinusoids
• Arterioluminal veins which connect the small arterial vessels of the inner myocardium with the cardiac chambers
• Arteriosinusoidal veins, which connect the small arterial vessels with the subendocardial sinusoids

The smallest cardiac veins of the left ventricle contribute to the physiological shunting of blood, as they lead to the mixing of the venous blood with the oxygenated blood that comes from the lungs. This is a normal process noted to explain why the blood from the left ventricle is less saturated with oxygen than the blood that it originally receives from the pulmonary veins.