- Veins possess all three basic histological layers of cardiovascular tissues (See: Cardiovascular Histology). Venous architecture is specially designed for accommodating large blood volumes and thus the venous system is used as a reservoir for blood. Additionally, veins possess special architectural features (unidirectional valves) that allow blood to drain back to the heart under very little pressure differential.
- Tunica Intima:
- The venous tunica intima is composed of a traditional continuous endothelium which rests on a thin collagenous meshwork. In veins larger than a few millimeters, outpouchings of the tunica intima form "Venous Valves" which prevent backward flow of venous blood and thus guarantee unidirectional return of blood toward the heart.
- Tunica Media:
- The venous tunica media is composed of a variably thick layer of vascular smooth muscle cells which may contain some elastic fibers in larger veins. Contraction or layer is used to modulate the physiological use of the reservoir of blood within the venous system (See: Venous Physiology).
- Tunica Adventitia:
- The venous tunica adventitia is generally the most prominent layer of these vessels and is composed of collagenous tissue which blends into that of the surrounding tissue. In larger veins the tunica adventitia may contain some vaso vasorum.