
Medium vein
Smooth muscle forms a more definitive and continuous tunica media in medium veins compared with small veins. Most named veins are medium veins. The valve in this vessel is formed by an extension of the tunica intima. Valves serve to prevent back-flow of blood and are most prevalent in the lower extremities. 200x

Endothelium
Smooth muscle forms a more definitive and continuous tunica media in medium veins compared with small veins. Most named veins are medium veins. The valve in this vessel is formed by an extension of the tunica intima. Valves serve to prevent back-flow of blood and are most prevalent in the lower extremities. 200x

Tunica media
Smooth muscle forms a more definitive and continuous tunica media in medium veins compared with small veins. Most named veins are medium veins. The valve in this vessel is formed by an extension of the tunica intima. Valves serve to prevent back-flow of blood and are most prevalent in the lower extremities. 200x

Tunica adventitia
Smooth muscle forms a more definitive and continuous tunica media in medium veins compared with small veins. Most named veins are medium veins. The valve in this vessel is formed by an extension of the tunica intima. Valves serve to prevent back-flow of blood and are most prevalent in the lower extremities. 200x

Valve flaps
Smooth muscle forms a more definitive and continuous tunica media in medium veins compared with small veins. Most named veins are medium veins. The valve in this vessel is formed by an extension of the tunica intima. Valves serve to prevent back-flow of blood and are most prevalent in the lower extremities. 200x