Veins

A) carry blood away from the heart.
B) carry blood under very high pressure.
C) may contain valves and are lined with endothelium.
D) are described as strong, rigid vessels that always carry oxygenated blood.
E) have thick, many layered walls.

Veins are characterized by the presence of valves that prevent backflow of blood and are lined with endothelium, a type of simple squamous epithelium. This contrasts with arteries, which carry blood away from the heart and often under high pressure. Veins, in general, carry blood back to the heart and do not always carry oxygenated blood, as this is the role of arteries except for the pulmonary artery. Veins also have thinner walls compared to arteries, not thick, many-layered walls.