Histology of the vein

A vein and an artery are built from the same three layers, called tunics. From the inside out: the tunica intima, which touches the blood; the tunica media, which holds the muscle; and the tunica externa, the outer wrapping. What differs between the two is not the list but the proportions.

In a vein the media is usually thinner than the externa, and it is made mostly of smooth muscle cells and collagen fibers, the fibers that provide support; the layer that bulks up is the externa, which usually ends up being the thickest layer of the wall. It is like a jacket whose outer fabric turns out to be the thickest of all while the middle layer stays thin. There is one more difference, and in a section you notice it by its absence: the internal elastic membrane, a band of elastic fibers at the edge of the intima, is not visible in a vein.

And there is one feature that, when present, is enough to recognize a vein. In medium-sized veins, the valves are what classically define them: one-way flaps, most abundant in the legs, that block backward flow, meaning blood that heads back the way it came.