Structure of Vein Walls

Steps:

• Recall that blood vessel walls have three layers: tunica intima, media, and externa.
• Identify tunica intima as containing endothelium.
• Note tunica media includes smooth muscle and elastic tissue.
• Confirm tunica externa consists of collagen fibers.

Why D is correct:

• Veins have all three tunics, so their walls include endothelium (intima), smooth muscle and elastic tissue (media), and collagen fibers (externa), per standard histological definition.

Why the others are wrong:

• A: Smooth muscle is present in the tunica media of veins.
• B: Elastic tissue is present in the tunica media; collagen fibers are in the tunica externa.
• C: Endothelium lines the tunica intima of all blood vessels, including veins.

Final answer: D