Which structures comprise the capillary wall?

• A. Arteries have thick tunica media with smooth muscle and elastic fibers; veins have thinner walls, larger lumens, less smooth muscle, and valves.
• B. Endothelium, basement membrane, pericytes, and glycocalyx on luminal surface.
• C. Choroidal epithelium, basement membrane, macrophages, tunica media.
• D. Endothelial cells only.

Answer: (B)

Capillary walls are built from four key parts: endothelial cells lining the lumen, a basement membrane beneath them, pericytes wrapped around the capillary, and a glycocalyx coating on the luminal surface. The endothelial layer forms the barrier and site of exchange for gases and nutrients. The basement membrane provides structural support and contributes to selective permeability. Pericytes help stabilize capillaries and regulate blood flow and permeability, especially important in microvessels. The glycocalyx is a carbohydrate-rich coat on the inner surface of the endothelium that influences shear sensing, prevents some protein leakage, and modulates interactions with circulating blood cells.

Describing arteries and veins with thick tunica media and valves doesn’t fit capillaries, since capillaries lack a muscular tunica media and valves. Descriptions that include choroidal epithelium or tunica media mix tissues not involved in capillary walls. Saying only endothelial cells would omit the basement membrane, pericytes, and glycocalyx, which together complete the capillary wall structure.