The correct option is Veins.
Explanation
The walls of major blood vessels (arteries and veins) are organized into three concentric layers: the tunica intima (inner lining), tunica media (middle muscular layer), and tunica externa (outer connective tissue). The structural variations in these layers correlate directly with the physiological function of the vessel.Detailed Analysis:
- Arteries: These vessels carry blood away from the heart under high pressure. To withstand this pressure and regulate blood flow, the tunica media is thick, consisting of abundant smooth muscle cells and elastic fibers.
- Veins: These vessels return blood to the heart under low pressure. While they possess the same three layers as arteries, the tunica media is comparatively much thinner, containing less smooth muscle and elastic tissue. This allows veins to be more distensible and hold larger volumes of blood (capacitance vessels).
- Capillaries: These are microscopic vessels designed for material exchange. They consist only of the tunica intima (endothelium) and a basement membrane. They lack a tunica media entirely. Therefore, in a comparative context regarding the "thinness" of an existing layer, the comparison is primarily between arteries and veins.
- Arterioles: These are small arteries that regulate resistance. They have a relatively thick tunica media in proportion to their lumen size to control blood pressure.
Key Takeaway:
The tunica media is the smooth muscle layer responsible for vessel contractility. It is thickest in arteries to manage high pressure and comparatively thinner in veins, which operate under lower pressure.