Final answer:
The portal vein, as a vein, contains valves that prevent the backflow of blood, a feature that arteries do not have. Arteries are built to withstand higher pressure, evident by their thicker walls, and under a microscope, their tunica media appears wavy due to muscle contraction, unlike the smooth appearance of veins.
Step-by-step explanation:
The portal vein is a blood vessel that transports blood toward the heart, unlike arteries which carry blood away from the heart. One significant difference between veins and arteries is that veins contain valves that prevent the backflow of blood, aiding unidirectional flow towards the heart. These valves are especially necessary in lower extremities where gravity could otherwise cause blood to pool. Since the portal vein is a vein, it has these valves, whereas an artery does not.
Under the microscope, the structure of veins and arteries can also be differentiated by their tunics. While both have endothelial inner linings known as the tunica intima, and surrounding layers of smooth muscle (the tunica media) and connective tissue (the tunica externa), the tunica media of arteries is typically thicker to accommodate the higher pressure of blood that has just been pumped from the heart. On the other hand, veins including the portal vein, which operates under lower pressure, have thinner walls, larger lumens, and are equipped with these critical valves. Interestingly, arteries exhibit a wavy appearance under the microscope due to the partial constriction of their smooth muscle, whereas the lumen and the entire tunica intima of a vein appear smooth because veins do not require the same degree of constriction for blood flow.