What characteristic differentiates veins from arteries?

Veins are differentiated from arteries primarily by the presence of valves. These valves play a critical role in ensuring unidirectional blood flow back to the heart, particularly in the limbs where blood must travel against gravity. When muscles contract during movement, they help push blood through the veins toward the heart, and the valves prevent backflow, ensuring that blood continues to move in the correct direction.

In contrast, arteries, which carry oxygenated blood away from the heart (except for the pulmonary arteries), do not have valves because the high pressure from the heart's pumping action is sufficient to propel blood forward without the need for this additional mechanism.

The other options focus on characteristics that do not specifically set veins apart from arteries. For instance, arteries are typically more muscular than veins because they need to withstand and regulate the higher pressure of blood pumped directly from the heart. While it's true that arteries carry oxygenated blood, veins primarily carry deoxygenated blood back to the heart, again illustrating a functional difference rather than a structural one. Additionally, while veins may often be located closer to the surface in the body, there are many veins located deeper, just as there are arteries that are also found superficially. Thus, the presence of valves is the most distinctive