Final answer:
Medium-sized veins have valves and are measured in millimeters, while large veins like the superior and inferior venae cavae have wider diameters to transport greater volumes of blood and are essential for unidirectional flow toward the heart.
Step-by-step explanation:
The difference between a medium-sized vein and a large vein lies in their structure and function within the circulatory system. Both types of veins transport blood toward the heart. However, large veins, such as the superior and inferior venae cavae, have a more significant role due to their size, facilitating the transport of a greater volume of blood from larger regions of the body.
While medium-sized veins have valves to prevent blood backflow and are measured in millimeters, large veins also contain valves and are characterized by their wider diameters, which enable them to accommodate the increased volume of blood flow. These large veins are essential for the unidirectional flow of blood to the heart, particularly from the extremities, where blood flow is sluggish due to lower pressure and gravity's effects.