Which vessels are thick-walled and elastic, carrying blood away from the heart?

The correct answer is arteries. Arteries are blood vessels that are characterized by their thick and elastic walls, which enable them to withstand and maintain the high pressure of blood that is pumped directly from the heart. This elasticity allows the arteries to expand when the heart contracts and then recoil to help push the blood forward during the relaxation phase of the heart's pumping cycle.

Veins, on the other hand, carry blood back to the heart and have thinner walls with less elastic tissue, as the blood pressure within them is much lower. Capillaries are the smallest blood vessels where the actual exchange of gases and nutrients occurs, and they have very thin walls to facilitate this exchange. Venules are small vessels that collect blood from capillaries before it enters the veins, and like veins, they also have thinner walls and lesser elastic properties compared to arteries.

Thus, the unique structure of arteries, taking into account their thickness and elasticity, is critical for their function in the circulatory system, making them the vessels that carry blood away from the heart effectively.