Which of the following statements regarding blood vessels is true?

Capillaries are indeed the site of gas exchange, making this statement accurate. This is the fundamental role of capillary networks within the circulatory system; they are the smallest blood vessels where oxygen, carbon dioxide, nutrients, and waste products are exchanged between blood and surrounding tissues. The walls of capillaries are extremely thin, consisting of only a single layer of endothelial cells, which facilitates this exchange process.

In contrast, arteries carry oxygenated blood away from the heart to the body's tissues, while veins return deoxygenated blood back to the heart. The structure of arteries, with thick muscular walls, allows them to withstand high pressure, while veins have thinner walls and valves that assist in the return of blood against gravity. Arterioles, which are smaller branches of arteries, play a crucial role in regulating blood flow into capillaries via vasoconstriction and vasodilation, thus disproving the statement regarding their inability to regulate blood flow. Therefore, the statement regarding capillaries being the site of gas exchange is the only true assertion among the options provided.