What branches out from a single arteriole and allows for diffusion of substances?

The correct answer is capillaries, which are small blood vessels that branch out from arterioles. Their primary function is to facilitate the exchange of substances between the blood and surrounding tissues. The walls of capillaries are composed of a single layer of endothelial cells, which allows for easy diffusion of gases, nutrients, and waste products.

Capillaries are the site where oxygen and carbon dioxide exchange occurs, along with the transfer of other essential substances like glucose and hormones. Their extensive network throughout tissues ensures that cells are adequately supplied with necessary nutrients and can efficiently eliminate waste.

In contrast, veins and arteries are larger vessels that primarily serve to transport blood; veins carry blood back to the heart, while arteries carry blood away from it. Venules are small vessels that collect blood from capillaries but do not facilitate the diffusion of substances in the same manner. Each of these vessel types plays a critical role in the circulatory system, but it is specifically the capillaries that enable the diffusion of substances at the cellular level.