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Capillaries are the blood vessels that form a network between arterioles and venules. They are the smallest and most numerous blood vessels in the body, facilitating the exchange of oxygen, nutrients, and waste products between the blood and surrounding tissues. This exchange process occurs because the walls of capillaries are incredibly thin, allowing for easy diffusion of substances.

Arteries are responsible for carrying blood away from the heart and are typically larger and more muscular than capillaries, primarily serving a different function in the circulatory system. Veins return blood to the heart and are also not involved in the capillary network. Arterioles are smaller branches of arteries that lead into capillaries but do not form the exchange network themselves. Thus, capillaries play a critical role in connecting the two and allowing for vital exchange processes in the body.