Final answer:
Tiny blood vessels between arterioles and venules are called capillaries, which are integral to the cardiovascular system for the exchange of substances with tissue fluid.
Step-by-step explanation:
Tiny blood vessels that connect arterioles and venules are called capillaries. Capillaries are the smallest type of blood vessel in the cardiovascular system, and they are essential for the exchange of substances between the blood and surrounding tissues. The process of this exchange is facilitated by the capillary bed, a network of capillaries that provides a large surface area for optimal transfer of nutrients, gases, and waste products. The walls of capillaries are only one cell thick, which allows for an efficient exchange through the thin endothelial layer. Nested between the resistance vessels (arterioles) and the small veins (venules), capillaries form an integral part of the microcirculation.
Arterioles, with their thinner walls and ability to regulate blood pressure through vasoconstriction and vasodilation, lead into the capillary beds. After the exchange of substances in the capillaries, the blood enters postcapillary venules. These then join to form larger veins, which carry the blood back toward the heart. The critical function of capillaries within the cardiovascular system is reflected in their uniquely adapted structure and role in perfusion and the exchange process.