Final answer:
Capillaries are the smallest blood vessels that connect arterioles to venules and facilitate the exchange of substances with tissue fluid in capillary beds.
Step-by-step explanation:
The blood vessels that connect arterioles to venules and exchange molecules with tissue fluid are known as capillaries.
These are the smallest blood vessels in the cardiovascular system, and they function as the primary sites for the exchange of substances such as oxygen, nutrients, and waste between the blood and the surrounding tissues. The walls of capillaries consist primarily of a single layer of epithelial cells which allows for such exchange. They form a network known as a capillary bed, which provides a large surface area for efficient material transfer. Capillaries eventually converge into venules, leading blood back towards the heart.