Final answer:
The smallest blood vessels in the human body are called capillaries. They connect arterioles and venules and facilitate the exchange of substances like oxygen between the blood and tissues.
Step-by-step explanation:
The term that describes the smallest blood vessels in the human body is capillaries. These tiny vessels are crucial for the exchange of substances, including oxygen and nutrients, between the blood and the surrounding tissues. Capillaries connect arterioles, which are small branches of arteries, and venules, which are small vessels that lead to veins. They form a network called a capillary bed, optimizing the surface area for efficient transfer of substances.
Arteries carry blood away from the heart into smaller vessels, while veins return blood back to the heart. Capillaries serve as the site where this exchange occurs, as they are thin enough to allow a single red blood cell to pass through at a time. The function of capillaries is vital for delivering oxygen to tissues and removing waste products.