Final answer:
Capillaries are the smallest blood vessels that allow the exchange of substances between the blood and cells. They have a microscopic size and form a capillary bed for efficient exchange.
Step-by-step explanation:
Capillaries are the smallest blood vessels that connect arterioles and venules, allowing the exchange of substances including oxygen and nutrients with cells. They have a diameter of 5-10 micrometers and form a capillary bed for efficient exchange of substances between the blood and surrounding tissues. Capillaries are essential for the perfusion process, where blood is supplied to the tissues.