Given a scientific passage and an answer, generate a question for the given answer.
Q: Passage: Blood vessels are long, tube-like organs that form a complex network throughout the body. They transport blood to all the body's cells. There are three types of blood vessels: arteries, veins, and capillaries. Transfer of substances between the blood and cells takes place across the thin walls of capillaries. Answer: capillaries
A:
Arteries and veins are two of the three types of blood vessels, what is the third?