What role did William Harvey think arteries and veins played in blood circulation?

William Harvey revolutionized medical understanding by demonstrating that blood flows rapidly through a single, continuous system of arteries and veins.1

Contrary to the prevailing belief of his time—which held that there were two separate blood systems and that blood was consumed by the body at the same rate it was produced—Harvey argued that the heart acts as a pump, expelling blood into the arteries. He concluded that because the heart pumps more blood than the body could possibly produce or consume, the blood must circulate throughout the entire body.1

While Harvey correctly identified this circulatory path, he was unable to observe the capillaries, the tiny vessels that link arteries and veins. Consequently, he was obliged to assume their existence to complete his theory of circulation, a link that was not confirmed until later in the 17th century by Marcello Malpighi.12