William Harvey

Harvey published De Motu Cordis in 1628, demonstrating that the heart pumps a fixed volume of blood in a continuous circuit through the body rather than the body continuously producing and consuming new blood, as Galen's model held. His case rested on a quantitative argument: he calculated the volume of blood the heart could pump in an hour and showed it was far more than the body could plausibly manufacture or absorb in that time, so the same blood had to be recirculating. He supported the conclusion with experiments tying off blood vessels in living animals and watching where blood pooled. The capillaries completing the circuit between arteries and veins were not directly observed until microscopy caught up, decades after his death. This description is adapted from Wikipedia contributors under CC BY-SA 4.0; changes were made. https://creativecommons.org/licenses/by-sa/4.0/