A doctor proved your blood moves in one closed loop, defying 1,400 years of belief

Physicians since Galen believed the liver constantly manufactured new blood that the body simply burned up. In 1628, William Harvey calculated that the heart pumps far more blood per hour than the entire body contains, proving blood must circulate in a closed loop and be reused. He worked it out from a pulse rate and simple arithmetic, decades before anyone could see a capillary.

— William Harvey, Exercitatio Anatomica de Motu Cordis et Sanguinis in Animalibus — published 1628