The English physician William Harvey forever changed the course of anatomical science in the first half of the 17th century by discovering that blood flows in a continual and repeating movement throughout the circulatory system. Harvey was interested in determining how organs worked together as a system and observed that the expansion of the arteries coincided with the contraction of the heart. He was the first to understand and prove that the blood flowed throughout the body in a continuous circuit and flowed out from the heart through the arteries and then returned through the veins.
Harvey ushered in a new approach to physiology by including mathematical measurements in his anatomical reasoning. By doing so, he was able to demonstrate that the previously held belief regarding blood was incorrect. Before Harvey's discovery, it was believed that blood was produced in the liver through the conversion of food and then distributed to other organs through the veins. It was also previously assumed that the blood the liver produced was absorbed by the organs rather than recirculated. Harvey showed, through mathematical calculations based on the amount of blood pumped by the heart, that if this were true, the liver would be producing 540 pounds of blood each day. This was also an impossible amount of blood to be absorbed by the organs. This led Harvey to the correct assumption that blood flowed in a continuous circuit and that the veins were the route taken for the return trip.