How did William Harvey prove that blood goes round in a circle, using arithmetic and a bandage?

Harvey did it with arithmetic, a great many animals and a tight bandage. In 1628 the English physician William Harvey published his treatise on the circulation of the blood. He was the first known physician to describe completely, and in detail, pulmonary and systemic circulation, as well as the process of blood being pumped to the brain and the rest of the body by the heart, though earlier writers had provided precursors to some of his theories.

The old view came from Galen. Until the 17th century, two separate systems were thought to be involved: venous blood that had its origin in the liver, and arterial blood and the "spirits" that flowed from the heart. Galen believed that blood passed between the two sides of the heart by means of invisible pores.

Harvey's sharpest weapon was a sum. He estimated the capacity of the heart, how much blood each beat pushes out, and how many times it beats, and he worked out that the liver would have to produce 498 pounds of blood in a day, more than the weight of the whole body. His estimates were purposefully low, so that people could see the vast amount of blood Galen's theory demanded. The only way the sum worked was if the same blood went round and round.

He then tied a tight ligature around a person's upper arm. The arm below was cool and pale, while above the ligature it was warm and swollen. Blood in a vein could be pushed up the arm but not down, past little valves discovered by his teacher Hieronymus Fabricius, which led him to believe that veins carry blood to the heart and the valves keep it moving one way.

Acceptance was slow, and it took twenty years for his theory to be generally accepted.