Strange as it sounds, European medical doctors didn't know, until the 1600s, that the heart pumps blood around the body. How they proved this seemingly simple fact is one of the most controversial (and gruesome) stories in medicine.
It's a shock, sometimes, to learn how much rudimentary knowledge — things any child would know — was an utter mystery to past generations. This isn't because past generations were stupid, but because current ones don't know the work that goes into proving even the most basic facts. Sometimes these proofs were made more difficult by prevailing theories. In the 1600s, people were taking 1,400-year-old advice from the teachings of the Greek physician Galen. He had believed that the lungs circulated the blood, or rather pushed the blood out into the body. Blood didn't circulate in the body, Galen claimed, but was consumed by the organs.
William Harvey, a court physician to two different kings, had another view of the situation. After dissecting both dead animals and humans, and examining their hearts, he realized that valves in the heart allowed the blood to move only one direction. Closer examination showed that the walls of the heart were solid, not punctured by capillaries which would allow blood to seep from one part of the heart to another - as Galen had claimed. Harvey began to realize that the heart, and not the lungs, might be responsible for moving the blood around the body.
Around and around and around the body, in fact. Harvey made an estimate of the amount of blood the heart pumped. Part of this involved ripping apart a human heart to see how much blood it could hold at any one time. (About two ounces!) Then Harvey made a study of the heart rate of living people, and some quick calculations. Human heart rates showed Harvey that the heart pumped blood so quickly that the human body couldn't possibly be making such a quantity of blood and serving it up for the rest of the bodily organs to "eat." Blood had to be circulated in the arteries and veins, continuously moving through the body.
This is where things got horrific. Harvey couldn't prove that the heart was a pump without observing, and sometimes demonstrating, circulating blood and a beating heart. He was one of the early practitioners of vivisection. After some practice, he found that a heart will keep beating even when removed from an animal's body. Most of the time, he simply observed and recorded the movements of a beating heart in situ, and noted that its function resembled that of a conventional pump. He did this for years, documenting all the experiments so he could make a summary of them and publish it.
Harvey was, in turn, ripped into by his Galenist critics. It took multiple decades for people to accept that the heart moves blood in a continuous loop around the body. As he was right, his theories won out. Harvey is looked to as the father of cardiology. He also proved that even the most basic facts can only be established with a lot of work — and sometimes some very nasty experiments.