It Took People Freakishly Long To Realize That Blood Circulates

Go back to the 1600s, and you’ll know most people thought that the Earth was in the center of the universe, while Galileo said the Earth revolved around the Sun, and people didn’t much care for that idea. At around the same time, science was digging through a controversy over how something else moved, something whose motion you’d think would be much easier to observe than a planet. We’re talking about blood, blood in the human body.

We’ve always known that blood is essential for life, since long before recorded history. But as for where blood comes from and what exactly it does, we were vague on that for a long time. Galen, a Greek doctor from the second century, believed that the liver produces blood continuously. This blood flows to the tissues, which eat the blood up, then the liver makes even more blood, which trickles to every extremity.

Of course, you know that that theory leaves out some crucial steps. Once blood reaches your fingers, toes, etc., it circulates right back to the heart that pumped it. Tissues absorb some nutrients from the blood, but the blood itself flows around the body over and over again. But they didn’t know that in the second century. In fact, we didn’t figure that out for another millennium and a half after that.

Through 1,400 years of primitive surgery, of people bleeding in wars, of anatomists like Leonardo, no one realized that blood circulates. They observed the heart, but they thought it combines a substance called pneuma from the lungs with blood from the liver; they didn’t realize that the heart takes in and pumps out blood through a system that’s a closed loop. 

In 1628, an English doctor named William Harvey published a book that stated the truth. He started with a calculation that anyone else could have made—he figured out that if the liver really does make blood continuously, then based on how quickly blood flows, it has to make four times a person’s weight in blood every hour, which is absurd. Then he dug into bodies, followed blood around, and found out where it all really went.

Everyone laughed at him. They called him a “circulator,” derisively, and not till some decades after his death did consensus flip his way.

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