How We Found Out Blood Goes Round

Step 1 of 5

For 1,400 years, blood was thought to be used up.

The accepted picture was that the liver made blood from food, the body consumed it like fuel, and more was made. It was detailed, internally consistent, taught everywhere, and completely wrong.

Step 2 of 5

Harvey measured how much the heart holds.

He worked on hearts from dead animals and found each beat pushes out a couple of tablespoons. That is a small, unremarkable number, and nobody had thought it worth combining with anything else.

Step 3 of 5

Then he multiplied by the pulse.

About seventy beats a minute, sixty minutes an hour. That comes to roughly 250 litres of blood pumped every hour — around three times the weight of the whole body, every hour, all day.

Step 4 of 5

So it must be the same blood, coming back.

That was the only possibility left. The heart is not a factory but a pump on a loop, and the blood in it now is blood that has been round before. He confirmed it with a simple test anyone can do.

Step 5 of 5

The veins gave it away.

Press a vein in your arm and slide your finger along it. It empties and stays empty until you let go — because valves inside it only allow flow one way, towards the heart. Blood in veins always travels back. It had been drawn in textbooks for years without anyone asking what one-way valves were for.

Harvey could not see the connection between arteries and veins — capillaries are too small. They were found with a microscope thirty years later, after he died.