How we learned blood circulates
For about 1,400 years, doctors believed the liver made blood fresh and the body used it up. One English doctor did the math and proved them wrong.
The short version
- In 1628, William Harvey showed the heart pumps the same blood around the body in a loop.
- His proof was a simple calculation: the heart pumps far more blood than the body could ever make.
- He couldn’t see capillaries. They were found a few decades later with a microscope.
The old idea
Since the Greek physician Galen in the second century, doctors taught that the liver turned food into blood, which flowed out to the body and was consumed, like fuel. Veins and arteries were thought to carry different substances.
What Harvey did
William Harvey, physician to King Charles I, published his book De Motu Cordis (On the Motion of the Heart) in 1628. He estimated how much blood the heart pushes out with each beat, then multiplied by the number of beats. Even with cautious numbers, the heart moved more blood in an hour than a person weighs. The body could not possibly make that much blood that fast. It had to be going around in a circle.
He backed this up with experiments, including tying off arm veins and showing their valves only let blood flow toward the heart.
The missing piece
Harvey couldn’t explain how blood got from arteries to veins. In 1661, the Italian anatomist Marcello Malpighi used a microscope to see capillaries in a frog’s lung, the tiny connecting vessels Harvey predicted.
Why it still matters
Harvey’s method (measure, calculate, test) is the backbone of the studies on this site. Every modern blood flow measure, from blood pressure to ultrasound, starts from the idea he proved.
Sources
Educational information, not medical advice. Read our medical disclaimer.