Cobra Effect · Body and medicine
The blood goes round
How a simple sum showed that the same blood goes round and round the body.
7 cards, read aloud in 3:07, with a test and sources.
For fourteen centuries, doctors believed the liver made blood that the body used up.
The Greek physician Galen, writing in the second century, taught that blood was made in the liver from food. It flowed out to the body and was consumed, like fuel, so it had to be made again and again. Galen’s ideas shaped medicine for well over a thousand years. An English doctor would test the idea by measuring.
William Harvey, physician to King Charles I, studied the heart as a pump.
Harvey had trained in Padua, in Italy, under the anatomist Fabricius. He watched the hearts of living animals, and saw each beat squeeze blood out into the arteries. He studied the valves in the heart and veins, which let blood flow only one way. In 1628 he published his conclusions in a short book on the motion of the heart and blood.
Harvey did a simple sum, and Galen’s idea could not survive it.
He estimated how much blood each heartbeat pushed out, and multiplied by the number of beats in an hour. Even with cautious guesses, the heart pumped more blood in an hour than the whole body weighs. No liver could make that much blood from food, hour after hour. The same blood had to be going round and round.
A band tied round an arm showed which way the blood flows in the veins.
Harvey tied a band tightly round a person’s upper arm, and the veins below it swelled. Small knots appeared along the veins, where the valves were. When he pressed on a vein and pushed the blood towards the heart, it moved easily. Pushed the other way, the valves stopped it. Blood in the veins was flowing back to the heart.
Blood leaves the heart in the arteries, and comes back in the veins.
One loop carries blood from the heart through the lungs and back again. Another carries it out around the body and back to the heart. Harvey argued that the same blood circulates, over and over. Many doctors resisted the idea for years.
One piece of the puzzle was missing, and Harvey never saw it.
Harvey could not find how blood crossed from the smallest arteries into the smallest veins. He died in 1657. In 1661 Marcello Malpighi, looking through a microscope at the lungs of a frog, found tiny vessels linking the two. They are called capillaries, and they closed the circle.
So when an old idea is widely believed, ask whether the numbers allow it.
Galen’s picture of blood being made and used up seemed reasonable for centuries. Harvey tested it with a rough calculation that anyone could follow. Even a cautious sum showed it could not be true. Sometimes the simplest arithmetic is the strongest evidence.
Sources
- William Harvey and the discovery of the circulation of the blood, Circulation Research. Harvey’s experiments, his calculation, and how the idea was received.
- Ceaseless motion: William Harvey’s experimentations in circulation, Royal College of Physicians Museum. An exhibition on Harvey’s experiments and his book of 1628.
- Marcello Malpighi and the discovery of the pulmonary capillaries and alveoli, American Journal of Physiology. How Malpighi found the vessels that closed Harvey’s circle.
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