16 Human Circulatory System
He began a series of animal experiments in which he tied off a single artery or vein to
see what happened. Sometimes he clamped an artery and later released it to see where this
surge of blood would go. He did the same with veins, clamping a vein and then releasing it.
Sometimes he clamped both vein and artery and then released one at a time. These experiments proved that arteries and veins were connected into a single circulatory system and
that blood always flowed from arteries to veins.
Harvey turned to the heart itself and soon realized that the heart acted as a muscle and
pushed blood out to lungs and out into arteries. Following blood as it flowed through various animals, Harvey saw that blood was not consumed, but circulated over and over again
through the system, carrying air and nourishment to the body.
By 1625 Harvey had discovered an almost complete picture of the circulatory system.
He faced two problems. First, he couldn’t figure out how blood got from an artery across to
a vein, even though his experiments proved that it did. (Harvey had no microscope and so
couldn’t see blood vessels as small as capillaries. By 1670—three years after Harvey’s
death—Italian Marcello Malpighi had discovered capillaries with a microscope, thus completing Harvey’s circulatory system.)
The second problem Harvey faced was his fear of mob reactions, Church condemnation when he said that the heart was just a muscular pump and not the house of the soul and
consciousness, and the press (scribes). He was afraid he’d lose his job with the king. In 1628
Harvey found a small German publisher to publish a thin (72-page) summary of his work
and discoveries. He published it in Latin (the language of science), hoping no one in England would read it.
News of Harvey’s book raced across Europe and made him instantly notorious. He lost
many patients, who were shocked by his claims. But Harvey’s science was careful and accurate. By 1650 Harvey’s book had become the accepted textbook on the circulatory
system.
Fun Facts: Americans donate over 16 million pints of blood each year.
That’s enough blood to fill a swimming pool 20 feet wide, 8 feet deep,
and one-third of a mile long!
More to Explore
Curtis, R. Great Lives: Medicine. New York: Charles Scribner’s Sons Books for
Young Readers, 1993.
Harvey, William. On the Motion of the Heart and Blood in Animals. Whitefish, MT:
Kessinger Publishing, 2005.
Power, D’Arcy. William Harvey: Master of Medicine. Whitefish, MT: Kessinger,
2005.
Shackleford, Joel. William Harvey and the Mechanics of the Heart. New York: Oxford
University Press, 2003.
Wyatt, Hervey. William Harvey: 1578 to 1657. Whitefish, MT: Kessinger, 2005.
Yount, Lisa. William Harvey: Discoverer of How Blood Circulates. Berkeley Heights,
NJ: Enslow, 1998.