Human Circulatory System
Year of Discovery: 1628
What Is It? The first complete understanding of how arteries, veins, heart, and
lungs function to form a single, complete circulatory system.
Who Discovered It? William Harvey
Why Is This One of the 100 Greatest?
The human circulatory system represents the virtual definition of life. No system is
more critical to our existence. Yet only 400 years ago, no one understood our circulatory
system. Many seriously thought that the thumping inside the chest was the voice of the conscience trying to be heard. Most thought that blood was created in the liver and consumed
by the muscles. Some still thought that arteries were filled with air.
William Harvey discovered the actual function of the major elements of the circulatory
system (heart, lungs, arteries, and veins) and created the first complete and accurate picture
of human blood circulation. Harvey was also the first to use the scientific method for biological studies. Every scientist since has followed his example. Harvey’s 1628 book represents the beginning of modern physiology.
How Was It Discovered?
Through the sixteenth century, doctors relied on the 1,500-year-old writings of the
Greek physician Galen, who said that food was converted into blood in the liver and was
then consumed by the body for fuel. Most agreed that the blood that flowed through arteries
had no connection with the blood that flowed through veins.
William Harvey was born in 1578 in England and received medical training at Oxford.
He was invited to study at Padua University in Italy, the acknowledged medical center of
Europe.
When Harvey returned to England in 1602, he married the daughter of Queen Elizabeth’s doctor, was appointed a physician in the court of King James I, and was then appointed personal physician to King Charles I in 1618.
While serving the English kings, Harvey focused his studies on veins and arteries. He
conducted extensive experiments with animals and human corpses. During these dissections, he discovered the series of flap valves that exist throughout the veins. He was not the
first to find these valves, but he was the first to note that they always directed blood flow toward the heart. Blood flowed in veins only from the arms, legs, and head back to the heart.
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