Harvey

In the second century ce , the Greek physician Galen theorized that blood is created in the liver, passes once through the heart, and is then absorbed by bodily tissues. Galen's ideas were widely accepted in European medicine until 1628, when William Harvey published a book describing the circulation of blood throughout the body. Through his observations of human and animal dissections, Harvey saw that blood flows from one side of the heart to the other and that it flows through the lungs and returns to the heart to be pumped elsewhere. There was one missing part of the cycle: How did the blood pumped to distant body tissues get into the veins to be carried back to the heart? As an answer, Harvey offered his own, unproven theory, one that has since been shown to be true: blood passes from small, outlying arteries through tiny vessels called capillaries into the outlying veins. Harvey's views were so controversial at the time that many of his patients left his care, but his work became the basis for all modern research on the heart and blood vessels.