William Harvey (1578-1657) was an English physician who described how blood is pumped in the heart and circulated throughout the body and returned to the heart via the lungs where it has picked up oxygen for re-circulation around the body.
There is controversy as to who first discovered the circulation of blood in the body. There is some tentative evidence that Muslim physicians knew something about blood circulation and, additionally, that some Italian physicians may also have learned about the nature of blood circulation in the body.
Harvey had studied at Padua, Italy where he might have learned about the nature of the heart, its valve systems and blood pumping system from his teacher, Hieronymus Fabricius.
Whereas all these may be true, in the English speaking world it was Harvey who first wrote about the circulation of blood (announced in 1616) and published in 1628 in his book Exercitatio Anatomica de Motu Cordis et Sanguinis in Animalibus. In that book, Harvey pointed out that blood was pumped to all parts of the body by the heart and that blood returned to the heart; blood, he pointed out, is circulated around the body.
Harvey had dissected animal and human bodies and actually observed how their hearts worked. Thus, he reached his conclusions on empirical observations, as opposed to earlier times when people merely speculated on what is the heart and blood and how they operated in the body.
Harvey correctly observed that blood flowed through the heart in two loops, one bringing blood to the heart and the other taking blood away from it. The names he gave to this mechanism are not important, what is relevant is his accurate description of what is happening in the heart and the heart’s role in circulating blood around the body.
The heart acts as a pump that forces blood to move through out the body; the outgoing blood carrying mostly oxygen and the retuning blood carrying mostly carbon dioxide.
Of course, Harvey’s work was rudimentary. For example, he did not show how blood was transferred from artery to vein, nor was he able to accurately describe the capillary network.
William Harvey. An Anatomical Exercise on the Motion of the Heart and Blood in Animals. (1628)
William Harvey. On the Motion of the Heart and Blood in Animals. London: George Bell and sons, 1889.