The route taken by blood after it leaves the heart was a point of much confusion until the seventeenth century. Chinese emperor Huang Ti (2697-2597 B.C.E.) correctly believed that it flowed in a complete circle around the body and back to the heart. But in the second century, Roman physician Claudius Galen argued that it flowed back and forth in the veins, like air in the bronchial tubes. He believed that the liver received food from the small intestine and converted it to blood; the heart pumped the blood through the veins to all other organs; and those organs consumed the blood.

Huang Ti was right, but the first experimental demonstration of this did not come until the seventeenth century. English physician William Harvey (1578-1657) studied the filling and emptying of the heart in snakes, tied off the vessels above and below the heart to observe the effects on cardiac filling and output, and measured cardiac output in a variety of living animals. He concluded that (1) the heart pumps more blood in half an hour than there is in the entire body, (2) not enough food is consumed to account for the continual production and consumption of so much blood, and (3) since the planets orbit around the sun and (as he believed) the human body is modeled after the solar system, it follows that the blood orbits around the body. Thus, for a combination of experimental and superstitious reasons, Harvey argued that the blood must return to the heart rather than being consumed by the peripheral organs. He could not explain how it did so, however, since the microscope had yet to be invented and he did not know of capillaries–later discovered by Antony van Leeuwenhoek and Marcello Malpighi.

Harvey published his findings in 1628 in a short but elegant book entitled Exercitio Anatomica de Motu Cordis et Sanguinis in Animalibus (Anatomical Studies on the Motion of the Heart and Blood in Animals). This landmark in the history of biology and medicine was the first experimental study of animal physiology. However, so entrenched were the ideas of Aristotle and Galen in the medical community, and so strange was the idea of doing experiments on living animals, that Harvey's ideas were rejected. Indeed, some of his colleagues regarded him as a crackpot because his conclusion flew in the face of common sense–if the blood was continually recirculated and not consumed by the tissues, they reasoned, then what purpose could it possibly serve?

Harvey lived to a ripe old age, served as physician to the kings of England, and later did important work in embryology. His case is one of the most interesting in biomedical history, for it shows how empirical science overthrows old theories and spawns better ones and how both common sense and blind allegiance to authority sometimes interfere with acceptance of the truth. But most importantly, Harvey's contributions represent the birth of experimental physiology–the method that generated most of the information in this book.