Discovery of the cardiovascular system: from Galen to William Harvey.

The goal of this review is to examine the events that led to discovery of blood circulation. The Ancient Greeks, including Hippocrates and Galen viewed the cardiovascular system as comprising two distinct networks of arteries and veins. Galen claimed that the liver produced blood that was then distributed to the body in a centrifugal manner, whereas air or pneuma was absorbed from the lung into the pulmonary veins and carried by arteries to the various tissues of the body. Arteries also contained blood, which passed from the venous side via invisible pores in the interventricular septum and peripheral anastomoses. This was an open-ended system in which blood and air simply dissipated at the ends of veins and arteries according to the needs of the local tissue. Blood was not seen to circulate but rather to slowly ebb and flow. This view would hold sway for 15 centuries until 1628 when William Harvey published his momentous 72-page book, On the Motion of the Heart and Blood in Animals. Harvey employed experiment and deductive logic to show that arteries and veins are functionally, if not structurally, connected in the lung and the peripheral tissues, and that blood circulates. The mechanical force of the heart replaced Galen's elusive attractive powers. Ultimately, Galenism would collapse under the weight of Harvey's evidence, and a new paradigm of blood circulation would prevail.