Blood Circulation
The heart and blood have been recognized since earliest times as vital elements of life, but the way in which they function was not understood until William Harvey's discovery of the mechanics of the circulatory system in the seventeenth century.
Various Greek physicians had distinguished arteries from veins, but each—Alcmaeon (born c. 535 B.C.) and Praxagoras (born c. 340 B.C.)--thought that arteries carried air, since arteries are usually empty in corpses (the source for most of these investigations). The Alexandrian anatomist Herophilus observed the arterial pulse, which he associated with the heartbeat, and held that arteries contain blood, not air. His fellow Alexandrian, Erasistratus, came close to a concept of the circulatory system, maintaining that both arteries and veins originated from the heart and divided repeatedly throughout the body, ending in fine capillaries. Erasistratus saw the heart as a pump, but he repeated the error of regarding arteries as conduits for air.
All these early ideas about the circulatory system were superseded by the medical system described by Galen, a schema that remained firmly in place until early modern times. According to Galen, the veins originated in the liver, where blood was formed, and flowed from there to all parts of the body, where it was consumed. Some blood was carried to the heart, where it passed from one side to the other through minute pores in the septum. In the left side of the heart, Galen maintained, the blood mixed with air, creating a vital spirit that was carried throughout the body by the arteries.
In the centuries after Galen, Christian, Jewish, and Muslim authorities firmly opposed postmortem dissection, so Galen's beliefs about anatomy and physiology stood virtually unchallenged for nearly fourteen centuries. One exception was the Muslim physician Ibn an-Nafis (c. 1205-1288), who accurately believed the right and left ventricles to be separate, and described movement of blood between them via the lungs. His work, however, passed into obscurity and was not rediscovered in the West until 1924.
The new scientific spirit that began stirring in the sixteenth century gave rise to questions about Galen's circulatory theory. Leonardo da Vinci (1452-1519) accurately drew cardiac valves. Andreas Vesalius rejected the idea of a permeable cardiac septum. In 1553, the Spanish theologian and physician Michael Servetus (1511-1553) published a theological treatise entitled Christianismi restitutio (The Restoration of Christianity) that, in describing the introduction and transmission of the divine spirit through the body, also described the "lesser circulation" of the blood--the passage of the blood from the left side of the heart through the pulmonary artery to the lungs and then back through the pulmonary vein to the right side of the heart. The theological views that Servetus espoused in his treatise resulted in his being burned at the stake as a heretic; almost all copies of his work were destroyed and his discovery of pulmonary circulation remained unknown until 1694. Although Servetus's idea about pulmonary circulation was correct, it was theoretical. An Italian anatomist, Realdo Colombo (1516?-1559), also described the "lesser circulation," his work being based on anatomical observations gained through vivisection, autopsy, dissection, and surgery. Colombo published his findings in 1559 in De re anatomica , and this work became widely known.
Two more discoveries paved the way for Harvey. The Italian physician-botanist Andrea Cesalpino (1519-1603) coined the term circulation, described the heart's valves in 1571, and emphasized the outflow of arterial blood and inflow of venous blood from and to the heart. Hieronymous Fabricius (1537-1619), who taught Harvey at Padua, discovered the valves of the veins and published his findings in De venarum ostiolis in 1603.
The man who finally unraveled the mysterious mechanisms of blood circulation was the English physician William Harvey. A doctor with a flourishing practice, Harvey was also an assiduous medical researcher. For his studies of the heart and blood vessels, he vivisected and dissected many species of animals as well as cadavers. In 1628, he published his ideas on the circulatory system in De motu cordis et sanguinis ( On the Motion of the Heart and Blood), which transformed medicine.
Harvey reached his revolutionary conclusions through sound reasoning based on detailed observation. He found that the heart acted as a pump, forcing blood through the arteries by its contractions. He noted that the valves of both the heart and the veins allowed blood to flow in one direction only. Harvey then calculated the amount of blood propelled out of the heart with every beat and found that the volume of blood pumped out of the heart every hour amounted to three times the normal weight of a man. The same blood, he reasoned, must be continuously moving through the body in "a kind of circular motion"--that is, circulating.
Harvey's theory, while it generated some controversy, was generally accepted during his lifetime. The final proof of the theory came in 1661 when Marcello Malpighi used the newly invented microscope to discover the capillaries, the previously invisible network of extremely fine blood vessels linking arteries and veins. Harvey's findings ended the era of Galenic and Greek medicine and ushered in the era of modern experimental physiology.
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