William Harvey's

On the Motion of the Heart and Blood in Animals.

Harvey then goes on to address one argument that could be posed against his theory that blood somehow soaks through the lungs tissue from the arteries to the veins: some may say that it would be difficult for the copious amount of blood pumped by the heart to pass so rapidly through such miniscule passages in the lung tissue. However, Harvey says, people don’t seem to have the same problem in believing that when one drinks large amounts of water that the water will be filtered through the liver and kidneys, sent to the bladder, and eliminated in a fairly short amount of time.

Harvey next discusses the valves of the heart and their functions. There are four valves in the heart: the two semi-lunar valves, one between the right ventricle and the pulmonary artery, and the other between the left ventricle and the aorta; the tricuspid valve between the right atrium and right ventricle; and the bicuspid, or mitral, valve between the left atrium and left ventricle. The function of each of these one-way valves is to close behind the filled chamber to prevent backflow into the chamber that emptied into it, and to open again when the second chamber in the sequence empties so that it call be filled for another cycle. This closing is passive, in that it depends only on the volume and direction of blood flow to operate the valves.

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