William Harvey's

On the Motion of the Heart and Blood in Animals.

Although the ancients called all blood vessels veins, Harvey explains the difference between arteries and veins, saying that, “the artery is the vessel which carries the blood from the heart to the body at large, the vein of the present day bringing it back from the general system to the heart; the former is the conduit from, the latter the channel to, the heart; the latter contains the cruder, effete blood, rendered unfit for nutrition; the former transmits the digested, perfect, peculiarly nutritive fluid.”

To further prove his assertion that the blood moves in a circular motion, Harvey gives us an example concerning the volume of blood pumped by the heart in each stroke. He says this volume is about 2 ounces, and that even if the heart only pumped out one eighth of its ventricular volume, it would pump out half an ounce of blood per stroke. Over the course of an hour, the heart will beat over one thousand times, so that means 500 ounces of blood would be pumped out of the heart every half hour, a larger volume than is contained in the entire body; therefore, the blood must be recirculated rather than being constantly produced from ingested food (which was the Galenistic, and thus prevailing, theory of the day) and pumped out to a blind end. Harvey observed this experimentally in sheep and dogs, the hearts of which, based on stroke volume and pulse rate, should pump out about three and a half pounds of blood every half hour, but when bled, their bodies produced no more than four pounds total.


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