continuously manufactured by the liver, as Galen and all ancient
physicians had always postulated, it was simply inconceivable to Harvey
that the flimsy world of Hippocratic medicine was genuine. The world’s
first physiological computation led to the obvious conclusion that Galen
was preposterously wrong, even before Lavoisier’s conservation of mass
would become law two hundred years later. There simply couldn’t be a fire
hydrant of blood gushing from the liver each day. Today, we know that an
average 150-pound human has a stroke volume of 70 milliliters, which
equates to a daily cardiac output of over 7,000 liters, or almost 2,000
gallons coursing through one’s heart. Harvey would surely pound his fist
on a table when realizing that over forty barrels of blood flows through our
hearts every day.
If the liver was not continuously producing blood, then where was it
coming from? And, perhaps more critically, what happened to the blood
once it reached its end destination? Did it indeed evaporate?
Harvey’s final experiment was likely triggered by his mentor in Padua,
Fabricius. Shortly after Harvey had left Padua, Fabricius published De
venarum ostiolis (The little doors of the veins), in which he investigated
the function of the valves. Not comprehending that the flow of blood in the
veins is from the extremities back to the heart, Fabricius had trouble
determining the function of the “little doors,” or valves of the veins. He
wrapped a cloth around a subject’s arm, causing the veins to swell;
Fabricius then applied fingertip pressure over a vein near the tourniquet,
but found that he was not able to force blood down toward the hand. He
concluded that the doors “held up and delayed” the flow of blood, a clear
misinterpretation of the venous valves. His blindness to the truth was
grounded in his false Aristotelian foundation, but his publication did
inspire his pupil to perceive the truth through a repetition of his
experiment.
Harvey repeated the experiment of his mentor, affixing a cloth
tourniquet around his servant’s arm, and confirming Fabricius’s
observation that blood could not be forced back into the hand because the
“little doors” prohibited backflow. Unencumbered by a blind belief in
Galenism, Harvey was free to interpret the findings of his little
experiment. (If you have visible veins in your arm, you can easily
reproduce the experiment by tightly wrapping a scarf or rubber band
around your limb above the elbow, and attempting to force the blood
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