William Harvey's
On the Motion of the Heart and Blood in Animals
Harvey begins his On the Motion of the Heart and Blood in Animals with
a flourishing letter of praise to Prince Charles and a dedication to
Doctor Argent. He then outlines a number of points he intends to expound
upon which sharply contradict Galenistic teachings: that the heart and
lungs have different ends; that arteries contain blood rather than air;
that arteries and veins contain the same blood, not different types;
that arteries dilate because they are filled rather expanding themselves;
that the right and left ventricles share the common function of propelling
blood from the heart; and that there is no blood flow between the ventricles
(through the septum).
He notes from his observations on dissected animals
that the heart has two phases: one of motion and one of rest; these
phases are termed systole and diastole, respectively. The heart also
becomes contracted and more firm during systole, just as the muscles
of the forearm do when the fist is clenched. He notes that the heart
becomes pale during systole and a richer red during diastole, which
he attributes to the fact that the heart is emptied during systole and
filled with blood during diastole.
Harvey observes during systole that when the left ventricle contracts,
the systemic arteries (those that supply the body systems with blood)
dilate and a pulse can be felt, and that at the same time, the right
ventricle contracts and the pulmonary artery (that which travels to
the lungs to oxygenate blood) dilates; he also notes that if these arteries
are severed, blood will be forcibly propelled from them upon systole.
He states that the diastole of the arteries coincides with the systole
of the heart, and that the arteries are passively filled by its pumping
rather that drawing in the blood. The pulse is felt simultaneously throughout
the body, but if blood flow is impeded or restricted in any way, it
will be felt less strongly in parts of the body distal to the impediment.
Harvey observes that the auricles, now called atria (atrium, sing.),
contract slightly earlier than the ventricles, and that they appear
paler after contraction. This is because the atria, being filled with
blood from the veins, contract to fill the ventricles with blood before
ventricular diastole, when the left ventricle sends blood out through
the aorta to the body systems, and the right ventricle sends it through
the pulmonary artery to the lungs. He relates this seemingly simultaneous
process of the heart contracting to that of a gun firing; although it
seems like all the events are taking place at once, it is really a sequential
process.
Finding the solution to a particularly difficult question that had baffled
physiologists for centuries, Harvey discovers that blood passes from
the veins to the arteries via the heart, which is easily observably
in animals without lungs, such as fish. In fish, all of the blood is
pumped out from the heart’s single ventricle through the arteries,
and it returns via the veins to the heart’s single atrium, where
the cycle repeats after the atrium fills the ventricle. Humans and other
mammals, however, have a four-chambered heart. We know today that the
right atrium and ventricle send blood to the lungs to be oxygenated
and then return to the left atrium and then to the left ventricle to
be sent out to the body, but the function of the pulmonary circuit,
or the right side of the heart and its vessels, was misunderstood up
to Harvey’s time.
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