In Harvey’s first steps at studying the circulatory system, he deduced that the heart was in fact a muscle, and that it did pump blood around the body and did not suck blood in like Galen had earlier thought. He also established that the arteries pulsed as a result of the hearts beating in pulses. Harvey’s main and most important discovery which was in fact what helped him on most in his study of the circulatory system was what entirely destroyed Galen’s theories. Harvey noticed that the volume of blood pumped out of the heart in one hour was much greater than the volume of the entire animal that the heart belonged to. This proved the blood must circulate, otherwise the arteries would explode under the pressure. Harvey wrote:
“Since all things, both argument and ocular demonstration, show that the blood passes through the lungs and heart by the force of the ventricles, and is sent for distribution to all parts of the body, where it makes its way into the veins and porosites of the flesh, and then flows by the veins from the circumference on every side to the centre, from the lesser to the greater veins, and is by them finally discharged into the vena cava and right auricle of the heart, and this in such a quantity or in such a flux and reflux thither by the arteries, hither by the veins, as cannot possibly be supplied by the ingesta, and is much greater than can be required for mere purposes of nutrition; it is absolutely necessary to conclude that the blood in the animal body is impelled in a circle, and is in a state of ceaseless motion”
Harvey also found that blood traveled out of the heart in the arteries and back to the heart via the veins. He experimented this by binding a person’s arm so that arterial blood could flow down it, but venal blood could not return back up the arm. Harvey saw that the veins had become swollen, meaning the blood had come from the arteries and out into the veins. This also meant that there had to be some kind of connection between the veins and arteries. These are capillaries – he knew they had to be present but he could not see them as they are so small. Harvey also understood how the valves inside veins stopped blood from flowing backwards (they close if blood flows the wrong way) and how the pressure in the arteries causes them to rapidly flow only one way. Another thing Harvey could now explain was how poisons could so quickly spread through the body.
To the heart is the beginning of life, the Sun of the Microcosm, as proportionally the Sun deserves to be called the heart of the world, by whose virtue, and pulsation, the blood is moved perfected, made vegetable, and is defended from corruption, and mattering; and this familiar household-god doth his duty to the whole body, by nourishing, cherishing, and vegetating, being the foundation of life, and author of all
-William Harvey
References: ‘William Harvey’ ()
‘Galen’ ()
http://www.williamharvey.org/
