The Bridge - Page 7

Science Today

What goes Around Comes Around

Sensational claims by William Harvey shed light on the circulation of blood in the body. A truly revolutionary book Exercitatio Anatomica de Motu Cordis et Sanguinis in Animalibus contains the a description of the circulation of the blood. His research indicates the action of the heart and the consequent movement of the blood around the body in a circuit. Harvey’s fundamental premise to his treatise, states that it was extremely important to study the heart when it was active in order to truly comprehend its true movement the movement of the heart was only to be comprehended by God. For I could neither rightly perceive at first when the systole and when the diastole took place by reason of the rapidity of the movement...

This initial thought led Harvey's ambition and assiduousness to a detailed analysis of the overall structure of the heart (studied with less hindrances in cold-blooded animals). Harvey analyses the arteries, showing how their pulsation depends upon the contraction of the left ventricle, while the contraction of the right ventricle propels its charge of blood into the pulmonary artery. Whilst doing this, the physician reiterates the fact that these two ventricles move together almost simultaneously and not independently as had been thought previously by his predecessors.

This discovery was made while observing the heart of such animals as the eel and several other types of fish; indeed, the general study of countless animals was of utmost importance to the physician: among the ones already cited, one can add the study of the snail, the invisible shrimp, the chick before its hatching and even the pigeon. A digression to an experiment can be made to this note: using the inactive heart of a dead pigeon and placing upon it a finger wet with saliva, Harvey was able to witness a transitory and yet incontrovertible pulsation. He had just witnessed the heart's ability to recover from fatigue.

William Harvey had already discerned the existence of the Ductus Arteriosus and explained its relative function. Here he says, "...in embryos, whilst the lungs are in a state of inaction, performing no function, subject to no movement any more than if they had not been present, Nature uses the two ventricles of the heart as if they formed but one for the transmission of the blood."[23] However, the apex of Harvey's work is probably the eighth chapter, in which he deals with the actual quantity of bloodpassing through the heart from the veins to the arteries. Coming into conflict with Galen's accepted view of the liver as the origin of venous blood, Harvey estimated the capacity of the heart, how much blood is expelled through each pump of the heart, and the number of times the heart beats in a half an hour. All of these estimates were purposefully low, so that people could see the vast amount of blood Galen's theory required the liver to produce. He estimated that the capacity of the heart was 1.5 imperial fluid ounces(43 ml), and that every time the heart pumps, 1/8 of that blood is expelled. This led to Harvey's estimate that about 1⁄6 imperial fluid ounce (4.7 ml) of blood went through the heart every time it pumped. The next estimate he used was that the heart beats 1000 times every half an hour, which gave 10 pounds 6 ounces of blood in a half an hour, and when this number was multiplied by 48 half hours in a day he realized that the liver would have to produce 540 pounds of blood in a day.[contradictory]

Having this simple but essential mathematical proportion at hand – which proved the overall impossible aforementioned role of the liver – Harvey went on to prove how the blood circulated in a circle by means of countless experiments initially done on serpents and fish: tying their veins and arteries in separate periods of time, Harvey noticed the modifications which occurred; indeed, as he tied the veins, the heart would become empty, while as he did the same to the arteries, the organ would swell up.