10. Birth of the theory (late 1610s–1620s): ‘I began to bethink my self’

DURING HIS PRIVATE investigations Harvey had been impressed, and no doubt troubled, by the force and the amount of blood expelled by the contracting hearts of the various dogs whose aortas he severed. Conventional wisdom could not account for its quantity or intensity. According to tradition, blood was continually produced in the liver and consumed by the organs, muscles and tissues, reaching them after slow progress through the vessels. Yet if Harvey’s observations regarding the large quantity of blood leaving the heart were correct, the liver would have to manufacture an inconcievably large amount of the liquid.

The vast amount of blood leaving the heart, along with its intensity, also seemed to lend credence to his idea that the organ’s active phase was systole, a forceful contraction being required to evacuate the organ of so much liquid. Perhaps with an eye to convincing his fellow physicians and the audiences of his lectures on this latter point, Harvey set about trying to quantify the amount of blood leaving the heart and entering the arteries with every heartbeat. If he could come up with an impressively large figure, others might be persuaded of his views.

Harvey estimated the amount of blood present in the left ventricle of the heart in passive diastole, then, deliberately keeping his calculations conservative, he speculated that only a fraction of that blood (roughly a drachm in weight) would be ejected at every systole. Harvey computed that in a single hour, during which the heart beat, at a low estimate, roughly 2,000 times, approximately 2,000 drachms would be expelled by the organ. This meant that over the course of one day, a little less than 50,000 drachms of blood would be ejected into the arteries.

Harvey simply could not believe that the liver was capable of producing so much blood; nor did he think it possible for men to consume the amount of food required (according to Galenic theory) to generate such an enormous quantity. Moreover, he could not understand either why the arteries did not swell, or even ‘burst with too much intrusion of blood’ from the contracting heart, or how the muscles, tissues and organs of the body could absorb it all. The body would surely become flooded with so much liquid, in the space of a few hours. Galen’s 1,500-year-old theory began to appear incongruous or impossible.

Harvey then focused on the question of where all the blood went after it entered the aorta. Tradition taught him, and observation seemed to confirm, that the liquid was transported throughout the body via the arteries. To test this, Harvey devised a number of experiments on humans using ligatures, which he had often used during his medical practice when letting the blood of his patients.

Harvey tied a ligature as ‘tightly as was bearable’ just above the elbow of one of his servants, who was instructed to clasp a piece of wood in his hand. He noticed that ‘beyond the ligature, towards the hand’ no blood was able to flow through the arteries. He also saw that blood began to build up in the arteries above the ligature ‘as if it were trying to burst through the passage and to reopen the channel.’ Releasing the ligature just a little, Harvey observed an ‘immediate coloration and distention of the whole hand.’ It was evident that blood was carried to the extremities of the body by the arteries, and that its flow could be impeded.

But what of the veins? At a certain point Harvey decided to conduct some tests on these vessels too, no doubt taking inspiration from his old mentor Fabricius. Fabricius had conducted trials on the veins at Padua, possibly in front of Harvey and his fellow students. He gave an account of these in print, in his 1603 treatise De venarum ostiolis (The little doors of the veins), the ‘little doors’ being ‘extremely delicate little membranes in the veins, occurring at intervals, singly or in pairs’.

The Italian investigated the function of ‘little doors’ by rubbing and squeezing, with his fingers, the distended veins in a man’s ligated arm. Fabricius ‘exerted pressure on the blood’ in the veins in a bid to ‘force it downwards’ towards the hand. Yet he found that the liquid would not flow in that direction because it was, so far as he could see, ‘held up and delayed’ in the veins by the ‘little doors’.

‘My theory’, the Italian concluded after these various trials, ‘is that Nature has formed the little doors to delay the blood, and to prevent the whole mass of it flooding to the feet, or hands and fingers, and collecting there. The little doors are thus made to ensure a fair general distribution of the blood for the nutrition of the various parts.’ This thesis made perfect sense within a Galenic view of the body in which the blood moved slowly around the vessels, in both directions, and needed to be sent out from the liver evenly and centrifugally to the various body parts.

Harvey repeated Fabricius experiments in his private chamber. Ligating his servant’s arm to swell the veins, he confirmed that blood was indeed prevented from flowing downwards in them by the ‘little doors’, no matter what pressure he exerted on it with his fingers. To test this further Harvey also ligated the exposed veins of vivisected animals, a little way above a pair of ‘little doors’.

Figures from De motu cordis.

Despite the vast build-up of blood in the veins between the ‘doors’ and the ligature, the ‘doors’ would not allow blood to flow back downwards.

From these tests Harvey deduced something quite different from Fabricius. He hypothesized that the ‘doors’ must function like valves, encouraging the flow of blood in the veins in one direction, not simply delaying its movement as the Italian had thought. That direction was obviously not downwards but upwards – something confirmed by movement of the blood towards the shoulder when the pressure of the ligature was released in his servant’s arm. Fabricius did not observe this unidirectional upward movement because his Galenic understanding of the blood within the body did not prompt him to look for it. Harvey, however, looked down at his servant’s arm with a mind far more sceptical to Galen’s theory.

At some point, Harvey had an inspiration which allowed him to look at the valves, and to consider their purpose, from an entirely original point of view. If they did not merely impede the downward flow of the blood ‘from the large veins into the smaller ones, or from the centre of the body to the extremities’ but actively facilitated its upward movement, then their central purpose might be to encourage the blood’s passage ‘from the extremities back to the centre’ – that is, to the heart.

Harvey’s understanding of the vast quantity and force of blood expelled by the contracting heart seems to have dovetailed in his mind with his attempt to map the blood’s flow within the arteries and the veins. In endeavouring to resolve one issue – where all the blood went to – perhaps he might be able find the solution to the other.

When [I] took notice that the valves in the veins … were so placed that they gave free passage of the blood towards the heart, but opposed the passage the other way: [I] was invited to imagine, that so provident a cause as nature had not plac’d so many valves without design: and no design seemed more probable, than that, since the blood could not well, because of the valves, be sent by the veins to the limbs, it should be sent [to the limbs] through the arteries [by the heart] and return [to the heart] through the veins … and when I had a long time considered with my self how great abundance of blood was passed through … I perceived that the veins should be quite emptied, and the arteries be burst with too much intrusion of blood, unless the blood did pass through arteries to the veins, and so return into the right ventricle of the heart.

The idea began to crystallize in Harvey’s mind that, after the venous blood had been vivified in the lungs and conveyed to the left side of the heart, it was ejected by the contracting organ into the aorta, which distributed it throughout the body via the arteries. The blood was then transferred from the arteries to the veins – how and where Harvey could not say – before the veins conveyed the same blood back again to the vena cava at the right side of the heart in a closed system. The blood thus traced a double ‘circuit’ – one through the lungs, and one throughout the body. ‘And so’, Harvey recalled, ‘I began to bethink my self if the blood might not have a circular motion …’ This was the closest Harvey ever came to a ‘eureka moment’.