Leonardo’s anatomical studies are more than just extensive, rather unprecedented. Not only did he draw muscles, nerves, vessels and organ correctly – he also did it aesthetically, artistically and accompanied the drawings with dense and precise commentaries, denoting a deep understanding on function. His anatomical dissections, more than thirty full bodies were autopsied into every minute detail during his career, often taking so long than they started to severely decompose before he was finished. He completed the dissections with parallel studies in a whole array of animals, and also increased his understanding of functional anatomy with his intense studies of live humans. For instance, he could note how all the muscles around the mouth interacted to form various grimaces, and which muscle combinations where not possible – this was then translated into his paintings, which always depict anatomically correct and realistic expressions, and made him fine tune every hint of expression. Leonardo presented the first scientific analysis of the human smile, at the time when he also worked on the Monalisa.
On Leonardo’s many pages covering heart anatomy is a section with drawings depicting the papillary muscle, accompanied with a description how it shortens and elongates when the heart beats.
Among Leonardo’s anatomical work his most sustained and successful scientific endeavours is the section encompassing the human heart. He entered the area with already masterful understanding of hydraulic engineering and a in depth fascination of fluid dynamics. This made it possible for him to make discoveries, neither fully understood, nor appreciated for centuries. In the medical science of the early1500s, the understanding of the heart was not more developed than what described in the 2nd century CE by Emperor Marcus Aurelius medical advisor Galen of Pergamum. Galen’s work was revived during the Renaissance and taught that the heart was not just a muscle, but made of a special life force driving substance, and that blood was made in the liver and distributed through the venous system. The heart’s produced vital spirit was distributed through the arteries. Veins and arteries were considered separate systems – no circulation was understood – rather it was believed the blood pulsed back and forth in both systems. Leonardo was among the first to understand the heart was the centre of the blood circulation, Although he was not able to dismiss the Galenian theory of the blood being pumped back and forth – the manifest theory of the time, and a very unusual case where Leonardo may have been blinded by contemporary book learning. Normally he would only base his believes on experiments; the full concept of blood circulation is a rare example of partial failure. A full explanation of the blood circulation would come with William Harvey a century later. But Leonardo was correct in nearly all other observations in this field. All the veins and arteries arise from the heart, in one of his anatomical notebooks.[i]
He accompanied the statement with minute drawings of how the vessels on both sides become smaller the farther away from the heart they were – he was the first to depict this, with splitting into invisible capillary level. Leonardo debunked Galen’s idea that the heart would be made of a special vital tissue but is rather a muscle. He was able to demonstrate how the heart, like all muscles, has its own blood supply, with own arteries and veins, and nerves. He also showed that Galenic medicine had wrongly believed there where only two ventricles – Leonardo showed there were four cavities: two lower and two upper ventricles. He reasoned that since these ventricles were separated with membranes and valves, they must have different functions.
Driven by his trademark curiosity to figure out how the four ventricles worked, he performed a dissection on a pig, which heart was still beating. He noted that the lower and upper ventricles opened at different times.
The Aortic Valve
Among all Leonardo’s anatomical contributions, his perhaps greatest is understanding of the functions of the aortic valve. His findings were confirmed to be correct in the 1960s. Contributing to his conclusions was his love and fixation for water eddies, swirls, spirals, studies of flowing liquids, cascades, hair curls and wind turbulences.
All this knowledge was used to analyse how blood in the Sinus Valsalva, a portion of the aorta, created fluid turbulent phenomena such as spirals and swirls, closing the valve in a beating heart. His profound analysis of hundreds of words and over twenty detailed and aesthetic drawings fill six full pages in this anatomical notebook. He described the swirls with mathematical equations, among other Fibonacci numbers.
Leonardo had spent years studying fluid dynamics. Around 1510 he studied how water in a pipe behaved when entering a tank. He noted the phenomenon called fluid drag, i.e., the water on the sides flows slower than the water in the middle. The reason for this is rubbing, or friction, against the pipe walls, or the riverbanks. This causes slowing of the flow, a slowing which is continuous so that the central string of water has the highest fluid speed.
When the pipe or river water enters a tank or a pond, the speed differences cause eddies and whirlpools. He noted all types of fluid phenomena associated with vortices, whirlpools and eddies, and also analysed what happens to the fluid when the passed surface is curved or when the width is narrowing or widening. He used these findings to understand cavitation and erosion of coasts, or how blood is pumped out of the heart. His conclusions are also to be found when applied to his artwork, e.g., background landscapes with rivers.
Leonardo eventually paid a lot of attention the triangular opening in the upper portion of the heart, where the blood is pumped out into the radix aortae. He noted that the central stream of the upward directed flow would reach much further, than the blood on the sides. Further he analysed how spiral eddies were formed, when the more rapid central blood flow enters the blood portion which is already in this wide initial portion of the aorta. This section is also called the sinuses of Valsalva, after Antonio Valsalva who described this aortic section in the 18th century, i.e., hundreds of years after Leonardo, who kept most of his discoveries to himself.
Closing Procedure of the Aortic Valve
The swirls per se caused by the blood pumped up into the root of the aorta has the effect that the three leaflets, i.e., the triangular shaped cusps of the tricuspid valve to spread out and cover the opening. It works exactly as when wind swirls fill a triangular sail on a boat and stretches out the corners, Leonardo noted with his typical sense for universal analogy. The revolving blood hits the sides of the valves and closes them so that no blood can descend back into the left chamber. The eddies caused by the next heart then opens the valve. Leonard understood the functions of the cords that open and shut the two sails. He used the word sails in his explanatory texts.
The common view among nearly all cardiologists until the 1960s, was that the aortic valve was closed from above when an exact blood volume had been pumped up into the arcus aortae and then attempted to drop back. This is the commonest modus operandi of nearly all other types of valves – they close upon the swing of the flow. In this way however, the aortic valve would not be closed in a sufficient manner. In the heart it is not the pressure of the blood in the aorta which closes the aortic valve. The amount of blood which tries to regurgitate, under normal conditions, when the heart valves open, is not the blood which closes the valves. Leonardo noted this would be impossible, since it would mean that a high-pressure wave of blood would forcibly beat at the folded and wrinkled heart valves, which would crush the membranes. Leonardo even depicted how the valve would collapse if faced with aortic pressure from above.
Based on analogies from his experience on air and water flow, with its eddies, spirals and vortices, he formed a hypothesis about the aortic valve, and designed an experiment to test it. He casted glass model of the heart and surrounding vessels, and was able to observe how, as he predicted, the blood would swirl and spiral into the aorta. With a bull’s heart as model, which he filled with wax, he was able to artistically cast a glass model of the heart, vessels and valves. He used the same sculptural techniques he had used for artwork and for the brain model he made of a cow’s brain, where also the cerebral ventricular system was modelled with wax. To make the intracardiac flow more visible he used water with an even distribution of grass seeds.
In the below image, Leonardo depicted the heart valves and the internal blood flow. He also describes how he used wax to form a mold, with which he cast a transparent glass model of the heart, including the aortic valve. He even presents a diagram in the upper right corner with a mould of gypsum for blowing up inside with thin glass… But first pour wax into the gate of an ox’s heart so that you may see the true shape of the gate. With this simple and ingenious experiment, he noted the widening just superior to the aortic valve, i.e., the aortic sinus. On the versal side he wrote that he could see in the glass what the blood does in the heart when it closes the little doors of the heart.
How we now know that Leonardo was right.
In the 1960s a team of medical scientists, Bellhouse et al,[ii] used a glass model, traceable dye and radiographic imaging and could confirm Leonardo’s explanation. The researchers revealed that a perfectly balanced fluid dynamic mechanism moved away the cusps from aortic wall, i.e., the most minute reversed flow closed the valve – they concluded that the swirls and vortices in the radix aortae, which Leonardo had observed 450 years earlier, were the phenomena responsible for the aortic valve closure. The blood swirl produces a push on the aortic valve sinus’ walls and on the valvular cusps – causing a synched and stable closure, exactly as Leonardo had explained.[iii]
In 1991 a Carolina Heart Institute team concluded that Bellhouse’s experiment was nearly a copy of Leonardo setup. As late as 2014 an Oxford team confirmed Leonardo’s theory with an experiment performed in a living human using magnetic resonance imaging (MRI).
As a summary – systolic vortices in the sinus of the aortic valve closes the valve, eventually confirmed in vivo using MRI, AI, computers and the latest technique in medicine of the 21st century.
In 2019, five years after Oxford team’s ultimate in-vivo confirmation of Leonardo’s explanation of the fluid dynamics and intracardiac pressure circumstances, and 500 years after his heart drawings and texts, emerged the AI-empowered concept of wireless, non-invasive, intracardiac pressure monitors (ICPM).
After half a millennium medical science, empowered with artificial intelligence, is ready to move on with intracardiac pressure and fluid dynamics, where Leonardo left us.
With non-invasive intracardiac pressure monitors we will be able to optimise heart failure treatment, amplify resources and widen our understanding and satisfy our creative curiosity – all in the spirit of Leonardo.