Blood circulation and the heart

Old engraving: forearms and hands, with swollen veins and points marked by letters. — Figure from William Harvey’s Exercitatio showing that blood circulates: when a vein in the arm is tightened with a tourniquet, it swells, because the blood can no longer return towards the heart. Unknown author, Public domain, source

Around 1242, a physician born near Damascus described the passage of blood through the lungs. Europe would not read his text for nearly seven centuries.

Nobody stole his text, as far as we know. Almost nobody read it.

It is a way of being erased that nobody ever thinks about, and it is perhaps the most common one.

The blood’s circuit

Blood does not wander around the body: it follows a closed circuit, always the same, driven by a pump that never stops.

The heart is that pump. It is the size of a fist and has four chambers: two atria at the top, which receive the incoming blood, and two ventricles at the bottom, more muscular, which expel it. Between the chambers, valves open and close in one direction only — it is mainly their closing that is heard as the heartbeat, more than the muscle. They close with every beat, more than one hundred thousand times a day, without interruption, for a whole lifetime.¹

Three kinds of vessel carry this blood, and their form matches their work.

Arteries leave the heart. They withstand high pressure and have thick, elastic walls.
Veins return to it. The pressure in them is low, the walls thinner, and many contain small valves that stop the blood from flowing back down when we are standing.
Capillaries connect the two. They are vessels so fine that a red blood cell has to pass through them in single file, one at a time, and their wall is only one cell thick. This is where, for the most part, exchanges with the cells take place — the rest of the network serves mainly for transport.²

The circuit has two loops, and this is the point that took longest to be understood. The small loop sends blood from the heart to the lungs, where it takes on oxygen and gets rid of carbon dioxide, then brings it back to the heart. The large loop then sends it through all the rest of the body, where the capillaries deliver oxygen and collect waste, before returning it to the heart. Two circuits for one complete round trip.³

The blood itself carries more than oxygen: nutrients from the gut, hormones, defence cells, heat, and waste to be taken to the kidneys and liver.

It is the small loop, and it alone, that is described by the text this page is about — three centuries before it was rediscovered in Europe.

What had been taught for a thousand years

Galen, a physician of Antiquity, held that blood passed from the right side of the heart to the left by crossing invisible pores in the wall that separates them.⁴

His authority lasted more than a thousand years.

⚠ Important note: a large part of his anatomical observations was made on animals, not on humans. So for a millennium, a description of the human body established elsewhere was taught.

What Ibn al-Nafis wrote around 1242

A physician born near Damascus, trained at the Nuri hospital, settled in Egypt from 1236, writes a commentary on a great treatise of medicine.⁵

He is twenty-nine years old.⁶

In it he rejects Galen’s pores, and he does so in a way that deserves to be read:

There is no passage between these two cavities; for the substance of the heart is solid in this region and has neither a visible passage, as was thought by some persons, nor an invisible one.⁷

The double negation is worth noting. Galen had proposed invisible pores — a convenient hypothesis, since one cannot show that an invisible thing does not exist. Ibn al-Nafis rejects both at once.

Then he describes the real route: the blood of the right chamber must reach the left, but with no direct passage. So it travels through the pulmonary artery to the lungs, is mixed with air there, and returns to the heart through the pulmonary vein.⁸

This is the first known description of what is today called the pulmonary circulation.

About three hundred years before the European authors to whom it is attributed.

He predicted something he could not see

This is the most impressive detail, and the entry would have been wrong to leave it out.

If blood passes from the pulmonary artery to the pulmonary vein through the lung, there must be a passage between the two. Ibn al-Nafis states it: there must be small channels connecting one to the other.⁹

He never saw them. The microscope did not exist.

These channels — the pulmonary capillaries — were observed four hundred years later.¹⁰

The approach is a familiar one: announce what must exist for the reasoning to hold, then wait for the instrument to arrive. This is what Mendeleev does with his empty boxes, and Darwin and then Wallace with the moth of Madagascar.¹¹

Here, the wait lasted four centuries.

One last detail: this text, written nearly eight hundred years ago, can still be read today by anyone who knows Arabic.¹²

What happened next

Nothing.

He wrote in Arabic, and before the invention of printing. His manuscript stayed in libraries. European medicine, as far as can be established, did not make use of it: nothing proves that it read it.

The heart is not ordered to beat

A surprising and fully established fact deserves to be highlighted.

Heart muscle contracts on its own. It waits for no signal from the brain.

The rhythm is produced inside the heart itself, by a small group of specialised cells that fire regularly, without being asked to do anything.¹³

The nervous system does not command: it moderates. It speeds the rhythm up during a run and slows it down during sleep. It adjusts a movement that already existed without it.¹⁴

This is why we speak of cardiac automaticity: the beat originates in the heart. The rhythm is in the muscle, not in the command.

How to check it

Not by guessing: by isolating.

Deprived of all nervous or hormonal control, this small group of cells, the sinoatrial node, would still start the beats, at about eighty to one hundred per minute.¹⁵

Remove the supposed command, and the phenomenon continues. It is the soundest reasoning there is: to know whether something is the cause, you have to remove it and observe.

What this says about the whole body

This is the fourth time, in this domain, that a part of the body acts without the brain having ordered it.

The hand pulls away from a burn before the pain is felt (see Entry 8.5). Gut bacteria do part of the work of digestion. The immune system sorts cells without consulting anyone. The heart beats by itself.¹⁶

The body is much less commanded than we think.

This is not worrying; it is even reassuring. A body that needed an order for every beat would be a very fragile body — a single interruption would be enough for everything to stop.

What keeps us alive is not only a conductor. It is also a set of things that each work on their own, and that the brain brings into tune.

The rediscovery, a story in itself

In 1924, an Egyptian physician, Muhyi al-Din al-Tatawi, is studying the history of Arabic medicine at a German faculty.

He works at the Prussian State Library in Berlin. There he finds the manuscript, until then unknown in the West.¹⁷

An orientalist, Max Meyerhof, then made translations of it himself into English, French and German.¹⁸

The question that must be left open

There remains the point on which this site refuses to come down on one side — and it needs explaining why.

Another work by Ibn al-Nafis, on compound remedies, was translated into Latin in 1547. Michael Servetus published his description of the pulmonary circulation in 1553. Colombo, his pupil Valverde and Vesalius, in the same years.¹⁹

Had the Europeans read it, then?

Specialists disagree.

Some believe that the European authors took this concept over in their “rediscovery”.
Another asks whether they knew of it and chose not to cite it.
Others stress that there is no proof that Servetus knew of it.

This site has a rule, and it applies here: faced with an origin attributed elsewhere, ask whether it has been shown or assumed.

This rule applies in both directions. Writing “Europe stole it” would be committing exactly the fault we criticise others for elsewhere. Writing “he had no influence” would be committing it in reverse.

Transmission is possible, plausible, and not established. That is what should be written.²⁰

A matter of fairness

Establishing that Ibn al-Nafis was the first to describe the pulmonary circulation takes nothing away from William Harvey, whose work, four centuries later, concerned the general circulation of the blood throughout the body, and rested on measurements.²¹

It is not the same discovery.

We make the same remark in our page on the great centres of learning: correcting an attribution does not mean taking from one to give to another. It means saying who did what.

⚠ Michael Servetus was burned along with his book in Geneva in 1553, for heresy — essentially for having denied the Trinity and infant baptism. His description of the pulmonary circulation, slipped into this theological work, was not the cause of his condemnation; it nevertheless almost disappeared along with nearly all copies of the book, which were burned too.

Note 1

The human heart consists of four chambers: The left side and the right side each have one atrium and one ventricle.

Betts, J. Gordon et al., Anatomy and Physiology, OpenStax, Rice University

A typical heart is approximately the size of your fist […] a human heart would contract approximately 108,000 times in one day