Does menstrual blood really attract bears? Why does blood look blue in your veins? And why were the first blood transfusions performed with animals? Here are 15 facts, historical and biological, you probably didn't know about blood.

Photo Credit: Steve Koukoulas | CC BY-NC-ND 2.0

1. A lot of people think that deoxygenated human blood is blue, turning red only when it spills from the body. In fact, though it is commonly coded as such in medical diagrams and anatomical models, blood is never blue. While your veins may appear to run blue with blood, this actually has to do with the way light interacts with blood and with skin.

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2. Horseshoe crabs, however, bleed blue. Certain leeches bleed green. And some species of marine worm – including penis worms – bleed violet.

3. The claim that bears are attracted to the smell of menstruating women can be traced to an attack on two female hikers in 1967 by grizzly bears. Scientists have since investigated whether bears are, in fact, interested in or attracted to menstrual blood. The studies haven't exactly been rigorous, but their results suggest the answer to these questions may actually depend on the species of bear in question. (Photo Credit: Jon Nickles / U.S. Fish and Wildlife Service)

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4. Some Ancient Greeks believed blood had curative properties that could be received by way of ingestion. Pliny the Elder (AD 23-79), an historian of the Roman Empire, describes how epileptic spectators would slurp blood from the bodies of recently fallen gladiators:

Epileptic patients are in the habit of drinking the blood even of gladiators, draughts teeming with life, as it were; a thing that, when we see it done by the wild beasts even, upon the same arena, inspires us with horror at the spectacle! And yet these persons, forsooth, consider it a most effectual cure for their disease, to quaff the warm, breathing, blood from man himself, and, as they apply their mouth to the wound, to draw forth his very life; and this, though it is regarded as an act of impiety to apply the human lips to the wound even of a wild beast! Others there are, again, who make the marrow of the leg-bones, and the brains of infants, the objects of their research!

5. Pope Innocent VIII is occasionally said to have been history's first recipient of a blood transfusion. But in July of 1492, when the Pope fell ill, it was still widely believed blood could be usefully introduced into the body by drinking it, so it's hard to say how his "transfusion" was any different from ones people with epilepsy self-administered in Ancient Greece.

Accounts of the "transfusion" vary from translation to translation, but the gist is usually that a physician of dubious reputation had the Pope drink the blood of three 10-year-old boys in a bid to save his life. The veracity of this story and its particulars have been debated by historians, (which you can read about here). Suffice it to say, the Pope died, and the boys probably did, too.

6. Prior to the discovery of the circulatory system, blood was thought to "burn up" in the heart, as one of the four humors — or bodily fluids — the condition and balance of which were thought to be reflected by a person's health.

7. The first person to describe the human circulatory system in detail was William Harvey. He did it in 1628. As Holly Tucker, Associate Professor in the Center for Medicine, Health & Society and the Department of French & Italian at Vanderbilt University, and author of the science-history book Blood Work explains in an essay for Scientific American:

His discovery did not abolish the notion of humors , which would continue well into the 19th century. But Harvey's theory of circulation (and it was initially just that, a theory) paved the way for natural philosophers to begin imagining the possibility of putting things into veins and arteries for the first time.

8. It's difficult to say when the first authentic attempt at blood transfusion – i.e. the passage of blood from one body to another by way circulatory passage, rather than ingestion – actually occurred. As Abdul Nasser Kaadan, Chairman of the History of Medicine Department at Aleppo University in Syria notes in his review of Blood Transfusion in History, "it is likely that all references to the transfusion of blood before 1628, the date of Harvey's discovery of systemic circulation, must be considered, at best, questionable."

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After 1628, the history of blood transfusion is similarly unclear. As Tony Long explains over at Wired:

Jean-Baptiste Denys, personal physician to France's Louis XIV, is generally credited with performing the first human blood transfusion, although some sources award that distinction to Englishman Richard Lower. What is not in dispute is the year — 1667 — and the patient — a 15-year-old boy who had been bled so much by his doctor that he required an infusion of blood.

The source is also not under dispute: Whoever the physician was, he used a sheep's blood. And, somehow, the kid recovered.

9. Why animal blood? Because it was thought to be more pure. Animals, Tucker explained in a 2011 interview with PLOS, "didn't drink, they didn't swear. Some of the first experiments were done with lambs' blood, [which had religious connotations], so that blood was considered to be more pure, more curing than human blood." [At left: "an early blood transfusion from lamb to man. Via Wellcome Images]

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10. On the other hand, some scientists feared infusing a human's blood with an animal's would create abominable human-animal hybrids. Tucker explains:

[It was feared] that humans would take on animal characteristics, that you could have quite literally humans who barked and that you could actually, as well, mutate identities. Samuel Pepys, a very well-known diarist in England, joked, "Well, I think it could be a pretty interesting experiment. What if we transfused the blood of an Archbishop into a Quaker?" And now you're getting into questions of: Could you not just mutate abilities? Could you be enacting some sort of change in beliefs in the very soul of the human being?

Some scientists so feared the possibility of human-animal hybrids that they attempted to sabotage later experiments of Jean-Baptiste Denys, dosing one of his patients with arsenic to ensure that his attempts to infuse the man with calf's blood would result in failure. The irony, of course, is that transfusing animal blood into human blood is a terrible idea, whether it's laced with arsenic or not. Why?

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11. Blood types wouldn't be formally identified until 1901, when Austrian researcher Karl Landsteiner observed that blood from different people would agglutinate, or clump, when mixed together. But today, we know that human blood types are distinguished by molecular differences on the surface of red blood cells, and that harmful and even fatal immune reactions when improperly mixed in the body of a blood recipient.

The four principle blood types in humans, first described by Czech serologist Jan Jansky in 1907, are A+/-, B+/-, AB+/-, or O+/-. Collectively, these make up the "ABO" blood group system.

12. There's more than one human blood group system. ABO is one of more than thirty major blood group systems recognized by the International Society of Blood Transfusion.

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13. There were other uses for animal blood during the Renaissance period, besides blood transfusions – in the theatre, for instance. According to James Shapiro – Professor of English and Comparative Literature at Columbia University and a specialist in Shakespeare and the Early Modern period – players in Shakespeare's time used real blood in their performances. As a young man, Shakespeare came to London as an actor, and, as Shapiro explains in a fantastic episode of Radiolab (titled, appropriately, "Blood"):

...as the youngest actor in the company, what [Shakespeare] was sent to do was to go to the shambles [a slaughterhouse]... and get a bucket of blood, so when they do the Spanish tragedy or any other [play]... they're not using fake blood, they're using animal blood.

14. Shakespeare would later use animal blood in his own plays. Here's Shapiro again:

On the way from where he lived to the theatre, [Shakespeare would] pass some kind of slaughtering area. I don't know how much it costs for a bucket of blood, but you need a bucket of blood for Titus Andronicus, you need a bucket of blood for Julius Caesar. In [Julius Caesar], Shakespeare has Mark Antony say 'I stand upon slippery ground,' I mean, that stage is covered in blood. He's slipping in this blood.

15. Shakespeare really had a thing for blood. According to Shapiro, "The word blood itself appears 673 times in 571 speeches in 41 of Shakespeare's plays and poems," including "37 times in King John; 28 times in Richard III, [and] 22 times in Henry IV Part I."

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