When was it discovered that life is in the blood

This Commentary emphasizes the fundamental contribution of William Harvey to the discovery of the circulation of the blood and his scientific and experimental approach to this matter.

Harvey was born at Folkestone, Kent, England, April 1, 1578. He received the degree of Medical Doctor from the University of Padua, Italy in 1602. After his return to England he became Fellow of the College of Physicians, physician to St. Bartholomew's Hospital, and Lumleian lecturer at the College of Physicians. In 1618, Harvey was appointed physician extraordinary to James I, and he remained in close professional relations to the royal family (Figure ​1). He died on June 3, 1657, at age 79. His last contribution was a book on the growth and development of the young animals entitled "De Generatione Animalium", published in 1651.

A portrait of William Harvey.

Harvey focused much of his research on the mechanics of blood flow in the human body. Most physicians of the time felt that the lungs were responsible for moving the blood around throughout the body. Harvey's famous "Exercitatio Anatomica de Motu Cordis et Sanguinis in Animalibus", commonly referred to as "de Motu Cordis" was published in Latin at Frankfurt in 1628, when Harvey was 50 years old. The first English translation did not appear until two decades later.

Harvey, observing the notion of the heart in living animals, was able to see that systole was the active phase of the heart's movement, pumping out the blood by its muscular contraction. Having perceived that the quantity of blood issuing from the heart in any given time was too much to be absorbed by the tissues, he was able to show that the valves in the veins permit the blood to flow only in the direction of the heart and to prove that the blood circulated around the body and returned to the heart. Fabricius, his teacher in Padua, had discovered the valves in the veins.

In Chapter 8 of "De Motu cordis", Harvey wrote how he hypothesized that the blood circulates: "In truth, when, from a variety of investigators through dissection of the living in order to experiment and through the opening of arteries, from the symmetry and magnitude of the ventricles of the heart and of the vessels entering and leaving (since Nature, who does nothing in vain, would not have needkessly given these vessels such relatively large size), from the skilfull and careful craftsmanship of the valves and fibres and the rest of the fabric of the heart, and from many other things, I had very often and seriously though about, and had long turned over in my mind, how great an amount there was, that is to say how great the amount of transmitted blood would be [and] in how short a time that transmission would be effected...I began privately to think that it might rather have a certain movement, as it were, in a circle..."

In Chapter 13, Harvey summarized the substance of his findings: "It has been shown by reason and experiment that blood by the beat of the ventricles flows through the lungs and heart and is pumped to the whole body. There it passes through pores in the flesh into the veins through which it returns from the periphery everywhere to the centre, from the smaller veins into the larger ones, finally coming to the vena cava and right atrium. This occurs in such an amount, with such an outflow through the arteries and such a reflux through the veins, that it cannot be supplied by the food consumed. It is also much more than is needed for nutrition. It must therefore be concluded that the blood in the animal body moves around in a circle continuously and that the action or function of the heart is to accomplish this by pumping. This is only reason for the motion and beat of the heart."

Harvey's predecessors and contemporaries believed the blood to be continually formed anew from the digested food, to be dissipated and used up in the tissues, and considered that the primary function of the heart was the production of heat. Blood was constantly being consumed in the periphery and replenished by ingested nutrients, and this was all carried out by the right ventricle and great veins. Harvey studied the hearts not only of various fishes, amphibian, reptiles, birds, and mammals, but also those of various other animal species. But most important, he not only compared these, he manipulated them in living as well as dead animals. He isolated parts of the heart; he ligated and divided arteries; he exerted pressure on veins on either side of the valves. His observations of dissected hearts showed that the valves in the heart allowed blood to flow in only one direction. Harvey measured the volume of the left ventricle and calculated that the amount of blood that passes through the heart of a man in an half hour and established that it was greater than the amount contained in the whole body. Direct observation of the heartbeat of living animals showed that the ventricles contracted together, dispelling Galen's theory that blood was forced from one ventricle to the other. Dissection of the septum of the heart showed that it contained no gaps or perforations. When Harvey removed the beating heart from a living animal, it continued to beat, thus acting as a pump, not a sucking organ. Harvey also used mathematical data to prove that the blood was not being consumed. Finally, Harvey postulated the existence of small capillary anastomoses between arteries and veins, but these were not discovered until 1661 by Marcello Malpighi.

The author declares that he has no competing interests.

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During my training as a hematologist at U.C.L.A., forty years ago, a senior faculty member introduced the program of study by citing a verse from Leviticus: “The life of the flesh is in the blood.” For the assembled young physicians, this was a biological truth. Red cells carry oxygen, required for our heart to beat and our brain to function. White cells defend us against invasion by lethal pathogens. Platelets and proteins in plasma form clots that can prevent fatal hemorrhages. Blood is constantly being renewed by stem cells in our bone marrow: red cells turn over every few months, platelets and most white cells every few days. Since marrow stem cells spawn every kind of blood cell, they can, when transplanted, restore life to a dying host.

In a wide-ranging and energetic new book, “Nine Pints” (Metropolitan), the British journalist Rose George examines not only the unique biology of this substance but also the lore and tradition surrounding it, and even its connections to the origins of the earth and of life itself. “The iron in our blood comes from the death of supernovas, like all iron on our planet,” she writes. “This bright red liquid . . . contains salt and water, like the sea we possibly came from.” George charts the distance that our blood (as her title suggests, we contain, on average, between nine and eleven pints of it) travels in the body every day: some twelve thousand miles, “three times the distance from my front door to Novosibirsk.” Our network of veins, arteries, and capillaries is about sixty thousand miles long—“twice the circumference of the earth and more.”

Ancient peoples knew none of this biology, but they were certain of blood’s importance and fascinated by its mystery. For them, blood was something hidden—visible only when flowing from a wound, or during childbirth, miscarriage, and menstruation—so it became a symbol both of life and of death. George returns often to this dichotomy, which she terms the “two-faced nature of blood” and sees as embodied in the figure of the Gorgon Medusa. In addition to her famous serpentine coiffure, Medusa was said to have two kinds of blood coursing through her veins: on her left side, her blood was lethal; on her right side, it was life-giving. To control blood was to master mortality, so it is unsurprising that blood features prominently in many religious traditions, and that, though our understanding of its functions is more sophisticated than ever, we remain in thrall to its primal mystique. The membrane between medicine and myth is thinner than we suppose, and blood is continually circulating back and forth across it.

In some cultures, blood loss is perceived as a danger not only to the individual but also to the larger community. George journeys to a remote Hindu village in western Nepal, where she finds Radha, a sixteen-year-old chau, which means “untouchable menstruating woman” in the local dialect. During her period, Radha can’t enter her family’s house or her temple, and she can’t touch other women, lest they be polluted. If she so much as consumes buffalo milk or butter, the buffalo themselves will get sick and stop producing milk. She can be fed only boiled rice, thrown by her little sister onto a plate from a safe distance, “the way you would feed a dog.”

Customs that denigrate women during menses are widespread. George notes that our word “taboo” is believed to derive from one of two Polynesian words: tapua, which means “menstruation,” or tabu, which means “apart.” Not long ago, in America, it was thought that “the curse” could cause women to spoil meat if they came in contact with it. But menstrual blood is not always seen as harmful, and menstrual segregation at its most benevolent can take the form of communality. Some three hundred miles northwest of where Radha lives, near the border between Pakistan and Afghanistan, menstruating Kalasha women “retire to a prestigious structure called the bashali, where women hang out, have fun, and sleep entwined,” George writes. “In this reading of menstrual seclusion, the woman is prized for her blood, because it means fertility and power.”

In Wogeo, an island off the north coast of Papua New Guinea, menstrual blood is held to be both lethal and cleansing, and men emulate menstruation by cutting their penises with crab claws. In ancient Rome, too, menstrual blood was not just a curse. Pliny the Elder wrote in his “Natural History” that when women had their periods they could stop seeds from germinating, cause plants to wither, and make fruit fall from trees. But their destructive power had its uses. A menstruating woman was able to kill a swarm of bees or ward off hail and lightning. Wives of farmers, Pliny suggested, could even offer a sort of pesticide: “If a woman strips herself naked while she is menstruating, and walks around a field of wheat, the caterpillars, worms, beetles and other vermin will fall from off the ears of corn.”

In Islam, menstruating women are forbidden to recite certain prayers and must refrain from vaginal intercourse. In Judaism, too, menstruation can be a cause of ritual impurity, as can childbirth. According to the rabbi and theologian Shai Held, “Childbirth takes place at—and to some degree unsettles—the boundaries between life and death: A new life comes into the world, but blood, considered the seat of life, is lost in the process.”

Observant Jews and Muslims alike follow dietary laws that forbid the consumption of blood. Both kosher meat and halal meat must be drained of blood, and kosher meat is also salted, to remove any residue of the substance. A tiny blood spot in an egg renders it inedible. While believers accept these prohibitions as divine edicts to prove devotion, some scholars speculate that they developed as health measures to prevent spoilage of meat, which is accelerated through oxidation and bacterial growth. These days, even meat that is not kosher or halal is drained of blood. People who say they like their steak “bloody” are actually responding to myoglobin, a red-pigmented protein that stores oxygen in muscle and brightens when exposed to air.

Yet, despite the firm proscription against ingesting blood, one breakaway Jewish sect of the first century A.D. made the idea of doing so central to its rituals. Its leader, Jesus of Nazareth, told his disciples that the bread and the wine at the Last Supper were his body and blood, and should be consumed thereafter in memory of him. The ritual of the Eucharist became a cornerstone of early Christianity, and with it the doctrine of transubstantiation—that a literal, not just figurative, transformation occurred during the sacrament.

“So then I’m, like, ‘Waaaaah! Waaaah!’ ”

Scholars have debated the reasons for this stark reversal—from forbidding the consumption of blood to sanctifying it as a bridge to the divine. Some speculate that, by the time of the Second Temple, many Jews were Hellenized, and early converts to Christianity were merely borrowing standard Dionysian rites. For instance, communal “agape feasts,” in which Christians symbolically ate their god’s flesh and drank his blood, resembled older Greek rituals.

David Biale, a professor of Jewish history at the University of California, Davis, has traced this divergence between Jewish and Christian traditions in his book “Blood and Belief” (2007). He cites a 1376 letter from the mystic Catherine of Siena to a disciple, in which she presciently warns of schisms within the Catholic Church and invokes the Eucharist as a symbol of unity. She argues that Christians, unlike Jews and Muslims, were “ransomed and baptized in Christ’s blood.” The notion of sacred blood as the vehicle for human salvation, Biale writes, justified the “literal interpretation of the Eucharist as dogma, popular celebrations of the Host that spread throughout Europe, and a new cult of blood relics.”

The blood of martyrs was also believed to cure disease. “The Golden Legend,” a thirteenth-century account of the lives of saints, attributed healing powers to the water used to wash the bloodstained clothes of Thomas à Becket, the Archbishop of Canterbury, who was murdered in 1170. The blood of St. Peter Martyr, who was killed by Cathar heretics in 1252, was also accorded medicinal properties. Blood became a central feature of Christian iconography—the stigmata, the Sacred Heart—and also a notable element in Christian anti-Semitism. In the late twelfth century, in England, a spate of pogroms occurred after Jews were accused of murdering Christian children to use their blood in the preparation of Passover matzo. It is tempting to wonder if this calumny, known as the blood libel, is connected in some way with Judaism’s highly exacting rituals designed to avoid the consumption of any kind of blood. The blood libel has proved startlingly resilient, recurring across medieval and early modern Europe, and reappearing in tsarist Russia in the early twentieth century. It has been revived as recently as last year—by Hamas, during its clashes with Israel.

For millennia, the human body was understood as a vessel for a quartet of liquids: yellow bile, black bile, white phlegm, and red blood. Each corresponded to one of the four classical elements—fire, earth, water, and air—from which it was thought everything in the cosmos was made. According to the second-century physician Galen, blood was made in the liver from food and drink carried from the digestive tract. This “natural” blood entered the veins and was transported, ebbing and flowing, to all parts of the body. Blood was believed to be constantly consumed by tissues and then replenished at each meal. The primary function of the heart was to generate heat. Galen thought that the blood in the left side of the heart came directly from the right side through pores in the septum, or through leaks from the lungs. The blood in the arteries was “vital,” its purpose to deliver spiritus vitalus to the flesh.

The key to good health was thought to be an ideal balance among the four humors. Danger arose if one predominated, so periodic voiding was crucial. For the most part, the body took care of this naturally: black bile, yellow bile, and phlegm were expelled through excrement, sweat, tears, and nasal discharge. But, other than menstruation, the body had no spontaneous way of getting rid of its blood, so, from ancient times until well into the modern era, bloodletting—using sharpened stones, fish teeth, and, later, lances and fleams—was a cornerstone of medical practice. George expounds on the work of the eleventh-century Persian scientist and philosopher Avicenna, whose “Canon of Medicine” includes an extensive guide to bloodletting treatments:

Different blood vessels served different purposes. Bleeding the veins between the eyebrows was good for long-standing headache, cutting the veins under the tongue—only lengthways, otherwise it was difficult to stanch—was useful for angina or tonsillar abscess. Opening the sciatic vein relieved podagra and elephantiasis; menstrual problems were alleviated by cutting the saphenous vein.

George becomes particularly enthralled by what she calls “an essential tool in the bloodletter’s armamentarium”—leeches. There are more than six hundred leech species: “Not all leeches suck blood and not all bloodsucking leeches seek the blood of humans,” she writes. “Many have evolved to have impressively specialized food sources: one desert variety lives in camel’s noses; another feeds on bats. Some eat hamsters and frogs.” She describes some of the earliest evidence of the human use of leeches—a painting on the wall of a three-thousand-year-old Egyptian tomb; representations of the Hindu god of healing, Dhanvantari, who is often shown holding a leech—and visits a contemporary leech-breeder, Biopharm, in Wales. The leeches raised there, destined for surgical use, are “freshwater, bloodsucking, multi-segmented annelid worms with ten stomachs, thirty-two brains, nine pairs of testicles, and several hundred teeth.” George compares the leech to an oil tanker. “The bulk of it is storage,” a Biopharm breeder tells her, with all its vital organs arranged around the periphery. He adds that, when feeding, a leech can increase its body weight fivefold—eightfold if it’s a small leech—and spend a year digesting one meal. Its bite is “spectacularly efficient,” causing far less trauma to the skin than a scalpel would, and it considerately injects its prey with anesthetic, making its feeding painless for the host.