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.

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.”