In the time it takes to read this sentence, your body will produce 17 million blood cells deep in its marrow. To put that in context, that’s as many cells as twice the population of New York City. Once created, those red blood cells move into the bloodstream—red blood cells, white blood cells, and platelets, all bobbing along in plasma through 60,000 miles of vessels in the human body.

And in the time it took you to read the first paragraph, all of the blood in your body has completed its regular journey—it has traveled from your heart to your extremities and returned, there and back again. Your blood knows your body better than your brain does, as your blood has seen all but the cornea, from the brain to the toes and everything in between. It has sailed on the quick current of the great arterial rivers and through the smallest cholesterol-clogged creeks. It has seen it all.

Because of this Hobbitesque journey, remnants of all of the battles waged by the white blood cells against the enemies of your body, foreign and domestic, persist in your blood. Evidence of aberrations developed over the course of your life lurks behind, indicating future problems on your health’s horizon.

Your blood is a biomarker. Biomarkers, according to epidemiologist Barbara Hulka, are “cellular, biochemical, or molecular alterations that are measurable in biological media such as human tissues, cells, or fluids.”

While your body has many types of biomarkers, blood is in many ways the most promising. Several drops of blood increasingly give doctors a multitude of physiological data about your health. It is thought that blood records all the biological events of your life.

For example, doctors can now analyze a drop of blood and determine whether or not a professional athlete has suffered a concussion and can return to the game.

The idea that your blood can tell you something about you is nothing new. As David Brown writes in The Washington Post: “Biomarkers aren’t new to medicine. What’s changed is medicine’s ability to measure more molecules, with greater precision and less cost, than was possible in the past.”

The idea has a longer history than modern medicine. In the ancient world, blood was a sign of health and life. The life of a person’s body is in the blood (Gen. 9:4; Lev. 17:11). When a person dies and their blood is spilled, the loss of blood is an indicator of non-life (Isa. 26:21).

In the minds of ancient people, blood was more than just a marker of physical health—it was also a marker of vitality and virtue. A courageous hero had courageous blood; whether the blood made the hero or the hero made the blood was not important. This is one reason ancient writers describe people drinking the blood of animals and people. In a notable example from the first-century writings of Pliny the Elder, Roman citizens with epilepsy drank the blood of dying gladiators in hopes of transferring vitality into themselves.

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Perhaps the temptation of people to steal the marks of life was one reason this was a practice forbidden in Israel (Deut.12:23). Or limited to only drinking God’s blood—after all, Christians are to drink the blood of Jesus if they are to have eternal life (John 6:53–54).

Blood is also the marker of humanity. As Homer noted, humans have blood flowing through their veins, but the gods only had ichor—a nectar or juice—since gods were immortal and bloodless. Just as the gods’ ichor was a sign of their lack of humanity, your blood is a marker of your humanity; it is, in the words of historian David Biale, “the [classic] way blood inhabits the imagination as both substance and symbol.”

That the blood is connected to human health and flourishing was clear to the earliest medical philosophers. Ancient Greek doctors believed that the condition of the blood was a key indicator of the health of the patient. If the patient was pale, it indicated that there was too little blood in the patient’s system, and the patient should eat liver. But if the patient was fevered, there was too much blood in the patient’s system, and a little bloodletting via leeches was in order.

Blood is a visible biomarker even in the stories about Jesus. He experienced anguish so great that his sweat became like a trickle of blood pouring to the ground (Luke 22:44), a sign of his decreasing health.

Trying to understand all of this led to various theories about the blood, such as the theory of humours, an idea first popularized by Hippocrates but enduring in some circles until the 19th century. Eventually, medical science began to progress as the study of the blood increased. In “Blood, Clotting and the Four Humours,” hematologist Hans Haak notes that by the second half of the 12th century, medical philosophers used the coagulation of a patient’s phlebotomized blood to detect health or disease.

Today the usefulness of blood as a biomarker is rapidly increasing. Medical science can derive detailed and complex information just from a few drops of your blood. As a biomarker, blood can be more predictive than CT scans. For example, newly discovered biomarkers in your blood can offer up a complete history of all of your viral illnesses.

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In fact, the information contained in your blood is seemingly limitless. It can identify the presence and type of multiple sclerosis, the detection of which required extensive testing in the past. Blood can pinpoint autism in children, serve as a warning system for the early detection of heart attacks, diagnose obstructive sleep apnea, verify the occurrence of Gulf War Illness, and will soon identify signs of neurodegeneration such as dementia.

And soon all of the information contained in your blood will be accessible via your smartphone. In the time it took you to read this far, one day soon your iPhone will have completed an exhaustive diagnosis of your health history with a prick of your blood.

The blood in your body is a treasure trove of information about all that you are; it is the peculiar example of the miracle of life. It reveals your past, and it is the sign pointing to your future.

When we are sick and need to know what can make us whole again, there is no other fount we know. Nothing but the blood.

Douglas Estes is an assistant professor and director of the DMin program at South University, Columbia in South Carolina. He is the author and editor of numerous books.

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