Through urination, the body manages the primordial sea inside all of us. (Christine Glade/Istock)

Learning about the body’s many excretions, secretions and suppurations in medical school, I realized that each medical specialty has its own essential effluent. And I heard that some physicians choose their careers based on the bodily fluid they find least revolting. Thus, a doctor disgusted by stool and pus but able to stand the sight of blood might end up a hematologist, while one repulsed by urine and bile but tolerant of sputum might choose pulmonology.

Many physicians are actively drawn to a particular bodily fluid, intrigued by its unique diagnostic mysteries. Each fluid that runs through the body is a language in which diseases speak to physicians, telling them what is wrong with a patient. And specializing means becoming fluent in one specific fluid’s dialect, learning to interpret its colors, textures and consistencies, and spending a career pondering its secrets.

As a medical student, I saw that a bodily fluid could shape a career. And though I resisted settling on just one (I remain a generalist), I have always been partial to pee.

I’ve studied all the body’s fluids and used each in diagnosing disease, and urine stands out in the wealth of information it grants about a patient’s condition. Conceived in the kidneys — a pair of bean-shaped organs tucked away in the abdomen’s rear — urine runs down the ureters and is conveniently stored in the bladder, from which it is gathered in plastic cups for testing. Urine analysis is performed frequently enough by physicians to have earned the shorthand “urinalysis” — no other bodily fluid can claim to be on a nickname-basis with the medical profession.

I remember the first time I watched a nephrologist turn a urine sample into a diagnosis. As a medical student at Cooper University Hospital in Camden, N.J., I followed behind as he carried a small, plastic urine cup to the microscope room in the nephrology department. He plunged a diagnostic dipstick into the fluid to reveal bits of blood and protein unseen by the naked eye. He then placed some urine into a centrifuge, which spun rapidly and concentrated floating cells into a sediment at the vial’s bottom. After peering through a microscope at a single drop of this stuff, noting stray bits of debris flung across the viewing field, the nephrologist wove a comprehensive diagnostic tale that encompassed all the patient’s symptoms and lab abnormalities. The diagnosis turned out to be glomerulonephritis, a rare form of kidney disease. He was able to look inside that patient with a clairvoyance that seemed positively sorcerous, with urine as his crystal ball. From that moment I was determined to learn urine’s subtle language.

In the months that followed, I practiced wielding the dipstick and centrifuge, and trained my eyes to recognize clues under the microscope. I befriended hospital lab technicians and had them save interesting specimens to broaden my experience of diseases. A designated cup with my name on it soon appeared in the laboratory fridge, where the technicians would leave their latest finds. When my patients needed a urinalysis, I performed it myself rather than sending samples to the laboratory only to receive back computerized results. With my own (gloved) hands, I carried the patients’ still-warm samples to the microscope lab and personally extracted their stories. Doing it myself gave me a deeper sense of the pathologies playing out inside my patients. And repeating the ritual of dipstick and centrifuge again and again created an intimacy with them and a growing respect for my favorite bodily fluid.

I began to comprehend urine’s enigmatic language. White blood cells on the dipstick speak of urinary tract infection, while crystals under the microscope explain some cases of flank pain. But more than other fluids, pee reveals hints not only about trouble with its own exit route through the urinary tract, but also about the body as a whole — even systemic, seemingly unrelated problems. Red urine can imply problems like kidney stones and bladder cancer, but it can also indicate a genetic defect in red blood cells, a breakdown of muscle cells or a recent meal of beets. Urine can uncover a cause of infection in the lungs, announce a recent drug ingestion or, in diabetics, confess the failings of a distant pancreas. In previous centuries, physicians made this last diagnosis by tasting urine for sweetness, a time when taste buds were the only assay. Today, thankfully, this is no longer necessary, though decoding urine still often feels like being a sommelier.

Years after medical school, I worked in an emergency room and discovered more fundamental reasons to appreciate urine. In questioning each ER patient, I always brought the conversation back to urine, no matter the primary problem. Whether the issue was vomiting, diarrhea or cough, I invariably asked how much patients had urinated lately. Though a measure far more elementary than a dipstick or a microscope, the simple volume of urine produced by the kidneys is an essential indicator of a person’s hydration level. When the body becomes dehydrated, through poor intake or fluids lost to vomiting or diarrhea, the normally surging bloodstream becomes sluggish, like a dried-up stream in a drought, and nutrient delivery to vital organs diminishes. The kidneys perceive slackening in the blood vessels and respond by turning off the urine faucet, preserving the body’s hydration while letting out only minuscule amounts of dark-yellow pee. This crucial message raises a red flag for more serious illness and suggests the need for further diagnostics and intravenous rehydration.

Beyond diagnostic utility, urine — and the way kidneys make it — carries a deeper story about the origins of human life. By finely calibrating the urine extracted from the bloodstream at every moment, kidneys safeguard not only the body’s hydration but also its salts. Though mostly water, urine carries salts as well, thereby maintaining the body’s essential balance of water and electrolytes. In illness, the kidneys work overtime to accomplish this, struggling in the face of dehydration to keep blood high in sodium and low in potassium — roughly the same salt proportions as the world’s oceans, from which humanity’s ancestors once crawled. By editing urine out of the bloodstream, kidneys preserve the primordial sea in our blood, maintaining the balance of salt essential to our survival. Without them, and without urine’s salubrious flow, our forebears could never have left the ocean to live on land, just as each newborn baby could never adjust to life outside its personal salty, amniotic sea — itself composed almost wholly from the unborn baby’s urine. So when urine flow slows in illness (when patients report poor urine output or parents tell of fewer wet diapers in sick infants), it is the body fighting to maintain the life-giving ocean inside each of us, our ancestral brine.

Without the careful harmony protected by urine’s flow, many bodily systems would fail, including the electrical activity responsible for every heartbeat, every muscle movement and every neuron firing in the brain, each a manifestation of the ebb and flow of salt ions through water. It is precisely because the kidneys receive blood and turn it into urine that the brain can receive blood and turn it into thought. Urine is more than a handy diagnostic tool — it keeps us human.

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