The kidneys maintain homeostasis by constantly filtering the blood. They regulate fluid volume, control electrolyte concentrations, and remove metabolic waste products. If the kidneys fail, the body loses this ability, leading to a dangerous buildup of toxins. Urinating without kidneys depends on the difference between the physical act of voiding and the biological production of true urine.
How Kidneys Create True Urine
Urine production occurs within the nephrons, the functional units of the kidney. The first step is glomerular filtration, where blood pressure forces water and small solutes from the blood into the nephron tubule, creating a preliminary filtrate. This initial fluid is essentially plasma, excluding large proteins and blood cells which cannot pass the filtration membrane.
Next, tubular reabsorption reclaims nearly 99% of the water, along with beneficial substances like glucose and electrolytes, moving them back into the bloodstream. This selective process retains valuable nutrients and maintains appropriate fluid levels. The final step is tubular secretion, where cells actively transport additional waste ions from the blood into the remaining filtrate.
The resulting fluid is true urine, a concentrated solution of approximately 95% water and 5% metabolic wastes. These wastes primarily include nitrogenous compounds like urea and creatinine. This three-stage process produces a waste product much more concentrated than the initial filtrate.
Anuria and Oliguria: The State of No or Little Output
When the kidneys fail, urine production ceases, leading to severely reduced output. Oliguria describes low urine output, generally defined as less than 400 milliliters over 24 hours in adults. This quantity is insufficient to effectively clear the body of daily metabolic waste products and excess fluid.
Anuria is a more severe state, defined as the production of less than 100 milliliters of urine per day. The kidneys have virtually stopped filtering the blood altogether. A person with anuria may still pass a small amount of residual fluid from the bladder, but this fluid is not the product of active kidney filtration.
The absence of urine production means toxic waste products and excess fluid accumulate in the bloodstream and tissues. This buildup of substances like urea and potassium can quickly become life-threatening. The failure lies in the kidney’s ability to create the necessary waste-carrying fluid, not the bladder’s function.
Life Support: Replacing Kidney Function
Since the body cannot survive long-term with toxic buildup from kidney failure, medical interventions are required to replace the lost filtration function. Dialysis is the common treatment that artificially cleans the blood using a semipermeable membrane to remove waste and excess fluid. There are two primary forms of this life-sustaining therapy.
Hemodialysis
Hemodialysis uses a machine to pump blood out of the body and pass it through an artificial kidney, known as a dialyzer. Inside the dialyzer, blood flows on one side of a membrane while a cleansing solution called dialysate flows on the other. Waste products move into the dialysate through diffusion, and excess water is removed by a pressure gradient.
Peritoneal Dialysis
Peritoneal dialysis utilizes the patient’s own peritoneal membrane as the natural filter. A sterile dialysate solution is introduced into the abdominal cavity through a catheter and allowed to dwell for several hours. Toxins and fluid pass from the blood vessels lining the peritoneum into the dialysate, which is then drained.
Ultimately, a kidney transplant is the only permanent solution that restores the body’s natural ability to produce true urine. This procedure involves surgically implanting a healthy donor kidney.