Many individuals undergoing dialysis treatments often cease producing urine. This article explores the physiological reasons behind this, detailing normal urine creation, what happens when kidneys fail, and how dialysis compensates for this loss.
Normal Kidney Function and Urine Production
Healthy kidneys perform continuous blood filtration to maintain the body’s internal balance. Each kidney contains millions of tiny filtering units called nephrons. Blood enters the kidneys through the renal artery, leading to these nephrons.
Within each nephron, a structure called the glomerulus acts as a primary filter, allowing smaller molecules, waste products, and excess fluid to pass into tiny tubes called renal tubules. Larger elements like blood cells and proteins remain in the bloodstream. As this filtered fluid travels through the renal tubules, the body reabsorbs essential water, minerals, and nutrients back into the blood. The remaining fluid, now concentrated with waste products like urea and creatinine, becomes urine. Healthy kidneys typically filter about half a cup of blood every minute, producing approximately 1 to 2 quarts of urine daily, which is then stored in the bladder until elimination.
Kidney Failure’s Effect on Urination
When kidneys lose their ability to function properly, a condition known as kidney failure or End-Stage Renal Disease (ESRD) occurs. This decline means the nephrons become increasingly damaged, impairing their filtration capacity. As a result, the kidneys can no longer effectively remove waste products and excess fluid from the blood.
The diminished filtration leads to a buildup of toxins like urea and creatinine in the bloodstream, a state called uremia. Without the kidneys’ ability to filter and excrete these substances, the body cannot maintain proper fluid balance. This often results in a significant decrease in natural urine output, a condition known as oliguria (reduced urine) or even anuria (no urine production). The accumulation of fluid can cause swelling, shortness of breath, and put strain on the heart.
How Dialysis Replaces Kidney Function
Dialysis is a medical treatment that artificially takes over the role of failed kidneys by filtering the blood to remove waste products and excess fluid. This intervention becomes necessary when kidney function is severely compromised. There are two main types: hemodialysis and peritoneal dialysis, both designed to mimic the kidney’s filtration process.
In hemodialysis, blood is circulated outside the body through a machine called a dialyzer. Within the dialyzer, a semipermeable membrane separates the patient’s blood from a cleansing fluid called dialysate. Waste products, such as urea and creatinine, and excess water move from the blood across this membrane into the dialysate. The cleaned blood is then returned to the body.
Peritoneal dialysis uses the lining of the patient’s abdomen, called the peritoneum, as a natural filter. A sterile dialysate solution is introduced into the abdominal cavity through a catheter. The peritoneal membrane allows waste products and excess fluid from the blood to diffuse into the dialysate. After a few hours, the fluid is drained and discarded.
Because dialysis efficiently removes excess fluid and waste, many patients no longer need to urinate, or their urine output is minimal. While some patients may retain residual kidney function and produce small amounts of urine, for many, dialysis fully replaces this function.