The kidneys filter the blood, removing waste products and excess fluid to maintain chemical balance within the body. When kidney function declines to End-Stage Renal Disease (ESRD), this natural filtration process fails, requiring patients to begin dialysis treatment. For many individuals with ESRD, one noticeable change is the near or complete cessation of urination. This outcome is a direct consequence of the physical destruction of the kidney’s filtering units, which dialysis cannot fully replicate.
How Healthy Kidneys Make Urine
Urine production in healthy kidneys is a continuous, three-step process carried out by approximately one million microscopic filtering units called nephrons. The first step is filtration, where blood pressure forces water and small solutes out of the blood and into the nephron’s capsule, forming filtrate.
Next, tubular reabsorption occurs as the filtrate travels through the nephron. The body reclaims almost all of the water, glucose, amino acids, and essential salts it needs, returning them to the bloodstream. Of the approximately 180 liters of fluid filtered daily, only about one to two liters ultimately leave the body as urine.
The final step, secretion, involves the active transport of additional wastes and hydrogen ions from the blood directly into the tubule. This process fine-tunes the body’s acid-base balance before the fluid exits as final urine.
Why Kidney Failure Stops Urine Production
Kidney failure progresses to End-Stage Renal Disease when nephrons are irreversibly destroyed, typically when kidney function drops to fifteen percent or less. Common causes of this damage include long-term conditions like diabetes and high blood pressure. This widespread destruction means the kidney can no longer generate the pressure needed for filtration or perform the selective reabsorption required to produce urine.
As the disease advances, a patient first experiences oliguria, a significant reduction in urine produced. Eventually, the patient may progress to anuria, defined as less than 100 milliliters of urine output over 24 hours. Once the filtering units are scarred and non-functional, the biological mechanism for extracting fluid and concentrating it into urine ceases.
Dialysis A Filter Not a Urine Creator
Dialysis, whether hemodialysis or peritoneal dialysis, takes over the mechanical, waste-clearing duties of the failed kidneys. The treatment uses a semipermeable membrane to passively filter metabolic waste products, such as urea and creatinine, and remove excess fluid from the blood. This process, known as ultrafiltration, mimics the waste and fluid removal aspect of the healthy kidney.
However, dialysis cannot replicate the active and hormonal functions of a natural kidney. The kidney is an endocrine organ that produces hormones like erythropoietin, which stimulates red blood cell production, and converts Vitamin D into its active form. It also regulates blood pressure and electrolyte balance through hormonal signaling, a level of biological complexity a mechanical filter cannot achieve. Because dialysis is a passive process performed intermittently, it removes fluid and waste but does not restore the biological machinery required to create urine.
Life Without Urine Output Managing Fluid Intake
For patients who are anuric, the inability to excrete excess water necessitates strict fluid management. Since the body cannot naturally remove water consumed between dialysis sessions, fluid contributes to overload. This excess fluid accumulates, leading to complications like hypertension and pulmonary edema, where fluid backs up into the lungs.
To prevent overload, anuric dialysis patients are placed on strict fluid restrictions, typically limiting daily intake to between 1.0 and 1.5 liters. They must also monitor their interdialytic weight gain, aiming to keep it below a target of 1.5 to 2.0 kilograms. Managing sodium intake is also important, as salt increases thirst, which complicates fluid restriction.