What are oliguria and polyuria during acute renal failure?

In the context of acute renal failure, oliguria and polyuria represent distinct phases of the condition's progression.1

Oliguria

Oliguria is a hallmark of the second phase of acute renal failure, known as the phase of established renal failure. It is defined as a urine volume of less than 500 milliliters per day. During this stage, the kidneys are unable to adequately eliminate waste products, water, and electrolytes, leading to their retention in the body. This volume is considered the minimum amount necessary to meet the body's ordinary excretory demands, which are often further increased by tissue breakdown during the illness.1

Polyuria

Polyuria characterizes the third or recovery phase of acute renal failure. As the kidney tissue begins to regenerate and function returns, there is an increasing output of urine. While this phase presents fewer management challenges than the oliguric phase, the recovering kidney may initially lose electrolytes and water at an unusual rate, necessitating careful replacement to maintain balance.1