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Acute Renal Failure - Postrenal Failure & How It Alters The Body

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Instructor: Artem Cheprasov

Artem has a doctor of veterinary medicine degree.

Postrenal failure is a type of acute renal failure that occurs when an obstruction blocks the outflow of urine. Learn the diagnosis criteria for postrenal failure, the definitions and alterative effects of azotemia, hydroureter, hydronephrosis, and anuria diagnoses, and explore treatments for patients with renal failure. Updated: 10/01/2021

Obstruction to Outflow

If a dam is built over a river, the water behind the dam backs up and expands outward. If the dam that is built isn't strong enough to absorb the force of the water backing up behind it, it can catastrophically crack, fail, and burst open. Well, this scenario can occur within your own urinary tract if the outflow of urine is obstructed for any reason. This obstruction to outflow can result in postrenal failure, among other things.

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  • 0:01 Obstruction to Outflow
  • 0:34 Postrenal Failure
  • 3:00 Consequences,…
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Postrenal Failure

Postrenal failure is a type of acute renal failure that occurs as a result of obstruction to urine outflow. This obstruction to urine outflow can occur anywhere after, or 'post-,' the kidneys. This means that if a ureter, bladder, or urethra is obstructed due to:

  • Stones
  • A tumor
  • An enlarged prostate (in men)
  • Or anything else that can compress these structures from the outside or obstruct them from the inside

then the kidneys can fail.

The kidneys fail because, just like water building up behind a dam destroys houses and villages behind the dam, so too does a kidney begin to flood with the urine it produces if urine outflow is obstructed. This results in kidney shutdown and, therefore, kidney failure. This occurs because as the pressure inside the kidneys increases from the obstruction, the glomerular filtration rate decreases.

The glomerular filtration rate is the speed with which your kidneys filter waste substances from the blood. It's a forward force that pushes stuff out of the blood and into the kidneys. But if the kidneys are flooded, then this flooding creates a force that opposes the glomerular filtration rate and brings it to a standstill.

To prove this to yourself, take out a straw and exhale through it. The air coming out is like the urine coming out of the kidneys through a ureter - the tube that connects the kidney to the bladder. Now, close the end of the straw with your finger. This will mimic a stone blocking the ureter. As you blow into the straw, the air will back up very quickly inside of it, opposing any forward flow of air, thereby making it impossible to push any air forward within and through the straw.

This means you can't exhale through it, bringing exhalation, our glomerular filtration rate so to speak, to a standstill. If the kidneys shut down, they cannot filter out waste substances, such as blood urea nitrogen (BUN) and creatinine. Elevations of BUN and creatinine are termed azotemia.

Consequences, Diagnosis & Treatment

Postrenal failure has other consequences beyond azotemia. Let's examine some of them. Let's say that a stone blocks the outflow of urine from a kidney because it has lodged itself in a ureter. Again, go back to our dam example. The water behind the dam backs up and widens out. So, the urine backing up behind the stone in the ureter causes the abnormal widening of the ureter, called hydroureter, and the dilation of the renal pelvis and calyces, known technically as hydronephrosis. The renal pelvis and calyces are the parts of the urinary system that help to channel urine into the bladder.

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