Acute Renal Failure - Prerenal Failure & Its Effects On The Body

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  • 0:01 Acute Renal Failure
  • 0:43 Prerenal Failure
  • 1:10 A Decrease of Blood Supply
  • 2:04 Other Problems Can…
  • 6:04 Signs and Treatment
  • 8:05 Lesson Summary
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Lesson Transcript
Instructor: Artem Cheprasov

Artem has a doctor of veterinary medicine degree.

This lesson will mainly focus on the prerenal causes of acute renal failure, why they occur, how they influence the kidneys, and how this in turn causes problems in the body.

Acute Renal Failure

Your kidneys have the important function of filtering waste products and toxins from your body into the urine and away from the body during urination. They are, in every sense of the word, your body's filters.

Acute renal failure, aka acute kidney injury, refers to a sudden and rapid decrease in the ability of the kidneys to filter the blood that develops in less than two days' time.

When the kidneys can no longer properly filter the blood then toxins build up in the body, and the water, electrolyte, and acid/base balance becomes skewed too much to one side, resulting in major organ dysfunction and death.

Prerenal Failure

One form of acute renal failure is known as prerenal failure. This refers to kidney failure as a result of a decrease in the blood supply to the kidneys. Prerenal failure alone implies that the urinary tract system (the kidneys, ureters, bladder, and urethra) are initially perfectly healthy. With time, however, the kidneys may become compromised if this condition is left untreated for too long.

Decrease of Blood Supply

To understand why, consider the definition I just gave. The reason prerenal failure occurs is because there's a decrease in the blood supply to the kidneys. This can occur because a person is dehydrated or bleeding out. Either leads to low blood volume. This leads to less blood reaching the kidneys. This is exactly like the problem desert communities face when there's little rain, leading to dehydration of the land, and therefore less water flowing through the pipes of the water supply system.

Another cause for a decreased blood supply to the kidneys occurs when the arteries supplying the kidneys with blood are partially or fully blocked due to an arterial plaque or blood clot. These are like a cork in a bottle. The cork is not going to let anything flow past it; neither is a blood clot.

Other Problems Can Cause Prerenal Failure

Finally, something known as effective blood volume depletion may occur as a result of organ malfunction. This term refers to the fact that the body actually has plenty, and in some cases more than necessary, amounts of blood volume. The reason this volume of blood doesn't get to the kidneys is because something responsible for the circulation of blood, something responsible for getting blood to the kidneys, is not functioning correctly. What could that be?

It's the heart, of course! The heart pumps blood to the kidneys. If a person has congestive heart failure, the heart doesn't pump adequate amounts of blood to the kidneys, resulting in prerenal failure. This is exactly like a tire pump failing. This means no air will get to the tires. So, if our body's pump fails, little blood will get to the kidneys.

Another cause of effective volume depletion, resulting in prerenal failure, is the stagnation of blood somewhere else in the body. Portal vein hypertension (the rise in blood pressure in the blood vessel that drains the gut), as a result of cirrhosis (or scarring of the liver), can cause this.

In any cause of prerenal failure, the kidneys are initially healthy. But because they do not receive an adequate supply of blood, they eventually begin to malfunction and die from oxygen starvation.

Furthermore, the lack of adequate renal blood flow decreases the glomerular filtration rate. This is the rate at which your kidney's glomeruli filter away waste products in the blood. That's why I like to think of it as a 'waste filtration rate' to remember its function a little bit easier. A decreased 'waste filtration rate' therefore leads to the accumulation of waste product and toxins within the body.

The Consequences of Prerenal Failure

Now let's examine what happens in the body as a result of prerenal failure. The first thing you need to know is that the kidneys believe (correctly or not) that there is not enough blood volume in the body since regardless of the cause, the kidneys are not getting enough blood delivered to them. This means the kidneys activate a sodium and water conserving system called the renin-angiotensin-aldosterone system (RAAS). Meaning, the kidneys will reabsorb as much blood volume-enhancing sodium and water back into the bloodstream as possible (this helps to explain why patients with effective volume depletion may have too much blood volume).

How will the activation of this system present itself during testing on a patient with prerenal failure?

Well, knowing what the renin-angiotensin-aldosterone system does to the body, it should come as no surprise that the percent of sodium filtered by the kidney's glomeruli that's actually excreted in the urine will drop, since the sodium is being reabsorbed back into the body.

Secondly, activation of RAAS results in urine that is very concentrated, since the water is also being reabsorbed.

This is why patients in acute kidney failure due to a prerenal cause may have oliguria, or low urine output. As a general rule, oliguric patients have a worse prognosis for survival than non-oliguric patients in acute renal failure.

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