Renal Function and Pathophysiology

After going through the three things that the kidneys filter, we’ll head on to what the kidneys produce that helps in creating red blood cells within the bone marrow. We’ll also tackle some of the kidneys’ main functions, with a bit of pathophysiology.

Let’s start.

Kidney Malfunction

If the kidneys are not working properly or are malfunctioning, there are a couple of components that will immediately get affected, mainly:

  • Hydrogen ions
  • Urea nitrogen (BUN)
  • Creatinine

These three components with an acronym, HUC, are expected to increase dramatically. Aside from these elements, hemoglobin and hematocrit levels will then decrease due to the inability of the kidneys to create erythropoietin.

Erythropoietin Defined

Erythropoietin is the hormone responsible for initiating the creation of red blood cells inside the bone marrow.

As nurses caring for clients who have renal failure and is undergoing peritoneal dialysis or hemodialysis, you have to anticipate that hydrogen ions, blood urea nitrogen, and creatinine are relatively high. The kidneys, due to its condition, are unable to get rid of these elements because urine is not coming out of the body.

High Blood Pressure

When a client with kidney disorder has increased blood pressure, it’s difficult to get rid of the fluid from the body to lessen the pressure. For this reason, hemodialysis is done.

Aside from hemodialysis, the client needs to be on fluid restriction because the fluid will stay inside the body and will not be eliminated immediately.

Understanding Kidney Structure

To know the primary function of kidneys, we must first go through its physiology.

As the filtering system of the body, you can consider the kidneys as washer machines. This washer machine filters the blood and gets rid of toxins and wastes that are excreted from the body in the form of urine. The nephrons are the basic working unit of the kidneys that do the filtering.

Filtering is done with the presence of glomeruli found inside the nephrons. These glomeruli branch out to the ends of the nephrons and are somewhat similar with an apple tree or bouquet of flowers.

Glomerular Filtration Rate

Blood goes through the nephrons and enters the glomeruli where they will be filtered. This process is called the glomerular filtration rate (GFR). Remember, the normal filtration rate is between 85 ml/min to 105 ml/min which translates to a healthy kidney that’s constantly working to filter hydrogen, urea nitrogen, and creatinine.

Inside the hospital, the bare minimum is around 65 ml/min.

The Geriatric Community

When geriatric clients reach the ages of 65 – 70, they tend to lose 3% of functioning nephrons yearly. Therefore, by the time they reach the age of 85, retention of proper kidney function will only be at 50%. This is the reason why the geriatric community experiences chronic health issues related to the heart, skin, and even the eyes.

Hyperglycemia and Hypertension

Hyperglycemia due to diabetes and hypertension are factors that precipitate kidney malfunction or breakdown.  

With hyperglycemia, the blood becomes thick and syrupy, like mud. If this mud-like consistency is pushed inside the kidneys and into the nephrons, there is an increased possibility that the kidneys (washer machines) will eventually malfunction.

Chronic hypertension, on the other hand, causes a consistent and long-term heightened pressure that affects the kidneys which can also lead to acute kidney failure.

In our next lecture, we’ll tackle the causes of acute kidney injury which leads a client to go into acute renal failure instantly.

Acute Renal Failure (ARF): BUN and Creatinine Interpretation

In our previous articles, we already discussed the phases of acute renal failure, namely:

  • Phase 1 – Oliguric
  • Phase 2 – Diuretic
  • Phase 3 – Recovery

We also talked about the three main causes of acute renal failure which are:

  • Low blood pressure
  • Low volume
  • Low oxygen

The mentioned causes are mainly due to low perfusion and low mean arterial pressure (MAP) that will cause the body to break down. So now, we’re going to assess how severe the acute renal failure is by determining your laboratory values.

Think of it this way: You’re driving your friend’s car, and you got into a car accident. The first thing that your friend is going to ask you once you break the news is, “How bad is it?” or “What’s the damage?” With acute renal failure, there are levels to determine the progression of the condition.                      

How to Diagnose ARF

To determine the severity of acute renal failure, doctors will request for specific tests for verification. Some of these tests may include:

  • Urine specific gravity
  • Blood urea nitrogen (BUN) and creatinine
  • Glomerular filtration rate – function test

Technically, this is breaking down the pathophysiology of kidney function, and we want to have a more vivid picture of the status of the condition, or how much of the kidney is no longer working or how much of it is still working.

Urine specific gravity

Urine specific gravity is going to show the concentration of the urine. If the test result indicates 1.030 or higher, you can expect very concentrated, dark, and stinky urine due to lack of water inside the body.

You will notice this with clients who are fond of drinking alcohol and soda but drink minimal water. Blood becomes concentrated, and it is turned into very dark urine output. In the phases of ARF, this is called as the oliguric phase.

BUN and Creatinine

As what was mentioned before, if BUN increases but the creatinine level is normal, the main reason is dehydration and NOT acute renal failure. Now, if the BUN and creatinine are both elevated, this means that you have a kidney problem.

Glomerular Filtration Rate

Glomerular filtration rate measures the creatinine in the blood. Having a GFR test result of below 65 is a sign of ARF. So remember the magic number – 65. Anything below 65 is a serious kidney issue, and anything above 65 is passing. The number that has to show on the GFR result should be above 85 to become normal.

Two Conditions of ARF

Now, we will determine how severe the condition is and in doing so, you have to know the two conditions of acute renal failure which are:

  • Extrarenal failure (prerenal) – 20:1 ratio for BUN/creatinine
  • Intrarenal failure – 10:1 ratio for BUN/creatinine

The difference between the two is their location. Extrarenal or prerenal failure means that the problem is outside the kidneys while intrarenal means the problem is inside the kidneys. To give you a more straightforward example, if there’s a fire (damage) and it happened outside the house (kidneys), it’s called prerenal. On the other hand, if the fire happened inside the house, it’s called intrarenal. Between the two, prerenal is more preferred than intrarenal to avoid serious damage.

Interpreting BUN and Creatinine

If your BUN is at 40, the expected creatinine will be at two; there’s enough space separating the BUN and creatinine. Since creatinine is the main factor that determines the severity of ARF, the moment it starts rising, and the numbers become closer, that’s a sign that therapeutic modality should be provided immediately.

Remember, if the “fire” is inside the kidneys, there will be depleted oxygen content and will start to suffocate the kidneys. Therefore, your kidneys need to be rebooted with treatment modalities that Mike likes to call “HDTV.”

To know more about these HDTV treatment modalities, check out our other nursing videos at See you there!

Renal Labs: Uncomplicated BUN and Creatinine Interpretation

Today, Mike Linares will teach you about one of the favorite topics in nursing exams – BUN and creatinine.

Let’s begin.

The kidneys’ primary function is mainly to filter the following:

  1. Hydrogen ions
  2. Urea
  3. Creatinine

You can easily remember this because your kidneys look like a hook; so that’s HUC (pronounced as “hook”).

Hydrogen ions are acidic. Therefore, with clients who have renal failure, they will be experiencing metabolic acidosis because the kidneys have increased hydrogen ions.

Urea is the by-product or waste product of ammonia that is detoxified into the liver, then goes into a portion of the portal vein and is finally sent to the kidneys for filtering. Once filtered, urea is excreted out of the body in the form of urine. BUN (blood urea nitrogen) technically means the amount of urea concentration in the blood.

Creatinine is the by-product of muscle breakdown. Creatinine is filtered by the kidneys and is also passed out from the body as urine.

Normal Levels

Kidneys filter out creatinine and BUN; this is evident in a metabolic panel. What we want to see in our metabolic panel for creatinine and BUN is a good ratio. This means that normal laboratory values should show as:

  • Creatinine – 0.7 to 1.2 mg
  • BUN – less than 20 mg/dL

Acute Renal Failure

In acute renal failure you have to identify the following indicators:

  1. Urine output – how much pee the client has excreted
  2. Glomerular filtration rate (GFR) – how fast glomeruli (the little washer machines) in the kidneys wash blood milliliters per minute. Normal GFR should be between 85 to 110 mL/min. Acute renal failure clients will show a GFR of less than 60.
  3. BUN and creatinine ratio – if creatinine and BUN have increased ratio. A creatinine that shows more than 1.2 mg will equate to a BUN that’s thrice as much. Example, if you have a creatinine level of 3 mg, your BUN will be 60 mg/dL.

To explain further, the body filters typically creatinine and BUN. However, the most significant indicator for kidneys failing in its primary function (filtering), is the presence of high creatinine. This is because BUN can be high in the body but not in the potty. This could just mean that you are just dehydrated. To remember this, keep in mind that your “BUNs get burned” when you are dehydrated.

Now, if your BUN and creatinine are both increased, you have kidney involvement. If in case you are still confused with kidney involvement, just remember this by recalling that you have two kidneys, therefore, two lab values should both be increased. If you only have “burned BUNs,” then you’re probably just dehydrated.

In the other lectures, we will have a comprehensive discussion on the topics of:

  • Acute renal failure
  • Chronic renal failure
  • Creatinine ratios
  • Oliguric phase
  • Diuresis phase
  • Recovery phase from acute renal failure
  • Recovery phase from intrarenal and extrarenal
  • ABGs with renal failure (sodium, calcium, potassium phosphate)

If you want to check out other topics that commonly show up in nursing exams, visit