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11.06 Renal (Kidney) Acid-Base Balance

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Overview

  1. Acid-Base Balance
    1. Normal pH of blood = 7.35 – 7.45
    2. Alkaline reserve in the blood
      1. Sodium Bicarbonate
    3. Urine tends to be acidic
      1. Approx pH 6.0
  2. Existing conditions
    1. In the cells of the proximal convoluted tubule (PCT) and distal convoluted tubule (DCT), the Carbonic Acid Reaction (CAR) creates H+ and HCO3.  
    2. Sodium Biphosphate filtered into the urine

Nursing Points

General

  1. Under NORMAL conditions
    1. Sodium biphosphate in urine (Na2HPO4)
      1. One sodium breaks off and is reabsorbed into bloodstream
      2. Becomes sodium phosphate (NaHPO4)
    2. CO2 undergoes the Carbonic Acid Reaction in the cells, creating HCO3 and H+
    3. Hydrogen secreted into urine
      1. Combines with sodium phosphate to be excreted in the urine
      2. pH 6.0
    4. Bicarbonate reabsorbed into bloodstream
      1. Combines with sodium
      2. Sodium bicarbonate = alkaline reserve
      3. pH 7.4
  2. Under ACIDIC conditions (blood pH < 7.35)
    1. Same normal processes occur, PLUS:
    2. Cells make Ammonia (NH3)
      1. Secreted into urine
      2. Combines with excess Hydrogen ions
        1. Makes Ammonium (NH4+)
    3. Ammonium combines with Chloride to be excreted in the urine
    4. Makes the urine more acidic to get rid of excess Hydrogen and increase the body’s pH.
  3. Under ALKALINE conditions (blood pH > 7.45)
    1. Carbonic acid reaction still occurs, EXCEPT:
      1. Hydrogen reabsorbed into bloodstream
      2. Bicarbonate secreted into urine
        1. Combines with sodium to be excreted
    2. Increases alkalinity of urine by dumping sodium bicarbonate
    3. Body’s pH decreases

References:

Betts, J.G., et al. (2017). Anatomy and physiology. Houston, TX: OpenStax, Rice University. Retrieved from https://openstax.org/details/books/anatomy-and-physiology?Book%20details

Study Tools

Video Transcript

This lesson is going to talk about the kidney’s impact on acid-base balance. If you’ve reviewed the respiratory lessons, you know that the lungs help with this as well – so we’re gonna talk about the kidney’s role and how powerful it is.
Let’s remember that the normal pH of the blood is 7.35 to 7.45 – everything we do is in an effort to help maintain this normal pH. There’s an alkaline reserve in the blood in the form of sodium bicarbonate that really helps maintain this pH. But the urine actually tends to be more acidic with a pH around 6. Now, a couple things to know that are happening all the time before we start looking at these process. One is that inside the cells in the nephron, the carbonic acid reaction is taking place. This is when CO2 and water come together and create hydrogen and bicarbonate or vice versa. It’s also important to note that there’s a compound called disodium phosphate, or Na2HPO4 that is filtered into the urine and already inside the urine when we start this process.

Okay, so let’s look at what happens under normal conditions. This is our bloodstream, this is our urine, and these are the cells of the tubules in the nephron. So remember that this Na2HPO4 is already here in the urine. One of these sodium ions breaks off and gets reabsorbed back into the blood – so now we’re left with just NaHPO4, which is sodium phosphate. Then remember that the carbonic acid reaction or CAR is happening here in the cells and making Hydrogen and bicarb or HCO3. The hydrogen will be secreted into the urine, combine with the sodium phosphate and that gets excreted out of the body as sodium biphosphate, or NaH2PO4 – so that’s how we get rid of the hydrogen. This bicarb that we released is reabsorbed into the bloodstream and combines with this sodium that we already reabsorbed – that turns into sodium bicarbonate, or NaHCO3 – which is the alkaline reserve in our systems. So this is under normal conditions. Let’s see what happens under different conditions.

What if our blood pH is acidic? That means there is too much hydrogen ion concentration in the blood and the pH is too low. Under acidic conditions – all of the same normal responses will still happen – so review the last slide if you need to. But then our bodies will do a bit extra. Under acidic conditions, these cells will make ammonia, or NH3. That NH3 will combine with the excess hydrogen from the blood to form ammonium – or NH4+. That will get secreted into the urine, combine with Chloride, and get excreted. So ultimately our goal here is to excrete those excess hydrogen ions, get rid of the extra acids, and increase the pH of our blood back to normal. So the big thing here is this ammonia to ammonium chloride process.

So what if we’re in alkaline conditions? That’s when we don’t have enough hydrogen ions in our blood. So our pH is too high. In this case, we still have the sodium that breaks off and leaves sodium phosphate, and we still have the CAR in the cells making Hydrogen and Bicarb. But instead of the normal processes occurring, we actually see the opposite. The hydrogen ions will get reabsorbed into the bloodstream, and the bicarb combines with the sodium and stays in the urine to be excreted. So basically we’re reversing the normal processes. The goal here is to excrete the bicarb, which is an alkaline substance, and retaining the hydrogen. The hope is that we can increase the hydrogen concentration and help to decrease the pH back to normal.
So let’s recap. When it comes to acid-base balance, the whole goal is to maintain the blood pH between 7.35 and 7.45, which means the urine is usually pretty acidic at a pH of 6. Under normal conditions, our body will excrete hydrogen as sodium biphosphate and reabsorb bicarb as sodium bicarbonate. Under acidic conditions – those normal processes still happen, AND we see ammonia created to bind the extra hydrogen and excrete it as ammonium chloride so we can increase our pH back to normal. And ander alkaline conditions, we essentially see the normal conditions reversed so that we can retain the hydrogen ions and excrete the bicarbonate – so we can decrease our pH back to normal.
As you think about all of the functions that the kidneys perform – and start to learn about disease processes – start to think about what would happen in our body if the kidneys weren’t working. They are a vital organ and involved in the majority of the regulatory processes in our bodies. Alright guys, check out all the resources attached to this lesson. And if you want to learn more about acid-base balance, make sure you check out the ABG course as well. Now, go out and be your best selves today. And, as always, happy nursing!

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