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How the Kidneys Regulate Acid Base Balance

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  • 0:05 Acid-Base Balance
  • 1:03 Protons and Buffers
  • 2:56 Metabolic and…
  • 3:58 Respiratory and…
  • 4:51 Long-Term Compensation…
  • 5:17 Lesson Summary
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Lesson Transcript
Instructor: Artem Cheprasov
Find out how your kidneys keep you alive by excreting acids and bases in conditions like respiratory acidosis, metabolic alkalosis, metabolic acidosis, and respiratory alkalosis.

Acid-Base Balance

Everyday processes like walking, the digestion of food, and the overall metabolism in your body produce a lot of acid as a byproduct. Because of this, you'd be a giant walking lemon if it wasn't for your kidneys. What I mean is, like a lemon, you'd be filled with acid if your kidneys weren't there to help you regulate your body's pH through something we call acid-base balance.

This is a process whereby receptors are able to determine the pH of your body and blood and do something about it if it's too acidic or too basic. If an imbalance in the pH is detected by your lungs, buffers, or kidneys, your body springs into action to take care of the problem. In this lesson, we'll focus in on how the kidneys help to control the acid-base balance in your body.

Protons and Buffers

Whereas the buffers in your body and your lungs are involved in the rapid adjustment of your blood's pH, the kidneys adjust the pH more slowly. Under normal conditions, the kidney's main role in acid-base balance is through the excretion of acid in the form of hydrogen (H+) ions.

The kidneys secrete excess hydrogen ions primarily in the proximal tubule. The interesting thing to note is that while the proximal tubule secretes a lot of acid, the tubular fluid's pH remains virtually unchanged. This is because buffers filtered by the glomerulus, including phosphate and bicarbonate, help to minimize the acidity of the tubular fluid. In fact, what's really cool is that the pH of the tubular fluid, by the time it reaches the collecting duct, is about 7.4, which is exactly the pH of normal blood.

The Collecting Duct

However, by the time urine is excreted out of the body, it can be acidic, basic, or neutral. This is because the end-all, be-all gatekeeper in determining the final pH of urine is not the proximal tubule - it's the collecting duct.

The collecting duct secretes hydrogen ions if blood is too acidic.
Collecting duct and hydrogen ions

The collecting duct can secrete hydrogen ions with a really high hydrogen ion concentration gradient if your body thinks its blood is too acidic. Therefore, acidic, or that is to say urine with low pH, will be excreted in that case.

Conversely, if your body thinks it's a bit too alkaline, then the collecting duct can secrete bicarbonate, which is a base, into the collecting duct and therefore cause the urine to become more alkaline.

Metabolic Alkalosis

While that was a basic overview, keep in mind there's a lot more to it than that.

In some conditions, your body may undergo something called metabolic alkalosis, which is a process whereupon decreased hydrogen ion, or increased bicarbonate, concentrations occur in the body. Since the blood pH is too basic in these conditions, the kidneys excrete bicarbonate to try and compensate. You can probably guess by now that the secretion of bicarbonate causes the formation of alkaline urine.

Respiratory Alkalosis

Alkaline urine is also formed by the excretion of bicarbonate in order to compensate for a condition called respiratory alkalosis. This is a condition whereby a prolonged increased breathing rate causes the blood's pH to rise. In addition, reduced hydrogen ion excretion occurs in this case in order to try and decrease the pH of the blood.

Respiratory Acidosis

On the flipside, respiratory acidosis is a condition whereby a prolonged decreased breathing rate causes the blood's pH to fall. Here, the kidneys will not only create acidic urine by getting rid of hydrogen ions, but they will also retain bicarbonate in order to help buffer the body's low pH.

Metabolic Acidosis

Increased hydrogen ions in the body will cause the kidneys to excrete more acids.
Metabolic acidosis

Finally, a low pH of the blood may also occur thanks to a process called metabolic acidosis. This is a condition whereby increased hydrogen ion, or decreased bicarbonate, concentration occurs in the body. When this occurs, the kidneys will do everything they can to conserve basic substances like bicarbonate in order to buffer the blood. They will also try and increase the excretion of acids.

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