GI tract acidity

[capn jazz]

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From EK 1001 Bio:

As a protective mechanism, mucus is secreted by the epithelium along the entire GI tract. The mucus usually contains bicarbonate, which serves what purpose?

I. acts as a GI buffer
II. digests proteins
III. prevents the digestive tract from becoming acidic.

A) I only
B) I and III only
C) II and III only
D) I, II, and III

The answer is B, mucus as a GI buffer and prevents the digestive tract from becoming acidic.

Explanation: Bicarbonate (HCO3-) is a very important biological buffer that buffers the blood as well as the GI tract. It is also a weak base that increases the pH of the GI tract. If the GI tract is too acidic (emphasis mine), pancreatic enzymes denature and cannot digest luminal contents. However, bicarbonate is NOT a digestive enzyme.

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Now, I chose A) I only. I know the function of the mucus is the a) lubricate, and b) protect the EPITHELIUM from the ACIDIC contents of the stomach (part of the digestive tract!). In fact, in the EK Bio book itself, it says "Mucus...lubricates the stomach wall so that food can slide along its surface without causing damage, and mucus protects the epithelial lining from the acidic environment of the stomach."

So bicarbonate will raise the pH if it is TOO acidic, yes, but parts of the digestive tract are, indeed, acidic! If bicarbonate PREVENTED the GI tract from BECOMING acidic, the stomach wouldn't be very functional, especially the pepsinogen-->pepsin conversion!

So for the MCAT, would this question actually be I only or would be it the answer EK gives??

 

[Venicat]

To me options I. and III. are very similar since by definition a buffer would prevent a large change in PH and III. insinuates that bicarbonate acts to prevent a pH change. If you know what a buffer does, then III. must also be correct. While true that the GI tract is a largely acidic environment, it must be kept within optimal physiological pH ranges, otherwise nasty things happen like protein denaturation.

 

[capn jazz]

I mean the pH of 2 in the stomach is nowhere NEAR physio pH. Even the small intestine, which is "neutralized" by bicarbonate has a pH of 6 which makes it acidic.

 

[JasoniusUW]

Only the stomach is really acidic, you might be generalizing the stomach environment to the entire GI tract. The stomach is just but one step.

 

[0Complications]

I can see why you're frustrated using the logic of Too acidic and just outright acidic, but I still have to side with the answer choice I and III.

It's important to ask yourself what a question is testing when answering it. This one is trying to test your understanding that the GI tract has varying pH levels that occur at differing locations and that bicarbonate has nothing to do with directly digesting proteins.

The bicarbonate released into the duodenum is important in raising the pH of the acidic chime. It also activates prohormones that aid in the release of bile and the emulsification of fats, etc. It's important to think about the question in perspective. Once the bicarbonate comes into contact with the acidic chime it prevents all future progression through the GI from becoming acidic.

Another way to look at it is that if for instance bicarbonate was released into the esophagus as the bolus moved towards the stomach (which it isn't this is just to illustrate the point) then the stomach pH would be lower than the pH in the esophagus and in this situation, after bicarbonate is introduced, the GI contents become more acidic making III not a viable option.

Now in the real scenario, bicarbonate isn't introduced until after the chime reaches the duodenum causing the pH to rise and after which the pH does not become acidic comparatively to where it is at in the duodenum. This also helps to prevents the small and large intestine from damage that would be caused by a reduced pH.

 

[capn jazz]

Thanks for the responses... I was taking the question and I thought "If I select 'prevents the GI from becoming acidic' then I bet the answer key will say 'but wait! the stomach is acidic!'"... I should probably stop approaching EK qs like they're trying to trick me 🙂

 

[0Complications]

Thanks for the responses... I was taking the question and I thought "If I select 'prevents the GI from becoming acidic' then I bet the answer key will say 'but wait! the stomach is acidic!'"... I should probably stop approaching EK qs like they're trying to trick me 🙂

I used to do this to, still do sometimes.

I then decided to just go with what I know to be true and assume they are not trying to trick me. I would also do that on AAMC exams. I missed a number of questions on my first practice exam because I bought into the hype that the questions are never straightforward. They're always a hook, but that's just not the case. Just remember to think about what the question is asking.