Hi folks, Dr. Nair here. And in this video here, we are going to, this will be the first video in two videos in the series here, that kind of marches through the digestive tract here and just talks about the various different digestive processes that happen along the GI tract here. And so in this video, we'll be really talking about the oral cavity and the stomach. So that kind of first. part or first half maybe of the digestive tract.
So first of all here on our right hand side we see that we have all our digestive processes and this is digestive with a capital D here and so we'll see that some of them are blanked out like absorption and defecation. When it comes to oral cavity there is no absorption and as you may guess there's no defecation here right. So really We'll start here with ingestion.
This is exclusively occurring in the oral cavity, as you may guess. If you want to eat food, if you want to start digestion, you've got to put food into your mouth, aka your oral cavity, and that is where we actually start taking in food here. So this, hopefully, is very self-explanatory. Then the other parts do need a little bit more explanation. So propulsion.
As I mentioned before, propulsion, or otherwise known as motility here, it, um... It happens all throughout the digestive tract here. And this is really about moving food essentially forward, moving it along the digestive tract here. And so here, to get food from the oral cavity into esophagus, towards the stomach here, we'll use the action of swallowing.
And so this is deglutination. This is basically the technical term for it, but if you say swallowing, everyone knows what you're talking about here. So that and...
When it comes to oral cavity, that is our form of propulsion. We also have mechanical digestion happening here. And that is being done basically by chewing the food. This is otherwise known as mastication here. And as we chew our food, no matter how we're chewing it, what teeth we're chewing it with, we are literally trying to break up the food into smaller pieces.
By just the fact that you're kind of breaking the food up with your teeth here, you're not doing anything. chemical to it per se. So this is why it's considered mechanical digestion. Another big point in mechanical digestion and chewing in the oral cavity is to help mix saliva and other things that are being secreted into the mouth with the food, kind of coating the food into what is going to be called a bolus here. And so that kind of leads us to chemical digestion.
Because in the saliva and the other thing that's being secreted into the oral cavity, those chemicals, those solutions, are going to start the chemical digestion process already. So even before food has hit our stomach here, there's already a little bit of chemical digestion going on here. And so here we're really looking at saliva and other secretions. So when it comes to saliva, the big thing here is we have an enzyme in saliva called amylase. So you can call it like salivary.
amylase. And so this amylase here is primarily here to break down starch. So if you remember what starch is, starch is going to be considered our polysaccharide, our very complex carbohydrate here. And so amylase's job here, amylase is an enzyme here, and it's going to start breaking apart the bonds, trying to start freeing up some of those monosaccharides from the polysaccharide here.
And so... Anytime you see the amyl here, we're going to be talking about something that's about carbohydrates. And then if you ever see kind of a protein word that ends with A-S-E, ACE, this is typically denoting that that protein is an enzyme. So amylase here is an enzyme that deals with carbohydrates. In this situation, it's going to break down starch.
There is another enzyme, though it's not secreted with the saliva. It's secreted in other places of the tongue. We have lingual lipase, lingual referring to the tongue because it's coming from the tongue, and lipase. So, lipe here is referring to lipids and ace, again, enzymes.
So, this is an enzyme that's dealing with lipids here. And the interesting thing is, though it's secreted and technically has the ability to break down lipids, that's what lipase does, break down lipids, it's not active yet. That's because this lingual lipase is only active at lower pHs. So I'll leave you with that bit here. Alright, so let's move on.
Now we have food that goes from the oral cavity, via swallowing, right, to the stomach here. And so we're not really talking much about the esophagus here, because there's nothing really interesting other than food passing through it that goes on the esophagus here. So the next kind of big physiological space that we have to consider when we think about digestion is the stomach here. And so the stomach here really focuses on three processes.
mechanical digestion, I guess it's four. Propulsion, mechanical digestion, chemical digestion, absorption here. Of course, there's no ingestion happening here. Of course, there's no defecation here.
So we're focusing on these four here. Now here, I kind of lump propulsion and mechanical digestion together. And the reason I'm doing that is because when the stomach kind of moves around and kind of pushes the food around, it's kind of doing both things at the same time. So there are these kind of waves of kind of you can say undulation or waves of contraction that pass through the stomach and Those waves are called the mixing waves and as the name denotes here It's these waves that pass through the stomach as I could contract to start kind of say like let's say at that kind of the Top here and then they move down the body in this direction So they'll go here and move this way and so by doing passing this wave across the stomach here of contractions remember using the muscularis layer of the stomach, it's mixing up the content of the stomach here.
And so by doing that mixing here, we are promoting some mechanical digestion as we're helping the food kind of break apart as we kind of mix it up, kind of helping it dissolve in the gastric juices here. We also have gastric emptying. So there's also these same mixing waves will help push the bolus, which turns into the chyme here, towards the end of the stomach, towards the pyloric region here, and then out the stomach this way.
And so there are also contractions that kind of push food down and out of the stomach here. And so this is kind of part of the propulsion portion here. In all honesty, it is kind of hard to look at the stomach and be like, aha, that's the mixing wave, or that's gastric emptying.
These are more kind of phenomena that happen as the stomach churns and rumbles. These are things that we kind of put together because of that reason. Then we have chemical digestion.
Oh boy, there's a ton happening in the stomach here. The stomach is one of the biggest places of digestion here. Of course, we have mechanical digestion, but when we talk about chemical digestion, we have digestion of lipids and protein.
So all the amylase that was in the saliva, by the time it comes to the stomach and it's introduced to this very low pH of the gastric juices, it kind of... gets denatured and turns off. But the lingual lipase that kind of stuck around from the oral cavity, now that's active. On top of that, we have gastric lipase.
So the stomach also secretes its own lipase. So the lingual lipase and the gastric lipase come together, well, literally, but they work in tandem to break down a bunch of fats in your food. On top of that, we have hydrochloric acid, abbreviated as HCL.
And that's secreted along with pepsin. And hydrochloric acid is kind of one of the big components of gastric juices here. With these two pieces together, we're also breaking down protein. And this actually needs a little bit more explanation. We'll get there in a moment.
But so overall, in chemical digestion, we're breaking up the bonds between lipids, making kind of larger lipids into smaller lipids. Same thing with the... protein. We're breaking up these large proteins into smaller chains of amino acids, which we can call peptide chains basically, or polypeptides. And to be honest, there's not a whole bunch of absorption happening, but there is a little bit.
This is one of the first opportunities for hydrophobic substances to be absorbed here. When I mean hydrophobic substances, I'm talking about the substances that normally kind of pass through cell wall membran-uh, cell walls. Uh, sorry, cell membranes. Oh jeez.
So remember in the previous talk how we talked about how small hydrophobic, electroneutral molecules can kind of pass through cell membranes easily? Yeah, I'm talking about those kind of molecules. Those molecules get a chance to get absorbed here at the stomach. Okay. by simply just diffusing through the cell membranes of the stomach of the stomach cells and making it into the bloodstream of the stomach here.
So there is a site for absorption here. This is a particular note, especially when you ingest things like alcohol or other drugs here that have that particular nature. They can get absorbed in the stomach and basically have effect on your body sooner. Okay, remember we were talking about hydrochloric acid and pepsin a little bit more. Here we go.
So here we're looking at... kind of a crypt in the mucosa of the stomach here. So we have this little dip in the mucosa here and the epithelial lining here. We have a bunch of glandular cells, cells that are gonna secrete stuff.
And two things that get secreted from these secretory cells of the mucosa of the stomach, here's hydrochloric acid and an enzyme called pepsinogen. So we've talked about pepsin, but this is pepsinogen. And when you see the kind of the suffix of ogen, This is basically saying that this is an enzyme and it is inactive.
Oops, I wrote that way too big, sorry. This enzyme here is inactive, so when you see o-gen, it's inactive. Now, here's the thing.
When you take hydrochloric acid and you combine it with this pepsinogen, it creates pepsin. Now, this is not an actual chemical reaction, even though I'm using that notation here. Just kind of go with it.
Don't try to balance this or whatever. Hydrochloric acid will take pepsinogen and activate it, essentially by kind of cleaving a part of the protein off. And now pepsin, this is the active part here.
This is the active part. Oh, I can spell active properly. There we go. And so it's really the combination of hydrochloric acid and pepsinogen that leads to pepsin.
And then pepsin is an enzyme that can break down protein. And this is a big thing. This kind of gets to the autodigestion issue, right? We don't want to hold pepsin, an active enzyme, inside these cells because it's just going to start dissolving up all the proteins within the cells. So what we do is we secrete an inactive version with it along with hydrochloric acid, and the hydrochloric acid will activate the pepsin only in the lumen of the stomach.
So here, this is one of the tricks we have in our digestive system to help prevent autodigestion. Here's another one. We have a mucus layer.
So some of the cells of our stomach mucosa produce mucus. It's kind of the name mucosa, right? And so it creates this layer of mucus here. And so in our picture, that's this kind of layer here.
Just think of this as goopy mucus here. And so everything here, the gastric juices here, they're at a pH two, and then we have this mucus. And this mucus here is made up of protein molecules, which we'll call mucin.
These are very long protein chains that kind of branch like this. And the interesting thing is if you take these, you mix it with water, It kind of makes this weird matrix thing where parts of it connect with each other and get all kind of knotted up. Just think if you put a bunch of strings together, they eventually get knotted together.
Same thing with these mucin chains here. And so when you take this, you're basically making something called mucin gel, which we call mucus. And so that's kind of the crux of any kind of gelatin, like gel or whatnot. It's just a bunch of long chain of molecules kind of tangled together. Anyways, so this mucous layer acts...
as a kind of a physical barrier, but it also is embedded with bicarbonate ions, which are going to act as a kind of a base here. And so as even though pH is really low here and the hydrogen ions may diffuse to get to these cells here, they're going to be neutralized by the bicarbonate hidden or kept in this mucus. So this mucus layer is acting as a physical barrier, but also like a pH barrier. So we also don't burn these cells with acid here. Hopefully that makes sense.
That's the end of our video here. I'll see you in the next video where we talk about the other half of the Digestor track. I'll see you then.