Hi folks, Dr. Nair here, and in this video we'll be talking about the digestive processes of the intestines, so both small and large intestines. So here we're starting with the small intestines. So as you may guess, there's no ingestion or defecation happening in the small intestines, but we do have propulsion in the form of peristalsis.
So hopefully you remember from anatomy that peristalsis is this kind of moving contraction wave that kind of pushes. food through the small intestines, right? Like basically pushing, you know, yogurt out of a tube or, you know, a popsicle out of its tube.
Kind of doing that pushing motion and kind of pushing it across the tube, that's peristalsis here. So that's our form of propulsion. But then we have forms of mechanical and chemical digestion, and boy, we have a lot of that happening still in the small intestines.
So first of all, kind of like the stomach, we have segmental contractions as our form of mechanical digestion. So at this point, you have to remember that food at this point here is now considered chyme. And chyme is very different than a bolus.
Bolas was, you know, solid and wet, but chyme is more of like a slurry, or kind of like a smoothie-esque, maybe more liquidy smoothie-esque kind of composition. And so to do mechanical digestion on something that's mostly liquid, it doesn't really seem the same as like what we did with chewing, mastication in the oral cavity. But in this sense, we are trying to do our best to kind of evenly distribute all the nutrients and enzymes, things we'll talk about in a moment, through the chyme here. In that sense, we are really kind of spreading out all the particles here.
That's our form of mechanical digestion here. We're still trying to break these things apart and try to suspend them as best we can in this fluid. Then we have a lot of chemical digestion by means of the pancreatic secretion, something we'll talk about in a moment.
We're breaking down things like polypeptides, carbohydrates, fats, etc. It's done by a whole bunch of enzymes, things we'll talk about in a moment. Then, of course, when it comes to intestines, as you may know what the intestines are known for, it for its absorption and it's absorbing things like amino acid, monosaccharides, fatty acids, water, vitamins, minerals, ions, the list goes on. Hopefully you notice there's a difference about what is being digested and what's being absorbed. What's being absorbed is like very simple units of these biomolecules, amino acids, monosaccharides, fatty acids, and water and minerals and ions, they're even smaller than those monomer versions of those biomolecules.
So we're absorbing things that are small, very small. And that's a point that we will kind of stress on at later videos. All right, so moving on, let's talk about the secretions that go into the small intestine. So there's a bunch of secretions that go into small intestines, and they all help with either digestion or absorption, or both. And so one of these secretions is comes from the liver and gallbladder.
This is bile. And so the bile of the fluid is actually 90... Oh, I forgot to write it here.
It is 90% water. Water, there we go, I show. And only 2% bile salts. So what is bile?
What is bile salts and what do they do? What's it about here? So remember, oils and water don't really mix, right? They separate... with each other because oil is non-polar, water is polar here.
However, if you add something like bile, bile salts, something that can emulsify, this is going to create a situation of emulsification. And so what happens is here we have our bottle here, oil, water. Normally if we shake this up, they will stay in two different layers, right? But if you add a little bit of this emulsifier here, What ends up happening is if you shake it up, it looks like the oil has dissolved in the water.
If you zoom up into it, it actually hasn't. It's just broken up into tiny little beads or spheres of oil that can sit within the water. The big reason that can do so here... is because of these biosolids. These biosolids are amphipathic.
They have a hydrophilic portion and a hydrophobic portion, right? So if you look at the chemical portion here, they have kind of this hydrophilic head here, and this is what looks almost like a steroid as its kind of base here. And so these biosolids can actually surround the oil, little droplets here, and then this part here can be submerged in water. Because here we have hydrophilic outside, hydrophobic inside. This works out, right?
It kind of looks like a phospholipid bilayer a little bit, right? And this emulsification process is vital. By breaking up or emulsifying our fats in water, we're doing a couple things.
One, that's actually a form of mechanical digestion. We've taken all this fat here and broken it up into smaller chunks here. Mechanical digestion.
That's one part. Two, in order to have... proper digestion and absorption, we need this oil to be as well dispersed in our water solution as best we can. Our water solution is basically chyme at this point, right? And by doing so, we're really increasing the surface area contact between the water and the oil, right?
Before, we just had this much, right? But now, since there's water surrounding every single oil droplet here, that... vastly increases the surface area, which means enzymes can get more of the oil and break them down, and we can have a better time absorbing them. So this is a big, big part of fat digestion, something we'll harp on a little bit more in another video.
What else is getting secreted into the intestines? We have pancreatic secretions, and there are two big things here. We have our bicarbonate ions coming in here, And as we talked about in the stomach video here, this is gonna neutralize the pH. This is basically molecules that are considered basic, and so if you mix it with that acidic chyme coming out of the stomach, it's gonna neutralize the pH, kind of bring that pH back up, which is good because the small intestines can't really handle a low pH. But another big thing here are a whole suite of enzyme, a whole bunch of different enzymes, in fact, families of enzymes.
come from the pancreas as well, enter the small intestines and start mixing with the, sorry, with the chyme. And it has a whole bunch of different types of enzymes. We have proteases, amylases, lipases, and nucleases. And so these are all ACE proteins. So these are enzymes mentioned before.
So they're here to break stuff down. And you can kind of tell by the first part of the name, what are they breaking down? So proteases, as you can guess, break down protein.
Amylase, we've seen amylase before, breaking down carbohydrates. Lipase we've seen before, that's going to break down lipids here. And then we have nucleases here that are meant to break down nucleic acids here, right?
And so let's talk about these enzymes, because, again, remember about our autodigestion issue. We don't want to create active enzymes that sit in our cells. So for the pancreatic secretions here, we're secreting out inactive... where it says zymogens, but they're basically inactive enzymes. So we have these inactive enzymes, and you'll see a lot of ogens here.
You'll see a lot of pro, so like pro-colipase. Pro is talking about pre. So this is a pre-lipase.
It's not a lipase yet, but it's going to be there. So a pro-colipase is an enzyme that's going to be called colipase later, but it's not activated yet. So it's the predecessor.
So ogen and pro basically mean the same thing here. And so the interesting thing here is pancreas solutions come with a whole bunch of these inactive enzymes, but also comes with an enzyme here called trypsinogen that's also inactive. Now here's the trick here.
Trypsinogen actually interacts with this apical layer of the intestinal mucosa, the epithelial tissue that lines the mucosa. And there are little enzymes here called enteropeptidase that actually will activate trypsinogen. Trypsinogen will become trypsin.
And then what happens, the trypsin will then go activate all these inactive enzymes, making them active enzymes. So you see, right, we went from prokaryote, colipase to colipase. And so now all these enzymes here can then do all the active stuff of breaking down lipids, carbohydrates, proteins, so on and so forth.
Furthermore, built into our small intestines, we have what we call pear patches. These are basically lymph nodules that are built into the mucosa of our intestines. Why do we have this? Again, to fight against pathogens.
I didn't mention this in the last video, but gastric juices also fight against pathogens by having hydrochloric acid. It turns out most microbes can't handle a pH of 2, meaning the ones that do are pretty particularly gnarly, right? Anyways, here we see another form of just pathogenic defense here, as we just have a whole bunch of lymph nodules filled as you may guess, lymphocytes here, just monitoring whatever's being absorbed by the intestines will go through here and has to go through all these payer patches, and so this is another way we can fight against pathogens. Okay, we're at the large intestines. We're basically at the end of the digestive tract here.
So of course, No ingestion here, but we stop repulsion and mechanical digestion in the form of peristalsis and segmental contractions, things we've seen in previous parts of the digestive tract. And then we do have some, but not a whole bunch, chemical digestion. We have carbohydrates being digested.
And particularly, we ourselves are not really doing the digestion. Our gut flora, so all the microbes that live in our large intestines, we call that the gut flora. Sometimes you might call it the... get microbes, or there are many names for these microorganisms that live in our large intestines.
They actually take up whatever leftover material that we haven't digested, and they will digest it themselves. And that's a whole bunch of things, but a key example here is fiber. There's a lot of fiber we don't digest, but who does?
It's the microbes in our large intestine that does so. Particularly here in the cecum, that's going to happen a lot here. And so... We still do a lot of absorption in the large intestines. At first, you know, researchers thought it was just water absorption, right?
Trying to deal with that mass balance problem. We put a whole bunch of fluid into the diatric, we're absorbing a lot of that back here. And that's true. Large intestines absorbs a lot, a lot of water. But we found that it absorbs a lot more than we initially thought.
It absorbs things like a lot of ions, minerals, and a lot of byproducts from the gut flora. So we have this very interesting relationship of the gut flora. gut flora eats whatever leftover material gets into the large intestines, they produce some of these byproducts, we absorb them, we use them. And so we have this very friendly relationship with our gut flora that way.
Then of course we have defecation. Defecation exclusively happens in the rectum here, we'll definitely talk about that. And here we go, we're going to talk about basically the reflex loop here that starts and stops defecation here.
And so we're talking about this roughly three-step process here. So first step here is feces distends the rectum. So by the time chyme gets here and gets kind of dehydrated by the large intestine, it becomes feces.
And that feces starts piling up in the rectum here. When we say distends, we're talking about the, that's the verb form of it, distension, that's the noun. And distension is basically just stretching of, you know, of some kind of organ or tube, something along those lines.
So when feces distends the rectum, I mean there's so much rectum in the feces, uh, there's so much feces in the rectum that it's literally kind of stretching out the rectum. There are stretch receptors that will be stimulated if the rectum gets stretched out long enough or large enough. Then what happens is sensory neurons from these stretch receptors will send that sensory information saying, ah, the rectum is really stretched, all the way to the spinal cord.
Then We'll have a spinal cord reflex, where a parasympathetic motor nerve will travel from the spinal cord back to the rectum and does a couple things. One, it does similar contractions of the rectum here, kind of pushing feces down the, you know, the length of the GI tract towards the anus. It's also gonna relax the internal anal sphincters, opening up that anus.
And now here's the third step. And perhaps the most important step here, when you are consciously ready, you will basically activate somatic motor neurons here, or actually you're going to inhibit somatic motor neurons here. That will relax the external anal sphincter here. That will fully open up the anus here, allowing basically feces to pass on through.
And ha, we have defecation here. And of course, with some somatic action, we can also do some further... pushing of the rectum here. But that's it. That is the defecation reflex.
That's it for this video. In the next video, we are going to start jumping into the various different macromolecules and talking about how they get digested and absorbed. I'll see you for that video.
Thank you.