Small Intestine - Online Tutor, Practice Problems & Exam Prep

The small intestine, approximately 6 to 7 meters long, is crucial for digestion and absorption. It consists of three sections: the duodenum, jejunum, and ileum. The duodenum mixes chyme with bile and pancreatic juice, while the jejunum is primarily responsible for nutrient absorption through microvilli. The ileum absorbs bile salts and remaining nutrients. Motility is facilitated by peristalsis and segmentation, enhancing digestion and nutrient uptake. Immune functions are supported by Peyer's patches and Paneth cells, which protect against pathogens, ensuring a balanced gut environment.

Functions of the Small Intestine

As we start talking about the small intestine, we want to take one of these steps back and just look at the functions of the small intestine generally, so that when we get into all the details that come up, we understand how they play into this big picture. So, we're going to say here that the small intestine is really maybe the major digestive organ responsible for both digestion and absorption. And when you think of the function of the digestive system, well, it's to break down food digestion so that you can absorb it. And this is the center of the digestive system. This is where so much of that happens. I do want to note that it's small because of its diameter and not because of its length. It's actually quite long: 6 to 7 meters in a cadaver, so somewhere in the range of 20 feet there. And we say in a cadaver because in a living person it's actually a little bit shorter because it's a muscular tube and those muscles are contracting, which shortens the overall length. But it's easily the longest part of this alimentary canal. Now when you're thinking about the diameter of the small intestine, you can think of something like a sausage, like a kielbasa or a bratwurst. Right? Because the traditional ways to make that is to take a pig intestine and stuff it with meat. And a pig intestine is about the same size as yours.

Alright. So, we have 3 functions here we want to talk about: digestion, absorption, and also immunity. Alright. So first up, digestion. In the small intestine, you're going to have both mechanical digestion and chemical digestion. Mechanical digestion via muscle contraction. Alright. Remember this is the alimentary canal is a muscular tube, so the small intestine is just going to sort of be squeezing down and breaking up any chunks of food that are left there and mixing it in with the chyme. And also chemical digestion. And really when you think of the small intestine, chemical digestion is mostly what we're talking about. And in the small intestine, this is going to be via enzymes. So we see this illustrated here. We see a big starch molecule being broken down by an enzyme here into its smaller subunit, so that it can eventually be absorbed. Well, that brings us to absorption. Well, in the small intestine, we are going to absorb virtually all the nutrients. Now, water is going to get absorbed in the large intestine, but if you're thinking of nutrients, pretty much all of it gets absorbed here in the small intestine. And to do that there is going to be a huge surface area because maximizing surface area makes absorption much more efficient. So, we're going to say here that that huge surface area maximizes absorption. And to illustrate that, we see some cells here that represent sort of the edge of that intestinal wall and we see one of these levels of the ways that it maximizes surface area. These will be the microvilli, these sort of cellular projections or what we call the brush border that increases surface area. We can see here some nutrients passing through a channel, through the cell, and eventually into the blood.

Alright. Our final function here, immunity. Now, I just want to make really clear digestion and absorption, those are the 2 big ones, but there is also going to be some immune function because, well, inside the small intestine is warm, wet, and filled with nutrients. That's a lovely place to live if you're a microbe and so we need to control those. So in there, we are going to have specialized immune cells that we will talk about and also cells that secrete antimicrobial agents. And to illustrate that, we have what looks like a white blood cell here punching out a bacteria or something like that. Alright. So again, digestion, absorption, and sort of as a side function, also immunity. We're going to look at all of this in a lot more detail coming up. I'll see you there.

Regions of the Small Intestine

Alright. We now want to look at the gross anatomy of the small intestine, and it's very likely you're going to be responsible for these three different sections of the small intestine. So that's what we're going to talk about. Now the regions of the small intestine. So we're just going to start out by saying that there are 3 sections of the small intestine, and in order they are the duodenum, the jejunum, and the ileum. And we can look over our image here. We have them labeled and color coded. You can see here in blue. That's going to be the duodenum. B in purple. That's all the jejunum. And C in this teal color. That is the ileum. Alright. So let's go through these one by one and see what we're talking about. So the duodenum, well that's that first section. We're going to say that this is the shortest section and it's just this sort of short C curve shaped section, and it's only about 10 centimeters long. Ten centimeters, that's something like 4 inches long. So we can see it in our image here coming right off the stomach off that pyloric sphincter valve. This sort of C curve there labeled A. That's going to be the duodenum. Now, in there are going to be some glands that are only present in the duodenum, and they are aptly named the duodenal glands. And the duodenal glands release an alkaline mucus. So remember the chyme coming from the stomach, it's going to have a very low pH. So you want an alkaline mucus to protect the wall of that duodenum, just like you had in the stomach, protecting the wall of the stomach. Now also coming into the duodenum, well remember we identified before the major duodenal papilla, that and that's where the pancreatic juice and the bile are going to come in. Now because you have the chyme, and the pancreatic juice, and the bile all coming in, that means we can sort of think of this as the mixing bowl of the intestine. It gets mixed in the duodenum and then it's going to move on to the jejunum. Alright. So the jejunum, again, that's that second region here in purple. And we're going to say that this is the middle section, and we mean middle both it's in between the other ones, but it's also sort of middle in terms of length. It's going to be 2 and a half meters, roughly. And 2 and a half meters is something somewhere around, like, 8 feet long. So it's a good size. Now the jejunum is going to be the site of most digestion and absorption. And when you think of the job of the small intestine, it is digestion and absorption. And so the jejunum is really doing most of that role. Alright. The kind that passes through the jejunum though is going to end up in the ileum, and the ileum is this third section down here labeled C. Now this is the 3rd section. It's also the longest section. Longest. Got to spell it right there. The longest section is something like over 3 and a half meters long, so something like 12 more feet of intestine there. And in the ileum, it's the longest section, but it's actually doing less than the jejunum. So it's going to absorb bile salts. So remember, bile entered into the duodenum. Now in the ileum, we're going to start picking up some of those bile salts so that they can be recycled and we can use them again. Now even more bile salts will get picked up in a large intestine later on. It's also going to absorb any remaining nutrients, but by this point there isn't a lot of nutrient left in the kind because most of it was absorbed in the jejunum. Alright. At the end of the ileum, we reach the ileocecal valve, and that's sort of right in here, on our image there. This is that sphincter separating the ileum from the cecum of the large intestine. The cecum is really that, sort of, the first section of that large intestine right there on our image. Alright. And the ileocecal valve, we can look at that word. It is the valve between the ileum and the cecum. Alright. So those are your major regions. Now I'll be honest, I get these regions switched in my head all the time, and so we have a little memory tool to keep it straight for you. I say DJ ileum feels the peristaltic pumps. Alright. Peristalsis, remember that's that wave of muscle contraction that's going to move chyme through the small intestine, and DJ ileum well, DJ that's the duodenum, the jejunum, and the ileum. Alright. There you go. We're going to look at this small intestine. Go inside and look at it in more detail. That's going to be coming up. But before that, we've got examples and practice. I'll see you there.

Small Intestine Example 1

Here it says in the table below, identify the three regions of the small intestine, then match each statement to one of the regions by writing the corresponding letter in the correct cell. Alright. So we have this little table here. It says first section, second section, and third section, and we have to name the region going down for each one. So let's do that first. Do you remember the names of those three regions of the small intestine in order? Alright. If you don't remember, remember we had that memory tool. We said DJ ileum has that peristaltic beat. Right? So DJ ileum. Well, the D stood for duodenum. And I always have trouble spelling duodenum, so I say it as duodenum. That's not the right way to say it, but it helps me spell it. So duodenum or duodenum. Help me spell it. DJ, jejunum, jejunum and ileum. Ileum. Alright. So those are my 3 segments. Now, I have to match the statement. So let's go through them one by one. The longest section of the small intestine. You remember which one that is? That's the ileum. Right? The shortest is going to be the duodenum. The longest is the ileum. Next, receives pancreatic juice and bile. Well, we said that the duodenum was like the mixing bowl of the small intestine, and so that's where those liquids enter. Next, we have the majority of digestion and absorption happens here. The jejunum. So that's going to be C. Whoops. Turn my pen. C is where that happens. Next, we have connects to the ileocecal valve. Well, ileocecal, that first part of the word is ileo. It refers to the ileum. Now on the other side of that valve is the cecum. Alright. So that is going to be D. At the end of the ileum, you reach the ileocecal valve. Then we have secretes alkaline mucus to neutralize stomach chyme. Well, the stomach chyme enters into the duodenum, so you're secreting that alkaline mucus in the duodenum. And then finally, we say receives chyme from the stomach. We just said that. That the chyme from the stomach goes into the duodenum, so I'm going to put F there. Alright. There we did it. We got more practice after this. Give it a try.

Which pathway correctly identifies the path of chyme as it moves through the small intestine?

• A. Jejunum → Duodenum → Cecum.
• B. Duodenum → Ileum → Cecum.
• C. Ileum → Jejunum → Duodenum.
• D. Duodenum → Jejunum → Ileum.

Where are bile salts reabsorbed in the small intestine?

• A. Duodenum.
• B. Jejunum.
• C. Ileum.
• D. Cecum.

As a treatment for certain diagnoses, a section of the small intestine may be removed from a patient. Based on your understanding of their respective functions, a portion of which section of intestine do you think could be removed with the fewest adverse side effects and why?

• A. The Ileum, very little chemical digestion or nutrient absorption occurs in the Ileum.
• B. The Ileum, the primary role of the ileum is to secrete digestive enzyme which can be supplemented.
• C. The jejunum, digestive enzymes secreted by the jejunum can be supplemented and most absorption takes place in the ileum.
• D. The jejunum, the primary role of the jejunum is to mix chyme and enzymes, which can occur in other regions.

Microscopic Anatomy

As we start talking about the small intestine, we want to discuss some microscopic anatomy. As we do this, we're going to start at the macroscopic level and work our way down to the microscopic. But before we do, let's set this up by saying that to digest and absorb, the small intestine needs a huge surface area. And that's really what we want to focus on today: how we maximize that surface area. We are going to say here that there will be three levels that increase surface area as we work our way down from the macroscopic to the microscopic. We also just want to note that in the small intestine, the amount of surface area is not equal throughout the whole thing. We have those three regions, and the amount of surface area is going to be the greatest in the duodenum, then a little bit less in the jejunum, and then the least amount of surface area, or at least surface area per length, is going to be in the ileum. Alright. Let's look at this.

We're going to start looking at circular folds. Circular folds are these folds of the mucosa and the submucosa, and we can see that in our image here. If we look here, we have the small intestine cut away, and we're really looking on the inside of the small intestine here. We see these ridges that are sort of making circles all the way around. It's even zoomed in here a little bit, so we can see some of those ridges here. These ridges are there to increase the total amount of surface area and also to decrease the flow rate of the chyme, which will increase absorption and digestion. So, what do we mean by slowing the flow rate of the chyme? Well, you can imagine if this was just a smooth tube, chyme would move through quite quickly. But here, the chyme is going to get caught up in all these little ridges, all these little spaces. It's going to get caught up, and that's going to slow down how it moves through.

We're going to zoom in again. We're going to focus on one of these edges of these circular folds, and next, we'll see the villi. As we zoom in here, we see that these villi, they're projections of the mucosa. You see these finger-like projections sticking out, and these projections are going to be sticking out all the way down the entire small intestine. Now here, those circular folds, those are macroscopic. You can see them. Here, these are microscopic, but really, it's kind of on the edge of microscopic. In the duodenum, these villi can be as big as 2 millimeters. In the ileum, they're much smaller, closer to half a millimeter.

Within each one of these little finger-like projections that reaches out into the lumen of the small intestine, there is going to be a capillary bed. We see in this image here, we see those blood vessels, capillaries, and a lacteal. A lacteal is a lymph vessel that is going to absorb lipids. Not part of the villi, but also visible in this image, we're going to have intestinal crypts. The villi are these projections that extend outward, whereas the intestinal crypts are areas where the mucosa dives down somewhat, and these are going to be small glands producing molecules that are released into the small intestine, a topic we'll discuss more later.

Our final level of increased surface area is the microvilli. Microvilli are membrane projections creating what we are going to call a brush border, and we can see that in our image here. We're going to zoom in on a few of these cells on these villi. These cells have a sort of rough edge, these projections sticking out. Those are actually projections of the cell membrane, which increase that surface area. We refer to it as a brush border because it looks similar to a brush's edge. This brush border increases the total amount of surface area to increase absorption, but it also contains brush border enzymes. These are enzymes that are fixed to the surface of this brush border. Much of the chemical digestion happens in the lumen of the small intestine, but the final step of diet chemical digestion for most macromolecules takes place at these brush border enzymes, where nutrients are then absorbed right into these cells and through into the capillary bed that we talked about. That would be on the other side.

With that, those are our levels of increase in surface area. Next, we're going to look at the different cell types that we find in this intestinal wall. We'll leave that coming up next, and I'll see you there.

Small Intestine Example 2

In this example, it says that the slide below shows the wall of the small intestine, and here is this image where it's a section of the small intestine that we're looking at through a microscope. Alright. So what it wants us to do here is first, it says to draw a box around 1 villus. So do you remember what a villus is, and where do you find it on this image? Well, remember a villus, those are those like finger-like projections that kind of reach out into the lumen, from the mucosa of the small intestine. And as I look at this image, well, here's a villus. Here's a villus. Here's another villus. Right? Each one of those things sticking up, those would be villi or one villus. So I'm just going to pick one of them, and I'll put a box around. How about this one? Alright. So there's 1 villus. That thing sort of sticking up to increase the surface area.

B says draw at least three arrows pointing to where you would expect to find microvilli. Alright. Where do you expect to find microvilli in this image? Remember, microvilli, well, those are even smaller than the villi. Those are the projections of the cell membrane of those cells lining the edge of the villi. So as we look here, we have these columnar cells all the way around the edge of all of these villi, and it's on the edge of those cells that you'd expect to find microvilli. So I'm just going to draw the arrows just anywhere to right on the edge. It says three of them right on the edge of these villi. That's where I'd expect to find microvilli.

Next, we want to draw a star, where you expect to find a lacteal. Alright. First, you've got to remember what a lacteal is, and where do you expect to find one. Remember a lacteal, that's a vessel of the lymph system, and in each one of these villi, there's going to be at least one lacteal running up the middle to absorb fats. So in anyone of these, I can put a star, right sort of in the middle of one of these villi because in each one of these, there's going to be one or maybe two lacteals just sort of running up the middle like that. Alright. My star is a little hard to see. Let me draw it in a little better. Gotta make a good star, you know. Alright. A little bit better there.

Okay. Next, we have a draw a triangle where you expect to find capillaries. Well, where would you expect to find the capillaries? Same thing. Those capillaries, you would find in those villi. So just anywhere here, I'll put a triangle, but those capillary beds are running up through the middle of those villi so that they can absorb those nutrients. You'd find it in every one of those villi. Alright.

Now one thing that we haven't identified in this image that we did talk about is those intestinal crypts, and you can see them here. You see these down here. These are intestinal crypts. Now, the intestinal crypts do connect to the surface, but when you take a cross-section, sometimes it doesn't all line up correctly. Now you can sort of see it here. This is sort of one of those intestinal crypts sort of going down there, but what you're seeing here, these sort of circles, those are the intestinal crypts. You're just not seeing it perfectly lined up, so you can see the whole thing leading up to the surface of the small intestine. With that, practice problems after this. Check them out.

A lacteal can be found within each:

• A. Intestinal fold.
• B. Brush border.
• C. Microvillus.
• D. Villus.

The brush border enzymes are associated with which level of the small intestine?

• A. Circular folds.
• B. Intestinal crypts.
• C. Microvillus.
• D. Villus.

Which of the following statements is true?

• A. The microvilli contain capillary beds so that nutrients can be absorbed efficiently.
• B. The villi are largest in the duodenum and decrease in size throughout the length of the small intestine.
• C. Lacteals are found in the intestinal crypts and are the location of fat absorption.
• D. The circular folds of the small intestine are found in the jejunum and the ileum but not in the duodenum.

Cell Types of the Intestinal Mucosa

Alright, folks. We now want to really zoom in on these cell types of the intestinal mucosa. Before we start naming the cells, though, let's first just orient ourselves to our image here. We can see this is the mucosa, and we see these villi, these sort of finger-like projections that reach up into the lumen of the small intestine there to increase surface area. We can see that we have a cross-section, so we can see the epithelial cells lining one of those villi. We see the lacteal inside and the blood vessels. Then we also see the crypts down here that dive into the mucosa, and below that, we see the submucosa with those blood vessels down in there. We're not going to talk about the submucosa here; that's really just there for reference.

Alright. So first, we're going to identify these epithelial cells. These are the cells lining the inside of the small intestine. The first one we want to note is called the enterocytes. We have those color-coded in pink here. So you can see they're really just all over the place. They're lining the villi. They're down in the crypts as well. Now these enterocytes are going to be most of the cells that are part of that epithelial layer. Now that makes sense if you break down the word enterocytes. Entero means gut and cytes means cells, so that really just means the cells of the gut. These are going to be simple columnar cells that are specialized for absorption, especially up on those villi. Because really, if I want to say anything is the number one job of the small intestine is to get those nutrients from your food like pizza, absorbed into the blood. It's these cells that are doing the absorption. Now the cells down in the crypts, though, these enterocytes down here, aren't really doing absorption. The enterocytes in the crypts are going to secrete intestinal juice. We're putting even more liquid into the chyme here, again, to aid in digestion.

Those are the enterocytes, but this is the intestinal mucosa. So we are also going to have goblet cells because goblet cells secrete mucus. And we can see these goblet cells are these light blue. They're just going to be spread out throughout. Now there is going to be some mucus lining here. It's not going to be anything like in the stomach where you have that real thick mucus lining, but all of the alimentary canal is going to have a little bit of mucus in it.

Alright. So our goblet cells secrete the mucus, and then we're going to have enteroendocrine cells. Our enteroendocrine cells, a lot of these are going to be down in the crypts. You see them in green down here. There's not going to be nearly as many of these. Now you should remember these from the stomach. Entero, that means gut and endocrine. Well, enteroendocrine, they're going to secrete things like hormones and paracrine signals. So secrete hormones and paracrine signals. Now the one that we just want to call out here is one we've talked about previously, that hormone cholecystokinin. Remember, cholecystokinin, we broke that word down. Chola meant bile. Cysto referred to the gallbladder and kinin means to move. So this is a hormone that tells the gallbladder to release its bile, and we abbreviate it as CCK.

As we move down, we reach the stem cells, and we see these in sort of this darker blue pretty far down in the crypts. And well, stem cells are cells that can divide and become other cells. So these divide in the crypts and they replace other epithelial cells as they die. Now you may wonder how the heck if a cell dies up here. Is it going to be replaced by one of these dividing stem cells? Well, these cells are sort of always migrating very slowly upwards. So the cell division happens down here, and then very slowly over time, these cells move upwards. And as those cells near the top start dying, well, cells below them sort of move up and replace them.

Alright. Now all the way down at the bottom, we are going to find the Paneth cells, and that's these yellow cells here. The Paneth cells are there to secrete antimicrobial agents. Antimicrobial agents. Things like lysozyme, an enzyme that kills bacteria, or defensins, those are small peptides that kill bacteria. Now your small intestine has bacteria living in it, but you don't want it sort of overrun by bacteria, and you want a lot less bacteria in there than, for example, you have in the large intestine. So the Paneth cells are going to help sort of keep those numbers down and protect against infection in that small intestine. Now I have a little memory tool here just to remember where those are. Those Paneth cells are down in the bottom of the pan. They're all the way down at the bottom of the crypts.

Alright. So those are the epithelial cells that we want to talk about, but we want to call out one sort of other tissue here and that is going to be this mucosa-associated lymph tissue or MALT. And this is something that you very likely learned about when you were learning about the lymphatic system. So in the small intestine, you're going to have these nodules of this lymph tissue that we see here in purple, and we're going to call those Peyer's patches. And Peyer's patches are going to be these aggregated lymphoid nodules, and they are there to protect against pathogens. Now they're going to do it in a more traditional way with, like, white blood cells, how the lymph system works, and we're going to say that they're going to be most numerous in the ileum. Now remember, the ileum is that last part of the small intestine that's up against the large intestine connected through that ileocecal valve. And the thought is that that large intestine has all sorts of microbes living in it, and you don't want all of those sort of moving in and colonizing the small intestine. So you have these Peyer's patches there to protect against that invasion of microbes from the large intestine.

Now as I look at all of this that we've now gone over, these first four, I can usually remember pretty well. It's the Paneth cells and the Peyer's patches that often get me a little confused. So we got one more memory tool for you here. And so for those, I say that P is for protection. These protect against those microorganisms. So both Paneth cells and Peyer's patches, P is for protection. Alright. We're going to practice this more, and examples, and practice problems to follow. I will see you there.

Small Intestine Example 3

Our question here says that on the line next to each cell type below, write an 'a' if that cell type contributes to absorption. Write an 's' if it contributes to secretion, and write an 'i' if it contributes to immune function. If the cell type contributes to none of these, write 'none'. Cells may contribute to more than one function. Alright, so let's take a look.

• Enterocytes: What do you think? Well, enterocytes, they line the small intestine. It's the most common cell type in the small intestine, in the epithelium of the small intestine, and they do absorption. So I'm going to put a big 'a' for absorption here.
• Enteroendocrine cells: Remember, 'entero' means gut and 'endocrine' refers to hormones. These cells make those paracrine molecules and hormones. So, I'm almost tempted to say 'none,' but they do secrete those signaling molecules. So I'm going to put an 's' here because what they're secreting are things like hormones.
• Goblet cells: What do you think about that? Well, goblet cells make the mucus that lines the small intestine. They secrete mucus, so I'm putting the 's' on the line.
• Mucosa associated lymphoid tissue: These cells we said are part of immune function. So I'm putting the 'i' on this line here.
• Paneth cells: Remember, 'p' was for protection. They are secreting some of those molecules that have antimicrobial agents. So that is immune function, but I said they're secreting those molecules. So I'm going to put an 's' for secretion and what they are secreting is molecules that have immune function. So I'm putting both 's' and 'i'.
• Stem cells: Well, stem cells divide to make all these other cells. So in terms of immune function, absorption, or secretion, they don't do any of them. They turn into cells that eventually do. So here I'm going to write 'none'.

Alright. Those are our cells of the intestinal epithelium for practice – after this, I'll see you there.

The most numerous type of epithelial cells in the small intestine are the

• A. Paneth cells.
• B. Enterocytes.
• C. Goblet cells.
• D. Enteroendocrine cells.

Both Paneth cells and Peyer’s patches are more numerous in the ileum than in the jejunum or the duodenum. Which statement below correctly describes why that may be?

• A. The ileum is adjacent to the large intestine where there are high levels of intestinal microbes.
• B. The ileum is responsible for absorbing any remaining nutrients not absorbed by the jejunum.
• C. The ileum has the highest pH of any region of the small intestine.
• D. The intestinal folds and villi are relatively smaller in the ileum than in the duodenum and jejunum.

Motility in the Small Intestine

As we talk about the small intestine, I think it's important to remember that we're talking about a 6 to 7 meter long tube. Something like 20 feet, and we need to move the chyme inside this tube. Now to do that, we are talking about motility in the small intestine. Now we're going to start out just by saying that it typically takes chyme about 3 to 6 hours to move through the small intestine. And that number, 3 to 6 hours, initially kind of sounds like a lot to me. But if I think of a coiled tube 20 feet long, the diameter of a sausage, and I need to move a slop through it using only muscle contraction, it sounds about right.

The way this works is a process called peristalsis, and peristalsis is a wave of muscle contraction. That wave of muscle contraction pushes chyme through the GI tract, starting at the esophagus all the way through to the large intestine. And the way this works, importantly, is that it uses both the circular and the longitudinal muscle layers of the muscularis externa. So, we can see this in our image here. We have this chyme here, and we can see both layers of this muscularis externa kind of squeezing in like a ring squeezing down. As it squeezes just kind of behind this chyme, that pushes that chyme forward. And then that squeeze kind of moves along in a wave, and so does the chyme get pushed forward, so on and so on all the way through. Just like if you had a tube of toothpaste and the toothpaste was all at the back of the tube, you squeeze in the back and just squeeze along and push it to the other end.

Now that's in contrast to segmentation. Segmentation is going to be non-adjacent contraction. So just kind of squeezing almost randomly back and forth. Now segmentation aids in mechanical digestion. Mechanical digestion, remember, that's sort of physically breaking up the food particles, and it's also going to mix in the enzymes into that chyme to really help with chemical digestion and make that chemical digestion more efficient. Now, different from peristalsis, this uses the inner circular layer of the muscularis externa only. So, if we look at this, we have the small intestine, and we have sort of blue and yellow bits of chyme, sort of showing how the chyme is separate from each other, and we can see just this inner layer squeezing in different places.

And as it squeezes in, this kind sort of squishes out both sides. And then as you do that again somewhere else, well, now it squeezes in and it squishes out in a different way down here. And as you keep sort of squishing back and forth, we go from these separate blue and yellow bits of chyme to a well-mixed green chyme down here at the bottom. Okay. You need to keep these separate in your head. You can remember the alliteration. Peristalsis is a push. It moves that chyme through the alimentary canal. Segmentation is a swish. It kind of swishes it back and forth and mixes it up.

We got practice after this, and give it a try.

Which of the following statements are true about motility in the small intestine:

I. Peristalsis will generally propel chyme towards the ileocecal valve.

II. The efficiency of chemical digestion can be increased through segmentation.

III. Peristalsis and segmentation use the same muscles, but contract in different patterns.

• A. I only.
• B. II only.
• C. I & II.
• D. I & III.
• E. II & III.

Putting it Together: Overview of Small Intestinal Function

To finish up talking about the small intestine, we want to sort of take one more look at the whole thing in this overview of small intestinal function. And so to do that, we have this illustration of the small intestines, this sort of cartoon illustration showing the small intestine as a disassembly line. So we're kind of thinking of it as a factory here, but instead of putting things together, we're taking these food molecules apart. Alright. So let's go through this section by section. We'll start with the duodenum. Remember the duodenum mixes. So in our illustration, we have the stomach and the chime leaving the stomach and entering into the duodenum there. But to get into the duodenum, it's going through this gate. And so that gate is going to represent the pyloric sphincter. So we're going to say here that the chyme enters through the pyloric sphincter. Now it's in the duodenum, this mixing zone. Well, what's getting mixed together? Well, we have the gallbladder here and the liver, and they're shooting in through that major duodenal papilla. They're shooting in the bile. So we're going to say the bile is added to emulsify fats. Remember, emulsify means to break up. So it mix in with the chyme better so that those digestive enzymes can actually break down the fat molecules. But we also see the pancreas here, and the pancreas is releasing juices that also enter through that major duodenal papilla and are going to get mixed in. So we're going to say that pancreatic juice is added or adds a majority of the digestion enzymes. So that's where a lot of those enzymes come in to actually break down those molecules. Now finally, not in our image, but in the text here we see duodenal glands. Duodenal glands are going to secrete an alkaline or basic mucus. So remember, this chyme coming from the stomach is going to have a really low pH coming from the stomach. And so until that pH is neutralized, we need to protect the duodenum, and so we have this alkaline or basic mucus to do that. Well, this chyme moves along on this conveyor belt and really move via peristalsis, but it moves down now. It's in that second region, the jejunum. And in the jejunum we say here, digests and absorbs. So we can see here this sign digestion in progress, and I see all these brushes along the edge here. Those are supposed to represent that brush border or the microvilli that are sort of those cellular extensions part of the cell membrane of all those cells lining the small intestine. Remember, as part of that brush border, there's enzymes that complete digestion of these food molecules. So we're going to say here that the brush border enzymes complete digestion. They break those molecules down to their final components that can actually be absorbed, and here we see the nutrient absorbers. So our nutrient absorbers here are sort of drawn as a vacuum sucking up these nutrients through the brush border there. So we're going to say that most of the nutrients are going to be absorbed here in the jejunum through that brush border. But the conveyor belt or that peristalsis keeps on moving this chyme along, and then it reaches the third and longest section, the ileum. And in the ileum, well, we see this bile absorber there. That's absorbing the bile salts. So absorbs bile salts. So remember the bile gets added in the duodenum, but then those bile salts get picked up again so that they can be recycled. Now also the remaining nutrients are absorbed. But remember, the vast majority of nutrients have already been absorbed back in the jejunum. And then finally, while this conveyor belt, this peristalsis keeps moving this chyme along, and eventually, it'll exit the small intestine, but we can see this gate that it has to go through to leave. That gate is between the ileum and the cecum of the large intestine. So we're going to say here to get out of the small intestine, the chyme exits through the ileocecal valve, and now it's in the large intestine. We have an example and practice after this. I'll see you there.

Small Intestine Example 4

Our example here says that for each of the structures or functions below, identify whether they are most closely associated with the duodenum, the jejunum, or the ileum by writing the correct letter on the line. Some structures or functions may be associated with more than one region of the small intestine. Alright. So we have the duodenum, jejunum, and ileum over here. That's where we've got to put our answers, and we have a through j here. So let's start going through it.

Alright. Pyloric sphincter. Which part of the small intestine do you think that is most closely related to? Well, pyloric sphincter is how you get into the duodenum. So I'm gonna put an a here. Pyloric sphincter is between the stomach and the duodenum.

What about b? Contains a brush border. What do you think about that? Well, we talked a lot about the brush border in relation to the jejunum, but the brush border is just the microvilli of the small intestine and that's gonna be everywhere. So the brush border, that's gonna be through the entire small intestine. I'm gonna put b on every one of these lines here.

What about the major duodenal papilla? Where would you find that? Well, remember the major duodenal papilla, that's what the bile and the pancreatic juice enter through, that sort of donut shape that you can see on the inside of the small intestine there. And well, the major duodenal papilla, it's in the duodenum. All right. So I'm going to put c on that line for the duodenum.

Next, we have site of most nutrient absorption. What's that going to be? Well, most nutrient absorption happens in the jejunum. So I'm going to put a d on that line.

Next, we have the ileocecal valve. What is the ileocecal valve most associated with? Well, it's the ileocecal valve. It's the valve between the ileum and the cecum. So I'm going to put an e on the line for ileum there.

Next, we have contains chyme. Which one of these contains chyme? Chyme is going to be throughout. Right? Chyme is just that partially digested food in the small intestine. So chyme or f, I'm going to put on each one of these.

Next, we have g, the entry point for the bile duct. Alright. Where does the bile duct come in? Well, we already put it on here. That bile duct comes in at the major duodenal papilla, and that major duodenal papilla we said is in the duodenum. So I'm going to put a g up here on the line for the duodenum.

Next, we have region where bile salts are absorbed. Well, bile comes in in the duodenum, but those bile salts are reabsorbed in the ileum so that they can be recycled. Alright. So I'm going to put h on the line for ileum.

Brings me to i, specialized glands produce alkaline mucus. Alright. Where is that alkaline mucus going to be made? Remember the acidic chyme is entering from the stomach into the duodenum. So you have that alkaline mucus protecting the duodenum from that acidic chyme. So I am going to put an i on the line for duodenum.

One more. It says the longest region of the small intestine. So which of our three regions is the longest? That's the ileum. So I'm gonna put a j on the line there. Remember, the duodenum is really short, just like 10 centimeters. The jejunum, that's a couple of meters, and the ileum is even longer.

With that, we've identified all our parts. We've matched them all up. Good work.

Celiac disease is a condition where the body has an abnormal immune response to the protein gluten found in wheat and other grains. A result of this, the body’s immune response can cause damage to the villi. Based on your knowledge of the small intestine, what function would this this most directly affect?

• A. Immune function in the ileum.
• B. Chemical digestion in the duodenum.
• C. Absorption in the jejunum.
• D. Motility in all three regions of the small intestine.

Gastric bypass surgery involves creating an alternate pathway for food that bypasses the majority of the stomach and a portion of the small intestine to help patients lose weight. A schematic is shown below.

Why do you think it is important to attach the duodenum to the jejunum even if food is no longer passing through it?

• A. The gastric juice produced in the bypassed portion of the stomach is needed for digestion in the small intestine.
• B. The duodenum produces brush border enzymes needed for chemical digestion.
• C. Peristalsis in the duodenum will propel chyme through the GI tract.
• D. The duodenum receives bile and pancreatic juices needed for chemical digestion.

Two chyme samples are taken: one from the stomach and one from the duodenum. How could you tell the samples apart using lab techniques?

• A. Test for the presence brush border enzymes; the duodenal chyme will contain brush border enzymes while the stomach chyme will not.
• B. Test for the presence of bile salts; the stomach chyme will contain bile salts, but the duodenal chyme will not.
• C. Test the pH of each solution; the stomach chyme will be more acidic than the duodenal chyme.
• D. Test for the presence of starch; starches are digested in the stomach, so only simple sugars are present in the small intestine.

Lactose, a sugar found in milk, can only be digested by lactase, a brush border enzyme. All babies produce lactase, but only some adults do. Adults who do not produce lactase are referred to as lactose intolerant, as the presence of undigestible lactose can lead to uncomfortable digestive side effects such as gas and diarrhea. Based on your understanding of the small intestine, where would you expect to find the most lactase in babies and those adults that can digest lactose?

• A. Duodenum.
• B. Jejunum.
• C. Ileum.
• D. Cecum.

Here’s what students ask on this topic:

What are the main functions of the small intestine?

The small intestine has three primary functions: digestion, absorption, and immunity. Digestion involves both mechanical and chemical processes to break down food into smaller molecules. Mechanical digestion is facilitated by muscle contractions, while chemical digestion is driven by enzymes. Absorption is the process of taking up nutrients from digested food into the bloodstream, which is maximized by the large surface area provided by structures like microvilli. Lastly, the small intestine plays a role in immunity by housing specialized immune cells and secreting antimicrobial agents to control microbial populations.

What are the three sections of the small intestine and their functions?

The small intestine is divided into three sections: the duodenum, jejunum, and ileum. The duodenum is the shortest section and acts as a mixing bowl where chyme from the stomach is combined with bile and pancreatic juice. The jejunum is the middle section and is primarily responsible for digestion and nutrient absorption. The ileum is the longest section and absorbs bile salts and any remaining nutrients. Each section has specialized structures and functions to optimize digestion and absorption processes.

How does the small intestine maximize surface area for absorption?

The small intestine maximizes surface area through three levels of structures: circular folds, villi, and microvilli. Circular folds are macroscopic ridges that slow down the flow of chyme, increasing contact time for absorption. Villi are finger-like projections of the mucosa that further increase surface area and contain capillaries and lacteals for nutrient absorption. Microvilli are microscopic projections on the surface of enterocytes, forming a brush border that contains enzymes for the final stages of digestion. These structures collectively enhance the efficiency of nutrient absorption.

What is the role of peristalsis and segmentation in the small intestine?

Peristalsis and segmentation are two types of muscle contractions that facilitate the movement and digestion of chyme in the small intestine. Peristalsis is a wave-like contraction that pushes chyme forward through the digestive tract, ensuring it moves from the duodenum to the ileum. Segmentation involves non-adjacent contractions that mix the chyme, enhancing mechanical digestion and mixing digestive enzymes with the chyme. While peristalsis primarily moves chyme along, segmentation aids in breaking it down and mixing it for more efficient digestion and absorption.

What are Peyer's patches and Paneth cells, and what are their functions?

Peyer's patches and Paneth cells are components of the small intestine's immune system. Peyer's patches are aggregated lymphoid nodules found primarily in the ileum. They contain immune cells that protect against pathogens by monitoring intestinal bacteria and preventing their overgrowth. Paneth cells are located at the base of the intestinal crypts and secrete antimicrobial agents like lysozyme and defensins. These agents help control microbial populations in the small intestine, ensuring a balanced gut environment and protecting against infections.