If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

Main content

Small intestine 1: Structure

Created by Raja Narayan.

Want to join the conversation?

Video transcript

Voiceover: Did you know that the stomach flu can make you temporarily lactose intolerant? It's true. So, in this video, we will talk about how that happens. And in detail, how the small intestine works. So, as a review, once you put food into your oral cavity or your mouth, it's chewed up and then sent down the esophagus where it ends up in the stomach where it's churned and then introduced to acid where it gets broken down into chyme and then delivered into the first part of your small intestine. Now, the small intestine has three different parts to it. So, let's take a better look. So, the first part of the small intestine is called the duodenum, the duo-denum. This receives the chyme that just got processed in the stomach and it's the part of the entire GI tract where the most digestion occurs. The most breakdown of food products will happen in the duodenum. All right, so the next part of the small intestine is called the jejunum. I'll just write that right here, the jejunum. And this is the part of the entire GI tract where the most absorption occurs, anywhere. So, the most absorption of nutrients is going to happen in your jejunum, the jejunum. Then, finally after your food passes to the jejunum, it gets to the last part of the small intestine and that's called the ileum, the ileum (I-L-E-U-M). And the ileum, now this doesn't have a superlative like the most digestion or the most absorption, but there are some pretty important things that are absorbed here. Things like vitamin B12, vitamin A, D, E, K. So, there are some important things that are absorbed here. So, I'm just going to write important absorption. There are some important things that are absorbed in your ileum. Now, the busiest part of your small intestine is the duodenum, because there are a bunch of things that are involved in this digestion process. So, there are four key things to keep in mind. First of all, your stomach is going to be delivering a bunch of chyme or processed food into the duodenum. So, you're going to be working with all this chyme here. In addition, you're going to have some stomach acid that process food into chyme. That's going to be present in the duodenum. In addition to the stomach, the liver and the gallbladder are also going to be important to deliver bile to your duodenum. So, they give bile. And as I'll talk about in a subsequent video, bile is composed of two different things. Bile salts and bile pigments. And beyond the liver and the gallbladder, the pancreas also delivers a couple of very important enzymes for digestion here. So, I'm just going to write enzymes for now. And in a minute, I'm going to go through and talk about which enzymes are delivered by the pancreas. And then finally the duodenum itself has what are called brush border enzymes, brush border enzymes. That are very important for activation of certain enzymes and also for digestion of several nutrients that we're going to discuss. So, let's talk a little more about this brush border. Now, if I would have make a little drawing of the duodenum right here. Remember that first part of our small intestine. I would draw just a little tube, connected right there. And then blow up the wall if I want to take a better look at what's going on right there. We would find then that there is a whole bunch of things than just meets the eye. First of all, the wall isn't just a straight line. There is actually a bunch of infoldings that are present on the wall to help increase surface area. Think about it. If we're trying to digest as much as we can here, we need to make sure that there are a lot of projections or a lot of space where we can make contact with the food that's passing by. So, if this is the inside of our duodenum, and this is the outside, just like how we drew up here. That's in and that's out. You can notice that this wall here has a whole bunch of projections on it. These projections are called villi (V-I-L-L-I), villi. And a single one of them is just called a villus, just a single villus. And these are just a couple of folds or these outpouchings that help increase the surface area of our duodenum. Now, that's not where the story ends. If we take a closer look at one of these villi, then we'd find that there are even more projections sitting on that, even smaller microscopic projections. So, for this single villus that had a horizontal line right here for its shape. Fair to draw it out here. You'd notice that it's not a straight line, but instead these also have a bunch of projections that are present on them. And if you were to guess why all these projections are in there? I'm sure you'd say to further increase our surface area. That's sort of the name of the game when we're in the small intestine. To increase our surface area. And so these little guys cutely enough are called microvilli, microvilli. And a single one is just called a microvillus. So, we've got these villi right here or the single villus that you can see if you just blow up the wall of the duodenum. And then if you blow up a single villus, you'll find that they have a whole bunch of microvilli that are found on them too, to increase our surface area because that allows for better digestion. And when we say brush border enzymes, these are a whole bunch of enzymes that are present on this brush border. You guys think about it. These villi and these microvilli, they're no different from bristles on a comb. They act to increase surface area or places where you're going to have interaction with food that you want to digest. And so there are enzymes that are present on this brush border. So, just to make the point, all of these microvilli and villi together, that's what makes up the brush border of our duodenum. The brush border, which is the increased surface area of the wall helps to digest food with our brush border enzymes that are present. And as we'll see later, in the jejunum, it helps for absorption. So, now let's talk about the digestion process in detail.