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.