Hello, everyone. In this lesson, we are going to be talking about the functions of the small intestine, and how nutrients like fats and sugars are going to be absorbed through the small intestine. Okay. So, in the digestive system, the absorption of the nutrients that comes from your food is going to happen in the small intestine. The small intestine's main job is to absorb all of the energy and nutrients from the food that you eat. And absorption is going to happen through the epithelium cells that line the small intestine. Now, these are very selective cells. They're only going to let through certain substances. And each substance like sugar, or fats, or proteins are going to have their own unique method of getting into these cells. They may have a specific cell receptor. They may have a way that a vesicle enters the cell. They're going to have unique forms of transport into these epithelial cells. But just know that these epithelial cells in the small intestine absorb very specific nutrients molecules. And they're going to require specific transport proteins to do these particular processes. Now, whenever we talk about the movement of nutrients into these cells from the small intestine, it's actually going to require energy. Now, remember there are 2 different types of active transport or transport that requires energy. One is going to be primary active transport where you utilize ATP. And the second one is going to be secondary active transport where you're going to utilize the potential energy of a molecule. Now, both of these types of active transport are going to be used in these cells, and I'll show you a more specific example of both of these types of transport. But just know that transport is active, and it does require an energy source of some type. Because you're generally going to be moving these substances against their concentration gradient. Okay?

Now, once all of these nutrients have been absorbed into these cells, these cells are generally going to give these nutrients to the bloodstream. And the blood vessels from the microvilli, or the villi that line the inside of the intestine cells are going to converge at the hepatic portal vein or hepatic portal system. So, a portal system is basically just a system of blood vessels. And the hepatic portal vein is going to be a portal system or a blood vessel system that is going to specifically take things to the liver. And the way that I knew this is because the prefix Hepat, h e p a t is actually Greek for liver. So, if you ever see that prefix, you know we're talking about the liver. And you guys can see that right here. The Hepatic Portal System is going to be the system that leads these blood vessels to the liver. So this is going to transport nutrients directly liver. Now, you got to think about why would we want nutrients to go to the liver. Well, generally, remember the functions of the liver. It is going to be able to detoxify any toxic substances that we may consume into our bodies, and it also stores a lot of things. It can store glucose. Remember, it stores glucose in the form of glycogen. It can also store iron. It can store copper. And it also stores a lot of vitamins. So the liver is going to be the first stop that these nutrients get to because it stores a lot of this stuff. It's going to store glucose, our main energy molecule, in the form of glycogen, and it also destroys toxins. Remember that our liver is our main detoxifier. So, if we eat anything that might be toxic, it's going to go straight to the liver, and the liver's going to do its best to destroy those toxins. So, generally, the things that we absorb in our small intestine are going to enter the blood vessels. They are going to travel through the hepatic portal vein right to the liver, so the liver can either detoxify whatever we've eaten, store some of it, or send it on its way. Okay?

Alright. So, now, let's look at how these epithelial cells actually work. So, we're going to go down a little bit, and we are going to look at these cells. We're going to talk about how these epithelial cells in the small intestine actually do absorb glucose, because glucose is what we're really interested in because this is going to be the molecule that we use for cellular respiration, the main molecule. So glucose utilizes something interesting. It utilizes secondary active transport to cross those epithelial cells. And it's going to do this in a very unique way. The way it's able to do this is because of the functions of the Sodium Potassium ATPase, or as more commonly referred to as the Sodium Potassium Pump. Now, generally, when you think of the Sodium Potassium Pump, you're thinking of neurons because it's utilized to create that membrane potential in neurons. But the sodium potassium pump has another function as well. Now, the sodium potassium pump is used in these epithelial cells to create these concentration gradients. So, let's have a look at these cells right here. So, these cells are going to be our epithelial cells. And that's going to be kind of like a chunk here. Let's say that we took this chunk of these epithelial cells inside of the small intestine and we expanded it so we could see these cells. That's basically what we're looking at. Now, remember, whenever we are talking about these epithelial cells in the small intestine, they're going to be very specialized cells, and they're going to have 2 different sides. And, these 2 different sides are going to be called the apical side, which is this side with the little tiny microvilli right here. This is the apical side, and this is the side that is going to face the lumen of the small intestine. That means the inside of the small intestine. So, basically, the apical side is looking into your small intestine. And then, we're going to have the basolateral side. So, the basolateral side of these cells is this side. And this side is going to face the extracellular matrix and it's going to face your blood vessels. So, this is where this is the side that the nutrients are going to exit whenever it's trying to go to those blood vessels. So, we're going to have these two sides of these epithelial cells. And they're going to have tight junctions right here that actually make it so none of the juices inside of your small intestine actually leak into your body. Thank goodness. And they have these 2 different sides of these cells cells because these 2 different sides are specialized in the proteins that they have in their cell membrane. So on the apical side, we're going to have the glucose transporter, and on the basolateral side, we're going to have the sodium potassium pump. So, in fact, whenever we're looking at this blue structure right here, this is the sodium potassium pump. These blue proteins that you guys see right here are the sodium potassium pumps. So, what do the sodium potassium pumps do? Remember, they're going to be pumping sodium out of the cell. They're going to be pushing sodium out of the cell, and they are going to be pushing potassium into the cell. So, what that means for our cells is we're going to have high potassium concentrations, and low sodium concentrations inside of this cell. Now, whenever we're talking about the apical side, we're going to have this protein which is going to be a sodium glucose cotransporter. So these red proteins are the sodium glucose cotransporter. The reason they're a cotransporter is because both the sodium and the glucose are going in the same direction. They're moving together. The sodium potassium pump, on the other hand, is an antiporter. They're going in opposite directions. Now, the sodium glucose cotransporter is going to move sodium and glucose into the cell at the same time. Now, how does this work? Well, because there's such a low concentration of sodium inside of these epithelial cells, the sodium is moving with its concentration gradient. But the glucose is going to be moving against its concentration gradient. So, basically, this is called secondary active transport because this type of active transport, it is utilizing the potential energy of the sodium to move the glucose. The sodium wants to move into the cell because it's moving down its concentration gradient, and when it does, it drags the glucose with it. So, glucose uses sodium's energy via its concentration gradient to pull itself into the cell, and then you are going to have glucose and sodium inside of the cell now. And then the sodium potassium pump is going to actively pump that sodium back out of the cell. So, now, you have actively absorbed glucose into these epithelial cells. Now, something else to note that is not drawn here is we're going to have these other proteins, which I'm going to draw in green right here. This protein is going to actively transport glucose out of this cell. So, we're going to have glucose exit the cell, and it's going to exit the cell via a glucose transporter. Sometimes these are just abbreviated GLUT, g l u t. But these are glucose transporters which are going to transport glucose into the bloodstream, and out of these epithelial cells. So, then glucose will enter the bloodstream right here. Now, this would be an example of a glucose transporter. Now, the glucose transporter isn't really going to require any energy because there's a very high concentration of glucose inside of these epithelial cells, the glucose is going to use the transporter to just go down its concentration gradient and exit these epithelial cells into the blood, but it can't simply diffuse through the membrane, so it's going to have to use these glucose transporters that you guys can see over here. So it's going to need its own protein on the basolateral side of these particular cells to actually exit the epithelial cells. So that's the basis of how absorption of nutrients works. This was specifically for glucose, but just so you guys know, most of the nutrients that you absorb is kind of going to happen this way. We'll talk about fats in just a second because they're going to have a special form of absorption, but most substances are going to be absorbed the way that glucose is going to be absorbed. So just to recap, glucose uses utilizes secondary active transport because it utilizes the potential energy of the sodium ions to enter the cell, and the potential energy of the sodium ions is built via the sodium potassium pump concentration gradient that it makes, because it's transporting sodium out of the cell, establishing a sodium gradient. And then, the sodium glucose cotransporter is going to utilize that potential energy. And then, glucose is carried over the basolateral membrane through a glucose transporter via facilitated diffusion, which doesn't require any energy. Okay, guys?

Alright. So, now, let's talk about how fats are going to be absorbed and broken down inside of our bodies because that is a little bit different. Because fats are going to be nutrients that we need, but they're very special nutrients because they're going to be nutrients that are hydrophobic, they're going to be broken down and absorbed a bit differently. Okay. So fats are going to be broken down by this very important substance called bile. It sounds really gross, but it's very, very important. Bile is going to be a substance that is basically going to break down these fats, these hydrophobic molecules by increasing their surface area and pulling them apart. So, bile and lipases, which are going to be the special proteins, are going to break down these fat molecules. And, then these fat molecules can be absorbed by the enterocytes. Now, enterocytes is just another way to say epithelial cells in the small intestine. They're the same thing. So fats are going to be broken down into the smaller components by bile and lipases, and they're going to be broken down into these smaller components or smaller pieces called micelles. So this is a micelle right here. And basically, a micelle is how these hydrophobic fats, hydrophobic lipid molecules are going to form this structure to decrease the amount of surface area that they have interacting with hydrophilic, the hydrophilic interior of our cells. So we're going to break these fats up into all these tiny pieces called micelles, and then our epithelial cells can actually absorb those micelles. Now, what's the point of bile? What does it actually do? Well, the cool thing to know about bile is it's going to increase the surface area of fats. And this is also called emulsification. So it's going to increase the surface area of these fats, these lipids. And then it just makes it easier for lipases or these, proteins that break down lipids to break them apart. It just makes it easier. So, bile, kind of, takes these fats and stretches them apart, and then the lipases come in and they break the fats apart. Now, whenever a fatty food enters your small intestine, your body needs to understand that it's going to have to start breaking down this food a little bit differently than it does with glucose. So we're going to have these special hormones. So you're going to have your small intestine say, hey. There are fats in here. We need our bile and our lipases, so we need to make this signal. And, it's going to be the CCK signal. And, this is going to be a hormone that stimulates bile production in the liver. And a good example of that is actually down here. So, let me go out of the picture so you guys can actually see it. So, this is going to be the diagram of your digestive anatomy, and right here in orange we're going to have the small intestine. And once the fats enter the small intestine, the CCK hormone is going to be released from the small intestine, and it's going to travel to the liver, which actually generates and creates bile. And it's going to travel to the pancreas, which can actually start breaking down sugars because it creates insulin, and it's going to travel to the gallbladder, which you guys can see is this little structure right here. And the gallbladder actually stores bile for the release into the small intestine. So, the CCK hormone is going to stimulate the release and the production of bile. Now, remember, like I said, bile is going to be produced in the liver, and it's stored in the gallbladder. So, you guys can actually see these bile ducts right here in green through the liver. You guys can see it's being made in the liver, and it's actually going to be stored right here in the gallbladder. Now, some people will have issues with their gallbladder, and your gallbladder can be removed. It is not necessary for you to actually create bile. It's just important to know that with people who have their gallbladder removed, they can't store bile. So they have to not eat as many fatty foods at one time because they won't be able to digest it. So you just have to change your diet a little bit, but people without a gallbladder still function just fine. They just can't eat as many fatty foods because they don't have any bile stored up to actually break down those fats. Now, bile salts are going to be an important component of bile. I just want you guys to know that these are going to be molecules that are amphipathic. That's always hard for me to say, meaning they're both hydrophobic and hydrophilic, basically just know that they help bile do its job by emulsifying those fats. So then, once the fats are emulsified, broken down, how are they going to enter those cells, those epithelial cells? Well, they're going to enter in this form called these chylomicrons. These are going to be a special way that we package fats for transport through the body. And these chylomicrons are going to be these lipoprotein-associated complexes. So this is a chylomicron, right here. Which you guys can see right here. And it is going to have lipids right here, and it is going to have proteins. And this is, basically, just a shipping container for these digested fats. So, the digested fats are going to be in here. So, those are the digestive fats. And they're going to be stored in these chylomicron, kind of, packages. And this allows them to be transported around the body. So, they're going to be transported into the lacteal. Just so you guys know, a lacteal is going to be a vessel, a lymphatic vessel, and the lymphatic vessel just simply leads to the blood. So to transport fats, which are very hydrophobic molecules in the hydrophilic blood, we're going to need to actually package them in these chylomicrons or they won't be able to transport in the hydrophilic blood. So we package them this way, and then it is able to go to the lacteal, which will place those chylomicrons in the blood, and then we can transport fats anywhere in the body that we need those fats. So that is going to be how fats are broken down and packaged and transported. So fats are going to be broken down by bile and lipase proteins, and they are going to be packaged and transported in these chylomicrons. So this lesson just went over how the small intestine does many of its important functions. Just remember, the small intestine is utilized for absorption of nutrients, including glucose, vitamins, and fats. Now let's go on, and let's talk about the functions of the large intestines and its importance in absorbing water.