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Voiceover: We've come a long way since we started talking about the gastrointestinal tract from our oral cavity. Now that we've made it all the way, down here, it's time to talk about the final part of our GI tract. The large intestine, or the colon, the rectum and the anus. Let's zoom in and focus on what happens after we absorb our food in the small intestine. It looks like our food passes into the large intestine, as you can see here. Keep in mind, when we call it the large intestine, that does not mean it's the long intestine. In fact, it's shorter than the small intestine we just passed through. It starts right here and continues on to about this point. The end of the small intestine is the ileo… the ileocecal valve. The ileocecal valve, and that's the point where our large intestine begins, and this makes sense because, remember, the last part of the small intestine is called the ileum. The first part of our large intestine is called the cecum. The cecum. You can notice this little tail-like guy hanging out right here. This doesn't look like it does much, right? It looks like some functionless structure and, in fact, this is one of the most common causes of surgery in the GI tract. This guy is your appendix. That's the appendix right here. This is the part where our large intestine begins. Just to finish up our road map, this part of the large intestine as it ascends upwards, called the right colon but, most specifically, we call it the ascending colon. This is the ascending colon, and it does exactly what its name suggests, it ascends upwards. It goes from the inferior point of your abdomen to a more superior point, or above, and then we go this way. We run transversely, right? The part of the large intestine that continues on after the ascending colon is called the transverse colon, another great name that describes how it runs. The transverse colon, because it runs transversely. Next, we'll get into this part here. If we saw this part ascending, then I guess we could call this part the descending colon. The descending colon. Great names here. Great names in the large intestine or the colon. This is the descending colon. The descending colon does what it says. It descends. The last part of our large intestine, where we're not really descending anymore, but we're, kind of, flipping around like this, we're going upwards, you see, sort of, an S shape sometimes. There's an S shape that occurs, and so this last part is called the sigmoid colon. Sigmoid, because it's got an S shape to it. That's your sigmoid colon. What are the things the large intestine is most responsible for absorbing? Let's make this list right here. What does it absorb most importantly, and what's the impact of that? The first thing you've got to think about is water. Water is the main thing your large intestine is going to be absorbing, and it's very important for regulating, sort of, the output that we're eventually going to get from down here at the anus. Now, what would happen if we absorbed too little water? Too little water. That would mean that our output would tend to be more watery, right, because we didn't absorb that into our body. Now, that is what would cause diarrhea. Diarrhea, because you're going to have too much water left over in your stool. Now, let's think about the opposite case. What if we absorb too much water, right? There's too much water that's taken, so your stool is not as soft anymore. Water, as we can think about, is, sort of, the component of your stool that makes it relatively softer, so if we have too much water absorbed in our large intestine or in our colon, we'll get the opposite of diarrhea, right? We're going to get constipated. Constipated. Constipation, which is not good either. The large intestine is very important for regulating exactly how much water needs to be removed. One of the other things that we can mention at this point also is the disease known as cholera. I'm sure you've heard of this before. It's this terrible disease, where you have a bacteria attack certain receptors or certain proteins that are on your intestinal lining, specifically in your large intestine, and what that means is you're going to start losing a tremendous amount of fluid, eventually leading to death by dehydration. The interesting thing here is that if you can keep a person hydrated for long enough, your GI will actually pass the bacterium out. If you can keep a person hydrated while they have cholera, you can get them past the disease to be able to survive. The next important thing we should talk about that gets absorbed in the large intestine are inorganic ions. Inorganic ions, we talked about earlier, when we discussed the small intestine. These include things like sodium or potassium and, remember, back then, sodium was absorbed as a co-transport mechanism with things like amino-acids to help in their absorption, and that occurs, to a smaller extent, here in the large intestine. Now, one of the interesting things that come up because of this, if you notice water and inorganic ions, things like sodium, potassium chloride, the large intestine is not the only place that it does that. Neither is the small intestine. In fact, the organ that's most responsible for the absorption of your water and your inorganic ions is the kidney. Because of that, if we lost the large intestine entirely, if you had some sort of accident, or if you had a disease like Crohn's Disease, or ulcerative colitis, one of the treatments for ulcerative colitis is a total colectomy, so you remove the entire colon so you don't have that inflammatory process. You won't be able to absorb water or inorganic ions in your large intestine, but what that means is you still have the kidney. The kidney can still fulfil this function. Now, another interesting thing that I'm going to mention about the large intestine is that it's a rich source of microorganisms, microorganisms like bacteria. This gets a little weird because the question might be why do we have bacteria hanging out in our colon? What's the purpose of that? Bacteria in our colon actually assist in the digestion of nutrients that we're not able to do because we lack those enzymes. They assist in digestion mostly of carbohydrates, and this is, kind of, a significant process. You know, I read somewhere that about five percent of your output, your stool at the end, is composed of microorganisms like bacteria, and these are very important for digesting things like carbs. I'll put carbs here in parentheses, and, oftentimes, the by-products you'll get of that are things like methane, CH4, and then the other thing is hydrogen sulfide, which you may not have thought of this way but, actually, sort of, explains a lot. Methane is something that causes a lot of flatulence, and then hydrogen sulfide is something that smells terrible, and one food that we often eat that gets a bad rep are beans that are pretty rich in carbohydrates that we're not able to absorb because we lack the enzymes. The microorganisms in our large intestine actually do most of the digestion of the carbohydrates that we have in our beans. You've probably heard that rhyme before, "Beans, beans good for the heart, "the more you eat, the more you produce "methane and hydrogen sulfide." All right, enough farting around. Let's talk about the last part of our GI tract finally. Now that we've passed through the large intestine, the next place we're going to go is the rectum. Now, the rectum isn't responsible for any absorption or any breakdown. The main thing that the rectum accomplishes for us is storage. Now, we don't really think about it, but this is actually a very important part of our GI tract because, once we've processed food, and we've taken out all the water, and now we've finally produced the waste product that we're going to take to the bathroom, we don't want it to come out right away, right? You want to hold on to it until it's a more appropriate time to expel that, and the rectum is key for this, because it's going to hold onto stool until it's an appropriate time to get to the bathroom. What about the last part of our GI tract? The anus. Now, the anus itself is composed of two sphincters. As you might recall from our discussion on the esophagus, sphincters are just muscles that close off a hole. There are two sphincters that are responsible for the anus. There's going to be the internal anal sphincter, and there's going to be the external anal sphincter. The internal anal sphincter is composed of smooth muscle. The external anal sphincter, the one that's, sort of, wrapping around the internal anal sphincter, is composed of skeletal muscle, skeletal muscle that we control. Smooth muscle is under involuntary control. We have stool present in our rectum, it pushes on the internal anal sphincter, the internal anal sphincter will relax and open and allow the stool to move forward. Now, that doesn't mean that everything comes out right away, because we have an external anal sphincter. This is something that's under our control, but then we finally get to the toilet, and it's time to relax that external anal sphincter and release food we ate in that first video. Ahh, and, finally, that we've got here, man, do we feel relieved? (toilet flushing)