结肠、直肠和肛门

so 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 so let's zoom in and focus on what happens after we absorb our food in the small intestine well it looks like our food passes into the large intestine as you can see here and 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 pass through it starts right here and continues on to about this point the end of the small intestine is the ilio 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 this cecum and 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 function less 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 so this is the part where our large intestine begins and just to finish up our roadmap this heart of the large intestine as it ascends upwards call 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 and so 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 and then next we'll get into this part here if we saw this part ascend 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 so the descending colon does what it says it descends and then 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-shaped 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 so that's your sigmoid colon so 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 well the first thing you 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 well that would mean that our output would tend to be more watery right because we didn't absorb that to 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 is 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 so we're gonna get constipated constipated constipation which is not good either and so the large intestine is very important for regulating exactly how much water needs to be removed one of the other things that we can sort of 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 this amount of fluid eventually leading to death by dehydration and the interesting thing here is that if you can keep a person hydrated for long enough your GI will actually pass the bacterium out so if you can keep a person hydrated while they have cholera you can get them passed 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 so 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 coach transport mechanism with things like amino acids to help in their absorption and so 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 does it 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 and so 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 all sort of colitis is a total colectomy so you remove the entire colon so you don't have that inflammatory process and so 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 fulfill this function now another interesting thing that I'm gonna mention about the large intestine is that it's a rich source of micro organisms microorganisms like bacteria so 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 well bacteria in our colon actually assist in the digestion of nutrients that were 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's about 5% of your output your stool at the N 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 byproducts 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 so you've probably heard that run 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 finally produce a waste product that we're gonna 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 on to 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 winter is composed of skeletal muscle skeletal muscle that we control smooth muscle is under involuntary control so 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 huh and finally that we've got here man do we feel relieved