Parts of the digestive system
Voiceover: So, the first place our food goes for digestion in the body is, of course, the mouth, and the mouth actually has a couple of names, believe it or not. Of course, we call it the mouth most commonly, but it's also known as the oral cavity or even the buccal cavity, the oral or the buccal cavity, and so, let's learn a little more about what's going on in the mouth and take a better look right here. The overall goal of the mouth is to take food and convert it into something called a bolus, and a bolus is just a sphere of food that is easier for us to digest, and now we do this in two main steps. So, the first thing we do is we chew our food, and this is something we're very familiar with. There's a very fancy scientific term for chewing, and it's called to masticate or mastication. That's just to chew your food, and there are two things in our mouth that allow us to chew. First, of course, are our teeth, and we'll talk about these extensively in another video, and the second is our tongue. The tongue, as you know, sits right here, and it has a fancy name as well, which would be important to know. It's called the lingula, the lingula, and that will come in handy in a little bit. The tongue is made up of two types of muscles. There are extrinsic muscles that sit outside of the tongue and anchor into the tongue, and there are also intrinsic muscles, and intrinsic muscles are those that only sit within the body of the tongue. They are only located in here. Extrinsic muscles let us do things like elevate or raise the tip of our tongue, and the opposite, depress or lower the tip of our tongue. It lets us protrude our tongue out of our mouth, and it also allows us to retract our tongue back into the mouth. On the other hand, intrinsic muscles, because they're only located within the body of the tongue, will change the size of the tongue as it contracts. So, there are types of muscles that can shorten and widen your tongue, and these are ones that run from the tip of the tongue to the back of your mouth, and so we'll say they run anterior (A) to posterior, and then there are also muscles in your tongue that can allow it to lengthen, lengthen and narrow, so, narrow because we're pulling the tongue out, and these are ones that run from your right side to the left side of your mouth, or side-to-side, So, we'll write right to left over here. All right, awesome, so after we've accomplished mastication, what's the next step? Well, the next step is that we break down the food particles by hydrolysis, and hydrolysis, as you might recall, is a process where enzymes break down macromolecules. Now, the enzymes that cause hydrolysis in our mouth come from glands, and there are a bunch of glands that sit in our mouth, and we'll talk about which ones. They each contribute a certain component of our saliva, that we'll talk about right here, and they do different things. So, one, our glands can primarily release serous content, so serous content means things that are rich in enzymes, enzymes and salts, all right. So, we'll focus on enzymes. The other type of content our glands can release are mucinous things. So, mucinous things which are mainly composed of mucin, which you might recall is sort of the same way we spell mucus, and the whole point of mucin is to wet the food, to make it easier to wrap up into a bolus and send to the back of our mouth to swallow. Enzymes, on the other hand, and I'll clarify this right here, will cut the food, will break it down by hydrolysis. So, the first gland I'll mention is the largest gland that we have. They're called the parotid glands, and they sit on the side of your mouth, right above the jawbone, and these release about 25 percent of what's in saliva, and because they're green, they're mainly serous, so they contain enzymes, and we'll talk about which enzymes in a second. The next type of gland that we have is kind of a mouthful, these are submandibular, submandibular glands, and these guys release about 70 percent of saliva, and they sit, as the name suggests, below the mandible, which is the jaw bone, and these are also mainly serous, but they certainly have some mucinous stuff that they release as well. So, the next gland that we have is called the sublingual gland, and just like the name suggests, this guy sits right below our tongue, or our lingula, as we just learned that term, and it makes up about five percent of our saliva, and as the color suggests, it mainly releases mucin. It does release some enzyme but less so than the mucinous stuff. And then finally, the last gland that we have that contributes to hydrolysis in our mouth is called Von Ebner's gland. So, Von Ebner's gland, which releases less than five percent of our salivary content, so very little. These guys are mainly located at the tip of our tongue, and what's special about Von Ebner's gland is that they release an enzyme called lingual lipase. Lingual lipase, which as the name suggests, break down lipids, or more specifically, triglycerides, and these triglycerides are broken down into free fatty acids, so free fatty acids, as well as diglycerides or monoglycerides, So, I'll just write DG and MG for diglyceride and monoglyceride, and so that's what Von Ebner's does, and that's why that's unique. The other three all release, to some extent, an enzyme that's called alpha-amylase, alpha-amylase, which is an enzyme that will take carbohydrates, like starch, which is just a big stack or chunk of carbohydrates, and break it down into smaller carbs. So, I'll just write smaller carbs here. So, it'll break starch down into smaller carbs, to help with digestion, and when I say digestion, I should clarify that the amount of hydrolysis or the extent of breakdown that we have from these enzymes here and also from the cutting of food is very insufficient for absorption of nutrients. The only reason that we actually have digestion occur by enzymes in the mouth is for taste. When we break down our starch, and when we break down our lipids, that helps us really appreciate some of the fats in our burgers or some of the sugar that's in our sodas. The problem, though, is when we have too much sugar in our mouth, because the mouth likes to sit around at a pH of about 7.0, pretty neutral, but when we have a lot of sugar, as you know, there are a bunch of hydroxyl groups that sit on your glucose molecules. Hydroxyl groups can be acidic, and if we drop the pH down to something like less than pH of 5.5, we can actually start to demineralize or break down teeth, because of the acidic nature of your mouth, and so I'm going to keep that in mind next time I reach for a soda.