- [Narrator] Starting with the beginning of the GI tract, we're looking at the mouth. And when we're in the mouth, we're dealing with teeth. So I'm pretty sure, I'm going to probably guess, OK, teeth are meant to chew food, meant to bite food, tear food off, you get the idea. But the different types of teeth, they're called incisors, canines, pre-molars, and molars. Now children, before they lose their baby teeth, are going to have 20 teeth, while adults, that means once they have lost the baby teeth and the new ones grow in, we're talking 32 permanent teeth. Does that mean everyone has 32 permanent teeth? No. That third molar, known as the wisdom tooth, sometimes it comes in, sometimes it doesn't. Sometimes it has to be pulled out. So these aren't hard and cold numbers, they don't have to be there. It' just the typical or the average numbers. The tongue itself, positioning the food. Well, hopefully positioning the food while not getting chewed on itself. But also it's tasting the food, because the tongue has several different types of taste buds on it. But the tongue is more than just moving food and tasting it, it's also dealing with speech. Have you ever tried talking with the tongue always at the bottom of your mouth? Or hold the tongue to the top of your mouth. It doesn't sound normal. Because the tongue is going to help and shape the air coming out of your lungs. To make sure the air flows in the proper direction, therefore helping the resonant sound flow in the proper direction. So the tongue, positioning food, tasting food, but also speech is an important part as well. So here you can see a picture of your lower and upper jaws. What we're going to notice is, we have a total of eight incisors, four on the bottom, four on the top. These are your front teeth. They're meant really for biting into, for tearing things off. Then a total of four canines, you're going to have two on the top, two on the bottom. These are those a little bit more pointy sharp teeth. Canine, think of cat. Then we have eight premolars. The total of four on top, four on the bottom again. These teeth are starting to get a little bit flatter, meant more for grinding. But the true grinding is in the molars. And we're noticing the same 12 molars, top and bottom, so six top, six bottom. That's if you have the wisdom teeth. That third molar, the one all the way to the back, sometimes it just doesn't even come in. So, you might have a total of eight molars, you might have nine, 10, 11, 12, it all depends on which molars grew in. Genetics is what controls which ones are to come in, and which ones don't. So here you can see a picture of a child's mouth. What you're noticing is, there's little tiny lines going to deciduous teeth. Deciduous teeth are known as baby teeth. The permanent teeth, which you can see above it, into the gums, are starting to push the baby teeth out. That means the formation of the permanent teeth are actually pushing the baby teeth out of the way to allow them to come in. So you have your deciduous teeth or baby teeth, but then the permanent teeth will start to replace them as you age. The tooth itself is going to have two major regions, referred to as the crown, which is what you can see, and the root. While the crown of the tooth, this is the natural crown, not a crown put on by a dentist, the natural crown is covered by this substance called enamel. Enamel is going to be the hardest substance in your body. But even though it's the hardest substance, can you still wear it down? Sure, you can still wear your teeth down. So enamel's hard, but it must be cared for. Below the enamel, inside the tooth is the dentin. This helps give more structure to the tooth itself, below that hard enamel. Within the dentin, you're going to find the pulp cavity. The pulp cavity has your blood vessels, it has your nerves, and those blood vessels and nerves run down through the root canal. If you've ever had a hole in the enamel, or somehow had something get into the pulp cavity, the root canal, you'll know how painful that is to have those bare nerves touch something. But the entire tooth itself is held within the bone via this periodontal membrane. Little tiny bits of lines or ligaments help hold the membrane and keep the tooth anchored in the bone. But even with this periodontal membrane holding the tooth in place, can your teeth move just a little bit? Sure, the teeth can move just a little bit here and there, the adult teeth. Maybe sometime in the morning, you woke up and you kind of closed your mouth and realized, "Huh. "My teeth don't quite align the right way." That means maybe you had a lot of stress the night before, and you were clenching and grinding your teeth overnight. You can actually move the tooth just a little bit. Thankfully, the tooth will usually go back to its original position, it just takes a little bit of time. Well, the teeth can't work alone. We also have to look at the saliva. The saliva is produced primarily by three main salivary glands. The parotid, submandibular, and sublingual. What's in the saliva, you start with a lot of mucin. Well mucin, it's like a mucus, but it's kind of watery. Then we have the salivary amylase, breaking down carbohydrates, bicarbonate, it's a buffer, and lysozyme, meant to get rid of bacteria. So a lot of different things in your saliva. So here you can see a view from the side of the head, or a lateral view. The parotid gland is located just below the ear. Sublingual, well lingual refers to tongue, so it's below the tongue, submandibular, well the mandible is your lower jaw bone, so submandibular, below the lower jaw. Now once we've chewed everything up, using our teeth and mixed it up with the saliva, we need to swallow. Swallowing is actually a two-phase process. You have the voluntary phase and the involuntary phase. The voluntary phase is when you're consciously thinking about using the tongue to push the food backwards. What will happen is, you'll take this bolus of food, this kind of chewed up mass of food, you'll consciously push it backwards towards the pharynx. The pharynx is the throat at the back of your mouth. Well, once it's in the pharynx, then you enter the involuntary phase. This is simply a swallowing reflex. What happens is, your body automatically knows which muscles to contract, which muscles to relax, to allow the food to go down the pharynx, and down the esophagus. But food is not the only thing traveling down the pharynx. Air, what you breathe, can travel through the pharynx. It doesn't matter if you breathe through your nose or your mouth, both the nose and mouth connect to the pharynx in the back. So we have air and food passing through. Well, somehow we need to figure out how to sort out air from food and water. The epiglottis is a structure that will help to sort out and make sure that the food and water goes down the esophagus and stomach, and the air heads down towards the lungs. What ends up happening is, the epiglottis is always opened when you're breathing, but once you start to swallow food or water, the food or water will actually push or close the epiglottis. That way, it prevents any of that food or water from going down the trachea to the lungs. So it's the actual action of swallowing that will close the epiglottis, allowing the food and water to head down to the stomach, and not into the lungs, thankfully. But once you've swallowed, and you're past the epiglottis, you're down in the esophagus. The esophagus will have these peristaltic contractions, these peristaltic contractions are rhythmic. You'll contract over and over, about once every twelve seconds or so, you'll go through this rhythmic contraction. Food passes then, once it leaves the esophagus, down through the lower esophageal sphincter. Now I'm willing to bet you guys can probably figure out where that is. If you've ever swallowed something, and it was a little bit too much, and you've felt that kind of pressure in your chest as it slowly moved its way down? A lot of times it happens when you're eating peanut butter for instance, or anything else very sticky and tacky. Well, slowly moves its way down, that's the peristaltic contractions moving the bolus of food down the esophagus. Then, all of a sudden, as it moves down, down, down, you feel it right about where your ribcage is ending, all of a sudden the pressure disappears. That's because the bolus of food went through the lower esophageal sphincter, and it entered the stomach. The stomach is larger, there's not as much pressure on the walls when the food enters. So you have that lower esophageal sphincter. It's right between the esophagus and the stomach. Its main function, prevents backflow. The lower esophageal sphincter is preventing the stomach acid and the stomach contents from coming up into the esophagus. Because, if this sphincter's not working, and the stomach contents are coming back up into the esophagus, what is that? Yeah, that's acid reflux, that's heartburn. So your body really wants to keep that contents of the stomach in the stomach. So hopefully this lower esophageal sphincter is working properly. So here's a little diagram showing the voluntary versus involuntary. You start off with the tongue, pushing the food back. Now, before swallowing, so while you're breathing, the esophagus is normally closed. There's no food or liquids going down there, so, no point having it opened. But then, once that food enters the posterior portion of the oral cavity, the back of your mouth, it's entered into the pharynx, and it pushes the epiglottis closed. Now if you notice, the epiglottis is that little kind of blue structure sticking out in the back of the throat. As that green bolus of food comes down, you can see how it pushed the epiglottis down. By pushing the epiglottis down, it covered the trachea, it covered the airway, so that food can only go into the esophagus. That's all done, because you pushed that food back, sealed off the nasal cavity, and started to swallow. So the food can only go one way, it can only go down. As it keeps going down, it travels down the esophagus, down, down, down through your peristaltic contraction. It'll pass the lower esophageal sphincter, and eventually enter into the stomach. Once it enters the stomach, now we're going to start mixing and churning it up, making sure all that gets mixed up with the stomach acid and the stomach enzymes.