One thing that students get super excited about when they come into the lab is when I do this. Show them this bouquet of guts, if you will. But all of us at some point have wondered about our digestive system.
Whether it's when things go wrong and we have to rush to the toilet, or just simply how amazing it is that we can break down a food item into small, tiny molecules that we eventually absorb into the bloodstream. And some of our most popular videos are about the digestive system. Specifically about this structure actually that we call the stomach.
So today we're going to show you how food gets down to the stomach, what it does to the food once it gets there, and how your stomach doesn't eat itself alive with its own hydrochloric acid. So let's get into this anatomical awesomeness. Now one other thing I do want to mention that's really cool, and you can actually see this on the tray here, this is what I do.
one continuous tube that we could actually string out in a straight line. And what's really cool is when you're inside mom, it sets up as just one straight tube that we call the gut tube. And the first part of the tube just becomes the esophagus.
Then the esophagus expands out and becomes the stomach that you can see here. Then the stomach shrinks down and becomes the small intestine. Not small because of its length, because this is actually the longest part of the digestive tract. It's small because of its diameter. And then we have the large intestine, which the diameter gets larger.
So really cool to kind of see that it's just one long straight tube. And so let's actually start at the beginning of this. And we'll start with actually digestion where it begins. And that is where you can see right here, this sagittal head dissection. There's a ton of awesome anatomy here.
If you've watched our YouTube videos, you've seen us talk about the spinal cord, the brain up here, the cerebellum, the smaller portion of the brain, even the brain stem. and some other cool structures here, but we're going to focus in here in the oral cavity. Now, a lot of people are very impressed with the tongue here, and the tongue is primarily made up of a skeletal, well, not primarily, it is a skeletal muscle with mucous membrane over the top where we have papillae that actually can grip and manipulate food, and obviously we'll have taste buds embedded in there. But this is where digestion begins, mechanically through chewing, and also, if I turn this agile head to section over, Chemically, we get chemical digestion. Look how cool this salivary gland here.
This is the parotid gland and if you've ever had a moment where you've put a piece of sour candy in your mouth and you felt that aching in your cheek, that's because the parotid gland is starting to secrete some of this saliva and it kind of gives that aching sensation here. That's one of your three main salivary glands and really cool might as well point this out you can see the duct right here that's actually going to take that secretion into the cheek. inside the mouth. If you look in the inside of your cheek in the mirror really closely, you might see a tiny little hole or orifice on the inside of your cheek where this gland is secreting its substances through this duct here. Now another salivary gland that we have is the submandibular gland that you can see right here.
Sub means below, mandibular is just referring to the mandible. I forgot to tell you the name of what parotid means. Parotid means beside the ear and so you can see obviously the ear right here. Now the other salivary glands we can't see because we haven't done a dissection on those on this particular sagittal head, but we've got the sublingual glands which would be underneath the tongue here. But again, digestion is starting in the oral cavity through chewing mechanically and through chemically with saliva.
Now what's really cool about the saliva, it actually has some antimicrobial substances that can actually kill bacteria, and it also has things like amylase which will break down. amylose and so you're starting to see some just some examples of some of the substances that are in the saliva to help with that breakdown process. Now once we're done chewing and mixing that food together in our mouth we move it to the back here which is called the pharynx that's just a fancy pants name for the throat and here we have a fork in the road we don't want the food to go anteriorly into the trachea or the windpipe here we want it to go posteriorly through the esophagus. Now this little cool flappy doodad that you've probably heard me call it a flappy doodad before, that's not truly what it's called, but this is called the epiglottis.
And when you swallow, the epiglottis closes off the airway, so the food and water will stay posterior into the esophagus. Now if you kind of watch on me closely, I'm going to be all nervous about this now, but hopefully I can swallow properly here. But if you watch my larynx when I swallow. You see it elevated up.
And so one of the things that essentially happens is when the larynx comes up, the epiglottis flops down and closes the airway, and then the food would go posteriorly again down the esophagus. Now, again, we know digestion starts in the oral cavity. There is some absorption there.
The one exception to our discussion about digestion versus absorption is the esophagus. The esophagus is primarily just a transport tube that's going to transport the food and water from the pharynx down into the stomach, which is now where we can go to on the next part of our little journey here. So here I'm going to slide it down so you can see it a little bit better. This is the stomach. Here's the most inferior portion of the esophagus.
There's actually a little sphincter embedded in the tissue there called the lower esophageal sphincter, or sometimes referred to as the cardiac sphincter because of its location to the heart. It has nothing really to do with the heart per se. It's just low or close to the heart, and that's why I prefer the lower esophageal sphincter name a little bit better. But sometimes people will have a little bit more of a relaxed lower esophageal sphincter.
And as many of us know, there's stomach acid, hydrochloric acid produced in the stomach. And people who get heartburn... can have some of that acid reflux and move upward into the esophagus.
And the esophagus is just not built to deal with acid. The mucosal lining of the esophagus just gets really irritated. And you've probably felt that retrosternal pain or that burning sensation. Sometimes if it gets bad enough, you kind of can get some regurgitation and get like an acidic or kind of a sour taste up into your mouth. But the stomach, the other portion that I want to show you here, we have another dissection here.
which is really cool. A lot of you have probably seen this before, but it never gets old. Look at the inside lining of that stomach.
So this is the tunic mucosa. The tunic mucosa, the inside, is the name of the inside lining of the whole digestive tract. But the tunic mucosa, if we were to zoom in and zoom in to the cells that make this up in the stomach, we'd see chief cells, parietal cells, different cells that secrete certain substances that are important for the functioning of the stomach.
And yes, we would have mucus secreting cells here because what's important about the stomach and its ability to essentially not eat itself alive with the acid it's producing is essentially it has some, the mucosa, mucus, and then the acid on top of that. And the mucus will protect the stomach from actually kind of digesting or getting irritated from that stomach acid. Now, the cells like the parietal cells and the chief cells, they are going to be secreting the things like hydrochloric acid.
intrinsic factor which is really important for vitamin b12 absorption. Maybe you've also heard of pepsinogen. Pepsinogen is when it hits the acid becomes pepsin and that helps to break down certain substances like proteins and things like that. So some other cool things with the just the features of the stomach here are we've got the greater curvature right here and we even have the lesser curvature here. Let me get that a little bit better into view.
Greater curvature and then the lesser curvature. And you can actually just see with the naked eye that the stomach, or at least the tissue underneath my thumb, is a little bit more robust there. And I can feel it with my fingers here.
This is the pyloric sphincter. And what the pyloric sphincter is, is essentially a thickening of the smooth muscle that is in the wall of the stomach that thickens right around the distal portion of the stomach that we call the pylorus. And this pyloric sphincter, pyloric actually means gatekeeper. And this slowly allows...
emptying of the stomach. So it only allows certain amount of chyme, which is the mixture of the food and the water and everything we ingest here with the gastric juices. It only allows a small amount to pass from the stomach into the next part, which is the small intestine here. So that's how you get food down into the stomach and continue that initial process of digestion before we start doing much more absorption in the small intestine, which will have that portion of the video edited for you pretty soon.
But as always, thanks for watching. If you think gastric roogey, are really cool, then hit that like and subscribe button. If you don't think gastric urea are cool, then I guess that kind of just hurts my feelings, but that just means I'll have to work harder to impress you with anatomical awesomeness in our next video.