Biology: Introduction to the Digestive System

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Let's talk about the digestive system from a structural perspective to begin with, and then we'll talk about what functionally happens in there at another time. But you've got to remember something; structure and function go hand-in-hand. So I can't really talk much about structure without throwing a few things in about function.
What's up with the digestive system? What is its purpose? Well, whether you happen to be a jellyfish or a human being, the digestive system's major role is the ingestion of food, the digestion of that food and the absorption of the products of that digestion. That's what this whole nutrition system is all about. You have to be adapted to take it in, your adaptations have to get it somehow to an area where it's going to be enzymatically hydrolyzed and, once it's enzymatically hydrolyzed, it has to be somehow put into the system. How's it going to get put into that system? That comes later. But remember systems are always talking to each other and communicating with each other in this whole attempt to maintain homeostasis.
Let's take a look at this guy right here, who is very handsome, because he's bald. And let's take a look at his digestive system. Ingestion starts at the mouth for humans and we're going to stay to human systems most of the time. I've got a few good stories I'll tell you later. But anyway, let's start right here. Now, in the mouth, you know that - it's obvious. You start to say, "Well, do I really need to talk about the teeth?" Well, not unless you're planning on going to dental school. Suffice it to say that teeth are there to grind the food up, but think of your teeth. You have several different forms of teeth. Why is that? Why don't you have the same kind of teeth as a cow? When you chew your food, why don't you go sideways, like a cow does? Why do you go up and down? Well, it has to do with structure and function, you see, because we have some grinding teeth, but we also have some ripping teeth, because we were adapted to be omnivores, which simply means that you have the biological adaptations to eat foods of all different sources. Sometimes we make decisions not to make those foods, but, generally speaking, our biology says omnivore. Our teeth are where that starts. We have teeth for ripping meat, we have teeth for tearing meat and we have teeth for grinding things like lettuce, grass - well, not grass. So we have teeth of all different forms. And that's all we're going to talk about that. I don't want to go into all the molars, and bicuspids, and tricuspids.
But that's not all. You have other things in your mouth, too. You have, in your mouth, a tongue. Now, we're going to do a little experiment together here, a little interactive moment. Swallow. What did your tongue do? That was pretty cool, wasn't it? Try it again, swallow. Do you feel your tongue pushing up against the roof of your mouth? As it pushes against the roof of your mouth, it's compacting, if you had food in there, the food into a ball called a bolace of food. And that's the function of the tongue. Here's another thing: are you chewing gum or do you have something you can chew on? Just chew on it. As you're chewing it, what is your tongue doing? Well, think about gum, gum is a great example. When you chew gum, you bite it, and then it goes boink and it squishes out from between your teeth. And then what do you do? Well, you've got to get it back under your teeth, and you don't think about this. You don't say, "Push it under my teeth," but your tongue automatically does that. Otherwise, you'd have to chew gum like this and keep wagging your head back and forth. So people without tongues can't chew gum and, in fact, they have other problems, too, because they can't push the food against the roof of their mouth. So, the tongue is a very important digestive organ, or at least initializing digestion.
So we start to initiate digestion with the tongue and the teeth and we add some chemicals, too, and these are called the salivary glands. And we'll talk about chemical digestion and the events of chemical digestion later. So we have the tongue, the teeth, the salivary glands, all three sets of salivary glands are going to pour saliva so that you can mush that food up, get it nice and juicy. They say, "You should chew your food until it swallows itself." Forget it. I don't even chew my food. I get yelled out about this all the time by my wife, but I eat too fast and that's why. But anyway, for those of you who eat properly, you're supposed to chew your food until it swallows itself and enters this organ right here. And that organ is called the esophagus. Now, the esophagus is a very cool organ, because it begins peristalsis. Peristalsis is something that is the rhythmic contraction of the digestive system. You see, you have all these muscles called smooth muscles that line your digestive system. They're involuntary muscles, which simply means that you don't control them. They are involuntary, they are controlled by a part of your brain that does not control your skeletal muscles. And when you start peristalsis, you start an action like this, it goes whoosh, whoosh, whoosh. And it squishes the food down into the stomach, where it pops into the stomach. And peristalsis continues throughout your entire digestive system. You know, the growling of your stomach, that's peristalsis occurring. Did you ever have reverse peristalsis? That's rhythmic, too. Did you ever notice that when you vomit, it's not like all at once? It comes in spurts. That's reverse peristalsis, it's squirt, squirt, squirt. That is very cool! I'm not trying to gross you out here. And even think of when you defecate. When you defecate, even that is rhythmic. The digestive system is so cool!
Anyway, so we go from the peristalsis, down the esophagus into the stomach. And the stomach is an interesting organ. The stomach is basically a hollow bag, and what's cool about the stomach is that it has within it ridges, these rugay, these ridges inside of the stomach. The stomach's role is to store food and to do mechanical digestion, as well as chemical digestion, which we'll talk about later. Mechanical digestion is that growling you here. And when your stomach is full, you don't hear that, but the stomach churns and it's taking the food and just grinding and mushing it up and turning it into something called acid chime. We'll get to that in a second. But did you ever wonder why, when your stomach is churning, why stuff doesn't just shoot out of your mouth? If the stomach is squeezing, shouldn't that create a big negative food pressure and shoot stuff out? And it would, except for the fact that the stomach has these very cool valves in it. And it has two valves, one near your heart, it's called the upper valve, right up in here. It's kind of being hidden by this thing, the liver, but it's up top. And we call that the cardiac - it has nothing to do with your heart, except it's right near it - sphincter. A sphincter muscle is a round muscle that closes like this, like the iris of your eye. And there are sphincter muscles in the stomach. And the sphincter muscles can close down, and that's why, when your stomach is churning food, food doesn't shoot up. And the other sphincter, the piloric sphincter, keeps it from going down into the intestine. So the stomach is going to mix this thing up with chemicals, which we'll see later, and perform mechanical digestion, as well as some chemical digestion, and then the valves are going to open up and shoot the stuff downward into the intestines.
And now, we come to the small intestine and the large intestine. The small intestine is really where most of your digestion occurs. The small intestine is really where most of your chemical digestion occurs and absorption. Again, one of the things that happens while you're digesting a meal is this peristaltic activity stops, and we'll see the reason for that later. There's a slight amount of movement in the small intestine, but it's not anything like to get the food down into the stomach. The other thing about the small intestine that I want you to understand is, besides the chemical digestion and the absorption, is it length. It's about 21 feet long. How can that be? Obviously, it must be all crunched in and, if that's the small intestine, what must the large intestine be like? And the answer is it's not large at all. The small intestine is called the small intestine because it's not very wide. Its lumen is small. The large intestine is much wider. And so, if we take a look at this, we see that the small intestine, which leaves the stomach, is all convoluted and twisted and it arrives into the large intestine. Interestingly enough, and you may want to link back to the discussion we had in another lecture about the secum and the appendix, but, where the small intestine meets the large intestine, there's an overhang. And that's called the appendix. And the appendix is a vestigial organ that, in our ancestral past, our mammalian ancestors actually had an extra structure there. More on that when we talk about herbivores at another time.
Well then, last, but not least as we continue our slide down through the alimentary canal, if you will, we're going to end up in the large intestine, otherwise known as the colon. And the large intestine has a simple function; its function is to basically create feces and to absorb water. It absorbs water. And a lot of us often get afflicted with dilemmas of the large intestine. By the way, I said it's the colon and you might have heard of you ascending colon, and your transverse colon and your descending colon, ending up in the rectum. Well, that's ascending, transverse, descending, rectum. And when the large intestine doesn't do it's job properly, say, it absorbs too much water, we get plugged up, and we call that constipation. And sometimes it doesn't absorb enough water and we have a watery group of feces, and that's called diarrhea.
Anyway, structure follows function. There are the structures, we'll talk about function later.
Animal Systems and Homeostasis
The Digestive System
Introduction to the Digestive System Page [2 of 2]