Introduction to the Digestive System - Online Tutor, Practice Problems & Exam Prep

Organs of the Digestive System

As we introduce the digestive system, I want you to think of a piece of pizza. And that's because this pizza is made up of bread and sauce and cheese, and maybe it has some meat and vegetables on top. And really the job of the digestive system is to get that pizza into your blood so that you can use the nutrients. So we're now going to go through the organs of the digestive system. We're going to take sort of this bird's eye view so that later on when we look at the details, you'll understand how it all fits together. But right now, as we go through these organs, I want you to be thinking about that pizza and think, what does this structure or organ have to do with getting that pizza into my blood? Alright. We're going to start just by saying that the organs of the digestive system can be divided into two groups, and the first of those groups are those organs that make up the alimentary canal.

The alimentary canal, also called the gastrointestinal tract or the GI tract, those terms can all be used basically interchangeably. This is the tube. It's this muscular 9 meter long tube running from the mouth to the anus. So you put the pizza in your mouth, and then as it's traveling through you, it's always going to be in the alimentary canal until anything that wasn't used gets passed out the anus. Alright. We also have accessory organs, though. These are additional structures and organs that aid in digestion, but importantly, they're not part of the tube. The pizza's never in those accessory organs.

Alright. So we're going to go through all these one by one, but first, we'll introduce you to our anatomical model here. We can see the body pulled away so that we can see all those structures and organs of the digestive system. We'll start by introducing the alimentary canal. Remember, this is the tube. I'm sure you're familiar with some of these structures. We got the mouth, the pharynx, the esophagus. Right? The mouth, that's where you chew that pizza up. The pharynx is just the space sort of behind the mouth that's part of both the respiratory and the digestive system. And then the esophagus, that's the tube that carries the pizza down from the pharynx into the stomach.

So then we've reached the stomach. The stomach, this sort of muscular bag-like organ here, this is where chemical digestion begins. And chemical digestion is going to be breaking those complex molecules of the pizza up into things that can be absorbed. Now in the stomach, this is going to happen through acid. Right? I'm sure that you know that the stomach has a very low pH that helps break down molecules. And also using enzymes, we want to call out here the enzyme pepsin. Pepsin's active in the stomach, and it digests proteins.

Well, in the stomach, that pizza is no longer really pizza anymore. It's now basically slop, and that slop moves from the stomach into the small intestine. The small intestine, that's going to be the longest part of the alimentary canal here. It really takes up the majority of your gut. In the small intestine, we're going to finish chemical digestion, breaking that pizza down into molecules like amino acids, simple sugars, lipids that can actually be absorbed into the tissues of the body. Now in the small intestine, that's going to happen entirely by enzymes. There's going to be a lot of enzymes in there that break down that food. And then we have once it's broken down, we're going to have that nutrient absorption.

But of course, not all of the pizza gets absorbed. If it doesn't get absorbed, it gets passed along with also most of the water into the large intestine. And we see the large intestine here. It sort of goes around the gut there. The large intestine, this, is active in water absorption. So you drink water, you drink fluids, but you also just put a lot of fluid into the digestive tract as you digest something because you want to be digesting slop, not like dry pizza. And so you put liquids in there to create that slop. Here in the large intestine, you absorb that water back up so you don't waste it. Now you're also going to have waste excretion. Again, you can't absorb all of the pizza. Some of it's no good to you, so here that just gets passed out of your body as feces.

Well, we said we also have accessory organs, and these importantly are not part of a tube. Alright. We have teeth for chewing. We have your tongue that manipulates the food. We have salivary glands that you can see here in yellow around the mouth. The saliva that's produced wets the food, so it's easier to swallow the pizza, and it's also going to contain some enzymes. We want to call out the enzyme amylase. Amylase is an enzyme that breaks down starch molecules.

Next, we'll call out the liver. The liver this is this large brown organ right here. Now the liver does a lot of stuff for your body. We want to call it here what it does for the digestive system specifically, and that is to produce bile. The liver produces the bile, and then the bile is stored in a small green organ right there, the gallbladder. So the gallbladder stores the bile, and the job of the bile is to help digest fats. Now bile doesn't actually break down the fat molecules, it just helps the fat spread out into the liquid in the gut more easily so that other enzymes can actually digest the fat molecules.

Our final accessory organ that we want to call out here is the pancreas. The pancreas, we see this sort of long yellow organ here kind of tucked under the stomach. The pancreas is there to secrete buffers, so that's going to sort of neutralize some of the stomach acid as it comes into the small intestine, and it's also going to produce a whole bunch of enzymes. Enzymes that break down basically all the different macromolecules in your food. Now the pancreas also produces hormones, and you should be familiar with that from the endocrine system.

The final vocab word that we want to call out here is the lumen. The lumen, well, this is a word that can be used in all sorts of different parts of the body. It just means sort of the hollow space in a tube or a duct. So for the digestive tract, that is that hollow space inside the GI tract that food can occupy. So when we're thinking of that pizza passing through our body, if it's not absorbed in the blood, that pizza is always going to be in the alimentary canal. It's in the lumen.

So that is the organs we got to know. Again, we're going to go into this in a lot more detail going forward. I'm looking forward to it. I'll see you there.

Introduction to the Digestive System Example 1

Our example tells us that some of the organs and accessory structures of the digestive system are listed below. Now for each organ, write a GI if the organ makes up a part of the gastrointestinal tract/alimentary canal. If the organ is an accessory organ of the digestive system, write an A. Then for each structure, write a Y if the organ comes in direct contact with the food you eat as it passes through the gastrointestinal tract, or write an N if it does not. Alright. So we have this list of organs or structures here in alphabetical order. I can tell right away that some are missing, but as we go through this, let's just go through them one by one and say, is this part of the alimentary canal or the gastrointestinal tract or is it an accessory structure? Now let's remember what the difference is. Remember, the alimentary canal or gastrointestinal tract is the tube through which the food is going to pass. The accessory structures or accessory organs are not part of the tube. Alright. So first up, the gallbladder. Part of the tube or not part of the tube? Not part of the tube. So I'm going to put an A. Large intestine. Part of the tube or not part of the tube? That is part of the tube. That's that last section of the alimentary canal. So I'm going to put a GI for the gastrointestinal tract. The liver, not part of the tube. So that gets an A. The mouth, that's the start of the tube. We give this one a GI. What about the pancreas? Pancreas, not part of the tube. Accessory organ, it gets an A. The pharynx. Alright. Pharynx, remember, that's part of the tube. That part of the tube that's actually part of the tube for both the digestive system and the respiratory system, so I'm giving it a GI. Salivary glands, not part of the tube. Get an accessory organ A there. Small intestine, definitely part of the tube, the longest part of this tube. So it gets a GI. And, finally, your tongue, well, not part of the tube. Right? So, the tongue gets an A. Okay. So now, whether or not it comes in contact with food, the first thing to do really easily is I know that everything that's part of the gastrointestinal tract, the alimentary canal, is going to come in contact with the food because that's how we defined it. The food is passing through that tube. So for all of those that say GI, I'm going to go ahead and put a Y next to it, and I figured those out. Alright. Now, the other ones. The gallbladder. Not part of the tube, but does food come in contact with it? No. The gallbladder is sort of set off to the side connected by a duct, so this one gets an N. No contact with the food. The liver, well, the liver is right up there by the gallbladder. Same thing. No contact with the food. The pancreas, No. Pancreas, same thing. Those three things are all sort of right in the same area. They enter into the small intestines. They have ducts that connect and enter into the small intestine in the same place. The food does not pass through them. Salivary glands. Well, salivary glands are in your mouth or connected to your mouth, but the food doesn't actually come in contact with the glands. The saliva is produced and then enters the mouth, so no. And your tongue? Well, my tongue comes in contact with the food, and that's how I taste things. So I give it a yes. Alright. So there we go. Remember, we're going to be looking at a lot of different structures and organs in here. That alimentary canal, that tube, is something we want to pay close attention to and we want to understand how those accessory organs are different from that. Alright. More practice after this. See you there.

Four organs are listed below. During food’s path through the alimentary canal, the partially digested food passes through the lumen of each of these organs except:

• A. Stomach.
• B. Gallbladder.
• C. Small intestine.
• D. Large intestine.

True or False: if false, choose the answer that best corrects the statement.

While the accessory organs contribute important digestive enzymes and fluids, all digestions and absorption of nutrients takes place in the alimentary canal/GI tract.

• A. True.
• B. False, while most absorption occurs in the GI tract, the pancreas also absorbs vitamin B₁₂.
• C. False, both the organs of the GI tract and the accessory organs contribute to producing digestive fluids and absorbing nutrients.
• D. False, water is absorbed in the GI tract, but nutrients are absorbed by the accessory organs.

Functions of the Digestive System

As we talk about the digestive system, we want to think about what it does for your body, or in other words, what are the functions of the digestive system? Now as we go through these different functions, we're going to relate each function to where it happens in the digestive system. And to do that, we have this sort of stylized drawing here of the digestive system pulled out of the body, not drawn exactly to scale, but we can see here at the top, we have the mouth with the salivary glands, which leads to the esophagus, the stomach, small intestine, large intestine. We can also see those accessory organs there. We have the gallbladder, the liver, and the pancreas.

Alright. So we're going to say the activity of the GI tract or that alimentary canal can be broken into several distinct functions. And the first one we're going to mention is ingestion. Alright. Ingestion, you can see we highlighted the mouth area up there on our drawing, and this is just because food and water enter through the mouth, and taking things in in that way is what we call ingestion. Alright. Pretty simple. Our next one, though, is going to be propulsion. Alright. Propulsion. Well, food moves through the canal. Right? Food comes in at the mouth, and eventually, waste product leaves through the anus. And things need to get through the entire canal. The way it does it is through propulsion. There's really 2 things we wanna mention here. The first is just swallowing. Swallowing is movement of food out of the mouth. Initiating swallowing is a voluntary process. But the next movement here, peristalsis peristalsis is involuntary, and peristalsis is just a muscular wave of contraction that pushes substances through the canal. And so you can see for propulsion, we've highlighted the entire alimentary canal here because food needs to go through the entire thing. Again, swallowing starts as a voluntary process in the mouth, but then as soon as you're in the esophagus, that muscular wave peristalsis is going to move that food the entire rest of the way through this alimentary canal.

Well, this is the digestive system, and our next function is digestion. Alright. Digestion, breaking food into absorbable molecules. Well, as I just said, this is the digestive system, but I actually think if there's one thing that that the digestive system is doing for your body, it's absorbing nutrients. But to absorb nutrients, well, you don't just eat nutrients, you eat something like pizza. We got to break that pizza into the nutrients that you can actually absorb through cell membranes. So there's going to be 2 ways that we break this down. The first is called mechanical digestion, and the mechanical digestion is that physical breakdown of food through chewing, wetting the food, churning it, and segmentation. We'll talk about segmentation later. That has to do with squeezing of muscles in the intestines. Alright. The whole point of mechanical digestion is to increase surface area. So, I'll indicate that with a little up arrow there. We wanna break that sort of food down into just mush and mix it around with all the liquid and the enzymes and whatnot so we can do chemical digestion. And that's our next one here. Chemical digestion, using acids and enzymes to break down those food molecules to be absorbed. So we're talking about breaking, like, starches down into simple sugars, proteins down into their amino acid components so that we can absorb them. Alright. Now for digestion, you can see we've highlighted here the mouth, the stomach, and the small intestine. Now, really, all three of those places do both mechanical digestion and chemical digestion, though they do them a little bit differently, and we'll get into all that later.

Next, we have absorption. As I just said, I think if there's one function of the digestive system, it's absorption, the movement of nutrients and water into the blood or the lymph. That is why we eat food, to get those nutrients into our body. Now most of the stuff goes into the blood, but fat gets absorbed into the lymph. Alright. Now we can see here on our image, absorption happens in the small intestine, in the large intestine. Really, nutrients get absorbed in the small intestine, and water gets absorbed in the large intestine. This brings us to our last one, defecation. Elimination of waste products as feces. Alright. The waste products that pass through your body, well, you put food in your body and you absorb absolutely as much nutrients as you can from them. But some stuff you just can't absorb. And that's what passes on through as feces, and the way we pass that feces out of our body, we call defecation. So for where that happens, we have the large intestine or the colon here highlighted. Alright. Those are our functions of the digestive system. For all of these locations and the functions, we're going to go into a lot more detail one by one coming up. I'll see you there.

Introduction to the Digestive System Example 2

It says here that fiber is any complex carbohydrate for which the body does not have enzymes to break down. Starches, on the other hand, are large carbohydrates the body can break down using the enzyme amylase. Freddo drinks 500 milliliters of water with 10 grams of powdered fiber mixed in and Sabina drinks 500 milliliters of water with 10 grams of powdered starch mixed in. Based on just this information, say whether you think Freddo or Sabina will perform more of each particular function listed below. We want to write a less than, greater than, or equal sign between Freddo and Sabina's names to indicate the answer, and then the first answer has been filled in for you. So our first one here says ingestion, and well, it says here that they both take in 500 milliliters of water with either 10 grams of starch or fiber mixed in. So we can see here that they're doing the same amount of ingestion because ingestion is just taking food into your mouth, basically. Alright, so from there, let's move on.

Propulsion. Who do you think is going to do more propulsion, Freddo or Sabina, or will it be equal? Alright. My first instinct is that it should be equal because they're taking in the same amount of food, and propulsion means to move this stuff through the alimentary canal through your digestive tract. But as I think of some other thing these things going on, I might actually want to change that answer. So I'm actually going to hold off on propulsion right now, and we're going to come back to it at the end.

Mechanical digestion. Who do you think will do more, Freddo or Sabina, or will it be equal? Well, mechanical digestion is sort of physically breaking food down into smaller pieces. And here, they're both drinking water with a powder mixed in, so they don't really have to do much mechanical digestion at all. So I think this should be equal. I'm going to put an equal sign there.

What about chemical digestion? Well, chemical digestion is using enzymes or acids to break down these molecules in the food, but we defined fiber as not being able to be broken down, and we define starch as being able to be broken down. So that means that Freddo, with the fiber, can't digest his food using chemical digestion, and Sabina can. So Sabina is going to do more chemical digestion than Freddo.

Well, that leads us to absorption. Who's going to do more, Freddo or Sabina? Oh, you can't absorb the nutrients unless they are broken down into small molecules that can be absorbed into the blood. And fiber and starch are too big to be absorbed, so because Freddo can't digest his food, it can't be absorbed. Starch can be broken down into simple sugars, so it can be absorbed. So therefore, Sabina is definitely going to do more absorption.

Defecation. Remember, defecation is just pooping. So who here do you think is going to do more of that? Well, right away the fiber. And fiber is actually sold as a supplement to help with things like constipation because if you can't break it down, if you can't absorb it, it means that it just has to pass all the way through that alimentary canal and out the other end by way of defecation. So Freddo's going to have basically everything he took in, except for the water, he's going to defecate out. Sabina, she can absorb all of that starch once it's broken down. So based just on this, there isn't really anything that has to pass out of her alimentary canal. So who's going to do more defecation? Freddo.

Alright. That takes us back to propulsion. Well, now that I think about that. Right? If Sabina's really absorbing everything that she takes in and Freddo can't absorb the fiber, well, that fiber has to move through the alimentary canal. And that's what propulsion means, to move this food through the alimentary canal. So because it has to move farther, it has to make it all the way to the other end. For Freddo, I think that he actually does more propulsion.

Alright. That looks like we worked it all out. We got more practice after this. I'll see you there.

Which of the following are true for digestion:

I. Chemical digestion begins in the stomach.

II. Digestion involves breaking down food both physically and chemically.

III. Mechanical digestion increases the surface area of ingested food.

• A. I & II.
• B. I & III.
• C. II & III.
• D. I, II, & III.

Which is more essential to life and why: chemical digestion or mechanical digestion?

• A. Mechanical digestion: mechanical digestion breaks down food molecules into their absorbable components.
• B. Mechanical digestion: mechanical digestion increases the surface area of consumed food, so that it may be absorbed.
• C. Chemical digestion: chemical digestion breaks down food molecules into their absorbable components.
• D. Chemical digestion: chemical digestion increases the surface area of consumed food, so that it may be absorbed.

Of the following functions of the digestive system, which function is the responsibility of only one organ?

• A. Absorption.
• B. Propulsion.
• C. Digestion.
• D. Defecation.

Names for Food as it Moves

Before we dive into all the nitty-gritty details about the digestive system, we just want to note that as food passes through the alimentary canal, we're going to call it different things, and you're going to want to be familiar with the names that we call it, so that you know what we're talking about. Alright. So we're just going to start by saying that as food moves through the GI tract, we call it different names, and you see those three different names here. We can call it a bolus. We call it chyme, or we call it feces, and it just depends on where it is in the digestive tract. So to help us out here, we have this sort of stylized image of the digestive tract here, but we've added in 3 places. We can see this food passing through it. Alright.

So first, let's define a bolus. A bolus is going to be the lump of chewed food mixed with saliva, and you can see that in the digestive tract here being swallowed in the esophagus. And usually, when we're talking about a bolus, we're talking about it going down the esophagus. So it's going to be found in the mouth and the esophagus, and it really just is that ball, or lump of food. Now the word bolus is actually derived from a root word that means something like a pill. But the way I remember it is that bolus kind of sounds like the word ball, and a bolus is that ball of food that you swallow. Alright. But that ball of food goes down the esophagus, then it reaches the stomach, and it gets all broken up and mixed with the stomach acid and whatnot. And then we are going to call it chyme. So chyme, that’s the slime that's in your stomach. The chyme is going to be the partially digested food mixed with gastric juices, and then it's going to stay chyme as it passes out of the stomach into the small intestine and get mixed with more juices like pancreatic juices, bile, and intestinal juices. So here we said it’s going to be found in the stomach and the small intestine. And in our drawing here, we can see this green slime in the stomach. There’s green slime in the small intestine. That’s the kind. Alright.

Once it passes out of the small intestine, we give it another name. Now we're going to call it feces. And feces, well, that's just another name for poop. Right? So feces is going to be this fully digested waste product. Basically, anything your body couldn't absorb just is going to pass through, and your large intestine is going to remove a lot of water, so it’s going to have low water content. And so we're going to find that feces in the large intestine or colon, and we can see that in our image right there. Alright. So be familiar with those names going forward because we’re going to use them, and I'll see you there.

Introduction to the Digestive System Example 3

This example says for each description below, write a b on the line if the description applies to a bolus. Write a c on the line if it applies to chyme, and write a f on the line if it applies to feces. You may write more than one letter on the line if appropriate. Alright. So quickly, let's just remember those definitions. Right? A bolus. Do you remember what that means? Well, a bolus, that was that sort of ball of food that's passing from the mouth down the esophagus and into the stomach. The chyme. The chyme is the slime that you find in the stomach and the small intestine. That bolus gets all broken up and mixed up with digestive juices, and then we have feces. Feces is just another word for poop. Alright. So as we go through here, what do you think is found in the large intestine? Well, we have this stylized image of our digestive system here. Right? Remember the large intestine is at the end and passing through there, that's the feces. So I'm going to put an f for feces. Now which one do you think is largely undigested food? Alright. For that, I'm going to put a b for bolus. Now the bolus you've chewed up, you've added some saliva too that does have some enzymes, so digestion has started, but it is the least digested part in this entire alimentary canal. It's just going to get more and more digested as it goes through. Alright. Which one do you think contains the most active digestive enzymes? Well, I'm going to say that that is the chyme. So I'm leaving it a c. That chyme, we add digestive enzymes in the stomach and the small intestine to break down those food molecules. Alright. Which one do you think has high nutrient content? Well, definitely the highest nutrient content is the bolus. So I'm going to put a b for bolus. I'm also going to put a c for chyme though, and now that nutrient content is going to get less and less as it passes through the small intestine because the nutrient is going to get absorbed, but at least at the very beginning, the nutrient content in the chyme is going to be very high when it's still in the stomach. So I'm putting a c there as well. Alright. Which one is going to be present in the small intestine? Well, in the small intestine, you will find the chyme. So I'll put a c there. Alright. Which one would have the highest water content? Highest water content, I think, would be the chyme. So remember that food you just eat is the bolus, and so that I mean, that may be something quite watery like watermelon, but even then, as it passes down, once it gets mixed in with the stomach, it's getting mixed in with digestive juices in the stomach. So no matter what it is that you eat, that bolus, once it gets turned into chyme, that water content is going up. Now in the large intestine, the large intestine is absorbing water, so the water content is going to go back down in the large intestine where you find the feces. So that's why I feel pretty confident that the chyme is going to have the highest water content. Alright. And what about low nutrient content? Well, low nutrient content, I am going to say feces. Right? The feces is basically what your body could not digest. It's what's there just weren't absorbable nutrients left, gets passed on, and so that's going to have the lowest nutrient content. Alright. So we solved the problem. More examples after this. I'll see you there.

A bolus is found in which alimentary tract organ or organ(s)?

• A. Small intestine.
• B. Esophagus.
• C. Stomach.
• D. Both A and C are correct.

In which the following would you expect the most chemical digestion to occur?

• A. A bolus.
• B. Feces.
• C. Chyme.
• D. Chemical digestion would occur equally in all three.