The Digestive System
IMPORTANT NOTE: You may be tempted to print out this material. While you're certainly allowed to do so, I would suggest that you NOT print it until 3 days before the test. Anytime prior to that, I may make changes to reflect what is going on in class. If you print it too soon, you will be missing information that may appear on the test.
Now that you have learned the important nutrients that should be part of a balanced diet, it's time to learn how those nutrients get from the food to your cells. You might remember that the very first day that we started learning about nutrition I asked "Why do we eat?" The answer was "Because our cells need energy."
Most people know that food needs to be digested. Lots of people know some of the places where digestion takes place. The stomach is often given as an example of where digestion happens. The intestines is another common example. Both those answers are right, but there is a lot more to the digestive system.
Before we look at the organs that form the digestive system, it's important to understand that the digestive system performs three related, but separate, functions.
- First, the digestive system needs to break down food into molecules that cells can use.
- Second, the molecules need to be transferred to the blood so that they can be carried to all of our cells.
- Finally, waste products need to be eliminated from the body.
Digestion: The name of the process for how foods are broken down into small molecules of nutrients that cells can use is called digestion. There are two types of digestion. One type of digestion is called mechanical digestion. In mechanical digestion, foods are physically broken down into smaller pieces. Chewing food is a good example of mechanical digestion. The second type of digestion is called chemical digestion. Chemical digestion is when enzymes and other chemicals produced by the body break down food into the smaller building blocks it is made of. One example is when starch, a complex carbohydrate, is broken down into the sugar molecules it is made of. Another example is when a protein is broken down into the amino acids it is made of.
Absorption: The name of the process for how nutrients are transferred to blood is absorption. In absorption, the completely digested nutrient molecules pass through the wall of the digestive system into the blood. The blood then carries the nutrient molecules around the body, reaching every cell.
Elimination: Not every material found in food is digested (fiber, for example, is not digested) and not every material found in food is absorbed. These materials need to be eliminated from the body as wastes.
This illustration shows some of the organs that are part of the digestive system.Let's begin our journey through the digestive system at the same place that most food begins its journey: the mouth.
The Mouth
Several different things happen to food when it is in the mouth. First, we use our teeth to chew food into smaller pieces. This is an example of mechanical digestion. Each piece of food is also covered in and mixed with saliva. Saliva is produced by the salivary glands and contains enzymes. One enzyme, called amylase, begins the process of breaking down carbohydrates. The digestion of carbohydrates in the mouth by amylase is an example of chemical digestion.
We all know how important it is to get proper nutrition. Since you were elementary school students, you've learned about making the right food choices, learned how to read the Nutrition Facts label found on food packages, and learned about the importance of choosing foods according to the Food Pyramid. In order to really understand the importance of nutrition, though, you need to understand what actually happens to your food, and everything that is in your food, as it moves through your digestive system.
The Esophagus
The esophagus is a tube that leads from your mouth to your stomach. But it's not just gravity that makes your food go to your stomach. Your esophagus has strong muscles that push food down using a process called peristalsis. Sometimes, the process goes backward. That's called reverse peristalsis, when the contents of your stomach move back up the esophagus to your mouth. Yuck! That's why vomiting is such an unpleasant experience!
It's not just food that goes into your mouth. You can breathe through your mouth, too. If you look at the illustration above of the digestive system, you'll see that the nose is also connected to the mouth. So even the air that you breathe through your nose "connects" with the air that you breathe in through your mouth. The air that you breathe doesn't go to your stomach, though. Instead, it needs to go to your lungs through a different tube called a trachea. What keeps food from going down the trachea? The epiglottis! The epiglottis is a small flap of tissue that covers the opening to the trachea every time you swallow. Because of the epiglottis, you can't breathe and swallow at the same time. Go ahead, try it! Just be sure that there's no food in your mouth when you do. You probably know from experience that if you take a breath with food in your mouth, some of it might get into your trachea. That's why people choke. If a large enough piece of food gets stuck in the trachea, your air supply is cut off and if you can't get the food out, you die.
The Stomach
You might think that chewing food in the mouth is the end of mechanical digestion. In fact, most mechanical digestion occurs in the stomach. Your stomach has three layers of strong muscle that contract, squeezing food and mixing it with digestive juices. You can even hear your stomach "growl" sometimes! As the food gets tossed and turned in your stomach, it breaks into smaller and smaller pieces.
There is also chemical digestion taking place in your stomach. An enzyme called pepsin digests proteins, breaking them down into the amino acids they are made of. Your stomach also contains hydrochloric acid. The acid has two purposes. First, it kills a lot of the bacteria in your food that might harm you. Second, it makes it easier for pepsin to do its job, because pepsin works best in an acidic environment.
You might not think that walking around with a stomach full of hydrochloric acid is such a great idea. It's the same acid that gets poured into your swimming pool. If you've ever spilled any on the ground, you've probably seen all the bubbling and smoking! So how come the acid doesn't burn a hole in your stomach! The answer may surprise you. The answer is mucous. The inside of your stomach (and the inside of lots of other organs) is coated with mucous. This keeps the acid from burning a hole in your stomach. If the mucous membrane in your stomach is damaged, then the acid will start to burn a hole. That's what an ulcer is. Sometimes, a little bit of your gastric juices come up into your esophagus. It hurts! We call that heartburn.
It takes the stomach a few hours to complete mechanical digestion of food. By this time, most complex carbohydrates have been broken down into sugar. Most proteins have been broken down into amino acids. The food has been transformed into a thick liquid. This liquid, called chyme, is then released into the small intestine. The chyme is able to pass into the small intestine when the pyloric valve, located between the stomach and the small intestine, opens. It is in the small intestine that the final stages of chemical digestion occur. It is also in the small intestine where nutrient molecules are transferred to the blood.
By the way, the word "chyme" is not pronounced the same way as "chime," which is the sound a bell makes. Instead, the "ch" is pronounced as a hard "k."
The Small Intestine
Let's get one thing straight: there is nothing small about the small intestine. It got its name because it has a smaller diameter than the large intestine. But, it goes on and on and on! The small intestine is about 6 meters long. That's almost 20 feet! To give you a point of reference, the largest 2015 Land Rover is 17 feet long.
The small intestine is long because it needs to be long. A lot needs to happen there. Most starches and proteins are partially broken down by the time they get to the small intestine, but lots of other nutrients haven't even started digestion.
Digestion in the Small Intestine
Food in the small intestine is mixed with different enzymes and juices. Some of these are produced by the small intestine and others are produced by the liver and the pancreas. The substances produced by the liver and the pancreas enter the small intestine through small tubes.
The Role of the Liver
The liver is the largest organ in the body, and it has many different jobs. Right now, we'll just focus on the role it plays in the digestive system. The liver produces a special digestive fluid called bile. Bile is used to break up fat into tiny droplets. The liver stores bile in an organ called the gall bladder. A small tube connects the gall bladder to the small intestine through which bile is transferred.
The Role of the Pancreas
The pancreas, like the liver, is a multi-purpose organ. It's job in the digestive system is to complete the digestion of starches, proteins, and fats.
Absorption in the Small Intestine
None of this digestion would matter if there weren't a way to transfer the nutrients from the small intestine to the blood. Remember that blood travels to every cell in the body. In addition to carrying oxygen, blood carries nutrients.
Inside the small intestine are millions of tiny structures called villi. Inside each villus (villus is singular and villi is plural) is a microscopic capillary (a capillary is a very small blood vessel). The nutrients in the small intestine "leak" into the capillary where they can be carried into larger blood vessels and, eventually, to every cell in the body.
The photo above is an actual picture of a villus taken through a microscope. If you look really closely at the capillaries, you can see the individual red blood cells inside of them. Remember that the inside of the small intestine is covered by millions of these tiny villi.
It's fine for me to say that the nutrients in the digested food moving past the villi "leak" into the capillaries. But how does that happen? The answer is diffusion!
How Diffusion Works
Diffusion is defined as the movement of particles from areas of high concentration to areas of low concentration. Got it now? I didn't think so! Let's look at an example of diffusion in real life. Just in case your parents or an older brother or sister is reading this, I won't use the example from class. No sense grossing them out! Instead, I'll use an example from the kitchen. Let's say someone is cooking something on the stovetop that has a strong odor. Maybe it's curry. Maybe it's fish. Maybe it's marinara sauce. Maybe it's bacon. Where is the smell the strongest? Right over the pot. After a while, though, you'll be able to smell what's cooking anywhere in the kitchen. The odor might not be as strong as it was right over the pot, but it's there. Keep cooking for long enough, and the whole house will smell. The smell won't be as strong as it was when it was just in the kitchen, but you'll be able to guess from your bedroom what you're having for dinner. That's diffusion. The odor moves from a place where it's concentrated (strong) to a place where it's not as concentrated. It moves all by itself. Once the concentration of the odor is equal everywhere in the house, the movement stops. If you were to open the door, the odor would start to move outside, trying to fill the world with the smell of bacon frying. By then, the little particles of odor would be so spread out that your nose would not be able to detect them. But they're there!
There are lots of nutrients in the digested food surrounding the villi, but no nutrients inside of the capillaries. So, the nutrients move from the area of high concentration to the area of low concentration.
The Importance of Surface Area
Do you know what surface area is? Think back to the little activity you did in class with the yarn and your hand. When you kept your fingers together, how long did the yarn have to be to surround your hand? What happened when you put your fingers apart so that the yarn had to go around each finger? Did it take more yarn? That's what surface area is all about.
A lot of nutrients need to get absorbed in the small intestine. The greater the surface area, the more nutrients can be absorbed. The small intestine is about 20 feet long. If you repeated the yarn experiment with a smooth small intestine, it would take 20 feet of yarn. But, suppose you had to wrap the yarn so that it went around every one of the millions of villi? It would be much, much longer than 20 feet. The villi are how the small intestine increases surface area without needing to be longer.
Surface area is a very important topic when we get to Biology in second semester. You'll see it again this semester when we study the respiratory system, so if you don't completely understand it, ask!
Celiac Disease
Celiac disease is a condition that damages the villi. If the villi are damaged, then nutrients cannot be absorbed. People with celiac disease need to avoid gluten, because gluten causes the body's immune system to attack the villi, causing the damage.
The Large Intestine
Once all of the nutrients have been removed from the food in the small intestine, it moves into the large intestine. In the large intestine, water is removed. Bacteria that live in the large intestine get their nutrients from the materials passing through. These bacteria do not cause disease. In fact, they are helpful; some of the bacteria that live in the large intestine produce vitamin K.
The waste moved from the large intestine into the rectum. It is in the rectum that the waste products are compressed into solid form and eliminated from the body through an opening called the anus.
Can Digestion Affect Evolution?
Have you ever looked at a chimpanzee? Most of us focus on their faces or their hands. We focus on these things because we're used to recognizing things by their faces and we like to see how similar the hands of chimpanzees are to our own. But what about their bellies? A chimp has a much larger belly than a human. That's because chimps have much more intestine in their bodies than we do. Scientists wondered why, in our evolution, we lost so much "plumbing" in our intestines. Some scientists believe that it's because we cook our food. It turns out that when food is cooked, it requires less energy and less work to digest it. So, we don't need as much intestine as a chimpanzee does. Chimps, of course, do not cook their food.
Because it is harder to digest raw food as compared to cooked food, humans are able to get more energy from the food they eat. Remember that it takes energy in the form of calories to digest food. There are even some foods that may require more energy to chew and digest than they provide! So when humans started cooking their food, they had "extra" calories. Some scientists believe that these extra calories is what allowed our brains to grow, making us the intelligent beings we are today.
Study Links
If you are interested in learning more about the digestive system, here are some links.
Just for fun, you can click here to watch Can I Eat That? You will not be tested on the material contained in this video.
Click here to watch the Bill Nye digestive system video that we watched in class at the beginning of this unit.
Click here to watch the Crash Course Biology video that we watched in class. Remember to pause the video often, as it was designed for students enrolled in a high school biology class. Pay especially careful attention to the information about surface area.
How small is a cell? You've heard me say that cells are tiny. But how tiny? Click here to visit a website that shows you just how small cells are!
Click here to view or download a copy of the textbook pages that this section of the Online Textbook is based on. These are not pages from your Teen Health textbook!
Study Guide - Click here to view or download a copy of the Study Guide.
Practice Test- Coming soon! Remember that you must have a valid student login for Jupiter Grades in order to access the practice test.
Narrated PowerPoint - The narrated PowerPoint is below. Click on the arrow to start the presentation. Be sure to make the presentation full screen so that you will be able to read it! Please remember that the PowerPoint is not intended to be a replacement for the text and the links above. Not every topic that you may be tested on is in the PowerPoint, and not every topic contained in the PowerPoint will be on the test.
