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How does the digestive system break down a bacon, lettuce, and tomato sandwich?

Now that you've learned what you need to eat, let's see what your body does to the food you eat. To learn about the process, think about factories and conveyor belts. Your food tube or digestive tract passes from your mouth to your anus. It works like a long assembly line-but in reverse. Let's see the conveyor belt principle in action first.

Let's say you build cars in a factory. The factory is equipped with all kinds of machines and people to make sure the cars are built correctly. At position A, the car body lands on the conveyor belt. The belt moves clockwise. By the time the car gets to position B, the doors and trunk are on. When the car reaches position C, the wheels are attached, and by position D the steering wheel is in! The conveyor belt moves the unfinished car to each workstation in sequence to add more parts.

Figure 3.1 A conveyor belt moves the car past assembly stations. At each station parts are added to the car.

Figure 3.2 This car had better be made of flour, milk, and eggs if he is going to eat it!

Your food tube works the opposite way the car production conveyor belt works. Let's look at a simple picture. If you ate cars instead of food to live, you might digest them in the opposite way they were put together.

You put the whole car into your mouth at point A. Your food tube moves the car along and takes off the steering wheel, then the windshield, then the tires, then the doors. These parts are absorbed into your body across the wall of the food tube. What's left of the car gets crumpled up (like a trash compactor crumples waste). At point D, your anus, you push the unused pans of the car out of your body.

You digest food by disassembling it into its component parts. The important idea is that your food tube works like an assembly line in reverse. Digestion involves both a physical and chemical breakdown of food. The physical breakdown includes chewing in your mouth and churning food in your stomach. The chemical breakdown involves different chemicals that work on the food at different places along the tube. The chemical breakdown reduces complex molecules in food to simple chemical units or building blocks that can be absorbed by the cells that line the food tube.

Figure 3.3 Your digestive systems works like an assembly line in reverse. As food, represented by the car, passes through your digestive tract, it is disassembled (digested). Only a part of the car leaves as waste.

What Do You Think?

Why do you think some birds need to swallow stones to help Them digest their food? Describe how you think their digestive systems work, considering the kinds of food they eat.

Now let's take a journey through the digestive system to see how this conveyor belt really works. Let's follow a meal through the long tube called the gastrointestinal tract, which begins with the mouth, includes the throat, stomach, and intestines, and ends with the anus. You will see what happens to food along the way.

Imagine that you are a traveler in an “inner” space capsule about the size of a vitamin pill. First, you make yourself really small. Then you get into your capsule. Look at the picture of you in Figure 3.4 (page 26) in your “inner” space capsule. The label on your uniform stands for Gastrointestinal. The G stands for gastro, and the I stands for intestinal.

Figure 3.4 Imagine that you are inside your “inner” space capsule ready to travel. You hold a magnifying glass. S is your searchlight. P is a pressure sensor. A is an acid sensor. G is a grabber for picking up samples.

Did You Know?

In the early 1800s doctors didn't know a lot about digestion. In 1822 a doctor in the U.S. Cavalry stationed at a remote fort treated a fur trapper who had been shot in the stomach. Although the stomach regained normal functions, the hole in the stomach would not heal. The doctor plugged the hole with easily removed bandages and spent the next eleven years observing his patient's stomach at work.

Map of Your Digestive Tract - Part I

Before you start the dark, tumbling ride, you need to see a map of where you will go. Only the major landmarks are on the map.

Figure 3.5 The entire GI (Gastrointestinal) tract.

The following link demonstrates the processes that occur in the stomach.

Did You Know?

The average person eats half of a ton of food each year. How much food will you eat if you live to age 87?

What do you think you will see from the capsule? You need the searchlight to find your way. You can see through the walls of the capsule. You have a pressure sensor to measure the amount of muscle squeeze. You also have a sampler and an acid sensor. You might want to make a chart of the points of interest so you can record your findings.

What Do You Think?

Have you ever watched a TV commercial for a hot, steaming pizza that made you hungry even though you weren't hungry a moment before? A Russian scientist named Dr. Pavlov did an experiment on salivation. Dr. Pavlov rang a bell whenever a dog was fed. The dog salivated because it smelled and tasted the food. After many feedings, the dog would begin salivating when the bell rang, even if there wasn't any food. What do you think this shows about how the brain connects to our digestive systems? Why do we sometimes convince ourselves we are hungry when our bodies don't agree, or that we aren't hungry when we really are? Is this healthy behavior? Why or why not?


Now we're ready to start the journey. A friend places your capsule in the middle of a bacon, lettuce, and tomato sandwich. You are lifted to a mouth and feel a sudden acceleration as in an elevator. Suddenly, it gets dark. Then you hear cutting and grinding noises. Your capsule slides over a rubbery surface that bobs up and down. It's a tongue! Globs of mayonnaise, chunks of tomato, hunks of lettuce, and pieces of bread bounce the capsule around. Through all of the splatter you see incisor teeth working as they bite off huge pieces of bread and lettuce.

Figure 3.6 An open mouth.

Can You Tell the Types of Teeth? Look in a mirror at your teeth. Find the incisors. Which are canines? Which are bicuspids? Which are molars?

Did You Know?

Your teeth do not grow after they break through your gums and reach full size. The enamel covering keeps them from getting any bigger. During your life you get two sets of teeth. Around age six, the small baby teeth begin to fall out. They are called deciduous teeth, because they fall out when the adult teeth below them push them up. The term deciduous is used for trees that lose their leaves in autumn.


Teeth cut up food. The front teeth are the incisors. They cut and slice. Upper and lower incisors work like a pair of scissors. They nip out bites of sandwich. Over to the sides of the mouth are sharp, pointed canine teeth. They are called canine teeth because they are like a dog's large pointed teeth. These teeth can gnaw meat from bones, as when you eat chicken. Behind the canines are the bicuspids, and then the molars. Bicuspids and molars crush and grind what incisors and canines bite off. Bicuspids have two cusps or grinder surfaces. Molars have more.

Can you tell what kinds of foods different animals eat by looking at their teeth? Describe some differences between animal teeth and how they match the diets of those animals.

Let's get back to the journey. Your capsule is still in the mouth. Sounds of crushing and grinding are now louder. Your capsule is near the flat-topped molars, grinding away. Molars crush and grind. The word molar is another name for the mill wheel that grinds grain to flour. The tomato, bread, and lettuce turn into pulp as you watch.

Salivary Glands

Now you turn the searchlight on to see one of the slimy moving walls near the second upper molar. You can see a hole. Every once in a while the hole squirts like a windshield sprayer. The juice hits against the wall of your capsule. You reach out and collect some of this juice, which, as you can guess, is saliva.

The sight or smell of good food can make you salivate. Another condition that causes you to salivate is nausea, for example, when you get seasick. What role do you think saliva can play when you are nauseated?

The saliva from a salivary gland contains an enzyme, called amylase. An enzyme is a protein in cells that affects chemical reactions. Amylase is the enzyme in saliva that breaks starch down into sugar. You can taste when amylase has worked in your mouth if you chew a soda cracker for 5 minutes because the cracker begins to taste sweet.

The starch in the cracker is really a long molecule-a string of sugar molecules. Your sweet taste buds can't taste the whole long molecule. You can taste sugar only when starch is broken up into smaller pieces. The enzyme amylase breaks the long starch chain into smaller pieces, some of which have only 2-sugar units.

From this observation, you see that digesting starch starts in the mouth, as amylase breaks the starch chains into shorter pieces. The short double units are maltose. Later, another enzyme breaks up the double units called maltose to make single units of glucose.

Figure 3.7 This illustration shows the process of starch digestion by amylase. Amylase breaks the chain at the points shown by the scissors.

Activity 3-1: Digestive Enzyme in Action


In this activity you observe the result of the enzyme action of lactase on lactose, which is a sugar found in milk.

Materials per team:

  • Milk, regular and lactose-free
  • Glucose test strips (4)
  • Lactase tablets (2)
  • Two small beakers
  • Masking tape
  • Activity Report


Step 1 Use masking tape to label one beaker #1. Pour a small amount of regular milk in the beaker labeled #1.

Step 2 Use masking tape to label the second beaker #2. Pour a small amount of lactose-free milk into the beaker labeled #2.

Step 3 Test the milk in each beaker for the presence of glucose with glucose strips. Record the results you observe on the Activity Report.

Step 4 Break up 2 lactase tablets and place one in each beaker. Stir.

Step 5 Test the milk in the beakers with a glucose test strip. Record the results you observe on the Activity Report.

Map of Your Digestive Tract - Part II


In Activity 3-1 you observed the action of a digestive enzyme. But what are enzymes? Enzymes are proteins. They often need the help of vitamins or minerals to do their work. Every time energy is released in the body, an enzyme is needed to help release the energy. Whenever a molecule is changed, made bigger, or made smaller, an enzyme is at work. There are thousands of different enzymes in your body, and each has a specific job. They have long names that end with ase. Their names can tell you where an enzyme comes from and what it does. For example, salivary amylase comes from the salivary glands and digests the starch amylase.

Did You Know?

Your salivary glands make about 4 cups of saliva a day. Saliva is made up of about 99% water. The rest includes various chemicals such as enzymes, which aid digestion. Saliva keeps your mouth and throat moist, helps food slide easily down your throat, helps warm or cool food as needed, and starts the digestive process. Have you ever noticed that sour or dry foods make you produce more saliva?

One function of enzymes is to break down food molecules and release energy in the body. If all the energy in a meal were released at once, we'd have an explosion. For example, the energy in a piece of fudge cake might be enough energy to raise a 100-pound person's body to 117^\circ F. That temperature would kill the person if it happened all at once. Enzymes let your body break things down one small piece at a time. Enzymes also let you trap the energy in food to do things that are more useful than just warming up. Chemists say enzymes are catalysts. Catalysts let chemical reactions happen more easily than they would without the catalyst. Also, enzymes and catalysts are not used up by the reactions they help cause. That's why one small enzyme can work over and over again-splitting starches into simple sugars, for example. The body doesn't need huge salivary glands, because a little enzyme can go a long way in splitting all the starch you eat. You make about a liter of saliva each day, and most of that is water.

Did You Know?

Choking happens when food gets stuck and blocks the airways, instead of going down the esophagus. The choking person can't talk, because the food is blocking the vocal cords. The person can't breathe, because the food is blocking the airway. Get immediate emergency medical help because the person's lungs are losing oxygen to the blood. When the person tries to breathe, the effort to breathe can cause the food to be sucked down harder, further blocking the person's airway. A trained person can perform the Heimlich maneuver (also called the Obstructed Airway maneuver). This procedure uses the air left in a person's lungs to force out an object blocking the airway. You can be trained to perform the Heimlich maneuver.

Figure 3.8 The esophagus is located behind the heart and lungs. It connects the mouth to the stomach.


Let's return to the journey. Your capsule is swallowed and you begin a roller coaster ride. The esophagus is a tube going from the throat to the stomach, straight down. As you can see, it goes behind the trachea, heart, and lungs and through the diaphragm. The tube is closed when it isn't swallowing or bringing up gas.


When the swallowing begins, your capsule is in the center of the tongue. Your pressure sensor, or gauge, goes up as the capsule is squeezed by the tongue against the roof of the mouth. The capsule slides to the back of the mouth. The soft part of the roof of the mouth closes off the passage from the nose. This action keeps the pressure of swallowing from pushing the food into the nose cavities. A flap called the epiglottis is positioned over the opening of the airways. As the swallowing begins, the epiglottis closes to block the airway. Bubbles of air surround you as you watch the epiglottis slam shut over the hole where air enters your trachea. Breathing has to stop for a few seconds to allow swallowing to occur. As your capsule slides down past the airway opening, a huge hole opens up. The capsule slides through the hole. Your capsule is now in the esophagus behind the trachea. Your pressure gauge still reads a high pressure.

Figure 3.9 Remember to pretend that you are in the capsule moving through the esophagus to the stomach.

Did You Know?

You have many sphincters in the gastrointestinal tract that control movement of food through the food tube. Muscles do the work of sphincters. Make a ring with your index finger and your thumb. Squeeze the index finger so the ring gets smaller and smaller. That's how a sphincter works.

The esophagus is like a dark, floppy rubber tunnel that allows your capsule to move down, then closes behind you. In fact, the closing of the esophagus behind you keeps pushing you down. The end of the esophagus is the stomach. Your pressure gauge reads a very high number before you pop through from the esophagus into the stomach. The pressure barrier you just passed through is called the lower esophageal sphincter. A sphincter is a muscular valve. This one keeps what's in the stomach out of the esophagus. It does this by squeezing the passage shut all of the time except when a bit of swallowed food needs to pass through.


You now find yourself inside the stomach. Your capsule lies in sloshing, gooey chyme. Chyme is the semifluid mixture of digestive juices and partially digested food in the stomach. You can still see pieces of lettuce and tomato and bread, but they are smaller now. Your pressure gauge reads a lower pressure than in the esophagus. There is less pressure now, because the stomach stretches out easily as food enters from above. Your acid sensor now registers high. Why does your stomach have acid in it?

The following link demonstrates the processes that occur in the stomach.

Stomach Acid

Acid helps the stomach break up food into smaller pieces. The stomach wall keeps this acid from breaking down and digesting the rest of the body. The stomach wall also has to be protected from this acid. Cells in the lining of the stomach secrete a slippery substance called mucus. Mucus protects the wall of the stomach from the acid in the chyme. If the mucus lining is damaged, the acid can eat away at stomach tissue, causing cramps and pain. Have you heard of stomach ulcers? Ulcers are areas where the stomach wall is damaged. Sometimes stomach ulcers bleed, and blood mixes with the chyme in your stomach. When the hemoglobin in this blood mixes and reacts with stomach acid, it turns dark brown or black. Anyone with black, sticky feces that look like oil or tar needs to see a doctor, because black feces can mean stomach bleeding or a stomach ulcer. Ulcers are most often caused by bacterial infections, but they can be due to excessive secretion of stomach acids that occurs when people are tense or worried or are using anti-inflammatory drugs. Smoking, stress, alcohol, coffee (both regular and decaffeinated), aspirin, or caffeine can damage the protection of the stomach and cause bleeding ulcers. Ulcers can usually be treated with a combination of antibiotics, drugs that neutralize stomach acid, and attention to certain lifestyle factors. People with ulcers need to learn to live without causing more bleeding. This means that they must change how they live, eat, and respond to stress. Stress makes your stomach produce more acid and can give you heartburn. Heartburn is caused when the lower esophageal sphincter doesn't prevent stomach acid from entering the esophagus. The backup of acid into the esophagus causes a burning sensation in the esophagus. Heartburn can be caused by eating too much, wearing tight clothing, changing positions, or a defect in the sphincter.

Figure 3.10 This is aside view of a stomach. There you are inside your capsule. Your voyage has now taken you from the esophagus into the sea of chyme in the stomach.

Think about how you feel when you are under stress. How does your stomach feel? Explain how you think an ulcer can be caused by a person being under a lot of stress. Describe several ways you could deal with the stress in your life to leave you feeling calm instead of upset.

Stomach acid also helps protect you from invading organisms. It would be impossible to not eat some germs along with your food. However, most germs are killed by stomach acid.

Digesting Protein

Now let's get back to the journey. The sandwich is continuing to break up in the stomach. Your capsule bobs around in the stomach in a slurry of food and digestive juices called chyme. The digestive juices are acid, enzymes, and mucus, which were produced by cells in the stomach wall. The enzymes produced by the stomach begin to break down the protein in the bacon, lettuce, and tomato. The stomach enzyme pepsin begins to digest proteins by breaking the long protein chains into shorter pieces. When they are broken down, the shorter pieces jostle around in the chyme with everything else. Further on in the food tube, other enzymes break the shorter chains into individual amino acids, which are the building blocks of proteins.

Figure 3.11 Enzymes in the stomach break down protein (a long chain of amino acids) into small chains of amino acids.

Did You Know?

What is a burp? Your stomach churns around the stomach chyme, mixing acid and enzymes with food to keep digestion going. Bubbles of gas are released from the chyme. This gas is air that was swallowed with a meal. As the bubble above the chyme gets bigger, it pushes on the sphincter at the bottom of the esophagus. Sometimes the sphincter lets some of the gas go up the esophagus. When the gas goes up, you burp. Burps can be smelly depending on what you ate and what the stomach acid has done to what you ate. It is polite to put your hand over your mouth when you burp.


Food moves through your gastrointestinal tract in a process called peristalsis. The entire food tube has layers of muscle tissue in its walls. When the wall muscles squeeze the tube, the tube gets narrower or shorter. The muscles squeeze sequentially along the tube so the food in your food tube must move in one direction. In the stomach, the wall muscles churn the food to break it down into smaller pieces and to mix up the chyme. Squeezing muscles act like rings of contraction that move from mouth end to anus end of the food tube, one after the other in sequence. This squeezing of the muscles moves chyme from the stomach to the anus along the food tube. In Activity 4-1 your fingers will act like these muscles to move food through a long tube.

Entering the Small Intestine

Your capsule leaves the stomach by squeezing through the pylorus, This muscular sphincter keeps chyme in your stomach long enough for digestion to start breaking long chains of protein into smaller ones. The pylorus keeps the acid chyme from leaving the stomach too quickly. Now your capsule enters the small intestine where most of the digestion and the absorption of nutrients take place. What happens to all the stomach acid in your chyme? What chemicals break down food in the small intestine? You will explore that in the next section.

Review Questions

  1. The three major components in food that require digestion are carbohydrates, fats, and proteins. How is digestion of these three components similar and/or different, until the time they reach the pylorus?
  2. Why is it important to chew food thoroughly?
  3. What role do enzymes play in digestion? Include two examples of enzymes. Explain where each enzyme is produced, where each does its work, and what it does.
  4. What moves food along the food tube? Explain what keeps food moving in the right direction.

Interactive Puzzles

Image Attributions




6 , 7 , 8

Date Created:

Feb 23, 2012

Last Modified:

Dec 11, 2013
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