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How does your body get the nutrients it needs?

The capsule has traveled from the mouth, through the esophagus, and into the stomach so far on your journey. In this section you will explore what happens to food in the intestines.

Did You Know?

Your small intestine is about 20 feet long (5-6 \ m). The large intestine is 5 feet (1.5 \ m) long. If your intestines were significantly shorter, food would pass more quickly, and the nutrients would have less time to be absorbed.

Small Intestine

The small intestine is about 6 meters long. It can't be measured exactly, because it shortens when it's active and lengthens when it's quiet. The small intestine digests and then absorbs the molecules in food. Humans have more small intestine than needed to absorb what is eaten. Some of the small intestine is in reserve. However, if a small intestine is shortened through surgery, fewer nutrients will be absorbed. People with short small intestines can even become malnourished.

The first section of the small intestine is called the duodenum. This is where the digestion of food really gets going. Digestion in the duodenum starts with the secretion of digestive juices that include lots of enzymes that break down the large, complex molecules in food.

Coil a Rope The intestines coil around in a similar way that a rope can be coiled to fit into a compact space. Find a thick (about 1 inch in diameter) rope about 25 feet long, and see how small a space you can coil it up in. What features of the rope would enable you to put it into a smaller space?

Digestive Juices

As your capsule passes a large opening in the wall of the duodenum, you see a blast of clear juice. Then a squirt of green bile smears up your capsule's window. Your acid gauge registers less acid and moves toward neutral.

The organs called the pancreas and the gall bladder connect with the small intestine at the duodenum. The first squirt came from the pancreas, which secretes digestive juices containing enzymes. These substances digest fats, carbohydrates, and proteins into smaller molecules. The bile came from the gall bladder. Bile is green. It is made by the liver and stored in the gall bladder.

Figure 4.1 If you stretched out that coil of small intestine, it would be longer than a giraffe is tall!

Bile helps digest fat. When fat is present in the chyme that squirts through the pylorus into the small intestine, the gall bladder releases bile by squeezing. This squeezing squirts bile through the duct leading to the small intestine. The squeezing of the gall bladder is timed by hormone and nerve signals. By timing its release, the bile meets with the fat as it enters the small intestine.

Shake It Up Place some oil (fat) and water together in a jar with a lid. Shake and observe what happens. Now remove the lid and add some liquid (clear) soap. Shake and observe again. What did the soap do? Why is this kind of action accomplished by bile in the small intestine important in fat digestion? Read the contents on the label of a jar of mayonnaise. What takes the place of soap in mayonnaise?

Bile works on fat in a similar way that dishwashing soap works on grease. The bile emulsifies fat. What does emulsify mean? When a glob of fat or grease is emulsified, it is broken up into tiny droplets or particles. How does bile do this, and why is it important? Let's see.

One end of a bile molecule can mix with water. The other end of the bile molecule can mix with fat. Therefore, when bile is mixed with fat globs, the bile molecules cover the surface of the globs with the ends that mix with water sticking out. The ends that mix with water repel other fat globs. This prevents the smaller fat globs from joining together into one big glob. Think of salad dressing made of oil and vinegar. The vinegar is a water solution, which stays separate from the oil until you shake the bottle. As you shake the bottle, the oil breaks up into smaller and smaller droplets. Eventually, the oil mixes with the vinegar. But what happens when you stop shaking? The oil collects into bigger and bigger drops until all of the oil is floating on top of the vinegar again. If you added bile to the salad dressing (Ugh!), the oil and vinegar would stay mixed after shaking. If you looked at the mixture under a magnifying glass, you would see thousands of tiny droplets of oil in the vinegar. It would be called an emulsion. Emulsifying fats, breaking them down, is important in digestion. It increases the surface area of fats so that their digestion by the enzyme lipase is more efficient.

Figure 4.2 You are watching bile molecules keeping fat droplets from congealing into globs.

What are gallstones?

Juices from the Pancreas

The juice from the pancreas contains lots of powerful enzymes that break down the various kinds of molecules in your food. Then why don't those enzymes digest your pancreas? After all, your pancreas, like the rest of your organs, is made out of the same kinds of molecules that make up your food-proteins, fats, and carbohydrates. The trick is that the pancreas produces these enzymes in an inactive form. Each enzyme is a protein, but when it is secreted by pancreatic cells, it has an extra sequence of amino acids that block its activity.

When the inactive enzymes reach the small intestine, they meet an enzyme produced by cells in the wall of the intestine. This enzyme takes the amino acid muzzles off the pancreatic enzymes and turns them loose to do their work in the small intestine. The small intestine is coated with mucus to protect it from the active enzymes.

What Do You Think?

Now that you know how different kinds of foods are digested, think about what kinds of foods you digest first and what kinds you digest last. Why do you think you are not supposed to eat fatty foods before you exercise?

Neutralization of Stomach Acids

Pancreatic juice contains more than enzymes. It also contains lots of bicarbonate. You can buy bicarbonate in the grocery store as baking soda. It is also the active ingredient in pills people take for an acid stomach. Bicarbonate can neutralize acids.

What does it mean to neutralize an acid? Neutralization takes the burning power away from the acid and makes it like water. Pancreatic juice in the intestines neutralizes the acid chyme coming from the stomach. The enzymes from the pancreas need a neutral, watery environment to do their job of digesting food molecules in the intestine.

Figure 4.3 These are three time frames each showing you in your capsule in the small intestine. In Second 1 your capsule moves forward. In Second 2 your capsule moves back. In Second 3 your capsule is propelled forward again, but farther than your position in Second 1. The back-and-forth “rubbing” allows for the movement and mixing of digestion contents (chyme) and enzymes.

Movements of the Small Intestine

Now, back to your journey. You notice your capsule moving back and forth in the duodenum. First, you move down, then you move up. You think, “How strange.” You watch a spot on the wall move past you one way and then the other, like the movement shown in Figure 4.3.

You pass the spot several times as chyme sloshes back and forth. You record the pressure as your duodenum squeezes. This movement, or sloshing, moves the food through the intestine and mixes it with digestive juices. The sloshing also helps the nutrient molecules become absorbed across the wall of the intestine into your blood. Sloshing helps absorption by bringing the nutrients into contact with the wall of the intestine. The cells lining the intestine have transporters that move certain nutrient molecules across the cell membrane. Other nutrient molecules are absorbed across the wall of the intestine.

Activity 4-1: A Journey through the Intestine (Peristalsis)


Think about how you feel if your “stomach is upset” or you have intestinal problems. In this activity you explore how materials move through the intestine, some causes for changes in these processes, and how these factors can affect your body's overall balance.

The simulation in this activity represents peristalsis. Peristalsis is the muscular action that is responsible for mixing and moving materials along the intestine. Your hands represent the muscles of the intestinal tract as they squeeze and push the materials through the tube. When you add food coloring or “digestive juices,” you see how the movements of your hands determine how well your simulated chyme gets mixed and moved. This movement simulates how the semiliquid chyme moves from the stomach into the small intestine. Remember that real chyme is composed of partially digested food particles as well as digestive juices and enzymes.


  • Tubing (2-inch diameter)
  • Simulated chyme (cooked oatmeal/rice)
  • Coloring material
  • Selected food items (grapes/lettuce)
  • Bucket of water and paper towels
  • Water bottle
  • Spoons
  • Gloves
  • Funnel with large opening
  • Activity Report


Step 1 Follow your teacher's instructions about how you will be doing this activity at your lab station.

Step 2 Put some simulated “chyme” into your tube.

Step 3 Think about how you can use your hands to mix and move chyme from one end to the other (see example below), Think of your hands as muscles that can only squeeze. They cannot slide along the tube! Experiment until you find the most effective method of mixing and moving chyme through your tube. Record your observations.

Figure 4.4 Squeeze to see what happens.

Step 4 Put some coloring agent on the chyme at the beginning of the tube. Move this colored chyme through the tube and watch carefully to see what happens.

Step 5 Now add a food item and observe what happens as you move it through the tube. Summarize your observations for Steps 4 and 5, and complete the Activity Report.

Step 6 When you are finished, follow your teacher's instructions for storing lab materials and cleaning the lab.

The Inner Wall of the Intestine

Why does the inner wall of the intestine look so bumpy? Let's take a closer look at the small intestine. As your capsule moves along the small intestine, you see that the wall of the intestine folds into mountains and valleys. And the folds look fuzzy. The wall of the intestine looks like a shag carpet with folds in it.

All these foldings and fingerlike projections are ways of increasing the surface area of the walls of the intestine. The large surface area allows enough digested food molecules to be absorbed to satisfy the needs of the body.

After plugging some crude measurements you made into your capsule's computer, you calculate the intestinal surface area. The measurements range from the size of a tennis court on the low end to the size of a football field on the high end. But you know one thing. There is a vast surface area for the absorption of the food you eat.

Figure 4.5 There you are in your capsule observing the fingerlike projections, called villi, of the intestine that increase the surface area of the intestine walls.

What Passes across a Membrane? Use the cell membranes (A, B, and C in the table) to predict the size of a substance moving across. One meter is equal to 1,000,000,000 nanometers (nm).

  1. Water is smaller than \underline{\;\;\;?\;\;\;} \ nm.
  2. Glucose is larger than \underline{\;\;\;?\;\;\;} \ nm but smaller than \underline{\;\;\;?\;\;\;} \ nm.
  3. An amino acid is larger than \underline{\;\;\;?\;\;\;} \ nm but smaller than \underline{\;\;\;?\;\;\;} \ nm.
  4. Starch is larger than \underline{\;\;\;?\;\;\;} \ nm.
  5. If membranes with different pore sizes were available, how could you determine the exact size of the substance glucose?
Membrane Pore Size Can substance pass through pore?
Water Glucose Amino Acid Starch
A 1 \ nm Yes No No No
B 3 \ nm Yes Yes No No
C 5 \ nm Yes Yes Yes No

Figure 4.6 Find the substances that can and cannot pass through a pore.

Activity 4-2: A Journey through the Intestine (Villi)


How is it possible for your small intestine to do its job? What does your small intestine look like? Your intestines are specialized to absorb the small molecules released from the breakup of large food molecules by digestive enzymes. The lining of your intestine is folded much like the corrugations of cardboard. This folding gets more surface area into a small space. The folds are covered by fingerlike projections called villi. The surfaces of the cells lining the villi are covered with many tiny projections called microvilli, making them look like little brushes. The microvilli, villi, and foldings increase the surface of absorption in the intestine up to 600 times.


  • Slides of intestine showing villi, and epithelial cells. (Electron micrographs of microvilli from textbooks)
  • Microscope
  • Model or diagram of human digestive tract
  • Corrugated box cardboard
  • Sheep, pig, or cattle intestine
  • Butcher paper and/or adding machine paper
  • Scissors, markers, tape, ruler
  • Calculator
  • Scalpel or razor blade
  • Gloves (latex or plastic)
  • Activity Report



CAUTION: You should wear gloves for Steps 1-3.

Step 1 Look at the piece of intestine. Slit a section and rinse. Observe the velvety texture of the inner lining. Notice how the lining separates easily from the muscular layer. Run your finger along the inside of the intestine along the villi. What makes the surface so soft? Record your observations and answers to questions 1 and 2 on the Activity Report.

Step 2 Obtain a 3-square-centimeter piece of corrugated cardboard. Peel off the surface layer. How is your intestinal lining like this cardboard? Why is this important? Answer question 3 on the Activity Report.

Step 3 Look at the microscope slide of villi under the microscope. Each villus is 0.5 to 1.5 \ mm long. Can you see how the absorptive area of the small intestine is increased by the villi? Draw 3 or 4 of the villi on your Activity Report. Answer question 4 on the Activity Report.

Day 2

Step 4 Assume that your small intestine is 5 meters long and 2 \ cm (.02 meters) in diameter (D). Think about how much surface area it would have. Keep this in mind when you answer question 5 on the Activity Report. How would you calculate the inner surface area? [Hint: \text{Circumference} = \pi D or 3.14 \times 2 \ cm. So the strip would measure 5 meters by 6.3 centimeters. (.063 meters)]

Step 5 Using adding machine paper, butcher paper, or tubing, measure and cut a strip that is 5 meters in length by 6.3 centimeters in width. This would be the size of the small intestine if it were slit open along the entire length and flattened out.

Considering the villi, microvilli, and the foldings of the intestinal wall, the absorptive area is 600 times greater than the strip represents. If you could set 600 of these strips side by side, you would have a rectangle that measures approximately 5 meters by 38 meters. THAT WOULD BE THE ENTIRE ABSORPTIVE AREA OF YOUR SMALL INTESTINE.

Step 6 Instead of making 600 strips and placing them side by side, measure the area of 5 meters by 38 meters with string or by walking the rectangle. You can use the 5-meter strip or a 5-meter piece of string for one side of the rectangle and measure off the other three sides by pacing. Answer question 5 on the Activity Report.

Diffusion and Osmosis

How is food absorbed into the bloodstream and from the bloodstream into the cells? Have you ever watched a crowd of people enter a public space? Think about a school at lunchtime. First, people are all crowded in a hallway or around a door. Then they naturally spread out to find their own spaces. So what started as a crowd is now spread out more evenly. The same type of thing happens with gases and food nutrients in your body. Suppose a membrane separates an area with a higher concentration of oxygen, glucose, carbon dioxide, or amino acids from an area with a lower concentration. The molecules will tend to move from the area of higher to the area of lower concentration, if they can get through the membrane.

This natural process of molecules moving from an area of higher to an area of lower concentration is called diffusion. Diffusion moves nutrients and gases (oxygen and carbon dioxide) around your body. Diffusion means that molecules or gases in higher concentration in one area will naturally flow into areas with lower concentrations, until an equilibrium (both areas have the same concentration of the molecules and gases) is reached.

Osmosis is a specific kind of diffusion. Osmosis involves the movement of water across membranes. If two solutions are separated by a membrane that will let water molecules pass through but won't let other molecules pass, water will move from an area where there is more water to an area where there is less water. In other words, the water moves from a region of higher concentration to a region of lower concentration of water.

Why do health workers not just run water into your blood through the IV?


After several hours, your food has reached the end of the small intestine and another sphincter muscle (remember the muscular squeezing rings?). Because of the larger diameter of the tube, you know you are entering the large intestine, or colon.

The sphincter between the small and large intestines keeps what is in the colon from washing backwards upstream into the small intestine. In the colon the sloshing around has stopped. You observe that the chyme has gotten thicker. Undigested parts of lettuce fiber are about all that's left of your sandwich. Your capsule moves back and forth very slowly now.


Where do you go from here? There are two options. If you look up, the big tube that is the colon stretches out of sight, and the chyme is slowly moving in that direction. But the other option is a small passage below you that is a dead end. Your capsule probably can't squeeze into it. This is your appendix. Has anyone you know had appendicitis?

Figure 4.7 Find your capsule surrounded by chyme particles.

Appendicitis starts with a very severe stomachache caused by an infection in the appendix. Infections are common in the appendix because it is such a tight, little, blind-end tube. Bits of indigestible material can get into it, irritate it, and make it possible for bacteria to cause an infection. That infection can get worse quickly, causing the appendix to burst. If that happens, the infection can spread throughout your body cavity. Therefore, it is important to consult a doctor when you have a bad stomachache. If the doctor decides you have appendicitis, the remedy is for a surgeon to remove the infected appendix.

Did You Know?

Another example of a vestigial organ is eyes in cave fish. The ancestors of species of fish that only live in caves came from the sunlit world and they had functional eyes. Once they colonized caves, however, they no longer experienced light, so there was nothing to see. Since there was nothing to see, mutations that occurred in genes controlling development and function of the visual system accumulated. A mutation is a permanent change in hereditary material that causes a change to the organism. As mutations accumulated over thousands of years, functional eyes were lost.

Can you get along without your appendix? There's no problem. Your appendix is a vestigial organ. That means it had a function in our distant past, but that function is no longer necessary. Since it is no longer needed, the organ has gradually decreased in size.

What was the function of this part of the digestive tract, which is now a shriveled projection at the beginning of the colon? In some vegetarian animals such as rabbits, this projection off of the colon is enormous and is called a cecum. The enzymes in the small intestine of these animals cannot digest some vegetable matter very well. When this partially digested matter moves into the cecum, bacteria can work on it and digest it more thoroughly. The bacteria can even produce important nutrients such as vitamins that were not in the original food. Now the question is, “How does the rabbit benefit from the nutrients generated by the bacteria in the cecum?” Remember that absorption of most nutrients occurs in the small intestine and not in the colon. The answer is recycling. From the matter in the cecum, the rabbit produces a special kind of feces that it then eats to recycle the nutrients. This interesting behavior is called coprophagy. Phagy comes from the Greek word phagos meaning one that eats. You can imagine what copro-means.

The Function of the Colon

Your body absorbs water and some salt in the colon. Chyme can stay in the colon for several days. Bacteria live in the colon. The bacteria make enzymes that digest what's left of the food. The bacteria also make gas. This gas contains marsh gas or methane. Doctors call this “flatus” or just “gas.” You have probably heard that beans can give you lots of gas. Why? Well, beans contain some complex carbohydrates that can't be digested by your pancreatic enzymes. But the bacteria in your colon can digest those carbohydrates and use them for energy. When bacteria metabolize these carbohydrates, the waste products include gases such as CO_2, methane, and hydrogen sulfide (a gas with a very strong odor that smells like rotten eggs).

The walls of the colon absorb water from the chyme. So after some time in the colon, what is left of the chyme is now firm. When the pressure gauge next measures a rising pressure, you feel the capsule enter the sigmoid colon or rectum.

Figure 4.8 Your capsule is near the end of the journey. It is entering the rectum from the sigmoid colon region (large intestine). Soon your capsule will be expelled from the digestive tract through the anus.

Sigmoid Colon

Sigmoid means shaped like the letter S. Your capsule twists and turns before it comes to rest. Your capsule is embedded in what looks like semisolid mud. Doctors call this mass stool or feces. You wait in your capsule for hours, but nothing happens. You look at your watch for the umpteenth time and see that it is early morning. Suddenly, the pressure rises again, and the defecation reflex starts. Your capsule passes with the stool through the muscular rectum and out the anus. The anus is two or more rings of sphincter muscles that are tightly shut except when defecation occurs. Your trip has ended, where it started, outside the body. However, your capsule was never really inside the body. Obviously, only the food molecules that can be digested and move through the wall of the food tube into your blood actually get into your body and into its cells.


Many people don't know how often it is necessary to defecate, or make bowel movements, to be healthy. A normal range for defecation or bowel movements is after every meal to only once every two or three days. The fiber in your stool helps you defecate. Cows eat lots of fiber or grass and they defecate often and have bulky, soft stools. The stool of someone who eats only processed food with little fiber can become hard, making it painful to defecate. As a result, the person can become constipated. Water and fiber keep stools normal. If you feel constipated, eat more fiber and drink more water! Fiber is in cereal, bran, fruits, and vegetables. Don't use laxatives unless your doctor tells you to. Laxatives are chemicals that can become habit forming and make the colon work less well than a normal colon. Too much fluid flowing into the colon from the small intestine can cause watery bowel movements called diarrhea. Remember that if you get diarrhea, you must drink more water to replace the water you are losing.

The Liver

Now you need to see where most food molecules go before they reach the cells in your body. Your blood moves nutrients from the intestines to your liver. Your liver is a storehouse for nutrients, a chemical manufacturing plant, and a sewage treatment station.

Did You Know?

People who abuse themselves by drinking alcohol or using drugs can poison their livers. Alcohol kills liver cells and changes liver tissue from being soft and rubbery to being hard and leathery. This change is called cirrhosis (si-ro-sis). Alcoholics sometimes bleed to death because their livers cannot make enough clotting proteins. Alcoholics also have trouble getting rid of wastes that accumulate in their blood. One waste chemical called bilirubin causes the skin to turn yellow if it is not excreted. The yellowing is called jaundice (jon-dis).

Your liver is located at the upper right side of your abdomen, below the diaphragm and behind the rib cage. Your liver is very complex and works for you in hundreds of chemical ways. Blood brings the molecules of digested food to your liver from your intestines. Then your liver stores the glucose molecules from your diet when you eat and gives them back to your blood as you need them. Your liver also stores some fats. More importantly, it packages fats with proteins and sends them out into the blood. These packages of fat go to muscles that use the fat for fuel, or they go to fatty tissue where excess fat is stored. Your liver manufactures bile and stores it in your gall bladder until you eat a fatty meal. Your liver also makes the proteins that cause clotting in the blood, as well as other blood proteins. The treatment by your liver takes foreign chemicals such as drugs and alcohol and other waste products from your blood and changes them into various forms that will not hurt your body and can be excreted. However, excessive use of alcohol and/or drugs can cause irreversible damage to the liver cells. Some of the waste molecules are sent from the liver along with bile into the duodenum. These waste chemicals then leave your body as bowel movements. Some molecules changed by the liver pass in your blood to the kidneys and then leave your body as part of urine.

Figure 4.9 Digested nutrients from the small intestine are stored in the liver. The stored nutrients are released from the liver as needed by the body.

Drawing the Actions of the Digestive System Create three drawings that illustrate the analogy of the digestive system to the steps involved in recycling old cars:

  1. Workers take apart old cars.
  2. Scavengers pick up old car parts.
  3. Parts are sorted and stored in warehouses.

Put It All Together

You have explored all the parts of the digestive system at this point. Now let's revisit the idea of the conveyor belt you read about in Section 3 to review what happens to the food you eat. Suppose you just ate a bite of chicken salad with celery in it. The chicken and the celery are like old cars. Your digestive system disassembles the food along the conveyor belt.

  1. You smash the chicken and celery by chewing.
  2. The digestive enzymes break down the complex molecules in the smashed food.
  3. The intestines take up the molecules from the food. These molecules are amino acids, fats, and sugars-the building blocks of your diet. These are the same molecules your cells use to get energy and to build new complex molecules that become part of you, such as protein.
  4. Your blood transports building block molecules from your intestine to the cells of your body. The liver takes up many molecules. In the liver the molecules can be sorted, stored, and converted into other molecules. Remember that your liver acts as a warehouse. When your cells need these molecules, the liver releases them and the blood transports them to the cells in need of them.
  5. Your cells take up the molecules from the blood. The genes in the nuclei of your cells direct the cells to use the building blocks to make the products that your cells need.
  6. While the molecules that are needed by the cells are put to use, the waste products of digestion are eliminated from the body.

Review Questions

  1. What is peristalsis and how does it help you process food in the small intestine?
  2. What is the role of the small intestine in digestion?
  3. When a person has had his or her gall bladder removed, which digestive process will be affected? How will that process be affected?
  4. What is bile? What role does it play in digestion?
  5. Why don't the digestive enzymes of the pancreas digest the pancreas?
  6. What is osmosis?
  7. Why is the liver important? Explain five important functions of your liver.

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Date Created:

Feb 23, 2012

Last Modified:

Dec 11, 2013
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