Digestion · Digestion Level W 10 Absorption The small intestine is not small at all . In fact, this coiled tube is 6 .1 meters (20 ft .) long in most adults . We call it small because

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Writing Write about the sequence of events that occurs during digestion from the point of view of food. Use information from the text and transition words in your sequence.

Science and Art Research to learn more about how food gives people energy. Design a poster and share your findings with your class.

Connections

Digestion A Reading A–Z Level W Leveled Book

Word Count: 1,433

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Written by Sean McCol lum

LEVELED BOOK • W

Digestion

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What is the process of digestion?

Focus Question

Written by Sean McCollum

Digestion

CorrelationLEVEL W

S4040

Fountas & PinnellReading Recovery

DRA

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Words to Know

abdomen absorption digestive system dissolving elimination enzymes

gastric microbes molecules nutrients saliva vital

DigestionLevel W Leveled Book© Learning A–ZWritten by Sean McCollum

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3

Table of Contents

Let’s Eat . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

Ingestion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Digestion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Absorption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

Elimination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

Food for Thought . . . . . . . . . . . . . . . . . . . . . . . . . 15

Glossary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

Digestion • Level W 4

Let’s Eat

Over the lips, past the gums, look out stomach, here it comes! This catchy saying captures the voyage of every piece of chicken, cheese, and chocolate we have ever put in our mouth .

Getting chewed and swallowed, though, are just the beginning . Every bite goes on a long, messy, slow-motion roller-coaster ride through the body’s digestive system .

From top to bottom, our digestive system is an astonishing network of organs . They work together to break apart what we eat into smaller pieces until molecules of food are turned into energy . Digestion also delivers the raw materials that make and build our body’s bones, muscles, skin, eyeballs, toenails, nose hairs, spit, and everything else .

Everyone knows how to eat . Here’s what happens afterward, from top to bottom .

Every time we eat, food begins an amazing journey through our bodies.

5

Ingestion

Recall a moment when a fresh pizza or other dish came out of the oven . The scent of a favorite food can make our mouth water, for a very good reason . Our body is getting ready to ingest, or eat .

The smell of tasty food gets the digestive system going before we ever take a bite . Glands in our mouth start oozing saliva, or spit . Saliva is almost all water . However, it also contains molecules called enzymes that start breaking down food .

Teeth serve as the shredder to make digestion easier . Our sharp front teeth are made for slicing into an apple or sandwich . Our strong, flat back teeth grind up harder foods, like nuts .

All the while, saliva is softening up every mouthful . Enzymes are especially good at dissolving bread, crackers, potatoes, and other starchy foods . Saliva also kills many germs in food before they have a chance to make us sick .

Taste buds on our tongue let us enjoy the flavors in our mouth . Now, though, the tongue has another job . It takes the mashed-up glob of food, called a bolus (BOH-luhss), and pushes it into the throat .

After we swallow, the journey continues .

Digestion • Level W 6

The Digestive System

Digestion

Digestion is the next stage . When we swallow, the food bolus enters the esophagus (ih-SOF-uh-guhss) . The esophagus is the tube that runs from the back of the mouth to the stomach .

Near the top of the esophagus is a leaf-shaped flap called the epiglottis (eh-puh-GLAH-tuhss) . It closes to keep food and drink out of the windpipe and lungs . Have you ever taken a swig of water and started coughing because some of it dribbled down the wrong tube? That happened because the epiglottis did not close in time .

The digestive system has many organs and other parts that work together to break food down into different things the body needs.

The Digestive System

esophagus

liver

large intestine

small intestine

pancreas

stomach

gallbladder

rectum

appendix

7

The esophagus is about 25 centimeters (10 in .) long . It runs down the back of the throat, behind the windpipe . The esophagus is lined with muscles that push the food toward the stomach . Usually, the trip down the esophagus to the stomach takes two to three seconds .

Digestion continues in the stomach . This organ is like a rubbery, comma-shaped sack . It produces gastric juices made mostly of acid and enzymes . The stomach is lined with gastric mucus . This gooey, slippery gunk protects the stomach from its own acid .

Lactose Intolerance Some of us have difficulty

digesting certain types of foods. Lactose intolerance is a common digestive disorder. Lactose is the main carbohydrate found in milk, cheese, and other dairy products.

Lactose intolerance happens when a person doesn’t produce enough of an enzyme called lactase. Lactase digests lactose, breaking it into glucose and galactose, two types of sugar the body can use. Without enough lactase, lactose goes undigested, which can make people feel ill and gassy.

Lactose intolerance is common. About 75 percent of the world’s population experiences some form of it.

Digestion • Level W 8

Gastric juices speed up the breakdown of food . Muscles around the stomach help by churning the mixture and turning solids into mush . A meal may take up to five hours to completely break down before leaving the stomach .

The digestive system works overtime to fight germs that might otherwise make us ill . Gastric juices can kill sickness-causing bacteria that might have hitched a ride on our food . The digestive system doesn’t always win, though . Our body may sense dangerous germs in our food, for example . Or we may have caught an illness that irritates the stomach lining . When that happens, our body may signal that what is in the stomach needs to come out . A muscle above the stomach, called the diaphragm (DY-uh-fram), squeezes down . The muscles of the abdomen squeeze, too . The contents of our stomach may then make a quick exit in the form of vomit . Vomiting is never pleasant . However, it is another way our digestive system protects us .

If digestion goes well, though, the stomach prepares a nutrient-rich smoothie . That chicken, cheese, or chocolate is now part of a gloppy mess called chyme (KYM) . Muscles pump this mixture into the next station in the digestive system: the small intestine .

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Nutrient Basics

The digestive system breaks food down into nutrients.

Carbohydrates fuel the brain, power the muscles, and help us fight off disease.

Fats help us absorb vitamins and minerals, maintain healthy blood, and build cells and muscle.

Minerals include calcium, iron, zinc, and others. Calcium helps build teeth and bones, and keeps them strong. Iron contributes to healthy blood. Zinc helps ward off illness and heal cuts and bruises. Minerals come from many foods, including green leafy vegetables.

Proteins supply the building blocks for the body, helping it grow and gain or maintain strength. We have proteins in every cell in our bodies—muscles, bones, and even hair.

Vitamins are valuable for fighting illness and disease. They contribute to healthy eyes, skin, and bones, and can help ease depression. Fruits, vegetables, and eggs are packed with vitamins.

Water is an essential nutrient for the healthy functioning of the entire body. For young people ages nine through thirteen, six glasses of water a day are recommended.

Digestion • Level W 10

Absorption

The small intestine is not small at all . In fact, this coiled tube is 6 .1 meters (20 ft .) long in most adults . We call it small because it is narrow—about 2 .5 centimeters (1 in .) from side to side .

The main job of the small intestine is absorption . Absorption is what a sponge does while sopping up spilled water or other liquid . In a similar way, the walls of the small intestine absorb nutrients from the chyme arriving from the stomach .

While the digestive system has many parts, food spends most of its time in the small and large intestines.

stomach

large intestine

small intestine

gallbladder

pancreasliver

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The first section of the small intestine is called the duodenum (doo-AH-dih-nuhm) . It features two openings, or ducts . They deliver chemicals—bile and digestive enzymes—that continue the digestive process . Bile is produced in the liver and stored in the gallbladder . It helps break down fats and vitamins . The pancreas produces digestive enzymes that break down carbohydrates, fats, and proteins .

Now the nutrients are ready for absorption, and the small intestine is where most absorption takes place . The lining of the small intestine is just one cell thick . Reduced to molecules, nutrients can easily pass through this lining and reach the bloodstream . The blood picks up the molecules and carries them to where they are needed .

How Does Food Give Us Energy?We need energy to run, think, and

carry our backpack. Where does that energy come from? Our food. Our body breaks down food into molecules, including glucose. Glucose is a simple yet power-packed form of sugar. Our body uses some glucose immediately for energy and stores any extra as glycogen (GLY-kuh-juhn) in the liver and muscles. If that extra glycogen goes unused for too long, the body will store it as fat for later use.

Digestion • Level W 12

The liver is another important organ along the digestive system . It acts like a filter to clear out harmful chemicals . It sends some nutrients onward to other organs and other groups of cells . The liver, though, is also where other vitamins and important sugars are stored until needed . These sugars are our body’s main fuel .

The digestive system hates wasting food . It uses every nutrient it can and stores others . Whatever the small intestine does not absorb gets pushed into the large intestine .

Too Much Sugar! Having too much glucose, or blood sugar, can be harmful to

our health. Insulin is a hormone that helps our bodies use and store glucose properly. Some people have bodies that don’t make or use insulin well, though. Those people can develop a disease called diabetes that poses serious health risks.

artery

liver

veins

gallbladder

The liver filters blood that’s picked up nutrients and chemicals from the digestive system.

13

Elimination

The large intestine is the last organ in the digestive system . In adults, it is about 1 .5 meters (5 ft .) long and about 7 .6 centimeters (3 in .) around . Whatever food the stomach and small intestine have not digested and absorbed ends up here .

The walls of the large intestine may absorb more water from the digested food . They may also draw out some remaining vitamins and minerals, such as salt . These nutrients pass through the walls to the bloodstream, just as they do in the small intestine . The large intestine prepares what remains for elimination as feces—better known as poop .

The large intestine is also the main home of a zoo of microbes, mostly bacteria . These microbes are not just good but vital to have inside us . They fight harmful germs and also produce vitamins . For example, gut microbes produce vitamin K, which builds stronger bones and healthier blood .

The large intestine is the body’s last chance to get nutrients out of digested food.

rectumanus

large intestine (colon)

Digestion • Level W 14

Strong muscles around the large intestine finally squeeze the poop into the last stop—the rectum . Nerves there signal when it’s time for the poop to exit through the anus .

What’s actually in our poop? Most of it is water . Some of it is food waste . However, about a third of the solid content of feces is bacteria from the small and large intestines . (These bacteria are also responsible for most of the gas we pass .) The typical brown color is the result of bile that came along for the ride .

Once again, the digestive system has done its job .

Washing your hands after going to the bathroom removes germs and helps keep you and others healthy.

15

Food for Thought

You can take care of your digestive system so that it can take better care of you . Here are three suggestions that make it easier for your stomach and intestines to do their jobs .

1 . Eat real food . That means cutting back on junky grub . Foods that are loaded with sugar and salt (sodium) may taste good, but they are not great for the body . If the ingredients list on a snack package is super long and full of chemicals you’ve never heard of, there are better options for your digestive system and overall health .

2 . Increase your fiber intake . That means eating more foods like oatmeal, beans, nuts, seeds, and vegetables . Eating a lot of fiber is like cleaning out the digestive system with a scrub brush .

3 . Slow down and chew . The more you use your teeth and saliva to mash up a mouthful of food, the easier it is to digest and absorb . Slow eating reduces indigestion, belching, and gas .

The digestive system is messy and sometimes smelly . It acts like a biological assembly line . It moves food from top to bottom while supplying your body with everything it needs to stay strong, healthy, and energized . It is truly a wonder of the human body .

Digestion • Level W 16

Glossary

abdomen (n.) the part of the body between the chest and the hips, where most of the digestive organs are located (p . 8)

absorption (n.)

the process of soaking up or taking in (p . 10)

digestive system (n.)

parts of the body that work together to break down food and absorb nutrients that help the body live and grow (p . 4)

dissolving (v.)

breaking down or disintegrating, usually because of contact with a liquid (p . 5)

elimination (n.)

the act of removing or taking something away (p . 13)

enzymes (n.) proteins in plants and animals that cause certain natural chemical reactions (p . 5)

gastric (adj.) of or relating to the stomach (p . 7)

microbes (n.) microscopic organisms (p . 13)

molecules (n.)

the smallest parts of a substance that can exist by themselves, made of one or more atoms (p . 4)

nutrients (n.) substances that living things need to live, stay healthy, and grow (p . 10)

saliva (n.) a liquid in the mouth that mixes with food to help begin digestion (p . 5)

vital (adj.) important or necessary to the success or survival of something (p . 13)