Your Digestive System 

and How It Works 

National Digestive Diseases Information Clearinghouse 

U.S. Department 

of Health and 

Human Services 

NATIONAL 

INSTITUTES 

OF HEALTH 

The digestive system is made up of the 

digestive tract—a series of hollow organs 

joined in a long, twisting tube from the 

mouth to the anus—and other organs that 

help the body break down and absorb food 

(see figure). 

Organs that make up the digestive tract 

are the mouth, esophagus, stomach, small 

intestine, large intestine—also called the 

colon—rectum, and anus. Inside these hollow organs is a lining called the mucosa. In 

the mouth, stomach, and small intestine, 

the mucosa contains tiny glands that produce juices to help digest food. The digestive tract also contains a layer of smooth 

muscle that helps break down food and 

move it along the tract. 

Two “solid” digestive organs, the liver and 

the pancreas, produce digestive juices that 

reach the intestine through small tubes 

called ducts. The gallbladder stores the 

liver’s digestive juices until they are needed 

in the intestine. Parts of the nervous and 

circulatory systems also play major roles in 

the digestive system. 

Why is digestion important? 

When you eat foods—such as bread, meat, 

and vegetables—they are not in a form that 

the body can use as nourishment. Food 

and drink must be changed into smaller 

molecules of nutrients before they can be 

absorbed into the blood and carried to 

cells throughout the body. Digestion is the 

The digestive system. 

Pancreas 

Liver 

Gallbladder 

Duodenum 

Stomach 

Transverse Colon Descending 

colon 

Ascending colon 

Cecum 

Appendix Rectum 

Small 

intestine Sigmoid 

colon 

Anus 

Ileum 

Jejunum 

Esophagus 

process by which food and drink are broken 

down into their smallest parts so the body 

can use them to build and nourish cells and 

to provide energy. 
How is food digested? 

Digestion involves mixing food with digestive juices, moving it through the digestive 

tract, and breaking down large molecules of 

food into smaller molecules. Digestion 

begins in the mouth, when you chew and 

swallow, and is completed in the small 

intestine. 

Movement of Food Through 

the System 

The large, hollow organs of the digestive 

tract contain a layer of muscle that enables 

their walls to move. The movement of 

organ walls can propel food and liquid 

through the system and also can mix the 

contents within each organ. Food moves 

from one organ to the next through muscle 

action called peristalsis. Peristalsis looks 

like an ocean wave traveling through the 

muscle. The muscle of the organ contracts 

to create a narrowing and then propels the 

narrowed portion slowly down the length of 

the organ. These waves of narrowing push 

the food and fluid in front of them through 

each hollow organ. 

The first major muscle movement occurs 

when food or liquid is swallowed. Although 

you are able to start swallowing by choice, 

once the swallow begins, it becomes involuntary and proceeds under the control of 

the nerves. 

Swallowed food is pushed into the esophagus, which connects the throat above with 

the stomach below. At the junction of the 

esophagus and stomach, there is a ringlike muscle, called the lower esophageal 

sphincter, closing the passage between the 

two organs. As food approaches the closed 

sphincter, the sphincter relaxes and allows 

the food to pass through to the stomach. 

The stomach has three mechanical tasks. 

First, it stores the swallowed food and liquid. To do this, the muscle of the upper 

part of the stomach relaxes to accept large 

volumes of swallowed material. The second job is to mix up the food, liquid, and 

digestive juice produced by the stomach. 

The lower part of the stomach mixes these 

materials by its muscle action. The third 

task of the stomach is to empty its contents 

slowly into the small intestine. 

Several factors affect emptying of the stomach, including the kind of food and the 

degree of muscle action of the emptying 

stomach and the small intestine. Carbohydrates, for example, spend the least amount 

of time in the stomach, while protein stays 

in the stomach longer, and fats the longest. 

As the food dissolves into the juices from 

the pancreas, liver, and intestine, the contents of the intestine are mixed and pushed 

forward to allow further digestion. 

Finally, the digested nutrients are absorbed 

through the intestinal walls and transported 

throughout the body. The waste products 

of this process include undigested parts of 

the food, known as fiber, and older cells 

that have been shed from the mucosa. 

These materials are pushed into the colon, 

where they remain until the feces are 

expelled by a bowel movement. 

Production of Digestive Juices 

The digestive glands that act first are in the 

mouth—the salivary glands. Saliva produced by these glands contains an enzyme 

that begins to digest the starch from food 

into smaller molecules. An enzyme is a 

substance that speeds up chemical reactions 

in the body. 

 Your Digestive System and How It Works 
The next set of digestive glands is in the 

stomach lining. They produce stomach acid 

and an enzyme that digests protein. A thick 

mucus layer coats the mucosa and helps 

keep the acidic digestive juice from dissolving the tissue of the stomach itself. In most 

people, the stomach mucosa is able to resist 

the juice, although food and other tissues of 

the body cannot. 

After the stomach empties the food and 

juice mixture into the small intestine, the 

juices of two other digestive organs mix 

with the food. One of these organs, the 

pancreas, produces a juice that contains 

a wide array of enzymes to break down 

the carbohydrate, fat, and protein in food. 

Other enzymes that are active in the process come from glands in the wall of the 

intestine. 

The second organ, the liver, produces yet 

another digestive juice—bile. Bile is stored 

between meals in the gallbladder. At mealtime, it is squeezed out of the gallbladder, 

through the bile ducts, and into the intestine to mix with the fat in food. The bile 

acids dissolve fat into the watery contents of 

the intestine, much like detergents that dissolve grease from a frying pan. After fat is 

dissolved, it is digested by enzymes from the 

pancreas and the lining of the intestine. 

Absorption and Transport of 

Nutrients 

Most digested molecules of food, as well as 

water and minerals, are absorbed through 

the small intestine. The mucosa of the 

small intestine contains many folds that 

are covered with tiny fingerlike projections called villi. In turn, the villi are covered with microscopic projections called 

microvilli. These structures create a vast 

surface area through which nutrients can be 

absorbed. Specialized cells allow absorbed 

materials to cross the mucosa into the 

blood, where they are carried off in the 

bloodstream to other parts of the body for 

storage or further chemical change. This 

part of the process varies with different 

types of nutrients. 

Carbohydrates. The Dietary Guidelines 

for Americans 2005 recommend that 45 to 

65 percent of total daily calories be from 

carbohydrates. Foods rich in carbohydrates 

include bread, potatoes, dried peas and 

beans, rice, pasta, fruits, and vegetables. 

Many of these foods contain both starch 

and fiber. 

The digestible carbohydrates—starch and 

sugar—are broken into simpler molecules 

by enzymes in the saliva, in juice produced 

by the pancreas, and in the lining of the 

small intestine. Starch is digested in two 

steps. First, an enzyme in the saliva and 

pancreatic juice breaks the starch into molecules called maltose. Then an enzyme in 

the lining of the small intestine splits the 

maltose into glucose molecules that can be 

absorbed into the blood. Glucose is carried 

through the bloodstream to the liver, where 

it is stored or used to provide energy for the 

work of the body. 

Sugars are digested in one step. An enzyme 

in the lining of the small intestine digests 

sucrose, also known as table sugar, into 

glucose and fructose, which are absorbed 

through the intestine into the blood. Milk 

contains another type of sugar, lactose, 

which is changed into absorbable molecules 

by another enzyme in the intestinal lining. 

 Your Digestive System and How It Works 
Fiber is undigestible and moves through the 

digestive tract without being broken down 

by enzymes. Many foods contain both 

soluble and insoluble fiber. Soluble fiber 

dissolves easily in water and takes on a soft, 

gel-like texture in the intestines. Insoluble 

fiber, on the other hand, passes essentially 

unchanged through the intestines. 

Protein. Foods such as meat, eggs, and 

beans consist of giant molecules of protein 

that must be digested by enzymes before 

they can be used to build and repair body 

tissues. An enzyme in the juice of the 

stomach starts the digestion of swallowed 

protein. Then in the small intestine, several 

enzymes from the pancreatic juice and the 

lining of the intestine complete the breakdown of huge protein molecules into small 

molecules called amino acids. These small 

molecules can be absorbed through the 

small intestine into the blood and then be 

carried to all parts of the body to build the 

walls and other parts of cells. 

Fats. Fat molecules are a rich source of 

energy for the body. The first step in digestion of a fat such as butter is to dissolve it 

into the watery content of the intestine. 

The bile acids produced by the liver dissolve 

fat into tiny droplets and allow pancreatic 

and intestinal enzymes to break the large 

fat molecules into smaller ones. Some of 

these small molecules are fatty acids and 

cholesterol. The bile acids combine with 

the fatty acids and cholesterol and help 

these molecules move into the cells of the 

mucosa. In these cells the small molecules 

are formed back into large ones, most of 

which pass into vessels called lymphatics 

near the intestine. These small vessels carry 

the reformed fat to the veins of the chest, 

and the blood carries the fat to storage 

depots in different parts of the body. 

Vitamins. Another vital part of food that 

is absorbed through the small intestine are 

vitamins. The two types of vitamins are 

classified by the fluid in which they can be 

dissolved: water-soluble vitamins (all the 

B vitamins and vitamin C) and fat-soluble 

vitamins (vitamins A, D, E, and K). Fatsoluble vitamins are stored in the liver and 

fatty tissue of the body, whereas watersoluble vitamins are not easily stored and 

excess amounts are flushed out in the urine. 

Water and salt. Most of the material 

absorbed through the small intestine is 

water in which salt is dissolved. The salt 

and water come from the food and liquid 

you swallow and the juices secreted by the 

many digestive glands. 

How is the digestive process 

controlled? 

Hormone Regulators 

The major hormones that control the functions of the digestive system are produced 

and released by cells in the mucosa of the 

stomach and small intestine. These hormones are released into the blood of the 

digestive tract, travel back to the heart 

and through the arteries, and return to the 

digestive system where they stimulate digestive juices and cause organ movement. 

4 Your Digestive System and How It Works 
The main hormones that control digestion 

are gastrin, secretin, and cholecystokinin 

(CCK): 

•	 Gastrin causes the stomach to produce 

an acid for dissolving and digesting 

some foods. Gastrin is also necessary 

for normal cell growth in the lining 

of the stomach, small intestine, and 

colon. 

•	 Secretin causes the pancreas to send 

out a digestive juice that is rich in 

bicarbonate. The bicarbonate helps 

neutralize the acidic stomach contents as they enter the small intestine. 

Secretin also stimulates the stomach to 

produce pepsin, an enzyme that digests 

protein, and stimulates the liver to produce bile. 

•	 CCK causes the pancreas to produce 

the enzymes of pancreatic juice, and 

causes the gallbladder to empty. It 

also promotes normal cell growth of 

the pancreas. 

Additional hormones in the digestive system regulate appetite: 

•	 Ghrelin is produced in the stomach 

and upper intestine in the absence of 

food in the digestive system and stimulates appetite. 

•	 Peptide YY is produced in the digestive 

tract in response to a meal in the system and inhibits appetite. 

Both of these hormones work on the brain 

to help regulate the intake of food for 

energy. Researchers are studying other 

hormones that may play a part in inhibiting 

appetite, including glucagon-like peptide-1 

(GPL-1), oxyntomodulin (+), and pancreatic polypeptide. 

Nerve Regulators 

Two types of nerves help control the action 

of the digestive system. 

Extrinsic, or outside, nerves come to the 

digestive organs from the brain or the 

spinal cord. They release two chemicals, 

acetylcholine and adrenaline. Acetylcholine causes the muscle layer of the digestive organs to squeeze with more force 

and increase the “push” of food and juice 

through the digestive tract. It also causes 

the stomach and pancreas to produce more 

digestive juice. Adrenaline has the opposite 

effect. It relaxes the muscle of the stomach and intestine and decreases the flow of 

blood to these organs, slowing or stopping 

digestion. 

The intrinsic, or inside, nerves make up a 

very dense network embedded in the walls 

of the esophagus, stomach, small intestine, 

and colon. The intrinsic nerves are triggered to act when the walls of the hollow 

organs are stretched by food. They release 

many different substances that speed up or 

delay the movement of food and the production of juices by the digestive organs. 

Together, nerves, hormones, the blood, and 

the organs of the digestive system conduct 

the complex tasks of digesting and absorbing nutrients from the foods and liquids you 

consume each day. 

5 	 Your Digestive System and How It Works 
For More Information

American Dietetic Association

120 South Riverside Plaza, Suite 2000 

Chicago, IL 60606–6995 

Phone: 1–800–877–1600 or 312–899–0040 

Fax: 312–899–4739 

Email: hotline@eatright.org 

Internet: www.eatright.org

You may also find additional information about this 

topic by visiting MedlinePlus at www.medlineplus.gov.

This publication may contain information about 

medications used to treat a health condition. When

this publication was prepared, the NIDDK included 

the most current information available. Occasionally, new information about medication is released. 

For updates or for questions about any medications, 

please contact the U.S. Food and Drug Administration at 1–888–INFO–FDA (463–6332), a toll-free call, 

or visit their website at www.fda.gov. Consult your 

doctor for more information.

National Digestive Diseases 

Information Clearinghouse

2 Information Way 

Bethesda, MD 20892–3570 

Phone: 1–800–891–5389 

Fax: 703–738–4929 

Email: nddic@info.niddk.nih.gov 

Internet: www.digestive.niddk.nih.gov

The National Digestive Diseases Information Clearinghouse (NDDIC) is a service 

of the National Institute of Diabetes and 

Digestive and Kidney Diseases (NIDDK). 

The NIDDK is part of the National Institutes of Health of the U.S. Department of 

Health and Human Services. Established in 

1980, the Clearinghouse provides information about digestive diseases to people with 

digestive disorders and to their families, 

health care professionals, and the public. 

The NDDIC answers inquiries, develops 

and distributes publications, and works 

closely with professional and patient organizations and Government agencies to coordinate resources about digestive diseases.

Publications produced by the Clearinghouse are carefully reviewed by both 

NIDDK scientists and outside experts. 

This publication is not copyrighted. The Clearinghouse encourages users of this fact sheet to duplicate

and distribute as many copies as desired.

This fact sheet is also available at 

www.digestive.niddk.nih.gov.

U.S. DEPARTMENT OF HEALTH 

AND HUMAN SERVICES

National Institutes of Health

NIH Publication No. 08–2681

April 2008