What's the first step in digesting food? Believe it or not, the digestive process
starts even before you put food in your mouth. It begins when you smell something
irresistible or when you see a favorite food you know will taste good. Just by smelling
that homemade apple pie or thinking about how delicious that ice cream sundae is going
to taste, you begin to salivate — and the digestive process kicks in, preparing
for that first scrumptious bite.
If it's been a while since your last meal or if you even think about something
tasty, you feel hungry. You eat until you're satisfied and then go about your business.
But for the next 20 hours or so, your digestive system is doing its job as the food
you ate travels through your body.
Food is the body's fuel source. The nutrients in food give the body's cells the
energy and other substances they need to operate. But before food can do any of these
things, it has to be digested into small pieces the body can absorb and use.
Almost all animals have a tube-type digestive system in which food enters the mouth,
passes through a long tube, and exits as feces (poop) through the anus. The smooth
muscle in the walls of the tube-shaped digestive organs rhythmically and efficiently
moves the food through the system, where it is broken down into tiny absorbable nutrients.
During the process of absorption, nutrients that come from the food (including
carbohydrates, proteins, fats, vitamins, and minerals) pass through channels in the
intestinal wall and into the bloodstream. The blood works to distribute these nutrients
to the rest of the body. The waste parts of food that the body can't use are passed
out of the body as feces.
About the Digestive System
Every morsel of food we eat has to be broken down into nutrients that can be absorbed
by the body, which is why it takes hours to fully digest food. In humans, protein
must be broken down into amino acids, starches into simple sugars, and fats into fatty
acids and glycerol. The water in our food and drink is also absorbed into the bloodstream
to provide the body with the fluid it needs.
The digestive system is made up of the alimentary canal and the
other abdominal organs that play a part in digestion, such as the liver and pancreas.
The alimentary canal (also called the digestive tract) is the long
tube of organs — including the esophagus, the stomach, and the intestines —
that runs from the mouth to the anus. An adult's digestive tract is about 30 feet
How Digestion Works
Digestion Begins in the Mouth
The process of digestion starts well before food reaches the stomach. When we see,
smell, taste, or even imagine a tasty snack, our salivary glands,
which are located under the tongue and near the lower jaw, begin producing saliva.
This flow of saliva is set in motion by a brain reflex that's triggered when we sense
food or even think about eating. In response to this sensory stimulation, the brain
sends impulses through the nerves that control the salivary glands, telling them to
prepare for a meal.
As the teeth tear and chop the food, saliva moistens it for easy
swallowing. A digestive enzyme called amylase (pronounced: AH-meh-lace),
which is found in saliva, starts to break down some of the carbohydrates (starches
and sugars) in the food even before it leaves the mouth.
Swallowing, which is accomplished by muscle movements in the tongue and mouth,
moves the food into the throat, or pharynx. The pharynx (pronounced:
FAR-inks), a passageway for food and air, is about 5 inches long. A flexible flap
of tissue called the epiglottis (pronounced: ep-ih-GLAH-tus) reflexively
closes over the windpipe when we swallow to prevent choking.
From the throat, food travels down a muscular tube in the chest called the esophagus
(pronounced: ih-SAH-fuh-gus). Waves of muscle contractions called peristalsis
(pronounced: per-uh-STALL-sus) force food down through the esophagus to the stomach.
A person normally isn't aware of the movements of the esophagus, stomach, and intestine
that take place as food passes through the digestive tract.
At the end of the esophagus, a muscular ring called a sphincter
(pronounced: SFINK-ter) allows food to enter the stomach and then squeezes shut to
keep food or fluid from flowing back up into the esophagus. The stomach muscles churn
and mix the food with acids and enzymes, breaking it into much smaller, more digestible
pieces. An acidic environment is needed for the digestion that takes place in the
stomach. Glands in the stomach lining produce about 3 quarts of these digestive juices
Most substances in the food we eat need further digestion and must travel into
the intestine before being absorbed. When it's empty, an adult's stomach has a volume
of one fifth of a cup, but it can expand to hold more than 8 cups of food after a
By the time food is ready to leave the stomach, it has been processed into a thick
liquid called chyme (pronounced: kime). A walnut-sized muscular tube
at the outlet of the stomach called the pylorus (pronounced: py-LOR-us)
keeps chyme in the stomach until it reaches the right consistency to pass into the
small intestine. Chyme is then squirted down into the small intestine, where digestion
of food continues so the body can absorb the nutrients into the bloodstream.
The Small Intestine
The small intestine is made up of three parts:
the duodenum (pronounced: due-uh-DEE-num), the C-shaped first
the jejunum (pronounced: jih-JU-num), the coiled midsection
the ileum (pronounced: IH-lee-um), the final section that leads
into the large intestine
The inner wall of the small intestine is covered with millions of microscopic,
finger-like projections called villi (pronounced: VIH-lie). The villi
are the vehicles through which nutrients can be absorbed into the body.
The liver (located under the ribcage in the right upper part of
the abdomen), the gallbladder (hidden just below the liver), and
the pancreas (beneath the stomach) are not part of the alimentary
canal, but these organs are still important for healthy digestion.
The pancreas produces enzymes that help digest proteins, fats, and carbohydrates.
It also makes a substance that neutralizes stomach acid. The liver produces bile,
which helps the body absorb fat. Bile is stored in the gallbladder until it is needed.
These enzymes and bile travel through special channels (called ducts) directly into
the small intestine, where they help to break down food.
The liver also plays a major role in the handling and processing of nutrients.
These nutrients are carried to the liver in the blood from the small intestine.
The Large Intestine
From the small intestine, food that has not been digested (and some water) travels
to the large intestine through a valve that prevents food from returning to the small
intestine. By the time food reaches the large intestine, the work of absorbing nutrients
is nearly finished. The large intestine's main function is to remove water from the
undigested matter and form solid waste that can be excreted. The large intestine is
made up of three parts:
The cecum (pronounced: SEE-kum) is a pouch at the beginning of
the large intestine that joins the small intestine to the large intestine. This transition
area allows food to travel from the small intestine to the large intestine. The appendix,
a small, hollow, finger-like pouch, hangs off the cecum. Doctors believe the appendix
is left over from a previous time in human evolution. It no longer appears to be useful
to the digestive process.
The colon extends from the cecum up the right side of the abdomen,
across the upper abdomen, and then down the left side of the abdomen, finally connecting
to the rectum. The colon has three parts: the ascending colon and transverse colon,
which absorb water and salts, and the descending colon, which holds the resulting
waste. Bacteria in the colon help to digest the remaining food products.
The rectum is where feces are stored until they leave the digestive
system through the anus as a bowel movement.
Things That Can Go Wrong
Nearly everyone has a digestive problem at one time or another. Some conditions,
such as indigestion or mild diarrhea, are common; they result in mild discomfort and
get better on their own or are easy to treat. Others, such as inflammatory bowel disease
(IBD), can be long lasting or troublesome. GI specialists or gastroenterologists (doctors
who specialize in the digestive system) can be helpful when dealing with these conditions.
Conditions Affecting the Esophagus
Conditions affecting the esophagus may be congenital (meaning
people are born with them) or noncongenital (meaning people
can develop them after birth).
Tracheoesophageal fistula (pronounced: tray-KEE-oh-ih-saf-uh-jee-ul
FISH-chuh-luh) and esophageal atresia (pronounced: ih-saf-uh-JEE-ul
uh-TREE-zhuh) are both examples of congenital conditions. Tracheoesophageal fistula
is where there is a connection between the esophagus and the trachea (windpipe) where
there shouldn't be one. In babies with esophageal atresia, the esophagus comes to
a dead end instead of connecting to the stomach. Both conditions are usually detected
soon after a baby is born — sometimes even beforehand. They require surgery
Esophagitis (pronounced: ih-saf-uh-JEYE-tus) or inflammation
of the esophagus, is an example of a noncongenital condition. Esophagitis is usually
caused by gastroesophageal reflux disease(GERD),
a condition in which the esophageal sphincter (the tube of muscle that connects the
esophagus with the stomach) allows the acidic contents of the stomach to move backward
up into the esophagus. GERD can sometimes be corrected through lifestyle changes,
such as adjusting the types of things a person eats. Sometimes, though, it requires
treatment with medication. Occasionally, esophagitis can be caused by infection or
Conditions Affecting the Stomach and Intestines
Almost everyone has experienced diarrhea or constipation at some point in their
lives. With diarrhea, muscle contractions move the contents of the intestines along
too quickly and there isn't enough time for water to be absorbed before the feces
are pushed out of the body. Constipation is the opposite: The contents of the large
intestines do not move along fast enough and waste materials stay in the large intestine
so long that too much water is removed and the feces become hard.
Other common stomach and intestinal disorders include:
Celiac disease is a digestive disorder caused by the abnormal
response of the immune system to a protein called gluten, which is found in certain
foods. People with celiac disease have difficulty digesting the nutrients from their
food because eating things with gluten damages the lining of the intestines over time.
Some of the symptoms are diarrhea, abdominal pain, and bloating. The disease can be
managed by following a gluten-free diet.
Irritable bowel syndrome(IBS) is a common intestinal
disorder that affects the colon. When the muscles in the colon don't work smoothly,
a person can feel the abdominal cramps, bloating, constipation, and diarrhea that
may be signs of IBS. There's no cure for IBS, but it can be managed by making some
dietary and lifestyle changes. Occasionally, medications may be used as well.
Gastritis and peptic ulcers. Under normal conditions, the stomach
and duodenum are extremely resistant to irritation by the strong acids produced in
the stomach. Sometimes, though, a bacterium called Helicobacter pylori or
the chronic use of certain medications weakens the protective mucous coating of the
stomach and duodenum, allowing acid to get through to the sensitive lining beneath.
This can irritate and inflame the lining of the stomach (a condition known as gastritis)
or cause peptic ulcers, which are sores or holes that form in the lining of the stomach
or the duodenum and cause pain or bleeding. Medications are usually successful in
treating these conditions.
Inflammatory bowel disease (IBD) is chronic inflammation of the
intestines that affects older kids, teens, and adults. There are two major types:
ulcerative colitis, which usually affects just the rectum and the
large intestine, and Crohn's disease, which can affect the whole
gastrointestinal tract from the mouth to the anus as well as other parts of the body.
They are treated with medications, but in some cases, surgery may be necessary to
remove inflamed or damaged areas of the intestine.
Disorders of the Pancreas, Liver, and Gallbladder
Conditions affecting the pancreas, liver, and gallbladder often affect the ability
of these organs to produce enzymes and other substances that aid in digestion.
is a chronic, inherited illness that not only affects the lungs, but also causes the
production of abnormally thick mucus. This mucus blocks the ducts or passageways
in the pancreas and prevents its digestive juices from entering the intestines, making
it difficult for a person to properly digest proteins and fats. This causes important
nutrients to pass out of the body unused. To help manage their digestive problems,
people with cystic fibrosis can take digestive enzymes and nutritional supplements.
is a viral infection in which the liver becomes inflamed and can lose its ability
to function. Some forms of viral hepatitis are highly contagious. Mild cases of hepatitis
A can be treated at home; however, serious cases involving liver damage may require
The gallbladder can develop gallstones and become inflamed — a condition
called cholecystitis (pronounced: ko-lee-sis-TYE-tus). Although gallbladder
conditions are uncommon in teens, they can happen when a teen has sickle
cell disease or is being treated with certain long-term medications.
The kinds and amounts of food a person eats and how the digestive system processes
that food play key roles in maintaining good health. Eating a healthy diet is the
best way to prevent common digestive problems.