Food is our fuel, and its nutrients give our bodies' cells the energy and substances
they need to work. But before food can do that, it must be digested into small pieces
the body can absorb and use.
The first step in the digestive process happens before we even taste food. Just
by smelling that homemade apple pie or thinking about how delicious that ripe tomato
is going to be, you start salivating — and the digestive process begins in preparation
for that first bite.
Almost all animals have a tube-type digestive system in which food:
enters the mouth
passes through a long tube
exits the body as feces (poop) through the anus
Along the way, food is broken down into tiny molecules so that the body can absorb
nutrients it needs:
Protein must be broken down into amino acids.
Starches break down into simple sugars.
Fats break down into into fatty acids and glycerol.
The waste parts of food that the body can't use are what leave the body as feces.
How Does Digestion Work?
The digestive system is made up of the alimentary canal (also
called the digestive tract) and other organs, such as the liver and pancreas. The
alimentary canal is the long tube of organs — including the esophagus, stomach,
and intestines — that runs from the mouth to the anus. An adult's digestive
tract is about 30 feet (about 9 meters) long.
Digestion begins in the mouth, well before food reaches the stomach. When we see,
smell, taste, or even imagine a tasty meal, our salivary glands in front of the ear,
under the tongue, and near the lower jaw begin making saliva (spit).
As the teeth tear and chop the food, spit moistens it for easy swallowing. A digestive
enzyme in saliva called amylase (pronounced: AH-meh-lace) starts
to break down some of the carbohydrates (starches and sugars) in the food even before
it leaves the mouth.
Swallowing, done by muscle movements in the tongue and mouth, moves the food into
the throat, or pharynx (pronounced: FAIR-inks). The pharynx is a
passageway for food and air. A soft flap of tissue called the epiglottis
(pronounced: ep-ih-GLAH-tus) 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 or valve 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 digestive juices that have acids and enzymes, breaking it into
much smaller, digestible pieces. An acidic environment is needed for the digestion
that takes place in the stomach.
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 valve
at the outlet of the stomach called the pylorus (pronounced: pie-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 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 blood. The blood
then brings these nutrients to the rest of the body.
The liver (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 essential
The liver makes bile, which helps the body absorb fat. Bile is
stored in the gallbladder until it is needed. The pancreas makes enzymes that help
digest proteins, fats, and carbs. It also makes a substance that neutralizes stomach
acid. These enzymes and bile travel through special pathways (called ducts) into the
small intestine, where they help to break down food. The liver also helps process
nutrients in the bloodstream.
From the small intestine, undigested food (and some water) travels to the large
intestine through a muscular ring or 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 job is to remove water from the undigested matter and
form solid waste (poop) to be excreted.
The large intestine has three parts:
The cecum (pronounced: SEE-kum) is the beginning of the large
intestine. The appendix, a small, hollow, finger-like pouch, hangs
at the end of the cecum. Scientists 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 the
transverse colon, which absorb fluids 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.
It takes hours for our bodies to fully digest food.