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HOW THE BODY DIGESTS FOOD Digestion is the body’s way of processing
food into smaller molecules that it can use for
energy and to get rid of waste. There are
several steps in the digestion process as food
passes through the body including ingestion,
secretion, mixing, digestion, absorption, and
excretion. Several organs and pathways (seen
in Figure 1) participate in the digestion process
both mechanically and chemically. By taking a
closer look into the steps in the digestion
process, we are better able to understand what
goes on in our bodies when we eat food.
Figure 1. Basic anatomy of the GI tract and organs
involved in digestion.1
Ingestion
Secretion
The digestion process begins in the mouth
where teeth start to chew up the food.
Meanwhile, enzymes from the salivary glands
help initiate the breakdown of food so that it is
lubricated as it travels down through the
pharynx and esophagus. Once the food is
chewed up and traveling down the pharynx, it
is called bolus. The esophagus carries the
bolus past the esophageal sphincter and into
the stomach. The sphincter’s function is to trap
the food in the stomach so that it isn’t
regurgitated back into the esophagus. There
are sphincters (pyloric, ileocecal) that have the
same function in other locations through the
GI tract that will be discussed.
Secretion starts the moment food enters the
mouth. The body secrets several liters of fluids
each day including saliva, mucus, hydrochloric
acid, enzymes, and bile. All of these fluids
play an important role in helping to break
down food. Saliva contains an enzyme called
salivary amylase, which lubricates the food
and begins the digestion of carbohydrates.
Mucus provides a protective barrier on the
inside of the gastrointestinal tract as the bolus
travels through, and hydrochloric acid from
the stomach digests the food chemically while
also killing any unnecessary bacteria.
Enzymes help to disassemble large
macromolecules into smaller molecules via
chemical reactions. Lastly, bile assists in
breaking down masses of lipids into tiny
globules that can easily be digested. There are
other fluids that are secreted by organs during
the digestion process, like pancreatic juice, but
the five mentioned are the major ones.
Movement and Mixing
Movement and mixing occurs in every
pathway through the GI tract as the bolus
travels down. The three methods the body uses
to mix and move foods are swallowing,
peristalsis, and segmentation. Swallowing is
the process of taking in the food where the
muscles work to move the food towards the
stomach. Peristalsis is the wave-like
movement that travels down the GI tract; it
starts at the esophagus and works all the way
down past the intestines to the end of the GI
tract. Figure 2 is a simple representation of
how the smooth and skeletal muscles move the
bolus in a wave-like pattern. Finally,
segmentation is short contractions that occur
in the small intestine to help increase
absorption of nutrients and mixing of fluids.
Swallowing, peristalsis, and segmentation are
all vital steps in the mixing and movement
portion of the digestion process.
Figure 2. Demonstration of peristalsis.2
Digestion
Digestion transforms the food into basic
compounds that will be used by the body or
stored for future energy. Small particles of
food can be broken down into both major and
minor nutrients. Major nutrients include
macronutrients, which must be broken down
enzymatically, and minor nutrients including
carbohydrates, proteins, and lipids. There are
two types of digestion, mechanical and
chemical (Figure 3), and both contribute
equally to the breakdown of food. Mechanical
digestion begins with the teeth chewing food
and then continues with the mixing and
movement of food, aided by contractions
(segmentation). Chemical digestion occurs
simultaneously with mechanical digestion
through the help of the bile produced by the
liver. Chemical digestion begins in the mouth
as well, where the enzymes in saliva assist in
breaking down carbohydrates. The enzymes
and acid in the stomach contribute to this
chemical breakdown, but most of the chemical
digestion occurs in the small intestine where
the pancreas secretes pancreatic juice.
Carbohydrates, for example, will be broken
down into monomers called monosaccharides.
Through the combination of chemical and
mechanical digestion, food will be
successfully broken down into four
components: amino acids, fatty acids,
monosaccharides, and nucleotides. These
four components are considered the basic
“building blocks” that the body uses for
energy.
content of the interstitial cavity where the bile
helps to further dissolve it into fatty acids or
cholesterol. After crossing the mucosa, the
small molecules are reformed into large
molecules and then travel through vessels to
be stored in different parts of the body.
Finally, water and salts are absorbed by the
small intestines and assimilation occurs, where
nutrients are delivered to other cells in the
body.
Figure 4. Monosaccharides broken down into glucose molecules via chemical digestion.4 Figure 3. Examples of chemical and mechanical digestion in the stomach.3
Absorption
Once broken down into the four building
blocks, the body is ready to absorb the food.
Absorption begins in the stomach with the
help of the secreted fluids. In the walls of the
small intestine, small blood and lymphatic
vessels pick up the amino acids, fatty acids,
monosaccharides, and nucleotides and carry
them to the rest of the body. The large
intestine participates in absorption by further
absorbing water and vitamins. Absorbed
materials cross the mucosa of the upper cavity
of the small intestine into the blood, and are
carried off to different parts of the body for
storage or usage. As seen in Figure 4,
monosaccharides are further broken down by
enzymes into glucose molecules; the glucose
molecules will then travel to the liver where
they will be stored or used by the body for
energy. Fats are dissolved into the watery
Elimination
Elimination is the final step in the digestion
process. This step helps excrete waste from the
body through a process called defecation.
Defecation is controlled voluntarily by the
brain so that the body eliminates indigestible
substances, preventing them from
accumulating inside the intestines. Solid
wastes exit the body in the form of feces. The
rectum allows for storage of the wastes until it
is ready to exit the body, either voluntarily or
involuntarily.
Conclusion
Through these five steps (ingestion, secretion,
mixing and movement, digestion, absorption,
and excretion) the body is able to process food
for energy and get rid of wastes. Ingestion is
the intake of food in the mouth, which ignites
the body to start secreting different fluids like
saliva, bile, acids, enzymes and mucus. These
fluids help to break down the food into smaller
molecules that can be easily digested. Mixing
and movement then occurs within the
pathways, followed by mechanical and
chemical digestion. The body then absorbs the
necessary nutrients it needs and uses is for
energy or stores it somewhere else in the body.
Finally, unnecessary wastes are excreted from
the body.
Image References
1.http://www.bing.com/images/search?q=ingestion+step+of+
digestion&view=detailv2&&id=2A2013AF799E542E94631
E996E8144261B53EC39&selectedIndex=3&ccid=IpF0YSp
E&simid=607997478140709452&thid=OIP.M229174612a4
4334543d8e563c07953eco0&ajaxhist=0
2.http://www.bing.com/images/search?q=perastalsis&view=d
etailv2&&id=5B9CA3F4F7A4772622BC98AD135AF5EAC
CA61982&selectedIndex=48&ccid=tnA5touN&simid=6079
98599127043670&thid=OIP.Mb67039b68b8d7afd0b223045
f2c68784o0&ajaxhist=0
3.https://www.google.com/search?q=ingestion&espv=2&biw
=1205&bih=757&site=webhp&source=lnms&tbm=isch&sa=
X&ved=0CAYQ_AUoAWoVChMItOLe8uriyAIVQjoCh1fRQf#tbm=isch&q=chemical+digestion&imgrc=xM5yYSHm4Px
2tM%3A
4.https://www.google.com/search?q=absorption+process&es
pv=2&biw=1089&bih=712&source=lnms&tbm=isch&sa=X
&ved=0CAYQ_AUoAWoVChMIp4rD1sDWyAIVCVUCh0vqgGc#tbm=isch&q=break+down+of+carbohydrates&i
mgrc=_n9jGZjYBqICHM%3A