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The Human Digestive System: From Food to Energy and Waste
Written By: Roxanne Light
Date: March 25, 2015
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Audience and Scope
This document offers a general, scientific overview of how the human digestive system works.
It is necessary for people to eat so that they obtain energy. Energy keeps the body functioning,
even during periods of rest and relaxation, and it allows people to actively participate in the
world. More specifically, it keeps peoples’ hearts beating, lungs breathing, muscles moving, and
brains working. As a person’s body processes food, not all of the food’s materials can be
converted to energy or used otherwise; therefore, digestion also produces waste. As all humans
have experienced, these wastes must eventually be excreted from the body.
Chronologically following the process’ events, this document begins analyzing digestion at the
moment enters a person’s mouth. It document explores the organs, enzymes, and hormones that
are involved in breaking down this food into both energy and waste. Moreover, it distinguishes
between the processing of carbohydrates, proteins, and fats, the energy-yielding nutrients found
within food.
Moreover, this document is intended for an audience of freshman or sophomore undergraduate
college students. Upon entering college, the audience most likely has little to no understanding
of digestion. However, this process is usually addressed in introductory biology courses, human
anatomy and physiology courses, and nutrition courses. This document is an informative
resource that offers these students a comprehensive overview of the human digestive system and
process. By reviewing this document, the audience may better review material they are learning
their undergraduate biology courses.
What is the Human Digestive System?
The human digestive system is the system of organs and glands within the human body that is
associated with the digestive tract and that aids in the process of digestion. Digestion is the
process of breaking down food, via mechanical actions and chemical reactions, into the
energy-yielding nutrients of carbohydrates, fats, and proteins and waste. The human digestive
system involves organs of two main groups: alimentary canal organs and accessory digestive
organs. Alimentary canal organs include the mouth, pharynx, esophagus, stomach, small
intestine, and large intestine. These organs make up what is known as the digestive or
gastrointestinal tract. Accessory digestive organs include the teeth, tongue, salivary glands,
liver, gallbladder, and pancreas. These organs aid in the body's digestion of food as it passes
through the digestive tract (Marieb and Hoehn 849-52).
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How does the Human Digestive System Work?
As people eat, the food they place into their mouths travels down their pharynx, upon
swallowing, and then, continues down their esophagus into their stomach and small intestine.
Throughout this journey, enzymes completely digest food's carbohydrates, fats, and proteins. In
the small intestine, the enzymes finish digesting the food's nutrients, which are then absorbed.
These absorbed nutrients travel via blood or lymph to other cells throughout the body that either
metabolize them for energy or use them as structural components. Remnants of food that are
neither digested nor absorbed continue through the large intestine to the rectum. Eventually, they
are eliminated as waste through the anus during defecation.
The Mouth
The mouth is the external opening through which people
provide their body food and the entrance to the oral
cavity. Therefore, the mouth is the beginning of the
human digestive system. Once food enters the mouth, it
succumbs to both mechanical actions and chemical
The teeth, including the incisors, canines, pre-molars,
and molars, work together to break down, cut, and grind
the food into smaller fragments (Marieb and Hoehn 85960). Additionally, the tongue helps to position food
between the teeth (Marieb and Hoehn 857; Whitney and
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Rolfes 71-2). At the same time, saliva produced by the
salivary glands enters the mouth (Freeman 847; Marieb
and Hoehn 858; “National Digestive Diseases”; Whitney
and Rolfes 72). Substances in saliva, including water, mucins, and digestive enzymes moisten
and begin to digest the food (Freeman 847; Marieb and Hoehn 859).
Salivary amylase, one digestive enzyme found in saliva, begins breaking down the food’s
starches, a type of carbohydrate, into smaller fragments. Lingual lipase, another digestive
enzyme found in saliva, begins breaking down the food’s fats into smaller fragments (Freeman
847; Khanacademymedicine; Marieb and Hoehn 859, 892-3; Whitney and Rolfes 72). Water and
mucins mix to form a substance (mucus) that coats the food, making it easier to swallow
(Freeman 847; Marieb and Hoehn 859).
Swallowing forces the moistened mass of broken-down food to begin its journey down the
digestive tract. The mass of broken-down food, which is now referred to as a bolus, continues
toward the pharynx (Marieb and Hoehn 857, 861-2; Whitney and Rolfes 72).
The Pharynx
The pharynx is a short tube that connects the oral cavity of the mouth with the esophagus and
trachea. The trachea is the passageway that enables oxygen to travel to the lungs and carbon
dioxide to travel from lungs and out of the body. It is covered by a flap of tissue made of
cartilage known as the epiglottis.
When a person swallows, the epiglottis closes over the
trachea, directing the bolus into the esophagus, not the
airway (Whitney and Rolfes 71-2). After swallowing,
the bolus continues to move through the digestive tract
via the phenomenon of peristalsis. Peristalsis involves
continuous muscular contractions of digestive tract that
push the bolus toward the rectum and anus (Marieb
and Hoehn 863-4; “National Digestive Diseases;
Whitney and Rolfes 73).
The Esophagus
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After passing through the pharynx, the bolus continues
down through the esophagus, a tube that connects the
mouth to the stomach (Freeman 847;
Khanacademymedicine; Marieb and Hoehn 862-4; “National Digestive
Diseases”; Whitney and Rolfes 72). To enter the esophagus, the bolus passes through the upper
esophageal sphincter, which opens upon swallowing (Whitney and Rolfes 72). Peristalsis
forces the bolus down the esophagus, through the lower esophageal sphincter, and into the
stomach (Marieb and Hoehn 862-4; “National Digestive Diseases”; Whitney and Rolfes 72).
The Stomach
The stomach is the most muscular organ of the digestive tract (Marieb and Hoehn 866). Upon
entering the stomach, the bolus is held in the stomach's upper portion until it is gradually
transferred into the lower portion (Whitney and Rolfes 72).
When the bolus enters the stomach, it signals the G cells of the stomach to secrete gastrin
(Marieb and Hoehn 870), a hormone that causes the parietal cells of the stomach to secrete
hydrochloric acid (HCL-). The parietal cells continue to secrete HCL- until the stomach
reaches a pH of approximately 1.5 (Freeman 849, 852; Marieb and Hoehn 866; Whitney and
Rolfes 74-75, 83).
Simultaneously, chief cells of the stomach produce some gastric lipases, which continue to break
down fats (Marieb and Hoehn 866), and pepsinogen, an inactive form of the enzyme pepsin.
HCL- activates pepsinogen, converting it to pepsin. Pepsin begins the breakdown of proteins
(Freeman 849; Marieb and Hoehn 866, 894). Additionally, the acidic pH, which was caused by
HCL-, inactivates salivary amylase. Because of the inactivation of salivary amylase, the
digestion of carbohydrates ceases in the stomach (Whitney and Rolfes 75).
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While all these actions occur, the muscular stomach contracts, mixing the bolus and gastric
juices. This mixing produces chyme (Khanacademymedicine; Marieb and Hoehn 864, 866;
“National Digestive Diseases; Whitney and Rolfes 72). The stomach's contractions push the
chyme downward toward the pyloric sphincter, which guards the entrance to the small intestine.
Periodically, the pyloric sphincter opens and closes, allowing small portions of chyme to enter
the small intestine (Khanacademymedicine; Whitney and Rolfes 72).
Note: Sphincters close immediately after the bolus or chyme passes through them to
prevent the substances from flowing backwards up the digestive tract (Whitney and
Rolfes 72-3). Heartburn results from the backflow of acidic chyme through the lower
esophageal sphincter. This condition may result from eating or drinking in excess
(Marieb and Hoehn 862-3; Whitney and Rolfes 92) or having a leaky or defective lower
esophageal sphincter (Khanacademymedicine; Whitney and Rolfes 92).
The Small Intestine
The small intestine is the 10 to 20 foot segment of
intestine that functions as the main site of digestion and
absorption (Marieb and Hoehn 875, 884; Whitney and
Rolfes 72). It is divided into three parts: the duodenum,
the jejunum, and the ileum (Khanacademymedicine;
Marieb and Hoehn 875; Whitney and Rolfes 72).
Chyme moves through the small intestine by the
phenomenon of segmentation, which involves muscular
contractions similar to peristalsis except slower and steadier.
The slower and steadier contractions allows more time for
the substance to be fully digested and absorbed (Marieb
and Hoehn 886). With help from the secretions of the
pancreas, liver, and gallbladder, all three energy-yielding
nutrients, proteins, carbohydrates, and fats, continue to be
chemically digested within the small intestine (Marieb and
Hoehn 884-5; “National Digestive Diseases”; Whitney
and Rolfes 75).
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The Completion of Digestion
As chyme enters the duodenum, it causes duodenum cells to release the hormone secretin into
the bloodstream. This hormone travels to the pancreas and signals for it to send its bicarbonaterich (HCO3-) juices to the duodenum via the pancreatic duct. The pancreas’ bicarbonate juices
neutralize the acidic chime, thereby enabling intestinal and pancreatic enzymes to function
(Freeman 852; Marieb and Hoehn 882-4; Whitney and Rolfes 84).
Moreover, if fat is present in the chyme, it causes the duodenum cells to release the hormone
cholestokinin (CCK) into the bloodstream. This hormone also travels to the pancreas and
gallbladder. CCK orders the pancreas to secrete its digestive juices into the duodenum. This
digestive juice includes inactivated enzymes that become activated upon entering the duodenum.
Additionally, CCK instructs the gallbladder to contract and send bile into the duodenum via the
common bile duct (Freeman 852; Marieb and Hoehn 882-4; Whitney and Rolfes 84).
Bile, which is made in the liver and stored in the gallbladder, is used to emulsify the fats
(Freeman 853; Khanacademymedicine; Marieb and Hoehn 881; “National Digestive Disesases”;
Whitney and Rolfes 75, 138-9). Emulsification breaks down the large fat globules into smaller
globules, increasing their surface area (Freeman 853-4; Marieb and Hoehn 881). The higher
surface area of these globules enables pancreatic lipases and intestinal lipases to attach to and
continue digesting the fats (Whitney and Rolfes 76, 138-9).
Note: Upon removal of the gallbladder, the liver will send bile directly to the duodenum
of the small intestine (Whitney and Rolfes 84).
As the lipases digest the fats, pancreatic amylases and intestinal amylases continue digesting
the carbohydrates (Whitney and Rolfes (75-6, 101). Additionally, pancreatic proteases and
intestinal proteases, including trypsin, chymotrypsin, elastase, and carboxypeptidase, continue
digesting the proteins (Freeman 852; Marieb and Hoehn 883-4; Whitney and Rolfes 76, 171).
Upon the completion of digestion, the energy-yielding nutrients are broken down into their
monomers. Amino acids, obtained from protein (Whitney and Rolfes 171), monosaccharides,
obtained from carbohydrates (Whitney and Rolfes 101), and monoglycerides, fatty acids and
glycerol molecules, obtained from fats (Whitney and Rolfes 138), are absorbed.
The Absorption of Nutrients
The small intestine structure is specialized to enhance
absorption. Its tissue is highly-folded into villi, which are
finger-like projections that contain hundreds of cells coated
with microvilli, or hairs. Villi and microvilli both function
to increase the surface area of the small intestine, thereby
increasing nutrient absorption efficiency.
Furthermore, villi also connect to both lymph and blood
vessels, enabling the energy-yielding nutrients to travel to
the body cells that need them (Freeman 851;
Khanacademymedicine; Marieb and Hoehn 876-7; Whitney
and Rolfes 77-8). Amino acids, monosaccharides, shortchain fatty acids, glyercol are carried by the hepatic portal
vein to the liver, and then, to body cells (Whitney and Rolfes
76, 103, 140, 171). Conversely, monoglycerides and longchain fatty acids are carried by lymph to the heart, then, to
body cells, and then, to the liver (Whitney and Rolfes 140).
The body cells where the nutrients end up either metabolize
the nutrients via cellular respiration to produce adenosine
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triphosphate (ATP), the cells’ main form of energy, or use them as structural components
(Freeman 153).
Some parts of food are not able to be absorbed, including several vitamins, minerals, and fibers
(Whitney and Rolfes 75-6). These substances travel the entire length of the small intestine, pass
through another sphincter, known as the ileocecal valve, and enter the large intestine (Marieb
and Hoehn 875; Whitney and Rolfes 72).
The Large Intestine
The large intestine, or the colon, is the second
and final portion of human intestine. It is divided
into three parts: the ascending colon, the
transverse colon, and the sigmoid colon
(Khanacademymedicine; Marieb and Hoehn
As the unabsorbed substances travel through the
large intestine, some of the body’s water is
reabsorbed, thus, compacting the unabsorbed
substances into a semi-solid waste product
(Freeman 854-5; Khanacademymedicine; Marieb
and Hoehn 887; “National Digestive Diseases;
Whitney and Rolfes 72-3). This waste continues
to the rectum, where it is held until defecation
(Freeman 854-5; Marieb and Hoehn 887;
Whitney and Rolfes 72-3). Upon defecation, the
waste is excreted through the anus after passing
through its two sphincters (Marieb and Hoehn 887;
Whitney and Rolfes 72).
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Note: The strong rectal muscles and two anal sphincters are important in preventing
uncontrolled, continuous elimination of waste from the body. The production and
elimination of waste is important as it helps the muscles of the digestive tract to maintain
a sufficient strength to perform peristalsis (Whitney and Rolfes 75).
Image courtesy of: Understanding Nutrition (13 ed.) by Ellie Whitney and Sharon Rady Rolfes
Summary and Conclusion
Digestion is a life-sustaining process that facilitates the absorption of nutrients, the production of
energy, and the elimination of waste. The human digestive system enables the digestion of food
through its several organs and glands and the help of various hormones and enzymes. Together,
the mouth, teeth, tongue, salivary glands, pharynx, esophagus, stomach, pancreas, liver,
gallbladder, small intestine, large intestine, rectum, and anus work with gastrin, secretin, CCK
and amylases, lipases, and proteases to complete this vital process. Because of digestion, a
person can obtain the energy their body needs to keep their heart beating, muscles moving, and
brain functioning and can actively participate in the world around them.
Works Cited
Freeman, Scott. Biological Science. 4th ed. New York: Benjamin Cummings-Pearson, 2011.
845-855. Print.
Khanacademymedicine. “Digesting Food.” Online video clip. YouTube. YouTube, 17 May
2013. Web. 1 March 2015.
Marieb, Elaine N. and Katja Hoehn. Human Anatomy & Physiology. 9th ed. New York:
Pearson, 2013. 849-905. Print.
National Digestive Diseases Information Clearinghouse. “Your Digestive System and How It
Works.” U.S. Department of Health and Human Services. The National Institute of
Diabetes and Digestive and Kidney Diseases, 18 Sept. 2013. Web. 1 March 2015.
Whitney, Ellie and Sharon Rady Rolfes. Understanding Nutrition. 13th ed. Belmont, CA:
Wadsworth, Cengage Learning, 2013. H-24, H-62. Print.