May 10, 2023 • 32 mins
This textbook is designed specifically for Kansas State's Biology 198 Class. The course is taught using the studio approach and based on active learning. The studio manual contains all of the learning objectives for each class period and is the record of all student activities. Hence, this textbook is more of a reference tool while the studio manual is the learning tool.
Authors: Robert Bear, David Rintoul, Bruce Snyder, Martha Smith-Caldas, Christopher Herren, and Eva Horne
Kansas State University Libraries
New Prairie Press
Bear, Robert; Rintoul, David; Snyder, Bruce; Smith-Caldas, Martha; Herren, Christopher; and Horne, Eva, "Principles of Biology" (2016). Open Access Textbooks. 1. https://newprairiepress.org/textbooks/1
The textbook was originally published and is also available to download at http://cnx.org/contents/db89c8f8-a27c-4685-ad2a-19d11a2a7e2e@24.1.It is licensed under a Creative Commons Attribution License 4.0 license.
Authors: Robert Bear, David Rintoul, Bruce Snyder, Martha Smith-Caldas, Christopher Herren, and Eva Horne
Kansas State University Libraries
New Prairie Press
Bear, Robert; Rintoul, David; Snyder, Bruce; Smith-Caldas, Martha; Herren, Christopher; and Horne, Eva, "Principles of Biology" (2016). Open Access Textbooks. 1. https://newprairiepress.org/textbooks/1
The textbook was originally published and is also available to download at http://cnx.org/contents/db89c8f8-a27c-4685-ad2a-19d11a2a7e2e@24.1.It is licensed under a Creative Commons Attribution License 4.0 license.
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(00:00):
Welcome to Principles of Biology. Thisbook was written by the Open Alternative Textbook
Initiative at Kansas State University and isbeing released as a podcast and distributed under
the terms of the Creative Commons AttributionLicense. Today's episode is Chapter twenty six
Digestive Systems. All hyperlinks, imagesand sources can be found at the link
(00:23):
to the book. In the description, if you were to open up a
baby's head, and I am notfor a moment suggesting that you should,
you would find nothing but an enormousdrual gland. Dave Barry. The salivary
glands aka drual glands of vertebrates arejust one part of the elaborate and integrated
organ system we call the digestive system. The digestive system allows organisms such as
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us to obtain their nutrition from theconsumption of other organisms. Depending on their
diet, animals can be classified intothe following categories plant eaters, herbivores,
meat eaters, carnivores, and thosethat eat both plants and as omnivores.
The nutrients and macromolecules present in foodare not immediately accessible to the cells.
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There are a number of processes thatmodify food within the animal body in order
to make the nutrients and organic moleculesaccessible for cellular function. As animals evolved
in complexity of form and function,their digestive systems, including the jewel glands,
have also evolved to accommodate their variousdietary needs. Herbivores, omnivores,
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and carnivores. Herbivores are animals whoseprimary food source is plant based. Examples
of herbivores as shown in figure includevertebrates like deer, koalas, and some
bird species, as well as invertebratessuch as crickets and caterpillars. These animals
have evolved digestive systems capable of handlinglarge amounts of plant material. Herbivores can
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be further classified into frugivores, fruiteaters, granivores, seed eaters, nectivores,
nectar feeders, and follivores leaf eaters. Left photo shows a male American
bison. Right photo shows a caterpillareating a leaf. Herbivores like this a
American bison and b pipevine swallowtail caterpillareat primarily plant material. Photocredit David a
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raintol. Carnivores are animals that eatother animals. The word carnivore is derived
from Latin and literally means meat eater. Wild Cats such as lions shown in
Figure eight and tigers are examples ofvertebrate carnivores, as are snakes and sharks,
while in vertebrate carnivores include sea stars, spiders, and ladybugs shown in
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Figure beat. Obligate carnivores are thosethat rely entirely on animal flesh to obtain
their nutrients. Examples of obligate carnivoresare members of the cat family, such
as lions and cheetahs. Facultate ofcarnivores are those that also eat non animal
food in addition to animal food.Note that there is no clear line that
differentiates facultate of carnivores from omnivores.Dots would be considered facultative carnivores. Left
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photo shows a male lion on thegrass. Right photo shows an ambush bug
on a golden robbed flower. Carnivores, like the a lion, eat primarily
meat. The b ambush bug isalso a carnivore that consumes small insects such
as flies. Credit a modification ofwork by Kevin Pluck. Credit be David
(03:22):
A. Ringtool. Omnivores are animalsthat eat both plant and animal derived food.
In Latin, omnivore means to eateverything humans. Bears shown in Figure
A and chickens are an example ofvertebrate omnivores. In vertebrate omnivores include cockroaches
and crayfish shown in Figure B.Top photo shows a bear. Bottom photo
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shows a crayfish. Omnivores like thea bear, end B. Crayfish eat
both plant and animal based food.Credit A modification of work by Dave Mankie.
Credit be modification of work by JohnSullivan. Types of digest systems,
there re two types of digestive systems, incomplete and complete. An incomplete digestive
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system has only one opening to thedigestive tract Figure A. Ingested food and
excreted waste products passed through the sameopening. Incomplete digestive systems are found in
nigerians and flatworms. The other typeis naturally called a complete digestive system,
and this type has two openings figure. Food is ingested through one opening and
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waste products are excreted through a separateopening. This type of digestive system is
found in all other phyla, includingvertebrates. The advantage of a complete digestive
system is that it allows animals tofeed continuously without waiting for the residues of
the previous meal to be released fromthe digestive tract. It also allows specialization
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of regions of the digestive tract,which means that different food types can be
digested more efficiently. A particular regionalong the digestive tract with different physical and
chemical conditions can be optimized for onetype of nutrient, and other regions can
be optimized for efficient metabolism of othertypes of nutrients. Invertebrate digestive systems invertebrates
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can have incomplete or complete digestive systems. As noted above, an incomplete digestive
system has a gastrovascular cavity and onlyone opening for digestion. Plate helmets,
flatworms, tinapra comb jellies and Nigeria, coral jelly fish, and sea anemones
of this type of digestive system.Gastrovascular cavities, as shown in Figure A,
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are typically a blind tube or cavitywith only one opening the mouth,
which also serves as an anus.Ingested material enters the mouth and passes through
a hollow tubular cavity. Cells withinthe cavity secrete digestive enzymes that break down
the food. The food particles areengulfed by the cells lining the gastrovascular cavity.
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A complete digestive system with an alimentarycanal. Figure bee is a more
advanced system. It consists of onetube with a mouth at one end and
an anus at the other. Earthwormsare an example of an animal with a
complete digestive system. Once the foodis ingested through the mouth, it passes
through the esophagus and is stored ina working called the crop. Then it
passes into the gizzard, where itis churned and digested. From the gizzard,
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the food passes through the intestine.The nutrients are absorbed and the waste
is eliminated as feces called castings.Through the anus partis shows a hydra,
which has a vase shaped body withtentacles around the rim. The hydrous mouth
is located between the tentacles at thetop of the vase. Next to the
hydra is a jellyfish medusa, whichis bell shaped with tentacles hanging down from
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the edge of the bell. Themouth in the lower middle part of the
body opens into the gastrovascular cavity.Part bee shows a nematode which has a
long tube like body that is wideat one end and tapers down to a
tail at the other. The mouthis in the center of the wide end.
It opens into an esophagus than apharynx. The faring sempties into a
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long intestine which ends at the anusa short distance before the tail. A
A gastrovascular cavity has a single openingthrough which food is ingested and waste is
excreted, as shown in this hydraand in this jellyfish Medusa B. An
alimentary canal has two openings, amouth for ingesting food and an anus for
eliminating waste, as shown in thisnematode. Vertebrate digestive systems. Vertebrates have
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evolved more complex digestive systems to adaptto their dietary needs. Some animals have
a single stomach, while others havemulti chambered stomachs. Birds have developed a
digestive system adapted to eating unmasticated foodmonogastric single chambered stomach. As the word
monogastric suggests, this type of digestivesystem consists of one mono stomach chamber.
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Gastric humans and many animals have amonogastric digestive system, as illustrated in Figure
eight B. The process of digestionbegins with the mouth and the intake of
food. The teeth play an importantrole in masticating, chewing, or physically
breaking down food into smaller particles.The enzymes present in saliva also begin to
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chemically break down food. The esophagusis a long tube that connects the mouth
to the stomach using peristalsis or wavelike smooth muscle contractions. The muscles of
the esophagus push the food towards thestomach in order to speed up the actions
of enzymes in the stomach. Thestomach is an extremely acidic environment, with
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a pH between one point five andtwo point five. The gastric juices,
which include enzymes in the stomach,act on the food particles and continue the
process of digestion. Further breakdown offood takes place in the small intestine,
where enzymes produced by the liver,the small testine, and the pancreas continue
the process of digestion. The nutrientsare absorbed into the blood stream across the
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epithelial cells lining the walls of thesmall intestines. The waste material travels onto
the large intestine, where water isabsorbed and the drier waste material is compacted
into feces. It is stored untilit is excreted through the rectum. The
basic components of the human and rabbitdigestive system are the same. Each begins
at the mouth. Food is swallowedthrough the esophagus and into the kidney shaped
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stomach. The liver is located ontop of the stomach and the pancreas is
underneath. Food passes from the stomachto the long winding small intestine. From
there it enters the wide large intestinebefore passing out the anus. At the
junction of the small and large intestineis a pouch called the secum. The
small and large intestines are much longerin rabbits than in humans, and the
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secum is much longer as well.A humans and herbert wars such as the
bee rabbit have a monogastric digestive system. However, in a rabbit, the
small intestine and siccum are enlarged toallow more time to digest plant material.
The enlarged organ provides more surface areafor absorption of nutrients. Rabbits digest their
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food twice. The first time foodpasses through the digestive system, it collects
in the siccum and then it passesas soft feces called secotrophies. The rabbit
reingests these seacotrophies to further digest them. Avian birds face special challenges when it
comes to obtaining nutrition from food dueto the constraints impost by having to be
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lightweight in order to fly. Theyhave lost some adaptations found in their dinosaur
ancestors. For example, they donot have teeth, and so their digestive
system shown in figure must be ableto process unmasticated food. Birds have evolved
a variety of beak types that reflectthe vast variety in their diet, ranging
from seeds and insects to fruits andnuts. Because most birds fly, their
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metabolic rates are high in order toefficiently process food and keep their body weight
low. The stomach of birds hastwo chambers, the provinriculus, where gast
or juices are produced to digest thefood before it enters the stomach, and
the gizzard, where the food isstored soapd and mechanically ground. The undigested
material forms food palates that are sometimesregurgitated. Most of the chemical digestion and
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absorption happens in the intestine, andthe waste is excreted through the cloaca.
Illustration shows an avian digestive system.Food is swallowed through the esophagus into the
crop, which is shaped like anupside down heart. From the bottom of
the crop, food enters a tubularprovinriculus, which empties into a spherical gizzard.
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From the gizzard, food enters thesmall intestine, then the large intestine.
Waste exits the body through the cloaca. The liver and pancreas are located
between the crop and gizzard. Ratherthan a single secum, birds have two
quica at the junction of the smalland large intestine. The avian esophagus has
a pouch called a crop, whichstores food. Food passes from the crop
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to the first of two stomachs,called the provinriculus, which contains digestive juices
that break down food. From theprovinriculus, the food enters the second stomach,
called the gizzard, which grinds food. Some birds swallow stones or grit
which are stored in the gizzard toaid the grinding process. Birds do not
have separate openings to excrete urine andfeces. Instead, uric acid from the
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kidneys is secreted into the large intestineand combined with waste from the digestive process.
This waste is excreted through an openingcalled the cloaca. Evolution connection avian
adaptations, birds have a highly efficient, simplified digestive system. Recent fossil evidence
has shown that the evolutionary divergience ofbirds from other land animals was characterized by
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streamlining and simplifying the digestive system.Unlike many other animals, birds do not
have teeth to chew their food.In place of lips, they have sharp,
pointy beaks instead of teeth. Theyhave a proventricolis or gizzard for griding
up food. The emergence of thesechanges seems to coincide with the inclusion of
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seeds and the bird diet. Seedeating birds have beaks that are shaped for
grabbing seeds, and the two compartmentstomach allows for delegation of tasks. Since
birds need to remain light in orderto fly, passage time in the gut
is very short, which means theydigest their food very quickly and need to
eat often. Contrast this with theruminants, where the digestion of plant matter
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takes a very long time and aheavy water filled digestive tract. What would
you predict would be some characteristics ofbirds that eat like a cow ingesting primarily
leaves and shoots ruminants. Ruminants areherbivores like cows, sheep, bison,
et cetera, whose entire diet consistsof eating large amounts of leaves and shoots.
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They have evolved digestive systems that helpthem digest the vast amounts of cellulose
and this diet. An interesting featureof some ruminants mouths is that they do
not have upper incisor teeth. Theyuse their lower teeth, tongue, and
lips to tear and chew their food. From the mouth, the food travels
to the esophagus and onto the stomachto help digest the large amount of plant
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material. The stomach of the ruminantsis a multi chambered organ as illustrated in
figure. The four compartments of thestomach are called the rumin reticulum, omasum,
and abomasum. These chambers contain manymicrobes that break down cellulose and ferment
in ingested food. The abomasum isthe true stomach and is the equivalent of
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the monogastrid stomach chamber where gastric juicesare secreted. The four compartment gastric chamber
provides a larger space and the microbialsupport necessary to digest plant material. In
ruminants, it is essentially a bacterialfermentation vessel. The fermentation process requires lots
of watery fluid and produces large amountsof gas in the stomach chamber, which
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must be eliminated. As in otheranimals, the small intestine plays an important
role in nutrient absorption and the largeintestine helps in the elimination of waste.
Illustration shows the digestive system of agoat. Food passes from the mouth through
the esophagus and into the rumen.It circulates clockwise through the rumin, then
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moves forward and down into the smallpouch shaped reticulum. From the reticulum,
the food, which is now cutis regurgitated. The animal choose the cud
and then swallows it into the quilledomasum, which sits between the reticulum and the
rumin. After circulating through the omasum, the food enters the small intestine,
then the large intestine waste is excretedthrough the anus. Ruminant animals, which
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as goats and cows, have afour chambered stomach. The first two chambers,
the rumin and the reticulum, containprocaryotes and protusts that are able to
digest cellulose fiber. The ruminant regurgitatescut from the reticulum, choose it,
and swallows it into a third chamber, the omasum, which removes water.
The cut then passes on to thefourth chamber, the abomasum, where it
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is digested by enzymes produced by theruminant parts of the digestive system. The
vertebrate digestive system is designed to facilitatethe transformation of food matter into the nutrient
components that sustain organisms. Oral cavity. The oral cavity or mouth, is
the point of entry of food intothe digestive system, illustrated in figure.
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The food consumed is broken into smallerparticles by mastication, the chewing action of
the teeth. All mammals have teethand can chew their food. The extensive
chemical process of digestion begins in themouth as food is being chewed, saliva
produced by the salivary glands mixes withthe food. Saliva is a watery substance
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secreted by salivary glands into the mouthsof many animals. Saliva contains mucus that
moistens food and buffers the page ofthe food. Saliva also contains immunoglobulins and
lysisymes, which have antibacterial action toreduce tooth decay by inhibiting growth of some
bacteria. Saliva also contains an enzymecalled salivary amalase that begins the process of
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converting starches and the food into adisachride called maltose. The chewing and wetting
action provided by the teeth and salivaprepare the food into a mass called the
bolus for swallowing. The tongue helpsin swallowing, moving the bolus from the
mouth into the pharynx. The pharynxopens to two passageways, the trachea,
which leads to the lungs, andthe esophagus, which leads to the stomach.
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The trachea has an opening called theglottis, which is covered by a
cartilaginous fly called the epiglottis. Whenswallowing, the epiglottis closes the glottis and
food passes into the esophagus and notthe trachea. This arrangement allows food to
be kept out of the trachea andlungs. Illustration A shows the parts of
the human oral cavity. The tonguerests in the lower part of the mouth.
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The flop that hangs from the backof the mouth is the uvula.
The airway behind the uvula, calledthe pharynx, extends up to the nostrils
and down to the esophagus, whichbegins in a nack. Illustration BEACH shows
the two salivary glands which are locatedbeneath the tongue, the sublingual and the
submandibular. A third salivary gland,the parotid, is located behind the pharynx.
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Digestion of food begins in the aoral cavity. Food is masticated by
teeth and moistened by saliva secreted fromthe b salivary glands. Enzymes in the
saliva begin to digest starches and fatswith the help of the tongue. The
resulting bollus is moved into the esophagusby swallowing credit modification of work by the
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National Cancer Institute. Esophagus. Theesophagus is a tubular organ that connects the
mouth to the stomach. The chewedand softened food passes through the esophagus after
being swallowed. The smooth muscles ofthe esophagus undergo a series of wave like
movements called peristalsis that push the foodtowards the stomach. As illustrated in figure.
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The peristalsic wave is unidirectional. Itmoves food from the mouth to the
stomach and reverse movement is not possible. The peristaltic movement of the esophagus is
an involuntary reflex. It takes placein response to the act of swallowing.
Photo shows food moving down the esophagus, which is a muscular tube. Muscles
constrict behind the food. The constrictionmoves down, pushing the food ahead of
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it from the mouth to the stomach. The esophagus transfers food from the mouth
to the stomach through peristaltic movements.A ring like muscle called the sphincter forms
valves in the digestive system. Thegastroesophageal sphincter is located at the stomach end
of the esophagus. In response toswallowing and the pressure exerted by the bolus
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of food. This sphincter opens andthe bolus enters the stomach. When there
is no swallowing action, this sphincteris shut and prevents the contents of the
stomach from traveling up the esophagus.Many animals have a trucephincter. However,
in humans there is no trucephincter,but the esophagus remains closed when there is
no swallowing action. Acid reflux orheartburn occurs when the acidic digestive juices escape
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into the esophagus stomach. A largepart of digestion occurs in the stomach shown
in figure. The stomach is asaclike organ that secretes gastric digestive juices.
The pH in the stomach is betweenone point five and two point five.
This highly acidic environment is required forthe chemical breakdown of food and the extraction
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of nutrients when empty. The stomachis a rather small organ. However,
it can expand to up to twentytimes its resting size when filled with food.
This characteristic is particularly useful for animalsthat need to eat when food is
available. Illustration shows the human lowerdigestive system, which begins with the stomach,
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a sack that lies above the largeintestine. The stomach empties into the
small intestine, which is a long, highly folded tube. The beginning of
the small intestine is called the duodenum, the long middle part is called the
jugunum, and the end is calledthe ilium. The ilium empties into the
large intestine on the right side ofthe body. Beneath the junction of the
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small and large intestine is a smallpouch called the secum. The appendix is
at the bottom end of the secum. The large intestine travels up the left
side of the body, across thetop of the small intestine, then down
the right side of the body.These parts of the large intestine are called
the ascending colon, the transverse colon, and the descending colon, respectively.
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The large intestine empties into the rectum, which is connected to the anus.
The pancreas is sandwiched between the stomachand large intestine. The liver is a
triangular organ that sits above and slightlyto the right of the stomach. The
gall bladder is a small bulb betweenthe liver and stomach. The human stomach
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has an extremely acidic environment where mostof the protein gets digested credit modification of
work by Marianna Ruis Vareal. Thestomach is also the major site for protein
digestion in animals other than ruminants.Protein digestion is mediated by an enzyme called
pepsin in the stomach chamber. Pepsinsecreted by cells in the stomach lining,
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breaks peptide bonds and cleaves proteins intosmaller polypeptides. Another cell type, parietal
cells, secrete hydrogen and chloride ions, which combine in the lumen to form
hydrochloric acid, the primary acidic componentof the stomach juices. The highly acidic
environment also kills many microorganisms in thefood, and, combined with the action
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of the enzyme pepsin, results inthe hydrolysis of protein and the food.
Chemical digestion is facilitated by the churningaction of the stomach. Contraction and relaxation
of smooth muscles completely mixes the stomachcontents about every twenty minutes. The partially
digested food and gastric juice mixture iscalled chyme. Chyme passes from the stomach
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to the small intestine. Further,protein digestion takes place on the small intestine.
Gastric emptying occurs within two to sixhours after a meal. Only a
small amount of chyme is released intothe small intestine at a time. The
movement of chyme from the stomach intothe small intestine is regulated by the pyloric
sphincter. When jesting protein and somefats, the stomach lining must be protected
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from getting digested by pepsin. Thereare two points to consider when describing how
the stomach lining is protected. Firstly, the enzyme pepsin is synthesized in an
inactive form pepsinogen, which is activatedby the acid and other proteases in the
stomach contents. This protects the cellswhich secrete pepsin because pepsinogen does not have
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the full enzyme functionality of pepsin.Second, the stomach has a thick mucous
lining that protects the underlying tissue fromthe action of the digestive juices. When
this mucous lining is ruptured, ulcerscan form in the stomach. Ulcers are
open wounds in or on an organcaused by bacteria Helicobacter pylori. When the
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mucous lining is ruptured and fails toreform. Small intestine chime moves from the
stomach to the small intestine. Thesmall intestine is the organ where the digestion
of protein, fats, and carbohydratesis completed. The small intestine is a
long, tube like organ with ahighly folded surface containing fingerlike projections called the
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villi. The apical surface of eachvillis has many microscopic projections called microvilli.
These structures illustrated in figure are linedwith epithelial cells on the luminal side and
allow for the nutrients to be absorbedfrom the digested food and absorbed into the
blood stream on the other side.The villi and microvilli, with their many
folds, increase the surface area ofthe intestine and increase absorption efficiency of the
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nutrients. Most absorbed nutrients, sugars, amino acids, and nucleotides, and
the blood are carried into the hepaticportal vein, which leads to the liver.
There, the liver regulates the distributionof nutrients to the rest of the
body and removes toxic substances, includingdrugs, alcohol, and some pathogens.
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Fatty acids resulting from digestion of fatand the small intestine do not enter the
blood stream directly, but are takenup by the lymphatic system and the small
intestine. These are eventually delivered tothe blood via the thoracic duct to be
metabolized by the liver, as isthe case for the other nutrients. Illustration
shows a cross section of the smallintestine, the lumin or inside of which
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has many fingerlike projections called vili.Muscle layers wrap around the outside of the
intestine, and blood vessels interact withthe muscle layer. A blow up shows
that capillaries and lymphatic vessels travel upinside the villi. The surface of each
villis is covered with hairline microvilli.Vili are folds on the small intestine,
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lining that increase the surface area tofacilitate the absorption of nutrients. The human
small intestine is over six m longand is divided into three parts, the
duodenum, the jugunum, and theilium. The C shaped fixed part of
the small intestine is called the duodenumand is shown in figure. The duodenum
is separated from the stomach by thepyloric sphincter, which opens to allow to
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move from the stomach to the duodenum. In the duodenum, chyme is mixed
with pancreatic juices in an alkaline solutionrich and bicarbonate that neutralizes the acidity of
chyme and acts as a buffer.Pancreatic juices also contains several digestive enzymes.
Digestive juices from the pancreas, liver, and gallbladder, as well as from
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gland cells of the intestinal wall itself, enter the duodenum. Bile is produced
in the liver and stored and concentratedin the gallbladder. Bio contains bile salts,
which emulsify lipids, while the pancreasproduces enzymes that catabolize starches, disaccharides,
proteins, and fats. These digestivejuices break down the food particles in
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the chym into glucose, triglycerides,and amino acids. Some chemical digestion of
food takes place in the duodenum.Absorption of fatty acids into the lymphatic system
also takes place in the duodenum.The second part of the small intestine is
called the jujuno shown in figure here. Hydrolysis of nutrients is continued while most
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of the carbohydrates and amino acids areabsorbed through the intestinal lining. The bulk
of chemical digestion and nutrient absorption occursin the jagunum. The ilium, also
illustrated in figure, is the lastpart of the small intestine, and here
the bile, salts and vitamins areabsorbed into bloodstream. The undigested food is
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sent to the colon from the iliumvia peristaltic movements of the muscle. The
ilium ends and the large intestine beginsat the iliosecal valve. The vermiform worm
like appendix is located at the iliosecalvalve. The appendix of humans secretes no
enzymes and has an insignificant role inimmunity. Large intestine. The large intestine
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illustrated in figure reabsorbs the water fromthe undigested food material and processes the waste
material. The human large intestine ismuch smaller in length compared to the small
intestine, but larger in diameter.It has three parts, the secum,
the colon, and the rectum.The secum joins the ilium to the colon
and is the receiving pouch for thewaste matter. The secum and colon are
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home to many trillions of bacteria orintestinal flora that aid in the digestive processes.
The colon can be divided into fourregions, the ascending colon, the
transverse colon, the descending colon,and the sigmoid colon. The main functions
of the colon are to extract thewater and mineral salts from undigested food and
to store waste material. Carnivorous mammalshave a shorter large intestine compared to herbivorous
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mammals due to their diet. Illustrationshows the structure of the large intestine,
which begins with the ascending colon.Below the ascending colon is the secum.
The vermiform appendix is a small projectionat the bottom of the secum. The
ascending colon travels up the right sideof the body, then turns into the
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transverse colon on the left side ofthe body. The large intestine turns again
into the descending colon. At thebottom, the descending colon curves up.
This part of the intestine is calledthe sigmoid colon. The sigmoid colon empties
into the rectum. The rectum travelsstraight down to the anus. The large
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intestine reabsorbs water from undigested food andstores waste material until it is eliminated rectum
in anus. The rectum is theterminal end of the large intestine, as
shown in figure. The primary roleof the rectum is to store the feces
until defecation. The feces are propelledusing peristaltic movements during elimination. The anus
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is an opening at the far endof the digestive tract and is the exit
point for the waste material. Twosphincters between the rectum and anous control elimination.
The inner sphincter is involuntary and theouter sphincter is voluntary. Accessory or
the organs discussed above are the organsof the digestive tract through which food passes.
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Accessory organs are organs that add secretionsenzymes that catabolize food into nutrients.
Accessory organs include salivary glands, theliver, the pancreas, and the gall
bladder. The liver, pancreas,and ball bladder are regulated by hormones in
response to the food consumed. Theliver is the largest internal organ in humans
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and It plays a very important rolein digestion of fats and detoxifying blood.
The liver produces bile, a digestivejuice that is required for the breakdown of
fatty components of the food and theduodenum. The liver also processes the vitamins
and fats and synthesizes many plasma proteins. The pancreas is another important gland that
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secretes digestive juices. The kind producedfrom the stomach is highly acidic in nature.
The pancreatic juices contain high levels ofbicarbonate, an alkali that neutralizes the
ass chime. Additionally, the pancreaticjuices contain a large variety of enzymes that
are required for the digestion of proteinand carbohydrates. The gallbladder is a small
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organ that aids the liver by storingbile and concentrating bile salts. When chime
containing fatty acids enters the duodenum,the bile is secreted from the gallbladder into
the duodenum. This podcast will bereleased episodically and follow the sections of the
textbook in the description For a deeperunderstanding, We encourage you review the text
(32:32):
version of this work voice by voicemakerDotaan. This was produced by Brandon Casturo
as a creative Common Sense production.
Welcome to Principles of Biology. Thisbook was written by the Open Alternative Textbook
Initiative at Kansas State University and isbeing released as a podcast and distributed under
the terms of the Creative Commons AttributionLicense. Today's episode is Chapter twenty six
Digestive Systems. All hyperlinks, imagesand sources can be found at the link
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to the book. In the description, if you were to open up a
baby's head, and I am notfor a moment suggesting that you should,
you would find nothing but an enormousdrual gland. Dave Barry. The salivary
glands aka drual glands of vertebrates arejust one part of the elaborate and integrated
organ system we call the digestive system. The digestive system allows organisms such as
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us to obtain their nutrition from theconsumption of other organisms. Depending on their
diet, animals can be classified intothe following categories plant eaters, herbivores,
meat eaters, carnivores, and thosethat eat both plants and as omnivores.
The nutrients and macromolecules present in foodare not immediately accessible to the cells.
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There are a number of processes thatmodify food within the animal body in order
to make the nutrients and organic moleculesaccessible for cellular function. As animals evolved
in complexity of form and function,their digestive systems, including the jewel glands,
have also evolved to accommodate their variousdietary needs. Herbivores, omnivores,
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and carnivores. Herbivores are animals whoseprimary food source is plant based. Examples
of herbivores as shown in figure includevertebrates like deer, koalas, and some
bird species, as well as invertebratessuch as crickets and caterpillars. These animals
have evolved digestive systems capable of handlinglarge amounts of plant material. Herbivores can
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be further classified into frugivores, fruiteaters, granivores, seed eaters, nectivores,
nectar feeders, and follivores leaf eaters. Left photo shows a male American
bison. Right photo shows a caterpillareating a leaf. Herbivores like this a
American bison and b pipevine swallowtail caterpillareat primarily plant material. Photocredit David a
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raintol. Carnivores are animals that eatother animals. The word carnivore is derived
from Latin and literally means meat eater. Wild Cats such as lions shown in
Figure eight and tigers are examples ofvertebrate carnivores, as are snakes and sharks,
while in vertebrate carnivores include sea stars, spiders, and ladybugs shown in
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Figure beat. Obligate carnivores are thosethat rely entirely on animal flesh to obtain
their nutrients. Examples of obligate carnivoresare members of the cat family, such
as lions and cheetahs. Facultate ofcarnivores are those that also eat non animal
food in addition to animal food.Note that there is no clear line that
differentiates facultate of carnivores from omnivores.Dots would be considered facultative carnivores. Left
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photo shows a male lion on thegrass. Right photo shows an ambush bug
on a golden robbed flower. Carnivores, like the a lion, eat primarily
meat. The b ambush bug isalso a carnivore that consumes small insects such
as flies. Credit a modification ofwork by Kevin Pluck. Credit be David
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A. Ringtool. Omnivores are animalsthat eat both plant and animal derived food.
In Latin, omnivore means to eateverything humans. Bears shown in Figure
A and chickens are an example ofvertebrate omnivores. In vertebrate omnivores include cockroaches
and crayfish shown in Figure B.Top photo shows a bear. Bottom photo
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shows a crayfish. Omnivores like thea bear, end B. Crayfish eat
both plant and animal based food.Credit A modification of work by Dave Mankie.
Credit be modification of work by JohnSullivan. Types of digest systems,
there re two types of digestive systems, incomplete and complete. An incomplete digestive
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system has only one opening to thedigestive tract Figure A. Ingested food and
excreted waste products passed through the sameopening. Incomplete digestive systems are found in
nigerians and flatworms. The other typeis naturally called a complete digestive system,
and this type has two openings figure. Food is ingested through one opening and
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waste products are excreted through a separateopening. This type of digestive system is
found in all other phyla, includingvertebrates. The advantage of a complete digestive
system is that it allows animals tofeed continuously without waiting for the residues of
the previous meal to be released fromthe digestive tract. It also allows specialization
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of regions of the digestive tract,which means that different food types can be
digested more efficiently. A particular regionalong the digestive tract with different physical and
chemical conditions can be optimized for onetype of nutrient, and other regions can
be optimized for efficient metabolism of othertypes of nutrients. Invertebrate digestive systems invertebrates
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can have incomplete or complete digestive systems. As noted above, an incomplete digestive
system has a gastrovascular cavity and onlyone opening for digestion. Plate helmets,
flatworms, tinapra comb jellies and Nigeria, coral jelly fish, and sea anemones
of this type of digestive system.Gastrovascular cavities, as shown in Figure A,
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are typically a blind tube or cavitywith only one opening the mouth,
which also serves as an anus.Ingested material enters the mouth and passes through
a hollow tubular cavity. Cells withinthe cavity secrete digestive enzymes that break down
the food. The food particles areengulfed by the cells lining the gastrovascular cavity.
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A complete digestive system with an alimentarycanal. Figure bee is a more
advanced system. It consists of onetube with a mouth at one end and
an anus at the other. Earthwormsare an example of an animal with a
complete digestive system. Once the foodis ingested through the mouth, it passes
through the esophagus and is stored ina working called the crop. Then it
passes into the gizzard, where itis churned and digested. From the gizzard,
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the food passes through the intestine.The nutrients are absorbed and the waste
is eliminated as feces called castings.Through the anus partis shows a hydra,
which has a vase shaped body withtentacles around the rim. The hydrous mouth
is located between the tentacles at thetop of the vase. Next to the
hydra is a jellyfish medusa, whichis bell shaped with tentacles hanging down from
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the edge of the bell. Themouth in the lower middle part of the
body opens into the gastrovascular cavity.Part bee shows a nematode which has a
long tube like body that is wideat one end and tapers down to a
tail at the other. The mouthis in the center of the wide end.
It opens into an esophagus than apharynx. The faring sempties into a
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long intestine which ends at the anusa short distance before the tail. A
A gastrovascular cavity has a single openingthrough which food is ingested and waste is
excreted, as shown in this hydraand in this jellyfish Medusa B. An
alimentary canal has two openings, amouth for ingesting food and an anus for
eliminating waste, as shown in thisnematode. Vertebrate digestive systems. Vertebrates have
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evolved more complex digestive systems to adaptto their dietary needs. Some animals have
a single stomach, while others havemulti chambered stomachs. Birds have developed a
digestive system adapted to eating unmasticated foodmonogastric single chambered stomach. As the word
monogastric suggests, this type of digestivesystem consists of one mono stomach chamber.
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Gastric humans and many animals have amonogastric digestive system, as illustrated in Figure
eight B. The process of digestionbegins with the mouth and the intake of
food. The teeth play an importantrole in masticating, chewing, or physically
breaking down food into smaller particles.The enzymes present in saliva also begin to
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chemically break down food. The esophagusis a long tube that connects the mouth
to the stomach using peristalsis or wavelike smooth muscle contractions. The muscles of
the esophagus push the food towards thestomach in order to speed up the actions
of enzymes in the stomach. Thestomach is an extremely acidic environment, with
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a pH between one point five andtwo point five. The gastric juices,
which include enzymes in the stomach,act on the food particles and continue the
process of digestion. Further breakdown offood takes place in the small intestine,
where enzymes produced by the liver,the small testine, and the pancreas continue
the process of digestion. The nutrientsare absorbed into the blood stream across the
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epithelial cells lining the walls of thesmall intestines. The waste material travels onto
the large intestine, where water isabsorbed and the drier waste material is compacted
into feces. It is stored untilit is excreted through the rectum. The
basic components of the human and rabbitdigestive system are the same. Each begins
at the mouth. Food is swallowedthrough the esophagus and into the kidney shaped
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stomach. The liver is located ontop of the stomach and the pancreas is
underneath. Food passes from the stomachto the long winding small intestine. From
there it enters the wide large intestinebefore passing out the anus. At the
junction of the small and large intestineis a pouch called the secum. The
small and large intestines are much longerin rabbits than in humans, and the
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secum is much longer as well.A humans and herbert wars such as the
bee rabbit have a monogastric digestive system. However, in a rabbit, the
small intestine and siccum are enlarged toallow more time to digest plant material.
The enlarged organ provides more surface areafor absorption of nutrients. Rabbits digest their
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food twice. The first time foodpasses through the digestive system, it collects
in the siccum and then it passesas soft feces called secotrophies. The rabbit
reingests these seacotrophies to further digest them. Avian birds face special challenges when it
comes to obtaining nutrition from food dueto the constraints impost by having to be
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lightweight in order to fly. Theyhave lost some adaptations found in their dinosaur
ancestors. For example, they donot have teeth, and so their digestive
system shown in figure must be ableto process unmasticated food. Birds have evolved
a variety of beak types that reflectthe vast variety in their diet, ranging
from seeds and insects to fruits andnuts. Because most birds fly, their
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metabolic rates are high in order toefficiently process food and keep their body weight
low. The stomach of birds hastwo chambers, the provinriculus, where gast
or juices are produced to digest thefood before it enters the stomach, and
the gizzard, where the food isstored soapd and mechanically ground. The undigested
material forms food palates that are sometimesregurgitated. Most of the chemical digestion and
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absorption happens in the intestine, andthe waste is excreted through the cloaca.
Illustration shows an avian digestive system.Food is swallowed through the esophagus into the
crop, which is shaped like anupside down heart. From the bottom of
the crop, food enters a tubularprovinriculus, which empties into a spherical gizzard.
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From the gizzard, food enters thesmall intestine, then the large intestine.
Waste exits the body through the cloaca. The liver and pancreas are located
between the crop and gizzard. Ratherthan a single secum, birds have two
quica at the junction of the smalland large intestine. The avian esophagus has
a pouch called a crop, whichstores food. Food passes from the crop
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to the first of two stomachs,called the provinriculus, which contains digestive juices
that break down food. From theprovinriculus, the food enters the second stomach,
called the gizzard, which grinds food. Some birds swallow stones or grit
which are stored in the gizzard toaid the grinding process. Birds do not
have separate openings to excrete urine andfeces. Instead, uric acid from the
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kidneys is secreted into the large intestineand combined with waste from the digestive process.
This waste is excreted through an openingcalled the cloaca. Evolution connection avian
adaptations, birds have a highly efficient, simplified digestive system. Recent fossil evidence
has shown that the evolutionary divergience ofbirds from other land animals was characterized by
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streamlining and simplifying the digestive system.Unlike many other animals, birds do not
have teeth to chew their food.In place of lips, they have sharp,
pointy beaks instead of teeth. Theyhave a proventricolis or gizzard for griding
up food. The emergence of thesechanges seems to coincide with the inclusion of
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seeds and the bird diet. Seedeating birds have beaks that are shaped for
grabbing seeds, and the two compartmentstomach allows for delegation of tasks. Since
birds need to remain light in orderto fly, passage time in the gut
is very short, which means theydigest their food very quickly and need to
eat often. Contrast this with theruminants, where the digestion of plant matter
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takes a very long time and aheavy water filled digestive tract. What would
you predict would be some characteristics ofbirds that eat like a cow ingesting primarily
leaves and shoots ruminants. Ruminants areherbivores like cows, sheep, bison,
et cetera, whose entire diet consistsof eating large amounts of leaves and shoots.
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They have evolved digestive systems that helpthem digest the vast amounts of cellulose
and this diet. An interesting featureof some ruminants mouths is that they do
not have upper incisor teeth. Theyuse their lower teeth, tongue, and
lips to tear and chew their food. From the mouth, the food travels
to the esophagus and onto the stomachto help digest the large amount of plant
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material. The stomach of the ruminantsis a multi chambered organ as illustrated in
figure. The four compartments of thestomach are called the rumin reticulum, omasum,
and abomasum. These chambers contain manymicrobes that break down cellulose and ferment
in ingested food. The abomasum isthe true stomach and is the equivalent of
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the monogastrid stomach chamber where gastric juicesare secreted. The four compartment gastric chamber
provides a larger space and the microbialsupport necessary to digest plant material. In
ruminants, it is essentially a bacterialfermentation vessel. The fermentation process requires lots
of watery fluid and produces large amountsof gas in the stomach chamber, which
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must be eliminated. As in otheranimals, the small intestine plays an important
role in nutrient absorption and the largeintestine helps in the elimination of waste.
Illustration shows the digestive system of agoat. Food passes from the mouth through
the esophagus and into the rumen.It circulates clockwise through the rumin, then
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moves forward and down into the smallpouch shaped reticulum. From the reticulum,
the food, which is now cutis regurgitated. The animal choose the cud
and then swallows it into the quilledomasum, which sits between the reticulum and the
rumin. After circulating through the omasum, the food enters the small intestine,
then the large intestine waste is excretedthrough the anus. Ruminant animals, which
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as goats and cows, have afour chambered stomach. The first two chambers,
the rumin and the reticulum, containprocaryotes and protusts that are able to
digest cellulose fiber. The ruminant regurgitatescut from the reticulum, choose it,
and swallows it into a third chamber, the omasum, which removes water.
The cut then passes on to thefourth chamber, the abomasum, where it
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is digested by enzymes produced by theruminant parts of the digestive system. The
vertebrate digestive system is designed to facilitatethe transformation of food matter into the nutrient
components that sustain organisms. Oral cavity. The oral cavity or mouth, is
the point of entry of food intothe digestive system, illustrated in figure.
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The food consumed is broken into smallerparticles by mastication, the chewing action of
the teeth. All mammals have teethand can chew their food. The extensive
chemical process of digestion begins in themouth as food is being chewed, saliva
produced by the salivary glands mixes withthe food. Saliva is a watery substance
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secreted by salivary glands into the mouthsof many animals. Saliva contains mucus that
moistens food and buffers the page ofthe food. Saliva also contains immunoglobulins and
lysisymes, which have antibacterial action toreduce tooth decay by inhibiting growth of some
bacteria. Saliva also contains an enzymecalled salivary amalase that begins the process of
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converting starches and the food into adisachride called maltose. The chewing and wetting
action provided by the teeth and salivaprepare the food into a mass called the
bolus for swallowing. The tongue helpsin swallowing, moving the bolus from the
mouth into the pharynx. The pharynxopens to two passageways, the trachea,
which leads to the lungs, andthe esophagus, which leads to the stomach.
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The trachea has an opening called theglottis, which is covered by a
cartilaginous fly called the epiglottis. Whenswallowing, the epiglottis closes the glottis and
food passes into the esophagus and notthe trachea. This arrangement allows food to
be kept out of the trachea andlungs. Illustration A shows the parts of
the human oral cavity. The tonguerests in the lower part of the mouth.
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The flop that hangs from the backof the mouth is the uvula.
The airway behind the uvula, calledthe pharynx, extends up to the nostrils
and down to the esophagus, whichbegins in a nack. Illustration BEACH shows
the two salivary glands which are locatedbeneath the tongue, the sublingual and the
submandibular. A third salivary gland,the parotid, is located behind the pharynx.
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Digestion of food begins in the aoral cavity. Food is masticated by
teeth and moistened by saliva secreted fromthe b salivary glands. Enzymes in the
saliva begin to digest starches and fatswith the help of the tongue. The
resulting bollus is moved into the esophagusby swallowing credit modification of work by the
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National Cancer Institute. Esophagus. Theesophagus is a tubular organ that connects the
mouth to the stomach. The chewedand softened food passes through the esophagus after
being swallowed. The smooth muscles ofthe esophagus undergo a series of wave like
movements called peristalsis that push the foodtowards the stomach. As illustrated in figure.
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The peristalsic wave is unidirectional. Itmoves food from the mouth to the
stomach and reverse movement is not possible. The peristaltic movement of the esophagus is
an involuntary reflex. It takes placein response to the act of swallowing.
Photo shows food moving down the esophagus, which is a muscular tube. Muscles
constrict behind the food. The constrictionmoves down, pushing the food ahead of
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it from the mouth to the stomach. The esophagus transfers food from the mouth
to the stomach through peristaltic movements.A ring like muscle called the sphincter forms
valves in the digestive system. Thegastroesophageal sphincter is located at the stomach end
of the esophagus. In response toswallowing and the pressure exerted by the bolus
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of food. This sphincter opens andthe bolus enters the stomach. When there
is no swallowing action, this sphincteris shut and prevents the contents of the
stomach from traveling up the esophagus.Many animals have a trucephincter. However,
in humans there is no trucephincter,but the esophagus remains closed when there is
no swallowing action. Acid reflux orheartburn occurs when the acidic digestive juices escape
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into the esophagus stomach. A largepart of digestion occurs in the stomach shown
in figure. The stomach is asaclike organ that secretes gastric digestive juices.
The pH in the stomach is betweenone point five and two point five.
This highly acidic environment is required forthe chemical breakdown of food and the extraction
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of nutrients when empty. The stomachis a rather small organ. However,
it can expand to up to twentytimes its resting size when filled with food.
This characteristic is particularly useful for animalsthat need to eat when food is
available. Illustration shows the human lowerdigestive system, which begins with the stomach,
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a sack that lies above the largeintestine. The stomach empties into the
small intestine, which is a long, highly folded tube. The beginning of
the small intestine is called the duodenum, the long middle part is called the
jugunum, and the end is calledthe ilium. The ilium empties into the
large intestine on the right side ofthe body. Beneath the junction of the
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small and large intestine is a smallpouch called the secum. The appendix is
at the bottom end of the secum. The large intestine travels up the left
side of the body, across thetop of the small intestine, then down
the right side of the body.These parts of the large intestine are called
the ascending colon, the transverse colon, and the descending colon, respectively.
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The large intestine empties into the rectum, which is connected to the anus.
The pancreas is sandwiched between the stomachand large intestine. The liver is a
triangular organ that sits above and slightlyto the right of the stomach. The
gall bladder is a small bulb betweenthe liver and stomach. The human stomach
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has an extremely acidic environment where mostof the protein gets digested credit modification of
work by Marianna Ruis Vareal. Thestomach is also the major site for protein
digestion in animals other than ruminants.Protein digestion is mediated by an enzyme called
pepsin in the stomach chamber. Pepsinsecreted by cells in the stomach lining,
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breaks peptide bonds and cleaves proteins intosmaller polypeptides. Another cell type, parietal
cells, secrete hydrogen and chloride ions, which combine in the lumen to form
hydrochloric acid, the primary acidic componentof the stomach juices. The highly acidic
environment also kills many microorganisms in thefood, and, combined with the action
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of the enzyme pepsin, results inthe hydrolysis of protein and the food.
Chemical digestion is facilitated by the churningaction of the stomach. Contraction and relaxation
of smooth muscles completely mixes the stomachcontents about every twenty minutes. The partially
digested food and gastric juice mixture iscalled chyme. Chyme passes from the stomach
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to the small intestine. Further,protein digestion takes place on the small intestine.
Gastric emptying occurs within two to sixhours after a meal. Only a
small amount of chyme is released intothe small intestine at a time. The
movement of chyme from the stomach intothe small intestine is regulated by the pyloric
sphincter. When jesting protein and somefats, the stomach lining must be protected
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from getting digested by pepsin. Thereare two points to consider when describing how
the stomach lining is protected. Firstly, the enzyme pepsin is synthesized in an
inactive form pepsinogen, which is activatedby the acid and other proteases in the
stomach contents. This protects the cellswhich secrete pepsin because pepsinogen does not have
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the full enzyme functionality of pepsin.Second, the stomach has a thick mucous
lining that protects the underlying tissue fromthe action of the digestive juices. When
this mucous lining is ruptured, ulcerscan form in the stomach. Ulcers are
open wounds in or on an organcaused by bacteria Helicobacter pylori. When the
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mucous lining is ruptured and fails toreform. Small intestine chime moves from the
stomach to the small intestine. Thesmall intestine is the organ where the digestion
of protein, fats, and carbohydratesis completed. The small intestine is a
long, tube like organ with ahighly folded surface containing fingerlike projections called the
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villi. The apical surface of eachvillis has many microscopic projections called microvilli.
These structures illustrated in figure are linedwith epithelial cells on the luminal side and
allow for the nutrients to be absorbedfrom the digested food and absorbed into the
blood stream on the other side.The villi and microvilli, with their many
folds, increase the surface area ofthe intestine and increase absorption efficiency of the
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nutrients. Most absorbed nutrients, sugars, amino acids, and nucleotides, and
the blood are carried into the hepaticportal vein, which leads to the liver.
There, the liver regulates the distributionof nutrients to the rest of the
body and removes toxic substances, includingdrugs, alcohol, and some pathogens.
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Fatty acids resulting from digestion of fatand the small intestine do not enter the
blood stream directly, but are takenup by the lymphatic system and the small
intestine. These are eventually delivered tothe blood via the thoracic duct to be
metabolized by the liver, as isthe case for the other nutrients. Illustration
shows a cross section of the smallintestine, the lumin or inside of which
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has many fingerlike projections called vili.Muscle layers wrap around the outside of the
intestine, and blood vessels interact withthe muscle layer. A blow up shows
that capillaries and lymphatic vessels travel upinside the villi. The surface of each
villis is covered with hairline microvilli.Vili are folds on the small intestine,
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lining that increase the surface area tofacilitate the absorption of nutrients. The human
small intestine is over six m longand is divided into three parts, the
duodenum, the jugunum, and theilium. The C shaped fixed part of
the small intestine is called the duodenumand is shown in figure. The duodenum
is separated from the stomach by thepyloric sphincter, which opens to allow to
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move from the stomach to the duodenum. In the duodenum, chyme is mixed
with pancreatic juices in an alkaline solutionrich and bicarbonate that neutralizes the acidity of
chyme and acts as a buffer.Pancreatic juices also contains several digestive enzymes.
Digestive juices from the pancreas, liver, and gallbladder, as well as from
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gland cells of the intestinal wall itself, enter the duodenum. Bile is produced
in the liver and stored and concentratedin the gallbladder. Bio contains bile salts,
which emulsify lipids, while the pancreasproduces enzymes that catabolize starches, disaccharides,
proteins, and fats. These digestivejuices break down the food particles in
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the chym into glucose, triglycerides,and amino acids. Some chemical digestion of
food takes place in the duodenum.Absorption of fatty acids into the lymphatic system
also takes place in the duodenum.The second part of the small intestine is
called the jujuno shown in figure here. Hydrolysis of nutrients is continued while most
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of the carbohydrates and amino acids areabsorbed through the intestinal lining. The bulk
of chemical digestion and nutrient absorption occursin the jagunum. The ilium, also
illustrated in figure, is the lastpart of the small intestine, and here
the bile, salts and vitamins areabsorbed into bloodstream. The undigested food is
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sent to the colon from the iliumvia peristaltic movements of the muscle. The
ilium ends and the large intestine beginsat the iliosecal valve. The vermiform worm
like appendix is located at the iliosecalvalve. The appendix of humans secretes no
enzymes and has an insignificant role inimmunity. Large intestine. The large intestine
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illustrated in figure reabsorbs the water fromthe undigested food material and processes the waste
material. The human large intestine ismuch smaller in length compared to the small
intestine, but larger in diameter.It has three parts, the secum,
the colon, and the rectum.The secum joins the ilium to the colon
and is the receiving pouch for thewaste matter. The secum and colon are
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home to many trillions of bacteria orintestinal flora that aid in the digestive processes.
The colon can be divided into fourregions, the ascending colon, the
transverse colon, the descending colon,and the sigmoid colon. The main functions
of the colon are to extract thewater and mineral salts from undigested food and
to store waste material. Carnivorous mammalshave a shorter large intestine compared to herbivorous
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mammals due to their diet. Illustrationshows the structure of the large intestine,
which begins with the ascending colon.Below the ascending colon is the secum.
The vermiform appendix is a small projectionat the bottom of the secum. The
ascending colon travels up the right sideof the body, then turns into the
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transverse colon on the left side ofthe body. The large intestine turns again
into the descending colon. At thebottom, the descending colon curves up.
This part of the intestine is calledthe sigmoid colon. The sigmoid colon empties
into the rectum. The rectum travelsstraight down to the anus. The large
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intestine reabsorbs water from undigested food andstores waste material until it is eliminated rectum
in anus. The rectum is theterminal end of the large intestine, as
shown in figure. The primary roleof the rectum is to store the feces
until defecation. The feces are propelledusing peristaltic movements during elimination. The anus
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is an opening at the far endof the digestive tract and is the exit
point for the waste material. Twosphincters between the rectum and anous control elimination.
The inner sphincter is involuntary and theouter sphincter is voluntary. Accessory or
the organs discussed above are the organsof the digestive tract through which food passes.
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Accessory organs are organs that add secretionsenzymes that catabolize food into nutrients.
Accessory organs include salivary glands, theliver, the pancreas, and the gall
bladder. The liver, pancreas,and ball bladder are regulated by hormones in
response to the food consumed. Theliver is the largest internal organ in humans
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and It plays a very important rolein digestion of fats and detoxifying blood.
The liver produces bile, a digestivejuice that is required for the breakdown of
fatty components of the food and theduodenum. The liver also processes the vitamins
and fats and synthesizes many plasma proteins. The pancreas is another important gland that
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secretes digestive juices. The kind producedfrom the stomach is highly acidic in nature.
The pancreatic juices contain high levels ofbicarbonate, an alkali that neutralizes the
ass chime. Additionally, the pancreaticjuices contain a large variety of enzymes that
are required for the digestion of proteinand carbohydrates. The gallbladder is a small
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organ that aids the liver by storingbile and concentrating bile salts. When chime
containing fatty acids enters the duodenum,the bile is secreted from the gallbladder into
the duodenum. This podcast will bereleased episodically and follow the sections of the
textbook in the description For a deeperunderstanding, We encourage you review the text
(32:32):
version of this work voice by voicemakerDotaan. This was produced by Brandon Casturo
as a creative Common Sense production.
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