the digestive system and human nutrition continue
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THE DIGESTIVE SYSTEM
AND HUMAN NUTRITION
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THE DIGESTIVE SYSTEM
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WHAT IS DIGESTION?
process by which food and drink are broken down into
their smallest parts so the body can use them to build
and nourish cells and to provide energy.
involves mixing food with digestive juices, moving it
through the digestive tract, and breaking down large
molecules of food into smaller molecules. Digestion
begins in the mouth, when you chew and swallow, and
is completed in the small intestine.
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DIGESTIVE
S YSTEM
a complex series of organs and glands
that processes food
body system tasked to break the food
down into smaller molecules that it can
process and, excrete waste.
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made up of the digestive tract: a series of
hollow organs joined in a long, twisting
tube from the mouth to the anus and
other organsOrgans that make up the digestive tract
are the mouth, esophagus, stomach,
small intestine, large intestine,
rectum, and anus.
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THE DIGESTIVE
PROCESS, PARTS AND FUNCTIONS
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SWALLOWING
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mouth: The digestive process begins in
the mouth. Food is partly broken down by
the process of chewing and by the chemical
action of salivary enzymes; these enzymes
are produced by the salivary glands and
break down starches into smaller
molecules.
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The pharynx facilitates the passage of the food bolus
into the esophagus. The pharynx is designed to direct
the food bolus in this direction. After the moistened
food bolus is moved to the back of the mouth by the
tongue, an involuntary swallowing reflex is triggered
which prevents food from entering the respiratory
tract
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The tongue closes off the mouth,
the soft palate blocks the nose,
and the larynx rises such that the
epiglottis closes off the trachea.
Food then moves from the pharynx
into the esophagus.
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esophagus: After being chewed and swallowed, the
food enters the esophagus. The esophagus is a long
tube that runs from the mouth to the stomach. It
uses rhythmic, wave-like muscle movements to force
food from the throat into the stomach. This muscle
movement gives us the ability to eat or drink even
when we're upside-down.
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At the junction of the esophagus and stomach,
there is a ring-like muscle, called the lower
esophageal sphincter, closing the passage between
the two organs
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NUTRIENTS
Carbohydrates spend the least amount of time in the
stomach, while protein stays in the stomach longer,
and fats the longest.
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Carbohydrates: The carbohydrates are broken into
simpler molecules by enzymes in the saliva, in juice
produced by the pancreas, and in the lining of the
small intestine.
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Starch: First, an enzyme in the saliva and
pancreatic juice breaks the starch into molecules
called maltose. Then an enzyme in the lining of the
small intestine splits the maltose into glucose
molecules that can be absorbed into the blood.
Glucose is carried through the bloodstream to theliver, where it is stored or used to provide
energy for the work of the body.
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Sugar: An enzyme in the lining of the small
intestine digests sucrose, also known as table
sugar, into glucose and fructose, which are
absorbed through the intestine into the blood. Milk
contains another type of sugar, lactose, which is
changed into absorbable molecules by anotherenzyme in the intestinal lining.
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Fiber: moves through the digestive tract without
being broken down by enzymes. Many foods
contain both soluble and insoluble fiber. Soluble
fiber dissolves easily in water and takes on a soft,
gel-like texture in the intestines. Insoluble fiber, on
the other hand, passes essentially unchangedthrough the intestines.
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Protein: Foods such as meat, eggs, and beans consist of
giant molecules of protein that must be digested by
enzymes before they can be used to build and repair
body tissues. An enzyme in the juice of the stomach starts
the digestion of swallowed protein. Then in the small intestine,
several enzymes from the pancreatic juice and the lining of
the intestine complete the breakdown of huge protein
molecules into small molecules called amino acids. These
small molecules can be absorbed through the small intestine
into the blood and then be carried to all parts of the body to
build the walls and other parts of cells.
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6. Fats: Fat molecules are a rich source of energy for the
body. The first step in digestion of a fat is to dissolve it into the
watery content of the intestine. The bile acids produced by theliver dissolve fat into tiny droplets and allow pancreatic and
intestinal enzymes to break the large fat molecules into
smaller ones. Some of these small molecules are fatty acids
and cholesterol. The bile acids combine with the fatty acids
and cholesterol and help these molecules move into the cells
of the mucosa. In these cells the small molecules are formed
back into large ones, most of which pass into vessels called
lymphatics near the intestine. These small vessels carry the
reformed fat to the veins of the chest, and the blood carries
the fat to storage depots in different parts of the body.
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Vitamins. Fat-soluble vitamins (A,D, E, K) are
stored in the liver and fatty tissue of the body,
whereas water-soluble vitamins (B and C) are not
easily stored and excess amounts are flushed out
in the urine.
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The digested nutrients are absorbed
through the intestinal walls and
transported throughout the body.
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DIGESTIVE SYSTEM OF
OTHER ANIMALS Animals, for the most part, ingest their food as large,
complex molecules that must be broken down into
smaller molecules that can then be distributed
throughout the body of every cell.
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Single-celled organisms can directly take in
nutrients from their outside environment.
Multicellular animals, with most of their cells
removed from contact directly with the outside
environment, have developed specialized
structures for obtaining and breaking down theirfood.
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Ingestive eaters, the majority of animals, use a mouth
to ingest food. Absorptive feeders, such as tapeworms,
live in a digestive system of another animal and absorb
nutrients from that animal directly through their body
wall. Filter feeders, such as oysters and mussels,
collect small organisms and particles from the
surrounding water. Substrate feeders, such as
earthworms and termites, eat the material (dirt or wood)
they burrow through. Fluid feeders, such as aphids,
pierce the body of a plant or animal and withdraw fluids.
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Two types of animal body plans as well as two
locations for digestion to occur: Sac-like plans are
found in many invertebrates, who have a single
opening for food intake and the discharge of
wastes. Vertebrates, the animal group humans
belong to, use the more efficient tube-within-a-
tube plan with food entering through one opening
(the mouth) and wastes leaving through another
(the anus).
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Where the digestion of the food happens is also
variable: Some animals use intracellular digestion,
where food is taken into cells by phagocytosis withdigestive enzymes being secreted into the phagocytic
vesicles. This type of digestion occurs in sponges,
coelenterates (corals, hydras and their relatives) andmost protozoans. Extracellular digestion occurs in
the opening of a digestive system, with the nutrient
molecules being transferred to the blood or some
other body fluid. This more advanced type of digestion
occurs in chordates, annelids, and crustaceans.
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Chemotrophs are organisms (mostly bacteria)
deriving their energy from inorganic chemical
reactions. Phototrophs convert sunlight energy into
sugar or other organic molecules. Heterotrophs eat
to obtain energy from the breakdown of organic
molecules in their food.
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STOMACH
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is a muscular sac that lies between theesophagus and the small intestine in the upper
abdomen.
the stomach is not the only part of your digestivesystem that absorbs food but rather is a part of the
digestive system and important for churning food
into a consistency that is easier to digest for therest of your intestines.
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THE STOMACH’S FUNCTIONS
The stomach is J-shaped and it can expand to
temporarily store food.
Partial digestion of the food takes place here. The
churning action of the stomach muscles physically
breaks down the food.
The stomach releases acids and enzymes for the
chemical breakdown of food. The enzyme pepsin is
responsible for protein breakdown.
The stomach releases food into the small intestine
in a controlled and regulated manner.
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Cardia
is the first portion of the stomach and is where food
content passes from the esophagus into the stomach.
The acids and enzymes referred to as the gastric
juices are manufactured in the cardia.
Fundus
stores undigested food and also the gases released
from the chemical digestion of food.
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Corpus
is the largest of the four parts that make up the
stomach. And this is where the bulk of the partial
digestion occurs.
Pylorus
is connected to the duodenum or the beginning of the
small intestine. The contents of the stomach move
into the small intestine via the pyloric canal.
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THE STOMACH WALL
Mucosa
consists mainly of the gastric glands that secrete the
digestive juices. It is covered by a layer of columnar
epithelial tissue.
Sub-mucosa
consists of dense connective tissue and has blood
vessels, lymphatic vessels, and nerves running through it.
The sub-mucosa supports the mucosa and allows it to
move in a flexible manner during peristalsis.
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Peristalsis- is the contraction and relaxation of the
stomach muscles to physically breakdown food
and propel it forward. These contractions are
created by the muscular wall of the stomach which
consists of inner circular and outer longitudinal
smooth muscle.
Serosa
consists of an epithelial layer and connective tissue
which connects to the surrounding organs.
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CELL TYPES IN THE STOMACH THAT HELP WITH
DIGESTION
Mucous cells secrete the alkaline mucous for
shielding the epithelium from hydrochloric acid.
Parietal cells, the acid activates release of pepsin
for protein digestion. The acid also kills micro-
organisms swallowed with the food.
Chief cells secrete pepsin.
G cells secrete gastrin which stimulates the
secretion of hydrochloric acid.
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LIVER
system is to process the nutrients absorbed from
the small intestine. Bile from the liver secreted into
the small intestine also plays an important role in
digesting fat.
In addition, the liver is the body’s chemical "factory."
It takes the raw materials absorbed by the intestine
and makes all the various chemicals the body
needs to function.
It breaks down and secretes many drugs.
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P ANCREAS
secretes digestive enzymes into the duodenum, the
first segment of the small intestine. These enzymes
break down protein, fats, and carbohydrates.
also makes insulin, secreting it directly into the
bloodstream. Insulin is the chief hormone for
metabolizing sugar.
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G ALLBLADDER
stores and concentrates bile, and then releases it
into the duodenum to help absorb and digest fats.
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SMALL INTESTINE
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is about 20 feet long and about an inch in diameter
absorb most of the nutrients from what we eat and
drink
Velvety tissue lines the small intestine, which is
divided into the duodenum, jejunum, and ileum.
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DUODENUM
The C-shaped first part of the small
It protects the duodenal lining from the acidity of the
chyme and raises the luminal pH to the optimum
level for pancreatic enzyme activity
It is also the point of entry for the bile and
pancreatic ducts, which penetrate the full thickness
of the duodenal wall
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FUNCTION
receives roughly digested and mixed food from the
stomach and neutralizes the stomach acid.
also receives bile from the galbladder and digestive
enzymes from the pancreas. The combination of
food, bile and enzymes are mixed with mucus and
passed into the jejunum. The intermixed structure
of protein, fats and carbohydrates of food is broken
apart as it passes through the duodenum.
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JEJUNUM
The passage that takes place between the
duodenum and jejunum
The jejunum is not only second in order but is also
second in length when talking of the small intestine,
measuring around 8.2 feet.
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FUNCTION
The combination of food, bile, enzymes and mucus
passes into the jejunum after exiting the duodenum.
The inner lining of the jejunum and the later section
the ileum are lined with villi, small fingers containing
capillaries that increase the surface area that can
absorb nutrients.
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The jejunum absorbs nutrients such as
carbohydrates that have been broken down into
simple sugars, proteins that have been broken
down into amino acids and many vitamins.
Much of this transport is accomplished by an array
of transporter proteins that shuttle the nutrients
from the intestines to the capillaries.
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ILEUM
the longest and the lowest part of small intestine,
coming in at eleven and a half feet.
The ileum is slightly paler in color when compared
to the jejunum and mostly absorbs fatty acids and
glycerol, besides glucose and amino acids.
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FUNCTION
Water, minerals and salts as well as fats and
remaining nutrients are absorbed by the ileum.
Where much of the initial transport of nutrients was
specific, the ileum takes small amounts of water as
well as the minerals and vitamins floating in it and
shuttles them into the capillaries.
Fats are able to pass directly from the intestines to
the bloodstream.
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MICROSCOPIC P ARTS OF SMALL INTESTINE
The membrane that connects all parts of the small
intestine is known as the mesentery. This membrane is
richly supplied by blood vessels in the form of small
capillaries which help in absorption of food.
The internal lining of the small intestine in the ileum
consists of glandular epithelium which is present in the
form of highly convoluted and folded microscopic
structures known as microvilli
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L ARGE INTESTINE
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The large intestine is the second to last part of the
digestive system—the final stage of the alimentary
canal is the anus —in vertebrate animals. Its
function is to absorb water from the remaining
indigestible food matter, and then to pass useless
waste material from the body.
The large intestine consists of the cecum and
colon. It starts in the right iliac region of the pelvis,
just at or below the right waist, where it is joined to
the bottom end of the small intestine.
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From here it continues up the abdomen, then
across the width of the abdominal cavity, and then itturns down, continuing to its endpoint at the anus.
The large intestine is about long, which is about
one-fifth of the whole length of the intestinal canal.
The large intestine takes 32 hours to finish up the
remaining processes of the digestive system. Food
is not broken down any further in this stage
of digestion.
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FUNCTION
The large intestine simply absorbs vitamins that are
created by the bacteria inhabiting the colon. It also
absorbs water and compacts feces, and stores fecal
matter in the rectum until eliminated through the anus
and thus is responsible for passing along solid waste.
Cecum The cecum marks the beginning of
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Cecum- The cecum marks the beginning of
the large intestine and is basically a big pouch that
receives waste material from the small intestine.
Ascending Colon- The ascending colon runsthrough the abdominal cavity, upwards toward the
transverse colon for approximately eight inches (20
cm).
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Transverse Colon- The transverse colon is the large
part of the colon that attaches the ascending colon
to the descending colon by crossing the abdominal
cavity.
Descending Colon- The descending colon traverses
inferiorly along the left abdominal wall to the pelvic
region.
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Rectum- The rectum is a short, muscular tube that forms
the lowest portion of the large intestine and connects it
to the anus. Feces collects here until pressure on the
rectal walls cause nerve impulses to pass to the brain,
which then sends messages to the voluntary muscles in
the anus to relax, permitting expulsion.
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Appendix- The appendix, also called the (veriform),
is a troublesome pouch attached to the first portionof the large intestine. Digestion takes place almost
continuously in a watery, slushy environment. The
large intestine absorbs water from its inner contentsand stores the rest until it is convenient to dispose
of it. The appendix has no function in modern
humans; however, it is believed to have been part
of the digestive system in our primitive ancestors.
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HOW DO THE WASTE MATERIALS MOVE
THROUGHOUT THE LARGE INTESTINE?
Peristalsis is the coordinated contraction of muscle
movement to move food or waste through a tube.
To simplify, food and waste particles are squeezed
through a tube-like structure by a squeezing
motion. Peristalsis takes place in
the esophagus when an individual swallows food. It
also takes place in the colon as it moves solid
waste through the large intestine to be expelled
from the body through the rectum.
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