The Human Digestion

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The Human Digestion
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AYAT B. ALGHAFARY Dr. Wadiah Backer
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The Digestive System
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INTRODUCTION
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Digestion is a biological process concern with
the initial breakdown of food into a form in
which it can be taken in The whole process of
digestion involves many different organs, which
are called the digestive system, and include the
mouth, esophagus, stomach, small intestines,
large intestines, rectum and anus.
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Other organs are involved in supporting the
digestive process as well, but are not
technically considered part of the digestive
system. These organs are known as accessory
organs are the tongue, the glands in the mouth
that produce saliva, the pancreas, liver and
gallbladder.
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• The digestive tract
• Ingests food
• Breaks down food into small molecules that can
  cross plasma membranes
• Absorbs these nutrient molecules
• Eliminates nondigestible remains.

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THE MOUTH
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Digestion begins in the mouth with the chewing of
food (mastication). Mastication not only breaks
down very large aggregates of food molecules into
smaller particles and allows saliva and enzymes
to enter inside the larger food complexes, but
also sets off a signaling message to the body to
start the entire digestive process
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Saliva is secreted by three pairs of salivary
glands by way of ducts into the mouth and
moistens the food to improve the chewing and
grinding.
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Saliva also contains some enzymes that begin the
breakdown of starches and fats. For example,
carbohydrate digestion begins with the salivary
enzyme alpha-amylase, and fat digestion begins
with the secretion of the enzyme lingual lipase
by glands under your tongue.intro_250_Swallow.swf

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salivary amylase starch H2O
-----------------------gtmaltose
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THE ESOPHAGUS
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The esophagus, sometimes called the gullet,
connects the mouth to the stomach. It delivers
the saliva-mixed food from the mouth to the
stomach and serves as an air lock between the
outside world and the digestive tract
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The layers of muscle in the gastrointestinal
tract wall contract in a coordinated fashion.
Once it is swallowed, food moves through the
digestive tract by peristalsis, waves of muscular
contraction. This also mixes the food with the
numerous secretions of the digestive process.
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STOMACH
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Stomach is a large chamber consisting of the
fundus, the body and then the antrum.
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The stomach is the primary place where proteins
are disassembled and broken down into small
peptides
Due to its acidic environment, the stomach is
also a decontamination chamber for bacteria and
other potentially toxic microorganisms that may
have entered your gastrointestinal system through
your mouth
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Stomach stores liters of partially digested food
freeing humans from continual eating. The gastric
secretions of the stomach contains the
following 1-Hydrochloric acid (HCl) lowers pH
of the gastric contents to about This acid kills
most bacteria and other microorganisms. And also
Low pH stops activity of salivary amylase and
promotes activity of pepsin.
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2-Pepsin The gastric fundus mucosa secretes the
enzyme pepsinogen, which is present in the
stomach much of the time but is inactive until
the acid is present, when it becomes activated as
pepsin. Pepsin acts on the denatured proteins by
hydrolyzing, or cutting, the bonds between amino
acids in the protein chain, resulting in several
smaller chains, or peptides as follow           
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 pepsin protein H2O
----------------gt peptides
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3- The Antrum or lower part of the stomach, is
the site for the stomachs grinding action and
contains a sensor mechanism, called gastrin, for
regulating the level of acid produced in the body
of the stomach. intro_247_Parietal.swf
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The antrum also controls the emptying of food
into the intestine through the pyloric sphincter.
This way the food can be delivered into the
intestine in a controlled manner. Once the
food-acid-enzyme mixture leaves the stomach, it
is called chyme
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The movement of chyme through the pyloric
sphincter stimulates the intestine to release the
hormones secretin and cholecystokinin, which
signal the pancreas to release its contents, the
pancreatic juice, inside the lumen (the lining)
of the duodenum (the first segment of the small
intestine).
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THE SMALL INTESTINE
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The small intestine, which is specifically
designed to maximize the digestion and absorption
process, contains three parts duodenum, jejunum,
and ileum.
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It also has an expanded surface area with inner
folds, called villi and microvilli, to increase
its surface area and enhance its ability to
absorb nutrients. All together, this surface is
called the brush border of the small intestine.
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Some enzymes are present on the surface of the
brush border, such as Disaccharidases like
sucrase, maltase, and lactase, which hydrolyze
disugars (sugars composed of two monosaccharides)
to their two individual sugar molecules such as
maltase maltose H2O ------------------gt
glucose
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Additional digestion is aided by secretions from
liver and pancreas. 1-The Liver It contains
the following 1- Bile is a secretion of liver
temporarily stored in gallbladder before sent to
duodenum Bile emulsifies fat bile is a green
byproduct of the breakdown of hemoglobin
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• Bile contains bile salts that help in
  emulsification of fat.
• Emulsification breaks fat globules into
  microscopic droplets
• bile salts fat ----------------gt fat
  droplets
• This increases fat digestion by increasing
  surface area of fat globules exposed to enzymes

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• 2-Pancreatic juice secreted by pancreas and
  contains the following
• Sodium bicarbonate NaHCO3 that neutralizes
  acidity of chyme pH of small intestine is
  slightly basic.

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• Pancreatic amylase that digests starch to maltose

Pancreatic amylase starch H2O
-----------------------------gt maltose
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• Trypsin and other enzymes that digest protein to
  peptides (we will take later about Protein
  digestion in details)
• trypsin
• Protein H2O ----------------gt peptides

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• Lipase that digests fat droplets to glycerol and
  fatty acids.
• Lipase fat droplets H2O --------------------gt
  glycerol fatty acids

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3- Intestinal secretions complete digestion of
peptides and sugars peptides are digested by
peptidases to amino acids      peptidases
peptides H2O ----------------gt amino acids
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and maltose from the first step in starch
digestion is converted by maltase to glucose.
maltase maltose H2O ------------------gt
glucose
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4- Large molecules of carbohydrates, proteins
and fats are broken into small molecules absorbed
by villi.
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A few nutrients, like iron and calcium, are taken
up most efficiently in the duodenum however,
the jejunum, the middle section of the small
intestine, is the place where most nutrients are
actively absorbed. The amino acids as well as
most vitamins and minerals are absorbed in the
jejunum. The process of absorption used by the
jejunum is called active absorption since your
body uses energy to select the exact nutrients it
needs
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Active fat absorption also occurs in the duodenum
and the jejunum, and requires that the fat be put
into small aggregates that can be transported
into your body directly. This process is
particularly important for the absorption of the
fat-soluble vitamins (vitamins A, D, E, and K),
and for cholesterol absorption.
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The ileum is as we said before the final part of
the small intestine. The ileum is responsible for
completing the digestion of nutrients and for
reabsorbing the bile salts that have helped to
solubilize the fats
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At the end of transport through the small
intestine, the chyme has been depleted of around
90 percent of its vitamins and minerals and the
majority of its other nutrients
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In addition, around eight to 10 liters of fluid
is also absorbed in the small intestine each day.
Complex carbohydrates that resist the enzyme
degradation, such as fiber and resistant starch,
remain, as do a small amount of other food
molecules and nutrients that have escaped the
digestion process. For example, about 3-5 of
ingested protein normally escapes digestion and
continues to the large intestine
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THE LARGE INTESTINE
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The large intestine which is formed from four
parts cecum, colon, rectum, and anal canal is
not designed for enhancing absorption but is
particularly specialized to conserve the sodium
and water that escape absorption in the small
intestine, although it only transports about one
liter of fluid per day
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The large intestine is about five feet long,
including its final segments, the colon and the
rectum
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It is interesting, given that most digestion and
absorption occurs prior to the large intestine
that food, which at this point is primarily
fiber, will spend more time in your large
intestine than anywhere else during digestion.
Food travels through the stomach in 1/2 to two
hours, continues through the small intestine over
the next two to six hours, and spends six to 72
hours in your large intestine
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Accessory Organs
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THE PANCREAS
Pancreas lies deep within abdominal cavity, just
below stomach, and rests on posterior abdominal
wall. The pancreas can be thought of as a
protein factory. It produces and secretes many of
the enzymes necessary for digestion, which
include the enzymes that digest protein (trypsin,
chymotryosin, carboxypeptidase, and elastase)
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enzymes that digest fat (lipase and
phospholipase), and the enzyme that digests
carbohydrate (alpha-amylase). The pancreas
releases these enzymes in a pancreatic juice,
which is enriched with bicarbonate. The
bicarbonate is used to neutralize the acid in
chyme
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The pancreatic enzymes that digest protein have
the ability to digest your own tissues. Your body
has an intricate protection from self-digestion
by these enzymes. The stomach and intestinal
tract lining have a mucous layer protecting them
from direct digestion by these enzymes. The
pancreas uses other mechanisms for protection.
Primarily, it produces the enzymes in an inactive
form, called zymogens or proenzymes.
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For example, trypsin is produced as the inactive
proenzyme trypsinogen. Trypsinogen is transported
to the intestine where it is activated to trypsin
by a protease enzyme on the brush border of the
intestinal cells. All pancreatic enzymes except
lipase and alpha-amylase are secreted as
proenzymes, and are therefore inactive within the
pancreas.
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THE LIVER
Liver is a large glandular organ that fills the
top of abdominal cavity, just below diaphragm.
Liver has numerous functions -It detoxifies
blood by removing and metabolizing poisonous
substances. -It makes plasma proteins including
albumin and fibrinogen.
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-Liver destroys old red blood cells -converts
hemoglobin to bilirubin and biliverdin in bile.
-It produces bile stored in gallbladder before
entering duodenum to emulsify fats. -It stores
glucose as glycogen breaks down glycogen to
maintain constant blood glucose concentration.
wholebodyzoom.swf -Liver produces urea from amino
groups and ammonia.
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THE GALLBLADDER
The gallbladder is the storage site for the bile
acids produced by the liver After a meal is
consumed, the gallbladder is signaled to release
its contents into the duodenum and jejunum, where
they are available for fat digestion.
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Healthy digestion
The types of food you eat, the amount of exercise
you get, your level of stress all can affect the
health of your digestive system Good lifestyle
habits Practice good eating habits Maintain
healthy weight Control stress Limit alcohol
and tobacco
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Proteins

• Digestion
• Absorption
• Metabolism in Liver

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Protein Digestion
As we said before the digestion of proteins is
not started in the mouth, it takes place in the
stomach under the effect of HCL and Pepsin.
Then the Chyme which is formed in the stomach
is delivered to the small intestine for further
digestion by many enzymes that are specific for
protein digestion as follow
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1- Enteropeptidase which is present in the
pancreatic juice is responsible for activation of
trypsinogen into trypsin
Enteropeptidase Trypsinogen-----------------------
----gt Trypsin
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Then trypsin do the Following reactions
Trypsin Chemotrypsinogen---------------gt
Chemotrypsin Chemotrypsin cleaves peptide bonds
next to the amino acids Phenylalanine,
Tryptophan, Tyrosine, Methionine, Asparagine,
Histidine
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Trypsin Procarboxypeptidase A -----------------gt
Carboxypeptidase A Trypsin Procarboxypeptidase B
-----------------gt Carboxypeptidase B
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Then Carboxypeptidase A cleaves amino acids from
the acid (carboxyl) ends of polypeptides and
especially Alanine, Isoleucine, Leucine and
Valine. While Carboxypeptidase B Cleaves
Arginine and Lysine just like trypsin.
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Elastase Collagenase Polypeptides---------------
------------------gt Tripeptides
Tripeptidases Tripeptides ---------------------
----------------gt Dipeptides Aminopeptidase Dipe
ptides -------------------------------gt Amino
Acids
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Protein Absorption
The cells of the small intestine absorb amino
acids and have peptidase enzymes on their surface
that split most of the dipeptides and tripeptides
into single amino acids.  A few dipeptides,
tripeptides, and even larger molecules sometimes
escape digestion and cross the digestive tract
wall to enter the bloodstream.
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Eating predigested protein (amino acid
supplements) saves the body from having to digest
proteins and keeps the digestive system from
overworking.  Such a belief grossly
underestimates the body's abilities.  As a
matter of fact, the digestive system handles
whole proteins better than predigested ones
because it dismantles and absorbs the amino acids
at rates that are optimal for the body's use.
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Digestion facilitated mostly by the stomach's
acid and enzymes, the body first denatures
proteins , then cleaves them into polypeptides,
then tri-, and dipeptides, and some amino acids. 
Intestinal enzymes split them to single amino
acids.  Then transporter proteins in the
membranes of intestinal cells transport the amino
acids into the absorptive cells of the small
intestine, where they are passed through the cell
and released into the bloodstream.
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Protein Metabolism in Liver
Within each cell, proteins are constantly being
made and broken down.  If amino acids are in
excess of the body's requirements, they are
metabolized to glycogen or fat and used for
energy metabolism. If amino acids are to be used
for energy their carbon skeletons are converted
to acetyl CoA, which enters the Krebs cycle for
oxidation, producing ATP.
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The final products of protein catabolism
include carbon dioxide, water, ATP, urea, and
ammonia.
The role of the liver in Protein metabolism is
shown in the following figure (http//www.beginner
triathlete.net)
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Transamination (2 steps) converts amino acids to
L-glutamate, which undergoes Oxidative
Deamination to form ammonia, used for the
synthesis of urea. Urea is transferred through
the blood to the kidneys and excreted in the
urine. Ti Amino Acids------------gt
Glutamate T2 Glutamate-----------------gt Aspartate
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http//www.sickkids.ca
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NITROGEN BALANCE If the body maintains the same
amount of protein in its tissues from day to day,
it is in nitrogen balance.  If the body adds
protein, it is in positive nitrogen balance if
it loses protein, it is in negative nitrogen
balance.  Normally, healthy adults receive
enough protein to meet their needs, and they
dispose of any excess.  Their nitrogen intake
equals their nitrogen output, and they are said
to be in  zero nitrogen balance, or nitrogen
equilibrium. 
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Growing infants and children, pregnant women, and
people recovering from protein deficiency or
illness are in positive nitrogen balance their
nitrogen intake exceeds their nitrogen output. 
In contrast, people who are starving or
suffering other severe stresses such as burns,
injuries, infections, and fever are in negative
nitrogen balance nitrogen output exceeds
nitrogen intake. 
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• The most critical aspects of protein metabolism
  that occur in the liver are
• Deamination and transamination of amino acids,
  followed by conversion of the non-nitrogenous
  part of those molecules to glucose or lipids.
  Several of the enzymes used in these pathways
  (for example, alanine and aspartate
  aminotransferases) are commonly assayed in serum
  to assess liver damage.

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• Removal of ammonia from the body by synthesis of
  urea. Ammonia is very toxic and if not rapidly
  removed from the circulation, will result in
  central nervous system disease.
• Synthesis of non-essential amino acids. They are
  responsible for synthesis of most of the plasma
  proteins. Also, the liver synthesizes many of the
  clotting factors necessary for blood coagulation.
  
• However, there is an interrelationship between
  carbohydrate, lipid, and protein metabolism in
  the human as follow (http//www.faqs.org)

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