Digestive System

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Digestive System General Functions of the
Digestive System 1. Motility a. Ingestion b.
Peristalsis rhythmic contractions moving food
through the gastrointestinal tract. c.
Defecation/excretion removal the unabsorbed
materials out of the body.
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2. Secretion Exocrine secretion mucus, HCl,
enzymes, H2O... - digestive juice, 2-3 L/day (70
is reabsorbed). Endocrine secretion
hormones that regulate digestion activities.
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3. Digestion Mechanical and chemical
decomposition of food. 4. Absorption transport
of digested products into blood and
lymph. Lymphatic system drain interstitial
fluid and return it into veins, absorb lipid
molecules from the digestive tract and send them
to blood stream.
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Anatomy 1). Primary organs gastrointestinal
tract Mouth ? pharynx ? esophagus ? stomach ?
small intestines ? large intestines ? rectum ?
anus (Food Bolus ? chyme ? feces) 2). Accessory
organs (2nd) Teeth, tongue, salivary glands,
pancreas, liver and gall bladder.
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Layers of GI tract Mucosa lines the lumen of
the tract and form folds. It contains
epithelial cells, connective tissue, lymph
nodules, capillary vessels and a thin layer of
smooth muscle. Submucosa capillary vessels,
lymphatic tube, glands and nerves (submucosal
plexus).
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Muscularis inner circular and outer
longitudinal layers of smooth muscles,
responsible for segmental and peristaltic
movement through the GI tract. Between the
muscle layers nerve fibers and ganglia of
sympathetic and parasympathetic systems. Serosa
connective tissue, forms the very outside
layer of the GI tract.
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Functions of each organ A. Mouth (mechanical
digestion) Saliva contains H2O, mucus, and
enzymes Amylase breaks down starch into
disaccharides, Lysozyme kills bacteria.
Immunoglobulin - IgA an antibody Functions
of saliva moistening food, dissolve chemicals
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Tongue The mucous membrane of tongue contains
taste buds, mucus glands, glands that secrete
lipase. Inside of the tongue there are
skeletal muscles that makes voluntary
movement. Lipid digestion begins in the mouth by
lingual Lipase. Functions of the tongue
participating swallowing.
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B. Pharynx Swallowing process is the result of
corporation of tongue, larynx and pharynx.
Tongue lifts up which causes the elevation of
larynx to close trachea and open esophagus. So
food enters the esophagus but not trachea.
This is a precisely controlled process.
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C. Esophagus The upper part contains skeletal
muscle the lower part contains smooth muscle.
Skeletal muscle near pharynx perhaps
participates in swallowing action. Lower
esophageal sphincter contracts after food passes
into the stomach, which prevents the food from
coming back to the esophagus.
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D. Stomach Anatomy Cardia - upper
opening Stomach body Pylorus - lower
opening Pyloric antrum - is the area near
pylorus. Pyloric sphincter - Smooth muscles
in the stomach body are oriented in different
directions.
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The epithelial cells of mucosa form gastric
glands and secrete different products to form
gastric juice. Goblet cells secrete
mucus Parietal cells produce HCL Chief cells
produce pepsinogen (a zymogen, inactive form
of pepsin) ECL cells secrete histamine and
serotonin G cells (endocrine cells) gastrin
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Intrinsic factor required for the absorption of
vitamin B12, is produced by parietal cells.
Table 18.1
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Gastric juice contains all the above substances
and water. Food is mixed with gastric juice
in stomach to form paste like substance called
chyme.
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Several important components in the gastric
juice Pepsin, a protease that cleaves peptide
bonds and digests food proteins. Its optimal
working pH is 1-2. Pepsin is first
synthesized as pepsinogen, which is cleaved and
activated after it gets into the lumen of the
stomach.
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HCL is secreted by parietal cells. H is
transported into the stomach against its
concentration gradient by H/K exchange pump.
While H is pumped into the lumen of the
stomach, K is taken up into the parietal cells.
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Five functions of HCL a. Kills microorganisms
b. Causes denature of proteins. c. Breaks down
cell walls of plants d. Activates pepsin, and
provides an optimal environment for pepsin
to function. Cooperative activities of
pepsin and HCl allows the partial digestion
of ingested proteins in the stomach. e. The
acidic environment is also good for the
absorption of Ca and Fe.
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Mucus produced by Goblet cells, contains HCO3-
and forms an alkaline coating to prevent the
inner lining of stomach from being digested by
HCl and pepsin.
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Digestion and Absorption in the Stomach Proteins
are partially digested in the stomach. The
polypeptide chains are cleaved by pepsin into
oligopeptides, tripeptides and dipeptides.
There is very little digestion for
carbohydrates and fats in the stomach. The
major digestion process is in the small
intestines.
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Absorption in the stomach is very limited. The
substances absorbed in stomach alcohol, water,
small amount of minerals (Ca, Fe, Na, Cl), and
certain drugs, such as aspirin and
anti-inflammatory drugs.
The stomach empty time for solid food is 4-6
hours water - about 30 min. Pyloric
sphincter controls the emptying speed of
stomach. It also prevents the intestinal content
from backing up into the stomach.
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Regulation of The Stomach Function (Nervous and
hormonal controls) The movement and secretion of
stomach are both stimulated by parasympathetic
nerves. Cholinergic receptor blocker can
decrease HCl secretion and smooth muscle
contraction
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Locally produced hormone, gastrin, can also
stimulate secretion and mobility of stomach.
Secretion of gastrin is triggered by food
entering the stomach. The effect of ACh and
gastrin on gastric secretion is indirect.
Both ACh and gastrin can stimulate ECL cells
to produce histamine, which causes parietal
cells to produce HCL.
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Gastritis and peptic ulcer Gastritis -
inflammation in the stomach mucous membrane, can
be induced by certain drugs (aspirin, steroids,
anti-inflammatory drugs) and alcohol.
Infection of a bacterium, helicobacter has been
identified, as a direct cause of gastritis and
gastric ulcer. Emotional and physical stress
also contribute to gastritis.
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Peptic ulcer or digestive ulcer, is open wound
on the mucus membrane of the stomach or
duodenum, often developed on the basis of
gastritis. The mucous membrane of the stomach is
digested by HCl and pepsin. General mechanism
combination of over production of gastric acid
and underproduction of the alkaline
mucus. (Helicobector is also involved.)
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More specifically, duodenal ulcer may be more
related with the over production of gastric
acid. Gastric ulcer is often due to
insufficient production of alkaline mucus.
Gastric ulcer is closely related with stomach
cancer.
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The traditional treatment for digestive ulcer is
acid control in combination with antacid drugs.
Cholinergic nerve blocker to inhibit ACh
release (Cimitidine). New generation
histamine receptors blocker Zantag.
Antibiotics are sometimes prescribed.
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E. Small Intestines The major site for digestion
and absorption The anatomy Approximately 3 M
long in a living person, but it will measure 6 M
in a dead body, because of the relaxation of
longitudinal muscles.
Small intestines are held by mesentery - a broad
sheet of connective tissue, which contains blood
vessels lymphatic tubes and nerves.
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Three segments duodenum, jejunum, and ileum.

• The walls of small intestines fold into a
  structure
• called plica (plicae), which further folds into a
  
• microscopic structure, villi.
• The epithelial cells of each villi form smaller
  folds
• microvilli.
• Due to the folding plus villi and microvilli, the
  
• total surface area is 2200 ft2, (otherwise 3.6
  ft2)

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Structure of Villus (Fig. 18.12) A finger-like
fold of mucosa that projects into the intestinal
lumen. Each villus is covered by columnar
epithelial cells with goblet cells in between.
In the center connective tissue, capillary
vessels, and a lacteal.
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The absorbed carbohydrates and amino acids enter
capillaries in the center of the villus, while
lipids enter the central lacteal. Microvilli
are brush like structure (brush border) formed by
epithelial cells on the surface of villus.
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Functions of small intestines 1), Receiving the
digestive juice from pancreas and liver (enzymes
and bile salts).
2), Secretion Intestinal juice, 1.8 L/day. It
moistens chyme and neutralize its pH. Before
absorption, pH of the food content must be
neutralized.
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a. NaHCO3 - is secreted by pancreas and small
intestines. NaHCO3 ? Na HCO3- H HCO3 ?
H2CO3 ? CO2 H2O Duodenum is the major location
of buffering action. Duodenum (10 inches
long), receives chyme from stomach and
neutralize it. The pH of chyme here changes
from 1 to 7.5.
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pH of the rest of the small intestines is 7.5,
which is optimal for most enzymes here. b.
Enzymes
Disaccharides, trisaccharides, dipeptides and
tripeptides are further broken down by enzymes
in the small intestines. Brush border enzymes
stick to the cell surface and are exposed to the
intestinal content.
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Enterokinase is a brush border enzymes that
activates trypsin that is produced by pancreas.
Sucrase, maltase, and lactase are also produced
by intestinal epithelial cells. (Lactase gene
may stop expressing in adults.) Intestinal
epithelial cells also make dipeptidase to cleave
dipeptides into single amino acids which can
then be absorbed.
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c. Small intestines also produce hormone that
regulate secretion of stomach, pancreas and
gallbladder. The arriving of chyme to
duodenum stimulates the secretion
of Cholesytokinin (CCK) Chyme contianing
lipids and proteins secretion of CCK, which
stimulates contraction of gall bladder and
secretion of pancreas.
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3). Regulation Arrival of chyme into duodenum
also causes local production of secrenin, which
stimulates the secretion of alkaline buffer,
Na2HCO3, to neutralize the acidic chyme.
Secrenin also inhibits gastric secretion and
mobility.
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4). Digestion and absorption The absorption of
carbohydrate, lipids, Ca, Fe mainly occurs in
duodenum and jejunum. Bile salts, water and
electrolytes are absorbed in ileum.
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Movement of small intestines Segmentation is
the major movement form in small
intestines. Small, periodic, ringlike
contractions cut chyme into segments and move it
back and forth. Peristalsis is weak in the small
intestines.
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The smooth muscles in the small intestines are
controlled by vagus. Smooth muscle cells have
their own pace maker to produce slow rhythmic
contraction even when there is no neural
stimulation.
Stimulation from parasympathetic system can
accelerate both peristalsis and segmentation.
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Diarrhea If small intestines are overdistended
or irritated, a strong peristaltic rush may pass
through the entire length, sweeping the contents
of the small intestines into the large
intestines so quickly, that water, electrolytes
and other substances that would normally be
absorbed are excreted through frequent
defecations.
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F. Large intestines Anatomy It has larger lumen
than small intestines and contains 4
parts 1.) Cecum, the beginning part of large
intestines and is a short blind pouch. The
nerrow tube hanging down - appedix. Appendix
is an immune organs, which contains many
lymphatic nodules and lymphocytes.
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2). Colon from right to left, it includes
ascending, transverse, and descending colons.
3). Rectum 4). Anal canal. The main function
of large intestines is the reabsorption of water
and electrolytes. There is 1500 ml of materials
enters the large intestines each day, but only
200 ml becomes feces.
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Other functions of large intestines Reabsorb
and excrete bile salts. E.coli that normally
residing in the large intestines produce vitamin
K and some B vitamins, which can be absorbed in
the large intestines.
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G. Liver, Gall bladder and Pancreas (accessory
organs) Liver located at the upper right corner
of the abdomen. Gall balder is attached to the
liver between its two lobes.
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Liver is full of blood supply. Liver cells are
called hepatocytes, which form hepatic plates
that are one to two cells thick. The spaces
between hepatic plates are called Sinusoids,
which mimic capillary vessels.
However, hepatic plates are more permeable. They
have large pores and lack basement membrane,
which allow passage of proteins, fat and
cholesterol.
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Within each hepatic plate, there are thin bile
tubes that collects bile juice produced by
hepatocytes. Kupffer cells ?
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Hepatic portal system All blood leaving the
small intestines is transported to liver through
hepatic portal vein. The blood is filtered in
the liver and then enters hepatic veins, which
drains to inferior vena cava. Hepatic portal
system vein ? capillaries ? vein.
The purpose is to extract nutrients and to remove
toxins before they enter systemic circulation.
(The liver also receives blood from hepatic
artery.)
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• Functions of Liver
• 1. Bile production and secretion
• The liver produces 250 - 1500 ml of bile per day.
  
• Major constituents of bile bile pigments, bile
• salts, phospholipids, cholesterol and ions.
• Bile pigment includes billirubin and billiverdin.
  
• Both are metabolic products of heme.

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• 2. Detoxication
• The liver can remove drugs and toxic molecules
• by
• excretion into small intestines
• phagocytosis by the Kupffer cells
• chemical alteration of these molecules.
• For example, ammonia produced in protein
• metabolism is converted into less toxic urea,
• which is then excreted by the kidneys.

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Another example, alcohol is hydrolyzed and
broken down by alcohol dehydrogenase in the
liver. Also, liver converts lactic acid into
glucose and toxic purines into uric
acid. Disadvantage drugs can be deactivated in
the liver.
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3. Participating the metabolism of glucose,
protein and lipids While being filtered by
liver, blood glucose level is monitored.
If blood glucose is high, liver absorbs some of
it for the synthesis of glycogen. When blood
glucose is low (fasting), glycogen is broken
down in to glucose, which is released into
blood.
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The liver monitors amino acids levels in
the blood. It removes excessive amino acids
and use them for the synthesis of new plasma
proteins or convert them to glucose or lipids to
be stored. When blood amino acids are low, the
liver will break down glycogen and triglycerides
to synthesize and release amino acids to
circulation.
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Similarly, liver controls blood levels of
triglyceride, fatty acids and cholesterol.
When these molecules are low, liver breaks
down the fat storage to release them to blood.
When they are high, they are removed from
blood to be stored in the liver as glycogen or
fat.
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4. Storage Besides glycogen and fat, vitamins
(A, D, E, K and B12) and Fe can also be stored
in the liver. Over ingestion of the lipid
soluble vitamins can cause liver damage.
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5. Biosynthesis Liver is a biochemical factory
where many plasma proteins are synthesized.
Albumin accounts for 70 of plasma proteins
and is important for maintaining blood osmotic
pressure. Thrombin and other clotting
factors Transport proteins LDL and
HDL Angeotensinogen
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Gallbladder Bile is produced by hepatocytes. It
contains bile salts, billirubin, billiverdin,
cholesterol and other compounds.
Bile is continuously produced by liver cells but
is not continuously released to small
intestines. After being produced, bile is
stored in gallbladder. It is only released into
duodenum after meals.
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When chyme reaches duodenum, CCK will be
produced which stimulates gallbladder to
contract, and bile will be ejected. When the
small intestine is empty, the sphincter of
ampulla at the end of common bile duct closes,
forcing bile into systic duct and gallbladder to
be stored.
Pancreas duct and bile duct join into one tube
that opens to duodenum (duodenal papilla).
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Bile juice produced by hepatocytes ? hepatic
duct ? cystic duct ? gallbladder ? cystic duct ?
common bile duct ? duodenum. The function of
bile salts is to assist the digestion of fat.

• Bile salts break the clumped fat molecules into
• small droplets, which can be mixed with water
• emulsification.
• After emulsification, the fat splitting enzyme
• - lipase can work on the fat molecules easily.

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Pancreas - endocrine and exocrine gland Located
in the upper right abdomen behind stomach. The
endocrine function is carried out by pancreatic
islet cells. Pancreas also secretes pancreas
juice into duodenum.
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Components of pancreatic juice (Table 18.5)
contains water, bicarbonate and many digestive
enzymes. a. Pancreatic amylase digest
starch b, Pancreatic lipase that splits
triglyceride into fatty acids and
glycerol. c, Proteinases (trypsin, chymotrypsin,
and peptidase) cleave polypeptides into
dipeptides and tripeptides. d, Pancreatic
nuclease cleaves nucleotide chains into single
nucleotides.
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Digestion and Absorption in Small Intestines
Carbohydrate digestion The digestion of
starch occurs in the mouth and small
intestines. Pancreatic amylase, which
cleaves starch into disaccharides and
trisacharides that are later digested by
brushborder enzymes Sucrase sucrose ?
glucose fructose Maltase maltose ? 2
glucose Lactase lactose ? galactose glucose

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• Protein digestion
• initiated in the stomach is completed in the
• small intestines.
• Polypeptide chains generated by pepsin are
• further digested by tripsin, chymotripsin and
• elastase into dipeptides, tripeptides and free
• amino acids.
• Dipeptides and tripeptides are digested by
• dipeptidase and tripepdidase (brush border)
• into single amino acids.

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Lipid Digestion Bile is secreted in response to
the arrival of lipids into duodenum. After
emulsification, pancreatic lipase hydrolyzes
triglycerides and phospholipids into glycerol,
fatty acids and lysolecithin. These small
subunits then associate with bile salts and
cholesterol to form mixed micelles.
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These micelles are little particles that have
hydrophilic portion outside and hydrophobic
portion in the middle, which can be easily
absorbed by epithelial cells. All digestive
enzymes are listed in table 18.8.
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Absorption of six categories of nutrients mainly
occurs in the small intestines Monosaccharides
(glucose) and amino acids are absorbed into the
intestinal epithelial cells and secreted into
capillaries through facilitated diffusion and
active transport.
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Fatty acids, lysolecithin, cholesterol and
glycerol contained in micelles can diffuse into
epithelial cells. Within the cytoplasm of
epithelial cells, triglyceride is resynthesized.
Triglycerides, together with cholesterol,
phospholipids are then coated with proteins to
form chylomicrons, which are secreted by
epithelial cells into lacteals through
exocytosis.
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Lymphatic fluid containing the absorbed lipids
will eventually join the blood circulation at
subclavical vein. Water soluble vitamins are
mostly absorbed in small intestines via
diffusion. Fat soluble vitamins are mixed with
triglycerides and are absorbed together with
them.
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Absorption outside of the small intestines
Stomach can absorb alcohol (diffusion), small
amount of water (by osmosis), vitamin B12
(active transport, bound with intrinsic factor),
Fe, and Ca (active transport). Absorption in
the large intestines Water (hand out), bile
salts, Vitamins K and Biotin (produced by
E.coli).
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Regulation of pancreas and gall bladder secretion
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