Post on 26-Jun-2020
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THE DIGESTIVE
SYSTEM (PART II)
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FUNCTIONS OF THE DS• Ingestion
• Secretion
• Mixing and propulsion
• Digestion
• Absorption
• Defecation
Ingestion• This process involves taking foods and liquids into
the mouth (eating).
Secretion• Cells within the walls of the GI tract secrete about
7 liters of water, acid, buffers, and enzymes into
the lumen of the GI tract daily.
Mixing and Propulsion• Alternating contraction and relaxation of smooth
muscle in the walls of the GI tract mix food and
secretions and propel them toward the anus.
• This is referred to as motility.
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Digestion• Mechanical digestion.
– The teeth cut and grind food.
– The smooth muscles of the stomach
and small intestine churn the food to
help it dissolve and mix with enzymes.
• Chemical digestion.– The complex carbohydrate, lipid,
protein, and nucleic acid molecules in
food are split into smaller molecules
by hydrolysis.
– Digestive enzymes produced by the
salivary glands, tongue, stomach,
pancreas, and small intestines
catalyze these catabolic reactions.
– Amino acids, cholesterol, glucose,
vitamins, minerals, and water can be
absorbed without chemical digestion.
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Absorption• Absorption is the entrance of ingested
and secreted fluids, ions, and small molecules that are products of digestion into the epithelial cells lining the lumen of the GI tract.
• The absorbed substances pass into the blood or lymph and circulate to all cells of the body.
Defecation• Substances that were not absorbed leave
the body through the anus in a process called defecation.
• These substances include wastes, indigestible substances, bacteria, cells sloughed from the GI tract, and digested materials that were not absorbed.
• The eliminated material is called feces.
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Gastric Emptying• Gastric emptying is the periodic release of
chyme from the stomach into the
duodenum.
• Stimuli such as distention of the stomach
and the presence of partially digested
proteins, alcohol, and caffeine initiate
gastric emptying.
• The enterogastric reflex ensures that the
stomach does not release more chyme than
the small intestine can handle.
• Stimuli such as distention of the duodenum
and the presence of fatty acids, glucose,
and partially digested proteins in the
duodenal chyme inhibit gastric emptying.
• Within 2 – 4 hours after eating a meal, the
stomach has emptied its contents into the
duodenum.
• Foods rich in carbohydrates spend the least
time in the stomach, proteins longer, and
fat-laden meals the longest.
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Vomiting• Vomiting or emesis is the forcible
expulsion of the contents of the upper GI tract (stomach and sometimes duodenum) through the mouth.
• Stimuli include the following:
– Irritation and distention of the stomach.
– Unpleasant sights.
– General anesthesia.
– Dizziness.
– Certain drugs (morphine, derivatives of digitalis).
Vomiting involves squeezing the stomach
between the diaphragm and abdominal
muscles and expelling the contents
through open esophageal sphincters.
Prolonged vomiting can lead to alkalosis
(higher than normal blood pH).
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THE SMALL INTESTINE
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THE SMALL INTESTINE
• The small intestine, averaging about 6 meters in length, is small in diameter.
• The first 25 cm is the duodenumthat receives bile from the gallbladder and pancreatic juice from the pancreas.
• Walls of the small intestine have finger-like projections called villiwhere nutrient molecules are absorbed into the cardiovascular and lymphatic systems.
• Villi have microvilli that increase the surface area available for absorption.
• The small lymphatic capillary in a villus is called a lacteal.
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PANCREAS• Stomach chyme passes into the duodenum
for chemical digestion. Activities of the
pancreas, liver, and gallbladder are
necessary for this chemical digestion to
occur.
• The pancreas is a retroperitoneal gland.
• It is about 12-15 cm long and 2.5 cm thick.
• It lies posterior to the greater curvature of
the stomach.
• The pancreas consists of a head, body and
tail.
• It is connected to the duodenum by 2 ducts.
– Pancreatic duct (duct of Wirsung) –
larger. In most people, the pancreatic
duct joins the common bile duct from the
liver and gallbladder and enters the
duodenum as the hepatopancreatic
ampulla (ampulla of Vater). The
ampulla opens at the major duodenal
papilla.
– Accessory duct (duct of Santorini) –
smaller.
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PANCREAS
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COMPOSITION OF
PANCREATIC JUICE• рН 7,8-8,4 1,5-2 l
Enzymes:
• Proteolytic (Trypsin, chymotrypsin,
elastase, a carboxypeptidase) – a
proteolysis and polypeptides to amino
acids
• Amylolytic (-amylase) – splitting of
carbohydrates to a glucose
• Lipolytic (lipase) – splitting of fats to
glycerinum and fatty acids
• Nucleic acid digesting enzymes
(ribonuclease, deoxyribonuclease).
Bicarbonates – buffers acidic juice in
chyme, stops the action of stomach
pepsin, and creates the proper pH for
digestive enzymes of the small intestine
Water – a dissolvent
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ENZYMES• The enterokinase transforms
trypsinogen to trypsin
• Trypsin transforms
chymotrypsinogen to
chymotrypsin
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CONTROL OF
PANCREATIC SECRETION
• Conditional (a view, a smell of food, etc.)
• Unconditional (mastication, swallowing)
• Nervous (VNS: PSVNS)
• Humoral – gastro hormones:SECRETIN and CHOLECYSTOKININ-PANKREOZIMIN (CCK) stimulate
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LIVER
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• The liver is the heaviest gland of the body,
weighing about 1.4 kg.
• It is the 2nd largest organ in the body after the
skin.
• It is inferior to the diaphragm and occupies most
of the right hypochondriac region and part of the
epigastric region.
• The liver is divided into a large right lobe and a
smaller left lobe by the falciform ligament.
• The falciform ligament also suspends the liver.
• The liver is almost completely covered by visceral
peritoneum.
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HISTOLOGY OF THE LIVER &
GALLBLADDER• The lobes of the liver are made up of
many functional units called lobules.
• The lobules contain hepatocytes
arranged in irregular, branching,
interconnected plates around a central
vein.
• The liver has large endothelial lined
spaces called sinusoids instead of
capillaries.
• Fixed phagocytes called stellate
reticuloendothelial (Kuppfer) cells destroy
worn out WBCs, RBCs, bacteria, and any
other foreign material in venous blood
draining from the GI tract.
• Bile is secreted from the hepatocytes and travels through the right and left hepatic ducts.
• These ducts merge to form the common hepatic duct, which later joins the cystic duct from the gallbladder.
• Bile is stored in the gallbladder for later release.
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FUNCTIONS OF THE
LIVER
• Carbohydrate metabolism.
• Lipid metabolism.
• Protein metabolism.
• Processing of drugs and
hormones.
• Excretion of bilirubin.
• Synthesis of bile salts.
• Storage.
• Phagocytosis.
• Activation of vitamin D.
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Gallbladder• The gallbladder is a pear-shaped sac that
is located in a depression of the posterior
surface of the liver.
• It is about 7-10 cm long and typically
hangs from the anterior inferior margin of
the liver.
• The parts of the gallbladder are the broad
fundus, the body (central portion), and
the neck (tapered portion).
• The gallbladder stores and concentrates
bile until it is needed in the small
intestine.
• In the concentration process, water and
ions are absorbed by the gallbladder
mucosa.
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SECRETORY FUNCTION OF
THE LIVER– The liver consists of secretory cells –
hepatocytes which produce bile.
– Bile is a yellow, brown, or olive-green liquid.
BILE COMPOSITION
рН 7.3-8.0 0.5-1 l
– Cholic pigments (bilirubin, a biliverdin)
– Cholic acids (cholic, chenodesoxycholic)
– Bicarbonates
– Water
– Cholesterol
BILE FUNCTIONS
– Bile salts play a role in emulsification, the
breakdown of large lipid globules into a
suspension of droplets.
– Dissolution of hydrolysates of fats. This also aids
in the absorption of lipids following digestion.
– These droplets increase the surface area
allowing pancreatic lipase to function more
efficiently.
– Activates lipases of an intestinal and a
pancreatic juice
– Activates peristalsis of an intestine
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Intestine
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Digestive Hormones
• Gastrin promotes secretion of
gastric juice, increases gastric
motility, and promotes growth of the
gastric mucosa.
• Secretin stimulates the secretion of
pancreatic juice and bile. Inhibits
secretion of gastric juice.
• Cholecystokinin stimulates
secretion of pancreatic juice and
causes ejection of bile from the
gallbladder. Enhances the effects
of secretin.
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SMALL INTESTINE (SI)• The major events of digestion and absorption
occur in the small intestine.
• The length of the SI provides great surface area for this to occur.
• Circular folds, villi, and microvilli also serve to increase the surface area of the SI.
• The SI begins at the pyloric sphincter of the stomach, coils through the central and inferior parts of the abdomen, and eventually opens into the LI.
• The SI is divided into 3 regions:
– Duodenum – shortest region,
retroperitoneal, starts at the pyloric
sphincter of the stomach.
– Jejunum – between the duodenum and
ileum.
– Ileum – the longest region, joins the large
intestine at the ileocecal sphincter.
• Circular folds – permanent ridges in the
mucosa. They enhance absorption by
increasing the surface area of the SI and by
causes the chyme to spiral, rather than move
in a straight line.
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FUNCTIONS OF THE SI
• Mixing chyme with digestive
juice and help food contact
with the mucosa for absorption.
• Peristalsis propulsion of food
through the SI.
• Complete digestion of
carbohydrates, proteins, and
lipids.
• Beginning and complete
digestion of nucleic acids.
• Absorption of approximately 90
% of nutrients and water.
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HISTOLOGY OF THE SI
• The same 4 basic layers that make up the rest of the GI
tract exist in the SI as well, with some basic exceptions.
• The mucosa forms a series of fingerlike villi (tufts of
hair) projections, which increase the surface area
available for absorption.
• Each villus contains an arteriole, a venule, a blood
capillary network, and a lacteal through which nutrients
are absorbed.
• The mucosa is simple columnar epithelium, which contains absorptive cells, goblet cells, enteroendocrine cells, and Paneth cells.
• The apical surface of the absorptive cells contains microvilli (bundles of actin filaments). Collectively, they are referred to as the brush border. They increase the surface area.
• The mucosa contains deep crevices lined with
glandular epithelium.
• Cells lining the crevices form the intestinal glands
(crypts of Lieberkuhn), which secrete intestinal juice.
• Paneth cells secrete lysozyme, a bactericidal enzyme.
• The lamina propria of the SI has an abundance of mucosa-associated lymphatic tissue (MALT).
• Solitary lymphatic nodules are present as well as groups of aggregated lymphatic nodules (Peyer’s patches).
• Duodenal (Brunner’s) glands of the submucosa secrete an alkaline mucus.
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Villi
Villi are finger-like projections in the small intestine.
Each villus is made up of connective tissue and
smooth muscles. The villi are lined by a single layer
of thin walled epithelial cells through which the
absorption takes place. The fats are absorbed into
the lymphatic system. In the centre of each villus, is
a lymphatic duct. It is also called sinus or lacteal. The
space between the lacteal and the epithelium is
criss-crossed by several small blood capillaries.
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INTESTINAL SECRETION
• Intestinal juice is a clear yellow fluid that contains water and mucus.
• It provides the absorption of substances from chyme.
COMPOSITION OF INTESTINAL JUICE
рН 7.2-8.6 1.5-2 l
Enzymes
• Proteolytic (enterokinase, a peptidase,aminopeptidase)
• Amylolytic (a maltose, a saccharase, alactase)
• Lipolytic (lipase)
Bicarbonates – provide of alkalinemedium
Water – as a dissolvent
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Brush Border
Enzymes
• The absorptive epithelial cells
synthesize several digestive
enzymes, called brush border
enzymes, and insert them into
the plasma membrane of the
microvilli.
• Consequently, some digestion
occurs at the surface of the
epithelial cells and not
exclusively in the lumen of the
SI.
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Mechanical Digestion
In The Small Intestine
• Segmentations – localized mixing contractions that occur in portions of the intestines distended by a large volume of chyme. Sloshes chyme back and forth.
• Migrating motility complexes –a type of peristalsis that moves the chyme down the length of the SI after segmentation has occurred.
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Chemical Digestion In The SI
• Chyme entering the small intestines contains partially
digested carbohydrates, proteins, and lipids.
• Pancreatic juice, bile, and intestinal juice complete the
effort of digestion.
• Pancreatic amylase, sucrase, lactase, and maltase complete the digestion of carbohydrates.
• These enzymes break complex carbohydrates into monosaccharides, which can be absorbed.
• Lactose intolerance occurs in people whose mucosal cells fail to produce enough of the enzyme lactase.
– Symptoms include siarrhea, gas, bloating, and abdominal cramps after the consumption of dairy products.
• Pepsin, trypsin, chymotrypsin, carboxypeptidase, elastase,
and peptidases complete the process of protein digestion.
• Proteins are broken down into single amino acids, which can
be absorbed.
• Lipases complete the process of lipid digestion in the SI.
• Bile salts increase the surface area of triglycerides through the process of emulsification. The globules are converted into droplets.
• Lipids are broken down into monoglycerides, which can then be absorbed.
• Pancreatic juice contains two nucleases: ribonuclease
(breaks down RNA) and deoxyribonuclease (breaks down
DNA).
• Brush border enzymes further break these down into
pentoses, phosphates, and nitrogenous bases, which can be
absorbed.
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TYPES OF INTESTINAL
DIGESTION• Cavitary – Provides initial hydrolysis of
alimentary substances to intermediate
products.
• It is characterized by that enzymes
synthesized in glandular cells are allocated as
a part of digestive juice in an intestine cavity
here again have the specific an effect on a
chyme.
• Near wall (membrane) – Provides hydrolysis
of intermediate products to monomers and
prepares transition for an absorption.
• It is carried out by the enzymes fixed on
cellular membranes of an intestine. Feature of
the given kind of digestion consists that it is
carried out on border of extracellular and
intracellular mediums.
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Figure . Localization of food
hydrolysis with various types of
digestion: (a) extracellular, (b)
intracellular, (c) membrane
mediated; (1) extracellular fluid,
(2) intracellular fluid, (3)
intracellular vacuole, (4)
nucleus, (5) cell membrane, (6)
enzymes
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ABSORPTION
– Represents transport process in a
blood and a lymph of various nutrients
through one or several layers of cells.
PASWAYS OF SUBSTANCES`
TRANSPORT
– Passive (Water, salts, low-molecular
bonds):
– Osmosis
– Filtration
– Diffusion
– The active:
– Primary transport (Amino acids)
– Secondary transport
(Monosaccharides)
– Endocytosis (Suspensions, emulsions)34
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Absorption In The SI• Forms that can be absorbed:
– Monosaccharides (glucose, fructose, and
galactose) from carbohydrates.
– Single amino acids, dipeptides, and tripeptides from
proteins.
– Fatty acids, glycerol, and monoglycerides from
triglycerides.
Mechanisms of absorption:
– Diffusion.
– Facilitated diffusion.
– Osmosis.
– Active transport.
• Passage of digested nutrients from the
gastrointestinal tract into the blood or lymph is
called absorption.
• About 90% of the nutrients are absorbed in the SI.
• The other 10% occurs in the stomach and the
large intestine.
• Any undigested or unabsorbed material passes
through to the LI.
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Absorption Of
Monosaccharides• All carbohydrates are absorbed as monosaccharides.
• They are absorbed via facilitated diffusion and active
transport.
• The SI can absorb up to 120 grams of carbohydrates
per hour.
Absorption Of Amino Acids,
Dipeptides, & Tripeptides• Most proteins are absorbed as amino acids via active
transport processes.
• About half of the amino acids come from food.
• The other half of the amino acids come from proteins in
digestive juices and dead cells that slough off the
mucosal surface.
Absorption Of Lipids• All dietary lipids are absorbed via simple diffusion.
• Adults absorb about 95% of the lipids present in the SI.
• Most dietary fatty acids require bile for adequate
absorption.
• When lipids are not absorbed properly, the fat-soluble
vitamins A, D, E, & K are not absorbed properly.
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Absorption Of Electrolytes• Most of the electrolytes absorbed by the SI come
from gastrointestinal secretions, and some come
from ingested foods and liquids.
• Active transport mechanisms are utilized to
absorb Na+ ions.
• Negatively charged bicarbonate, chloride, iodide,
and nitrate ions can passively follow Na+ or be
actively transported.
• Iron, potassium, magnesium, and phosphate ions
are absorbed via active transport.
Absorption Of Vitamins
• The fat-soluble vitamins A, D, E, & K are included with dietary lipids and absorbed via simple diffusion.
• Most water-soluble vitamins are absorbed via simple diffusion.
• Vitamin B12 combines with intrinsic factor and the combination is absorbed via active transport.
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Absorption Of Water• The volume of water in the SI (about 9.3 liters daily)
comes from ingested liquids and gastric secretions.
• The SI absorbs about 8.3 liters of it.
• 90% of the remaining water (about 0.9 liters) is
absorbed in the large intestine.
• Water absorption occurs via osmosis.
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Absorption Of Alcohol
• Alcohol is lipid soluble and begins
to be absorbed in the stomach.
• There is greater surface area for
absorption in the SI; therefore, the
longer alcohol remains in the
stomach, the more slowly blood
alcohol rises.
• Fatty acids in the chyme slow
gastric emptying; therefore, eating
fatty foods with alcohol will cause a
slower rise in blood alcohol.
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LARGE INTESTINE
(COLON)
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• The large intestine is the terminal portion of the GI
tract.
• It is divided into four principal regions.
Major Regions Of The LI
• Cecum.
• Colon.
• Rectum.
• Anal canal.
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LI Anatomy• The LI extends from the ileum to the anus.
• It is about 1.5 m long and 6.5 cm in diameter.
• It is attached to the posterior abdominal wall by
mesocolon.
• The iliocecal valve guards the opening to the LI
from the ileum.
• The cecum hangs inferior to the iliocecal valve.
• The appendix is a twisted, coiled tube hanging on
the cecum.
• The open end of the cecum merges with the
colon, which is divided into ascending,
transverse, descending, and sigmoid portions.
• The last 20 cm of the GI tract make up the
rectum.
• The anal canal is the termination of the rectum.
• The anus is the opening of the anal canal to the
exterior.
• This opening is guarded by an internal anal
sphincter (involuntary) and an external anal
sphincter (voluntary).
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Structure of colon
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• No villi or permanent circular folds are
found in the mucosa of the large
intestine.
• The epithelium contains mostly
absorptive and goblet cells. The
absorptive cells participate mainly in
water absorption.
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Histology Of The
Large Intestine• The submucosa is similar to that found in the rest
of the GI tract.
• The muscularis consists of an external layer of longitudinal smooth muscle and an internal layer of circular smooth muscle. This forms three conspicuous longitudinal bands called the teniae coli.
• Tonic contractions of the bands draw the LI together into pouches called haustra.
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Large Intestine Functions• Completion of absorption (water, ions, and
vitamins).
• Production of some B vitamins and vitamin K by
bacteria in the LI.
• Formation of feces.
• Expulsion of feces (defecation) from the body
through haustral churning and peristalsis.
• The iliocecal sphincter regulates the passage of
chyme from the ileum into the cecum.
• Normally, the valve remains partially closed to limit
passage of chyme.
• After a meal, the gastroileal reflex intensifies ileal
peristalsis.
• The hormone gastrin also relaxes the sphincter.
• Haustral churning – The haustra remain relaxed and become distended while they fill up. When the distention reaches a certain point, the walls contract and squeeze the contents into the next haustrum.
• Peristalsis occurs.
• Mass peristalsis is a movement that occurs at the middle of the transverse colon and quickly drives contents into the rectum.
• Food in the stomach initiates the gastrocolic reflex.
Mechanical Digestion In LI
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Motor activity of an intestine
Types of contractions:
1. The rhythmic
2. Like pendulum
3. Peristaltic (very slow, slow, fast, prompt)
4. The antiperistaltic
5. The tonic.
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Chemical Digestion LI
• Mucous is secreted by the glands
of the LI, but no enzymes are
secreted.
• The bacteria of the LI perform the
final stages of digestion.
• Bacteria ferment any remaining carbohydrates, which releases hydrogen, carbon dioxide, and methane gases. These gases constitute the flatus (gas) in the colon. When the gas is excessive it is termed flatulence.
• Bacteria convert any remaining proteins into amino acids.
• Some B vitamins and vitamin K are produced by the bacteria in the colon.
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Absorption & Feces
Formation In The LI
• Water is absorbed from the chyme over a period of 3-10 hours. As the water becomes absorbed it becomes solid or semi-solid and is termed feces.
• The feces consists of water, inorganic salts, sloughed off epithelial cells, bacteria, products of bacterial decomposition, unabsorbed digested materials, and indigestible parts of food.
• The LI absorbs water, vitamins and ions (I.e. sodium and chloride).
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Dietary Fiber
• Dietary fiber consists of indigestible plant
carbohydrates such as cellulose, lignin,
and pectin. These are found in fruits,
vegetables, grains, and beans.
• Insoluble fiber does not dissolve in water.
Insoluble fiber speeds up passage of
materials through the track.
– Woody or structural parts of plants
include the skins of fruits and
vegetables and the bran coating
around wheat and corn kernels.
• Soluble fiber dissolves in water and forms
a gel that slows passage of material
through the tract.
– Soluble fiber is found in beans, oats,
barley, broccoli, prunes, apples, and
citrus fruits.
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Defecation Reflex
• Mass peristalsis movements push
fecal material from the sigmoid
colon into the rectum.
• This causes distention of the rectal
wall, which stimulates stretch
receptors and initiates a
defecation reflex that empties the
rectum.
• This reflex opens the internal anal
sphincter.
• The external anal sphincter is
voluntarily controlled.