Download Digestion

Digestive system
Is a disassemble line to break nutrients down into forms that can be used by the
body
for cells to perform metabolic reactions the cells need oxygen and organic
molecules that can be broken down to release the energy in there bounds
the digestive system provides these organic molecules
digestive processes
1. ingestion
2. propulsion
voluntary swallowing
involuntary peristalsis
alternate eaves of smooth muscle contraction and relaxation
3. mechanical digestion
chewing and mixing food
segmentation- rhythmic local contractions of intestines
4. chemical digestion
food to monomers using digestive enzymes
5. secretion
release of water, acids, enzymes, and salts
6. absorption
end products from GI tract into blood or lymph
7. excretion or defecation
elimination of indigestible substances
digestive system is separated into two main groups
1. the alimentary canal or gastrointestinal tract
includes
mouth
pharynx
esophagus
stomach
small intestine
large intestine
2. accessory digestive organs
includes
teeth
tongue
gallbladder
salivary glands
liver
pancreas
histology of alimentary canal
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from the esophagus to the anal canal the walls of the GI tract are
made of four layers or tunics
mucosa
submucosa
muscularis externa
serosa or adventitia
1. mucosa
is the moist epithelial membrane that lines the lumen of the
GI tract
functions
1. secretion of mucus, digestive enzymes, and
hormones
2. absorption of digestive produces
3. protection against infectious diseases
three layers of the mucosa
1. epithelium
oral cavity, pharynx, and esophagus
stratified squamous
most of remaining simple columnar
epithelium
lots of goblet cells
produces mucous
liberates food
protects against enzymes
some sites (stomach small intestine) contain
glands that produces
1. digestive enzyme
2. hormone from secreting cells
called
enteroendocrine cell
coordinate activates of
digestive tract and accessory
glands
2. lamina propria
underlies the epithelium
is loose areolar connective tissue
contains
2
1. capillaries and lymphatic vessels
for nutrients and absorption
2. lymph nodules for protection
3. sensory nerve endings
4. smooth muscle (very thin)
3. muscularis mucosae
thin layer of smooth muscle
adds strength and some movement
2. submucosa
moderately dense connective tissue
contains with much elastic fibers
1. blood and lymphatic vessels
traveling to lamina propria
2. lymph nodules
3. glands
4. nerve fibers
produces the submucosal nerve
plexus
regulates the activity of
glands in the mucosa and
submucosa
layer
3. muscularis externa
is an inner circular layer and outer longitudinal
of smooth muscle
also contains myenteric nerve plexus
regulates the contraction of smooth
muscle
responsible for peristalsis to propel foodstuff
in
4. serosa
is the outermost protective layer of organs located
the abdominopelvic cavity
is the visceral peritoneum
mesothelium
prevents organs from sticking to each other
not seen in oral cavity, pharynx, esophagus
they have an adventitia
is fibrous connective tissue no
epithelium
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oral cavity
digestive processes occurring in the mouth
1. analysis of materials before swallowing
2. mastication (chewing of food)
3. lubrication
mixing with mucus and salivary secretions
4. limited digestion by saliva
saliva
salivary amylase
breakdown of polysaccharides into smaller
fragments
salivary lipase
breakdowns triglycerides
only works at low pH so works in the stomach
97% water
moistens food
lysozyme
destroys bacteria
IgA antibodies
blocks infection
mucin which forms thick mucus when water is added
lubricates food
growth factors
release of saliva
secretion of saliva are controlled by ANS
salivatory nuclei of the medulla oblongata sends
actions potentials down the facial and
glossopharyngeal nerves to stimulate salivation
both sympathetic and parasympathetic nervous
system stimulates salivation
thus always have a flow
phases of saliva release:
1. oral phase
chemoreceptors
most strongly stimulated by
acidic substances
pressure receptors
presence of food in mouth
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results in the activation of the
parasympathetic nervous system to
produce abundant watery saliva rich
in enzymes
2. cephalic phase
thought, sight, smell of food
also stimulates the parasympathetic
nervous system to produce abundant
watery saliva rich in enzymes
sympathetic stimulation
results in the production of a smaller viscous
saliva with little enzyme content
due to constriction of blood vessels
feeding the glands
Absorption of oral cavity
very little if any absorption occurs in the mouth
some lipid soluble substances
stomach
functions:
1. function is primarily as a food storage organ
50 ml empty
4 liters extremely full
will extend nearly to the pelvis
2. also mechanically breaks up food particles
3. liquefies the food to produce chyme
4. some limited digestion of proteins and lipids
5. helps sterilize the food
acidity of stomach
actions of digestive enzyme pepsin
histology of stomach
stomach is lined by a simple columnar epithelium
is a secretary sheet
produces a carpet of mucus to protect the stomach for acid
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the epithelium is organized into shallow depressions called gastric
pits
pits are lined with columnar epithelium called mucous
epithelium
release mucin
cell at base of pit undergo mitosis to replace cells
at the base of a gastric pit will be two or three gastric glands
cells of gastric glands
1) mucous neck cells
located at the top of the gland where it opens
into the pit
produces mucin
2) parietal cells
produce intrinsic factors
required for the absorption of
B12 by the intestine
B12 need to produce RBC
Release drops as you age increasing
chances of pernicious anemia
produce HCL
1. CO2 diffuses into cell from blood
2. carbonic anhydrase produces H
and HCO3
3. H/K ATPase pump out H in
exchange for K
4. Cl shifts into cell as HCO3 goes to
blood
alkaline tide
function of HCL
kills micros
denatures proteins
breaks down plant cell
wall
activates pepsin
converts Fe3 to Fe2
can be absorbed
3) chief cells
produce pepsinogen
is converted to pepsin by
HCL
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function of pepsin
pepsin digests proteins to
shorter peptide chains
produce rennin and gastric lipase in
infants
milk digestion
4) enteroendocrine cells
produces hormones
gastrin
produced by G cells
stimulates secretion by parietal
cells and chief cells
somatostatin
produced by D cells
weekly inhibits gastrin release
5) regenerative cells
located at the base of the gland
undergoes mitosis to replace old cells
Digestive process of the stomach begin with the production of a gastric secretion
gastric secretion contains mucus, acid, enzymes, and hormones
regulation of gastric secretion
release is controlled by both nervous and hormonal mechanisms
three overlapping phases
cephalic, gastric, and intestinal
1. cephalic phase
directed by CNS to prepare the stomach to receive food
occurs before food enters the stomach
triggered by sight, smell or thought of food
PNS nerve fibers traveling down the vagus nerve stimulates
the mucous cells, chief cells, parietal cells, and G cells
2. gastric phase
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occurs once food has reached stomach
results from
1)neural stretch reflexes
2)gastrin stimulation
most important stimuli of the gastric phase are distention,
peptides and low acidity
neural response
distention of stomach by the food activates
local stretch reflexes stimulate the parietal
cells to make HCl and the chief cells to
release pepsinogen and stimulate the G cells
to release gastrin
hormonal response
gastrin released do to low acidity and
peptides and by stretch
gastrin effects
1. gastrin stimulates the release of
pepsinogen from chief cells
2. main effect of gastrin is the
simulation of HCl from the parietal
cells
more acid = more gastrin
which = more acid (positive
feedback loop)
gastrin release is inhibited
when pH drops lower then 2
this stimulates release
of somatostatin and
provides a negative
feed back control
3. intestinal phase
starts when chyme enters the SI
main function is to control the entrance of acidic
chyme into the duodenum
neural response
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stretch on SI receptors by the chyme inhibits
gastrin production by the stomach
hormonal response
four hormones are involved
gastrin released from the small
intestine if pH of chyme is 3 or above
stimulates gastric secretion
secretin released from the SI if pH of
chyme is less than 3
inhibits gastric secretion
cholecytokinin (CCK)
gastric inhibitory polypeptide (GIP)
released from SI due to the presence
of fatty acid and lipids in the chyme
both inhibit gastric secretions
regulation of gastric emptying
periodic gastric emptying must be closely regulated by neural reflexes and
by hormones
too fast and chyme is not formed
too slow and stomach too acidic
regulation is primarily accomplished by controlling the opening and
closing of the pyloric sphincter
stimulation of gastric emptying
1. stretch of the stomach mildly relaxes the pyloric
sphincter
2. gastrin released form the stomach due to stretch
acid and amino acids relaxes the pyloric sphincter
inhibition of gastric emptying
1. stretch and chemical receptors (acid, amino acids
and lipid) in the small intestine activate neural
reflexes that slow gastric emptying by contracting
the pyloric sphincter
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2. hormones of small intestine
acid, amino acids and lipid in the small
intestine also stimulate the release of
secretin, CCK and GIP from the small
intestine
inhibit emptying by contracting the pyloric
sphincter
small intestine
site of most digestion and almost all absorption
averages 10 feet in a living person (21 in a cadaver) and is 1 inch in
diameter
three regions
1. duodenum
smallest region
10 inches
is retroperitoneal
receives:
1. stomach contents
acid chyme
2. pancreatic juice
digestive enzymes and bicarbonate solution
3. bile
fat emulsifiers and waste products
site where ducts from gall bladder and pancreas enter
have a common entry point called the common bile ducts
controlled by a valve called the sphincter of Oddi
closed between meals
2. jejunum
3 feet long
more digestion then absorption
3. ileum
6 feet long
more absorption then digestion
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to perform the roles of digestion and absorption there are specializations to
increase surface area
1. plica circulares
deep folds of the inner surface (submucosa) of the intestine
their shape causes the chyme to spiral
not found in the distal half of the ileum
2. villi
fingerlike projections of the mucosal surface of the
intestine
3. microvilli
tiny projections of the plasma membrane of the
epithelial cells (absorptive cells)
also called the brush borders
has brush border enzyme
mostly for carbohydrates and
proteins
become progressively smaller in more distal regions of SI
histology of small intestine
cells of the mucosal epithelium
cells of the villus
simple columnar cells called absorptive cells
contain brush borders
mirovilli
1. produce digestive enzymes
brush border enzymes
2. contain transporters for nutrient uptake
goblet cells
secretes mucus
intestinal crypts also called intestinal glands
start at the base between two villi
secret intestinal juice
cell types
absorptive cells
goblet cells
enteroendocrine cells
secretes hormones
S cells = secretin,
CCK cells= CCK
G cell = gastrin
paneth cells
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secretes lysozyme and is
phagocytic
contains the epithilial cells that
renew the old cells
Peyer’s patches
mucosa-associated lymphoid tissue
(MALT)
structures of the submucosa
duodenal glands or Brunner’s glands
secrete an abundant alkaline mucus
neutralize stomach acid
lymphatic nodules like Peyer patches
more numerous closer to large intestine
intestinal secretion
1. largely water and mucus
1.8 liters per day
mainly secreted by duodenal glands and goblet cells
water is necessary for acid hydrolysis
most comes in to SI by osmosis
2. is enzyme poor
most enzymes of SI are brush border enzymes
3. is slightly alkaline to buffer acid
release of SI secretions
cephalic phase
ANS
parasympathetic stimulation stimulates
release of secretions before food arises
mucus will protect the walls from
enzymes
sympathetic stimulation inhibits release of
mucous so get duodenal ulcers
intestinal phase
local reflexes
distention by the acid chyme
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irritation of the intestinal wall by the acid
chyme
hormones
gastrin, CCK and secretin
released due to the presence of acid, amino
acids and lipid
small intestine is the site of most chemical digestion
this requires
1. brush border enzymes
2. juices from the gall bladder
3. juices from the pancreas
pancreas
produces enzymes that break down all categories of foodstuff
the enzymes are delivered to the duodenum by two pancreatic ducts.
The larger one which fuses with the bile duct to form the common
bile duct
controlled by a valve called the sphincter of Oddi
the smaller one (accessory duct) empties directly into the small
intestine
physiology of pancreas
endocrine pancreas
islets of Langerhans
beta cells release insulin
alpha cells release glucagon
exocrine pancreas
organized into small clusters of glandular epithelial cells
called acini
composition of pancreatic juice
produce about 1.5 qt. Per day
1. mainly water
2. electrolytes
most is sodium bicarbonate
3. is alkaline pH8
this is to neutralize the acid chyme from stomach
provides optimal environment for enzymes
4. enzymes
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most are release in inactive form
proteases
trypsinogen is activated to trypsin by enterokinase
enterokinase is a brush border enzyme
procarboxypeptidase = carboxypeptidase
chymotrypsinogen = chymotrypsin
proelastase= elastase
starch enzymes
pancreatic amylase
lipid enzymes
pancreatic lipase
DNA and RNA enzymes
nucleases
regulation of pancreatic secretion
1. neural stimulation
parasympathetic activity during cephalic
phase of gastric secretion stimulates section of pancreases
2. hormone control
due to food in the small intestine
1. CCK (released in response to fats in SI)
stimulate the release of pancreatic enzymes
2. secretin (released in response to HCL in SI)
stimulate pancreatic duct cells to release bicarb rich
juice
liver and gallbladder
functions of the liver
carbohydrate metabolism- maintaining normal blood
glucose
lipid metabolism- produce lipoproteins which traffic lipids
amino acid metabolism- deaminate amino acids
removal of waste products
vitamin storage
mineral storage
drug inactivation
phagocytosis and antigen presentation
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Kupffer's cells
plasma protein synthesis
removal of circulation hormones
removal of antibodies
removal and storage of toxins- bilirubin for example
synthesis and secretion of bile
liver’s role in digestion is the production of bile for export to the
duodenum
bile is a yellow-green alkaline solution
its role in digestion is as a fat emulsifier (meaning it breaks
up fat into tiny particles)
gallbladder is storage organ for bile
composition of bile
1. bile salts
1. cholesterol derivatives
mostly cholic acid and chenodeoxycholic
acid
2. emulsify fats
breaks them up into many small droplets
3. help the absorption of fats and cholesterol
bile salts are recycled by the enterohepatic
circulation
1. bile salts are reabsorbed by a specific
transporter by distal part of SI
2. return to the liver by the hepatic portal
vein
3. transported to gall bladder for release
2. phospholipids
important in fat emulsification and absorption
3. bile pigments
most is bilirubin -- a waste product of heme
bacteria breakdown to urobilinogen which
gives feces a brown color
4. cholesterol
5. electrolytes
mostly sodium bicarbonate
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regulation of bile release into SI
cephalic phase
parasympathetic impulses via the vagus simulate
release
not real strong stimulator
hormonal phase
1. CCK stimulates gallbladder contraction and
relaxation of sphincter of Oddi of the common bile
duct
is released due to presence of fats in the
chyme entering the SI
2. Secretin released by intestinal cells due to
presence acid in the chyme entering the SI
stimulates the bile duct cells to produce a
watery bicarbonate-rich juice
large intestine
about 5 feet long and 2.5 inches in diameter
four regions
cecum
the ileocecal sphincter allows material in from the ileum
2.4 inches long
off to the side of the cecum is the appendix
3 inches long
colon
about four feet long
ascending, transverse, descending, and sigmoid
rectum
about 8 inches long
final 3cm is the anal cannel
histology
not as complicated as the small intestine
no villi
no plicea circulares
few glands
epithelium
simple columnar with numerous goblet cells
columnar cells are also called absorptive cells
mainly absorb water
muscularis
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longitudinal muscles are thickened into three conspicuous
longitudinal bands called taeniae coli
contraction forms a series of pouches called haustra
functions of the large intestine
chemical digestion in the large intestine
the finial stage of limited chemical digestion occurs here by
bacteria
1. ferment some of the indigestible carbohydrates
flatus
hydrogen sulfide (H2S), H2, N2, CO2,
methane (CH4) two amines
indole and skatole
together with H2S = odor of
flatus and feces
the
2. synthesize B complex vitamins and most vitamin K
B vitamins necessary for some enzymes and DNA
production
K necessary for formation of clotting proteins by
liver
absorption
water
very important
some electrolytes
vitamins
vitamin K released from bacteria
required to synthesize four clotting factors
biotin
required for the mitochondria to convert
pyruvate (from anaerobic glucose
metabolism) to oxaloacetate during aerobic
metabolism
vitamin B5 (pantothenic acid)
is a component of coenzyme A (CoA)
required for fats, polysaccharides,
and proteins to enter the citric acid
cycle
for production of steroid hormones
and some neurotransmitters
remaining bile salts
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elimination of fecal material (defecation)
Chemical digestion and absorption
1. catabolic process
2. breakdown foodstuff into their monomers which can be absorbed
3. accomplished by enzymes
intrinsic enzymes found on surface (brush border)
secreted enzymes from accessory glands
4. enzymatic breakdown is called hydrolysis
part of a water molecule is added to each broken bond
the diet mainly contains carbohydrates, proteins, and fats
digestion of carbohydrates
carbohydrates in the diet
1) starch,
most of the digestible dietary carbohydrate
are long glucose polymers
2) glycogen
not much in diet
digested like starch
are glucose polymers
3) sugars
disaccharides
sucrose
lactose
small amount of maltose
monosaccharides
glucose
galactose
fructose
these are ready to be absorbed (monomers)
4) cellulose
can not be digested = dietary fiber
all carbohydrates must be broken down into monomers
mostly
glucose
fructose
galactose
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these are quickly absorbed
digestion of carbohydrates
starches
1. salivary amylase
works in mouth and center of food while in
stomach
2. pancreatic amylase
breaks down starches into smaller pieces.
Only takes 10 minutes to work in small intestine
yields
1. maltose (two glucoses)
2. 3 to 9 glucose chain polymers
(oligosaccharides)
3. dextrinase and glucoamylase found on brush border of
SI
works on small glucose polymers made of three or
more sugars
yeilds the sugars glucose and maltose
4. maltase found on the brush border of SI
splits maltose into two glucose molecules
sugars (disaccharides)
major dietary sugars are
sucrose (glucose + fructose)
lactose (glucose + galactose)
maltose (glucose + glucose)
disaccharides are digested by brush border enzymes of SI
1. maltase
breaks down maltose to two glucose molecules
2. sucrase
breaks down sucrose to glucose and fructose
3. lactase
breaks down lactose to galactose and glucose
absorption
glucose is absorbed
1) transcellular by sodium-glucose transport proteins
secondary active transport coupled to sodium ions
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2) paracellular by solvent drag
galactose is absorbed
1) transcellular by galactose-sodium transport proteins
secondary active transport coupled to sodium ions
2) paracellular by solvent drag
fructose is absorbed
1) transcellular by facilitated diffusion that is not sodium
coupled
fructose is quickly converted to glucose inside the
cell so there is always a strong gradient
2) paracellular by solvent drag
protein digestion
1. stomach
1. pepsinogen secreted by chief cells cleaved to pepsin by
acid
2. works in acidic conditions pH 1.5 to 3.5
stops in small intestine
3. attacks bonds involving tyrosine and phenylalanine
so get smaller polypeptides and some free amino
acids from terminal tyrosine and phenylalanine
digests 10 to 15% of dietary protein into shorted
polypeptides
particularly effective in digesting collagen which is poorly
digested by other enzymes
prepares meat for other enzymes
2. small intestine
1. pancreatic enzymes
trypsin
chymotrypsin
reduces size of polypeptides by attacking
internal peptide bonds
so have many smaller peptides
carboxypeptidase
removes single amino acids from the
carboxyl end (-COOH)
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2. brush border enzymes
carboxypeptidase
removes single amino acids from the
carboxyl end
aminopeptidase and dipeptidase
removes single amino acids from the amine
end (-NH2)
these work best in neutral pH
absorption
there are specific amino acid-sodium transport proteins
transporters for each class of amino acids
classes are
neutral charged amino acids
positively charged amino acids
negatively charged amino acids
there are also specific sodium dependent transporters for some di
and tripeptides
once in the cell they are broken down
infants can absorb proteins by pinocytosis
allows IgA from breast milk to inter blood stream intact
absorbed proteins also may triggers some food allergies
lipid digestion
almost all occurs in the small intestine
very small amount by salivary lipase
fats (triglycerides) are insoluble in water
so enzymes can not get at them
and form large droplets
bile salts from gallbladder act as detergents
bile salts have a polar end with combines with water and a
nonpolar end which combines with the fats
bile salts pull off small droplets from larger
thus emulsifies fats
this increases surface area of the droplets so enzymes can
get at them
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pancreatic lipases can now cleave two fatty acid chains leaving
fatty acids and monoglycerides
absorption
lipase activity breaks down the triglycerides
the fatty acids and monoglycerides combined with
phospholipids from bile and with bile salts to form
even smaller micelles
micelles have a core of lipids shielded
from water by a surface of bile salts
also in the micelle is cholesterol and fat sol
vitamins
the micelles come in contact with the cell surface and melt
into the plasma membrane releasing the contents into the cell
once inside the cell the free fatty acids and monoglycerides
are resynthesized into triglycerides
the triglycerides combined with phospholipids and cholesterol and
are surrounded with a coat of proteins to form a spherical particle
called a lipoprotein
the lipoprotein formed in the guy is called a chylomicrons
this occurs in the endoplasmic reticulum with final
assemble occurring in the Golgi and then exported from the
cell
most chylomicrons go to lacteals
some to capillaries
the chylomicrons enter the blood stream and lipoprotein lipase
found on the surface of capillary endothelium break the
triglycerides down to fatty acids and glycerol with can be
used by the tissue for energy or stored as fat in adipose tissue
the left over chylomicrons are taken up by the liver
nucleic acid
pancreatic nucleases
hydrolize RNA and DNA to nucleotides
Brush border nucleosidase and phosphatases
hydrolize nucleotides to their free base, pentose sugars and
phosphate ions
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absorption
specific carriers of purines and pyrmidines in the SI
vitamin absorption
1. fat-soluble vitamins
A, D, E, K
incorporated into micelles
2. water soluble vitamins
are small and easily absorbed by simple diffusion
except B12 which is large
binds with intrinsic factor from stomach
then binds to a specific receptor in the ileum
is endocytocytosised
electrolyte absorption
sodium
most absorbed coupled to active absorption of glucose and
amino acids
potassium
moves by simple diffusion
as water is absorbed potassium is concentrated and
moves into cells
anions like Chloride passively follow the sodium and potassium
iron
1. is actively transported into the GI cells where it is stored
by binding to ferritin
2. when needed it is released into the blood and bound to
transferritin
calcium
absorption when active form of Vitamin D is present
parathyroid hormone converts vitamin D to active form
and stimulates kidneys to reabsorb calcium
water absorption
9 liters of water enter the GI tract daily
most absorbed in SI due to uptake of nutrients
i.e. follows the glucose and sodium
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