Download lecture #17

The Digestive System
• Mouth---bite, chew, swallow
• Pharynx and esophagus---transport
• Stomach----mechanical
disruption; absorption of water
& alcohol
• Small intestine--chemical &
mechanical digestion &
absorption
• Large intestine----absorb
electrolytes & vitamins (B and K)
• Rectum and anus---defecation
24-1
Types of Digestion
• Mechanical – mouth, stomach,SI, LI
• Chemical – mouth, stomach, SI
24-2
Layers of the GI Tract
1. Mucosal layer
2. Submucosal
layer
3. Muscularis layer
4. Serosa layer
24-3
Mucosa
•
•
•
Epithelium
– stratified squamous (in mouth, esophagus & anus) = tough
– simple columnar in the rest
• secretes enzymes and absorbs nutrients
• specialized cells (goblet) secrete mucous onto cell surfaces
• enteroendocrine cells---secrete hormones controlling organ function
Lamina propria
– loose connective tissue
– contains BVs and lymphatic tissue
Muscularis mucosae
– thin layer of smooth muscle
– causes folds to form in mucosal layer
– responsible for local food movements
24-4
Submucosa
• Loose connective tissue
– containing BV, glands and
lymphatic tissue
• Meissner’s plexus
– part of the enteric nervous system - “brain of the gut”
– parasympathetic division only - sensory and motor neurons
• vasoconstriction of blood vessels to gut
• controls contraction of muscularis mucosa
• controls the secretory cells of the mucosal epithelium
• connected to the myenteric plexus (in the muscularis layer)
24-5
Muscularis
• Skeletal muscle = voluntary control
– in mouth, pharynx , upper esophagus and anus
– control over swallowing and defecation
• Smooth muscle = involuntary control
– inner circular fibers & outer longitudinal fibers
– mixes, crushes & propels food along by peristalsis
• Auerbach’s plexus (myenteric plexus)
– both parasympathetic & sympathetic innervation of circular and
longitudinal smooth muscle layers
– part of the Enteric nervous system
– controls overall GI tract motility
24-6
Serosa
• An example of a serous membrane
• Covers all organs and walls of
cavities not open to the outside of
the body
• Covering the digestive organs in the
peritoneal cavity = visceral
peritoneum
• Secretes a serous fluid
• Consists of connective tissue
covered with simple squamous
epithelium
24-7
Peritoneum
• Peritoneum
– visceral layer covers
organs
– parietal layer lines the
walls of body cavity
• Peritoneal cavity
– potential space containing
a bit of serous fluid
24-8
The Mesenteries of the GI tract
• Mesentery – small intestines  parietal
peritoneum
• Mesocolon – large intestine  parietal
peritoneum
• Lesser omentum – liver  stomach
• Greater omentum
24-9
The path of food:
oral cavity/teeth/salivary glands
oropharynx/epiglottis
esophagus
stomach
small intestine: duodenum
small intestine: jejunum
small intestine: ileum
large intestine: ascending colon
large intestine: transverse colon
large intestine: descending colon
sigmoid colon
rectum
anus
24-10
Mouth
• Oral cavity proper---the roof = hard, soft palate and uvula
– floor – geniohyoid, mylohyoid – contains the tongue
– lips and cheeks-----contains buccinator muscle that keeps food between upper &
lower teeth
– Vestibule---area between cheeks and teeth
• Lined with an oral mucosa (stratified squamous epithelium & lamina propria)
– Lining of the cheeks = buccal mucosa
– Lining of the maxilla and mandible = alveolar mucosa (gingiva)
• Landmarks: lingual frenulum, labial frenulum, uvula
24-11
– Shortened lingual frenulum can impede movement of the tongue within the mouth
• physiological functions
– manipulation of food for chewing
and swallowing
– production of digestive enzyme
– speech
Tongue
• made of extrinsic and intrinsic
muscles that control movement and
the shape of the tongue
• extrinsic – control the movement of
the tongue in and out of the mouth,
manipulates food, hold the tongue in
position and forms the floor of the
mouth
• intrinsic – originate from and insert
into the connective tissue of the
tongue, alter the shape and size of
the tongue for speech and
swallowing
• lamina propria layer of the mucosa
contains the lingual glands for the
secretion of mucus and lingual
lipase
24-12
•
Salivary Glands
•
•
Parotid Gland – development at 4 to
6 weeks
– duct = Stensen’s duct
– blood supply – external carotid
artery
– parasympathetic nerve supply
from IX
– sympathetic innervation by
superior cervical ganglion
Submandibular gland – development
at 6 weeks
– duct = Wharton’s duct
– opens lateral to the lingual
frenulum
– blood supply from facial and
lingual arteries
– parasympathetic supply – facial
nerve
Sublingual gland – development at 8
to 12 weeks
– small ducts = ducts of Rivinus
– larger duct = Bartholin’s duct
– empty with the SM duct at the
sublingual caruncle
– blood supply – sublingual and
submental arteries
24-13
– facial nerve innervation
Salivary Glands
•
•
minor salivary glands
– 600 to 1000 glands
– small aggregates of secretory
tissue present in the submucosa
– not found in the gingiva and
anterior hard palate
– predominantly mucus glands
– except the lingual glands (von
Ebner’s glands) found on the
tongue – open into troughs
surrounding the circumvallate
papilla
labial, lingual, palatal, buccal,
glossopalatine and retromolar glands
24-14
Saliva
• 600-1000ml/day
• Wet food for easier swallowing
• Dissolves food for tasting
• Bicarbonate ions buffer acidic foods
– bulemia---vomiting hurts the enamel on your teeth
• Chemical digestion of starch begins with enzyme (salivary
amylase)
• Protects mouth from infection with its rinsing action---1 to
1 and 1/2qts/day
• oral fluid is mixed or whole saliva – includes secretions
from all three major glands + minor glands plus
desquamated epithelial cells, microorganisms, food, debris,
inflammatory cells and serum components
24-15
Saliva
•
•
•
•
•
Components:
1. water
2. enzymes: amylase, lipase, lyzozyme
3. mucins, mucus
4. multiple electrolytes: sodium, postassium,
chloride, calcium, magnesium
• 5. glucose, amino acids, urea, uric acid and lipids
• 6. secretory Igs – IgG and IgM and IgA
• 7. growth factors & hormones: EGF, insulin,
24-16
Saliva
• functions:
– 1. buffering – bicarbonate, phosphate ions
• protection against demineralization caused by bacterial
acids resulting from the breakdown of sugars
– 2. Pellicle formation
• many salivary proteins bind to the surface of the teeth and
oral mucosa – forms a thin film = salivary pellicle
• several of these proteins bind calcium to protect the tooth
– 3. maintenance of tooth integrity
• saliva is saturated with calcium and phosphate ions
• at the tooth surface the high concentration of calcium and
phosphate results in maturation of the enamel – increases
surface hardness
24-17
– 4. antimicrobial action
• barrier function provided by mucins
• saliva also contains a spectrum of proteins with
antimicrobial activity – histatins, lysozyme, lactoferrin and
peroxidase
• also the presence of antibodies – IgA is the major salivary
Ig (results in agglutination of microbes and prevents their
adherence to oral tissues)
– 5. tissue repair
• variety of growth factors are present in saliva
• many promote tissue growth and differentiation
– 6. digestion
• amylase and lipase
– 7. taste
• solubilizes food substances – allows binding to taste
receptors located in taste buds
24-18
Salivation
• controlled by the ANS
• parasympathetic system provides a constant supply of saliva to
keep the mucus membranes moist and to lubricate the food
• Increased salivation – parasympathetic system
– stimulation of taste receptors are conveyed to the cerebral cortex to the
salivatory nuclei in brainstem
– returning impulses via the parasymp system travel via CN 7 & 9
• Decreased salivation – sympathetic system
24-19
Digestion in the Mouth
• Mechanical digestion = mastication or chewing
• breaks food into pieces
• mixes food with saliva so it forms a bolus
• Chemical digestion
– salivary amylase
• breakdown of complex carbohydrates into smaller pieces
• activity dependent upon pH
– lingual lipase
• secreted by glands in tongue
• begins breakdown of triglycerides into fatty acids and
glycerol
24-20
Teeth:
-grinding, tearing and shearing of food
-two main divisions: crown and root
-crown: above gum-line
-innermost layer - pulp (nerves/blood vessels)
-outer covering of specialized calcified connective tissue - dentin
-covered with a layer of enamel
-root: entry of nerves and blood vessels
-secures the tooth into the jaw (cementum)
-covered by a periodontal membrane - unites with gums
24-21
Primary and Secondary
Dentition
-primary: 20 teeth starting at 6 months
-secondary/adult: between 6 and 12 years = 32 teeth
8 incisors - biting
4 canines (cuspids) - tearing
8 premolars (bicuspids) - grinding
12 molars (tricupids) - grinding
** third pair of molars (wisdom teeth) may not erupt -impacted
24-22
Pharynx
• Funnel-shaped tube extending from internal
nares to the esophagus (posteriorly) and larynx
(anteriorly)
• Skeletal muscle lined by mucous membrane
• Deglutition or swallowing is facilitated by saliva
and mucus
– starts when bolus is pushed into the oropharynx
– sensory nerves send signals to deglutition center in
brainstem (medulla oblongata)
24-23
Esophagus
•
•
•
•
•
Collapsed muscular tube
In front of vertebrae
Posterior to trachea
Posterior to the heart
Pierces the diaphragm at hiatus
• Mucosa = stratified squamous
• Submucosa = large mucous glands
• Muscularis = upper 1/3 is skeletal,
middle is mixed, lower 1/3 is
smooth
–
upper & lower esophageal
sphincters are prominent circular
arrangements of smooth muscle
Physiology of the Esophagus - Swallowing
•
•
•
•
1. Voluntary phase---tongue pushes food to
back of oral cavity
stimulates receptors in the oropharynx
message travels to the deglutition center in
the MO and lower pons
2. Pharyngeal stage (involuntary)
– breathing stops & air passages are closed
– vocal cords close
– epiglottis becomes bent over airway (glottis)
as larynx is lifted
– controlled by autonomic nervous system
•
3. Esophageal phase (involuntary) Peristalsis pushes food down
– circular fibers behind bolus contract
– longitudinal fibers in front of bolus also
contract to shorten the distance of travel
and widens the espophagus
•
lower esophageal sphincter relaxes as
food approaches the stomach
24-25
Anatomy of Stomach
• Size when empty
– large sausage
– stretches due to rugae
• Parts of stomach
–
–
–
–
cardic portion
fundus
body
pyloric portion – narrows at the
pyloric sphincter
• bolus mixes with gastric juice
in the stomach to form chyme
• stomach empties as small
squirts of chyme leave the
stomach through the pyloric
sphincter
24-26
Muscularis
• Three layers of
smooth muscle-outer
longitudinal,
circular & inner
oblique
• Permits greater
churning &
mixing of food
with gastric juice
Serosa
• Simple squamous
epithelium over a bit of
connective tissue
• Also known as visceral
peritoneum
24-27
Histology of the Stomach - Mucosa
• simple columnar epithelium with embedded
surface mucus cells
• epithelial cells form columns of secretory cells =
gastric glands that line narrow channels called
gastric pits
• glands - for the secretion of gastric juice
• mix of water, HCl, enzymes and hormones
24-28
• Hydrochloric acid (parietal cells) converts
Pepsinogen (from chief cells) to the
enzyme pepsin = protein digestion
• HCl from the parietal cells is secreted as
H+ and Cl-ions
• the H+ comes from the absorption of water
into the parietal cell
Gastric Mucosa
– combination of water and CO2 by carbonic
anhydrase creates carbonic acid which
dissociates into HCO3- and H+ ions
• proton pumps actively pump H+ into the
lumen of the stomach
• the bicarbonate is pumped into the blood in
exchange for Cl-ions
• Cl- channels then allow the diffusion of
these Cl- into the lumen
• Intrinsic factor (parietal cells)
– absorption of vitamin B12 for RBC
production
24-29
Gastric Mucosa
Secretions
• Parietal cells: HCl and Intrinsic Factor
• Chief cells: enzymes: pepsin and gastric lipase
– pepsin is secreted as the inactive form
pepsinogen
– converted by contact with HCl
– gastric lipase – splits short-chain triglycerides
(e.g. in milk) into two fatty acids and a
monoglyceride (glycerol + one FA)
24-30
Stomach--Chemical Digestion
• Protein digestion begins
– HCl denatures (unfolds) protein molecules
– HCl transforms pepsinogen into pepsin that breaks
peptides bonds between certain amino acids
• Fat digestion continues
– gastric lipase splits the triglycerides in milk fat
• HCl kills microbes in food
• Mucous cells protect stomach walls from being
digested
24-31
Absorption of Nutrients by the Stomach
•
•
•
•
Water especially if it is cold
Electrolytes
Some drugs (especially aspirin) & alcohol
Gastric mucosal cells contain alcohol dehydrogenase that
converts some alcohol to acetaldehyde
– more of this enzyme found in males than females
– 5 different forms of ADH – some more efficient then others
24-32
Gastric phase of digestion
• starts once food reaches the stomach
• neural and hormonal regulation to promote gastric
secretion and motility
– neural regulation:
• distension of the stomach stimulates stretch mechanoreceptors –
stimulates production of gastric juices
• impulses travel via the parasympathetic neurons – stimulates
the flow of gastric juice, causes waves of peristalisis to mix the food
with the juice and move food into the SI
• chemoreceptors monitor pH of the stomach chyme as juice is
being made
• as the pH of the stomach chyme decreases (becomes more acidic) –
creates a negative feedback loop
• as the food leaves the stomach and the stretching of the wall lessens
– this inhibits this path
– hormonal regulation: digestive hormones
• gastrin, secretin, CCK, GIP
24-33
Anatomy of the Small Intestine
• 20 feet long----1 inch in diameter
• Large surface area for majority of
absorption
• 3 parts
– duodenum---10 inches
– jejunum---8 feet
– ileum---12 feet
• ends at ileocecal valve
24-34
Small
Intestine
• SI has specific structures
that increase surface area
– plica circularis
• permanent ½ inch tall
folds that contain part
of submucosal layer
• not found in lower
ileum
• cannot stretch out like
rugae in stomach
– villi
• Contains vascular
capillaries and lacteals
(lymphatic capillaries)
– microvilli
Functions of Microvilli
•
•
• produces a cell surface
feature known as
brush border
Absorption and digestion
Digestive enzymes found at cell surface on microvilli - digestion occurs at
cell surfaces
Intestinal
Glands
Small Intestine - Mucosal
layer:
Epithelial layer
• Absorptive cells –absorb nutrients in the
chyme
• cells at the bottom of the mucosa form
Intestinal Glands
• Goblet cells - Unicellular glands that are
part of simple columnar epithelium
• Enteroendocrine cells
– found within the Intestinal glands
– secretin
– cholecystokinin
– gastric inhibitory peptide
• Paneth cells
– secretes lysozyme
– kills bacteria
24-36
• Absorptive cells:
– production of brush-border
enzymes (“intestinal juice”)
-1 to 2 liters per day
-enzymes are made and inserted
into the microvilli of the
absorptive cells
-BB enzymes: enzymes for the
digestion of carbohydrates,
proteins (peptidases) and nucleic
acids
-
24-37
SI mucosal layer cont....
• lamina propria of the SI contains areolar
connective tissue
• plus an abundance of mucosa-associated
lymphatic tissue – MALT
– solitary lymphatic nodules in the distal part of
the ileum
– groups of nodes in the ileum – Peyer’s patches
24-38
Mechanical Digestion in the Small
Intestine
• 1. Weak peristalsis in comparison to the
stomach---chyme remains for 3 to 5 hours
– starts at the lower portion of the stomach and
pushes the chyme forward
– reaches the end of the ileum after 90 – 120
minutes
– then another wave starts in the stomach
• 2. Segmentation---local mixing of chyme
with “digestive juices” in the SI
– does NOT push the food through the tract
– move chyme back and forth over the lining of
the SI
– done in specific segments of the SI
– most rapid in the duodenum and slows at it
reaches the ileum
24-39
Small Intestine-Chemical Digestion
-protein and carbohydrate digestion via synthesis of
the brush border enzymes by the intestinal glands
-enterokinase
-maltase, sucrase, lactase, a-dextrinase
-aminopeptidase, dipeptidase
-phosphatases and nucleosidases
-enzymes are expressed on the surface of absorptive
cells – external digestion
-resulting monosaccharides, amino acids and
nucleotides are internalized by the absorptive cell
-SO carbohydrate and protein digestion stops in
the interior of the absorptive cell
24-40
Small Intestine-Chemical Digestion
– BUT - duodenum is also the site for secretion of the
pancreatic juice:
– pancreatic amylase, pancreatic lipase + 4 proteases:
trypsin, chymotrypsin, elastase, carboxypeptidase
• 1-2 qt./day------ at pH 7.6
• proteases are made in the pancreas as inactive forms
– eg. Trypsinogen, chymotrypsinogen, proelastase,
procarboxypeptidase
• Trypsinogen converted to trypsin by the brush border
enzyme enterokinase
• Activated trypsin then converts other three proteases into
their active forms
24-41
SI: Absorption of digested
nutrients
• occurs via diffusion, facilitated transport,
osmosis and active transport
• water: 90% absorption occurs in the SI –
10% in the stomach and LI
• carbohydrates – absorbed as
monosaccharides by either facilitated or
secondary active transport
– fructose passes through the apical
membrane of the absorptive cells via
facilitated transport
– glucose and galactose are transported via
secondary active transport created by the
active transport of Na+ (eg. Na/glucose
symporter)
– glucose and galactose compete for the
saccharide site on this transporter
– all three monosacarrides leave through the
basal surface of the absorptive cell and
enter the blood via facilitated transport
24-42
SI: Absorption of digested
nutrients
• proteins – absorbed as amino
acids by active transport in the
duodenum and jejunum
– amino acids absorbed come from
the food and from the digestive
enzymes and the degrading
absorptive cells
– different transporters carry different
amino acids
– some enter through Na+/amino acid
symporters others through H+
amino acid symporters
– enter into the blood via diffusion
- 95-98% of the proteins
present in chyme in the SI are
24-43
digested and absorbed
SI: Absorption of digested
nutrients
Fat globule
• fats – absorbed via simple diffusion
– 95% absorbed in the small intestine
– bile induced emulsification results in breakdown of
a fat into 2 fatty acids + a monoglyceride (one fatty
acid + glycerol)
– the fatty acids are either short-chain or long chain
– short chain fatty acids and cholesterol move into
absorptive cell via diffusion
– long chain fatty acids and monoglycerides require
combination with bile in the form of a micelle
– micelle “ferries” the fatty acids and MGs into the
absorptive cells
•
bile returns to the SI lumen to “pick up” more FAs
and MGs
– once inside the cell – FAs and MGs are reassembled
into triglycerides
– these fats aggregate with phospholipids,
apolipoproteins and cholesterol in the absorptive cell
to form chylomicrons which enter the lacteal
– large pores of the lacteal allow for the passage of
chylomicrons
Bile salts
Fat droplets
coated with
bile salts
Epithelium
of small
intestine
Epithelium
of lacteal
Micelles made
up of fatty acids,
monoglycerides,
and bile salts
Lacteal
24-44
Fat Processing
– chylomicrons enter the blood at the subclavian veins
– removed from the blood as they pass through the liver
– liver expresses lipoprotein lipase (LPL) – breaks down the
chylomicron into fatty acids, monoglycerides and cholesterol
– cholesterol is used to make bile
– cholesterol, TGs and apoplipoproteins are assembled into
HDL or VLDL
• VLDL breaks down into LDL (by LPL enzyme)
– adipose cells are also capable of taking up chylomicrons and
breaking them down via the LPL enzyme
24-45
Absorption in the SI
• Blood that enters the SI villi is rich in oxygen,
poor in nutrients
• Blood that exits the villi is poor in oxygen and
rich in nutrients
• BUT: venous blood may also be contaminated
with toxins
• SO: blood from SI flows into the liver via the
hepatic portal vein – to be filtered prior to
emptying via the hepatic vein into the inferior
vena cava
24-46
Absorption of Water
• 9 liters of fluid dumped
into GI tract each day
• Small intestine reabsorbs
8 liters
• Large intestine reabsorbs
90% of that last liter
• Absorption is by osmosis
through cell walls into
vascular capillaries inside
villi
24-47
Anatomy of Large Intestine
•
•
•
•
•
5 feet long by 2½ inches in diameter
Ascending & descending colon are retroperitoneal
Cecum & appendix
Rectum = last 8 inches of GI tract anterior to the sacrum & coccyx
Anal canal = last 1 inch of GI tract
– internal sphincter----smooth muscle & involuntary
– external sphincter----skeletal muscle & voluntary control
24-48
Histology of Large Intestine
• Muscular layer
– internal circular layer is normal
– outer longitudinal muscle
• taeniae coli = shorter bands
• tonic contractions of these bands
puckers the LI into pouches =
haustra (pouches) formed (also
called diverticulum)
• epiploic appendages
• Serosa = visceral peritoneum
• Appendix
– contains large amounts of
lymphatic tissue
24-49
Histology of Large Intestine
•
•
Mucosa
– smooth tube -----no villi or plica folds
– mostly contains absorptive cells with microvilli in its epithelial component + goblet
cells
– both absorptive and goblet cells are located in long tubular structures called Intestinal
glands
– absorptive cells are for the absorption of water and salt ONLY!
– goblet cells secrete mucus
24-50
Submucosa & mucosa contain lymphatic nodules – contribute to immunity
Mechanical Digestion in Large Intestine
• Smooth muscle of muscularis = mechanical digestion
• Peristaltic waves (3 to 12 contractions/minute)
– haustral churning----relaxed pouches are filled by muscular
contractions in the haustra below it
– gastroilial reflex = when stomach is full, gastrin hormone
relaxes ileocecal sphincter so small intestine will empty and
make room for new chyme
• Also intensifies the peristaltic waves in the ileum and the older chyme
then enters the caecum
– gastrocolic reflex = when stomach fills, a strong peristaltic
wave moves contents of transverse colon into rectum
24-51
Chemical Digestion in Large Intestine
• No enzymes are secreted only mucous – by the
goblet cells in the intestinal glands
• BUT - chyme is aacted upon by the action of
bacteria
– bacterial based chemical digestion NOT human-based
• Bacteria ferment:
– undigested carbohydrates - carbon dioxide & methane
gas
– undigested proteins - simpler substances (indoles,
skatoles, hydrogen sulfide)----odor
– turn bilirubin into simpler substances that produce color
of feces
• Bacteria also produce vitamin K and B in colon
24-52
Absorption & Feces Formation in the
Large Intestine
• food has now been in the GI tract for 3 to 10 hours
• solid or semisolid form in the LI due to water reaborption =
feces
• feces – water, salts, sloughed-off epithelial cells, bacteria,
products of bacterial decomposition, unabsorbed and
undigested materials
• 90% of all water absorption takes place in the SI – 10% in the
LI
• but the LI is very important in maintaining water balance
– Excess water in feces = diarrhea
– Insufficient water in feces = constipation
• also absorbs some electrolytes---Na+ and Cl• can also absorb some vitamins
24-53
Fiber
• feces also contains materials undigestible by human enzymes =
Fiber
• Three types of fiber:
• dietary fiber - indigestible plant carbohydrates (cellulose, lignin and
pectin)
• soluble fiber – dissolves in water (beans, barley, broccoli, prunes,
apples and citrus)
– forms a gel that slows the passage of materials through the colon
– also helps to lower blood cholesterol – binds to bile salts to prevent their
reabsorption
– liver must make more bile – so it gets the cholesterol component of bile from
LDL in the blood – lowers LDL levels
– also acted upon by bacteria – carbon dioxide and methane gas
• insoluble fiber – woody or structural parts of the plant (skins of
fruits and vegetables, coatings around bran and corn)
– passes though the colon relatively unchanged
24-54
Defecation
• Gastrocolic reflex moves
feces into rectum
• Stretch receptors signal
sacral spinal cord
• Parasympathetic nerves
contract muscles of rectum
& relax internal anal
sphincter
• External sphincter is
voluntarily controlled
24-55
Digestive Hormones
• production of digestive hormones
– gastrin – by the G cells of the stomach lining
• stimulates production of gastric juice and encourages emptying of the stomach
• also relaxes the sphincter between the ileum and cecum (ileocecal)
– gastric inhibitory peptide – antagonist to gastrin
– CCK – by the enteroendocrine cells of the SI (presence of fatty
acids)
• CCK stimulates the release of pancreatic juice and bile (synthesis and
increased gallbladder contraction)
• also slows the emptying of the stomach
• decreases gastric juice production
– secretin – by the enteroendocrine cells of the SI
• secretin stimulates the release of bicarbonate from the pancreas – neutralizes
chime
• decreases gastric juice production
• decreases gastrin production and release
• increases pepsinogen
• role in water regulation – activates the release of ADH from posterior
pituitary
Digestive Feedback systems
• emptying of the stomach:
– production of gastrin – stimulates emptying
– production of GIP/enterogastrone and CCK – inhibits emptying
• pancreatic juice production:
– secretin and CCK – stimulation of production
• bile production:
– CCK – stimulation of secretion
Key
Liver
Enterogastrone
Gallbladder
Stimulation
Inhibition
Gastrin
CCK
Stomach
Pancreas
Secretin
Duodenum
CCK
Anatomy of the Pancreas
• 5" long by 1" thick
• Head close to curve in
C-shaped duodenum
• pancreatic duct joins
common bile duct from
liver
• Opens 4" below pyloric
sphincter
24-58
Histology of the Pancreas
• Acini- dark clusters
– 99% of gland
– produce pancreatic
juice
• Islets of Langerhans
– 1% of gland
– pale staining cells
– produce hormones
– alpha cells, beta cells,
delta cells, F cells
24-59
• Endocrine cells secrete near a capillary
• 1 to 2 million
pancreatic islets
• Contains 4 types of
endocrine cells
•
•
•
•
Alpha cells (20%) produce glucagon
Beta cells (70%) produce insulin
Delta cells (5%) produce somatostatin
F cells produce pancreatic polypeptide
24-60
Composition and Functions of Pancreatic Juice
• 1 + 1/2 Quarts/day at pH of 7.1 to 8.2
• Contains water, enzymes & sodium bicarbonate
• Digestive enzymes
– pancreatic amylase, pancreatic lipase, proteases
–
–
–
–
–
trypsinogen---activated by enterokinase (a brush border enzyme)
chymotrypsinogen----activated by trypsin
procarboxypeptidase---activated by trypsin
proelastase---activated by trypsin
trypsin inhibitor---combines with any trypsin produced inside
pancreas
– ribonuclease----to digest nucleic acids
– deoxyribonuclease
24-61
Anatomy of the Liver and Gallbladder
• Liver
–
–
–
–
weighs 3 lbs.
below diaphragm
right lobe larger
gallbladder on right
lobe
– size causes right
kidney to be lower
than left
• Gallbladder
– fundus, body & neck
24-62
Histology of the Liver
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•
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Hepatocytes arranged in lobules comprised of
hepatocytes
Lobules are surrounded by branches off the hepatic
portal vein
Running between the hepatocytes of the lobules are
blood filled spaces = sinusoids
Also running between hepatocytes of a lobule are bile
canals = bile canaliculi – join to form the bile ducts of
the liver
Kupffer cells phagocytize microbes & foreign matter
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Gallbladder
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Simple columnar epithelium
No submucosa
Three layers of smooth muscle
Serosa or visceral peritoneum
Bile Production
• One quart of bile/day is secreted by the liver
– yellow-green in color & pH 7.6 to 8.6
• Components
– water & cholesterol
– bile salts = Na & K salts of bile acids
– bile pigments (bilirubin) from hemoglobin
molecule
Flow of Bile
• Bile capillaries
• Hepatic ducts connect to
form common hepatic duct
• Cystic duct from gallbladder
& common hepatic duct join
to form common bile duct
• Common bile duct &
pancreatic duct empty into
duodenum
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Bile functions
• emulsification – breakdown of fats into fatty acids (long or short
chain) + monoglycerides
• make the long-chain fatty acids and monoglycerides (which
are large) more soluble in the watery environment of the chyme
• bile salts surround the long-chain fatty acids and form tiny
spheres called micelles
– amphipathetic nature of bile salts – hydrophobic portion interacts with the
fatty acids
• micelles are absorbed into the absorptive cell
– the long-chain fatty acids and monoglycerides then separate from the bile
and move into the absorptive cells cytoplasm – leaving the micelles behind
(move back into the chyme)
– micelles act as a lipid “ferry”
– also solubilize other large hydrophobic molecules like the fat-soluble
vitamins (A, D, E, K) and cholesterol
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Liver Functions--Carbohydrate
Metabolism
• Turn amino acids into glucose (gluconeogenesis)
– Alanine and glutamine
• Turn triglycerides into glucose
Liver Functions--Protein
Metabolism
– Turns glycerol into glucose (gluconeogenesis)
– Turns odd number FAs into glucose (gluconeogenesis)
• Turn excess glucose into glycogen & store
• Deamination = removes NH2
in the liver
(amine group) from amino acids
• Turn glycogen back into glucose as needed
• Converts resulting toxic ammonia
(NH3) into urea for excretion by the
Liver Functions --Lipid
kidney
Metabolism
• Synthesizes plasma proteins utilized
in the clotting mechanism and
• Synthesize cholesterol - Synthesize
immune system
lipoproteins----HDL and LDL (used to
transport fatty acids in bloodstream)
• Convert one amino acid into
another
• Stores some fat
• Breaks down some fatty acids for
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energy production (ATP)
Other Liver Functions
• Detoxifies the blood by removing or altering drugs &
hormones (thyroid & estrogen)
• Releases bile salts help digestion by emulsification
• Stores fat soluble vitamins-----A, B12, D, E, K
• Stores iron and copper
• Phagocytizes worn out blood cells & bacteria
• Activates vitamin D (the skin can also do this with 1 hr
of sunlight a week)
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Chemical Digestion
Digestion of Carbohydrates
• Mouth---salivary amylase
– inactivated in the stomach by
the low pH
• Esophagus & stomach--nothing happens
• Duodenum
– secretion of pancreatic juice
(amylase)
– synthesis of the brush border
enzymes (maltase, sucrase &
lactase) act on disaccharides
• produces monosaccharides-fructose, glucose &
galactose
• lactose intolerance (no
enzyme; bacteria ferment
sugar)--gas & diarrhea
Digestion of Proteins
• Stomach
– HCl denatures or unfolds
proteins
– secretion of pepsinogen and
activation by HCl - pepsin turns
proteins into peptides
• Pancreas
– secretion of pancreatic juice
which contains trypsin,
chymotrypsin etc…
• Intestines
– synthesis of brush border
enzymes-----aminopeptidase or
dipeptidase------split off amino
acid at amino end of molecule
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Digestion of Lipids
• Mouth----lingual lipase
• Small intestine
– emulsification by bile
– pancreatic lipase within
the pancreatic juice--splits into 2 fatty acids + a
monoglyceride
– no lipid-digesting enzymes
made by the brush border
Digestion of Nucleic Acids
• Pancreatic juice contains 2 nucleases
– ribonuclease which digests RNA
– deoxyribonuclease which digests DNA
• Nucleotides produced are further
digested by brush border enzymes
(nucleosideases and phosphatases)
– break the nucleotide into pentose,
phosphate & nitrogenous bases
• Absorbed by active transport
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