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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
– thin layer of loose connective tissue
– contains BV and lymphatic tissue
Muscularis mucosae---thin layer of smooth muscle
– causes folds to form in mucosal layer
– increases local movements - increasing absorption
with exposure to “new” nutrients
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 divisions
– innervation by sensory and motor neurons
• vasoconstriction
• local movement by
muscularis mucosa
smooth muscle
• supply the secretory cells of the mucosal epithelium
• connected to the myenteric plexus (in the muscularis layer) by a series of interneurons
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 GI tract motility
Serosa
•
•
•
•
An example of a serous membrane
Covers all organs and walls of cavities not open to the outside of the body
Secretes a serous fluid
Consists of connective tissue covered with simple squamous epithelium
24-6
• Peritoneum
– visceral layer covers organs
– parietal layer lines the walls of
body cavity
Peritoneum
• Peritoneal cavity
– potential space containing a bit of
serous fluid
•
•
•
•
•
Mesentery – small intestines
Mesocolon – large intestine
Lesser omentum
Greater omentum
Peritonitis = inflammation
–
–
–
–
trauma
rupture of GI tract
appendicitis
perforated ulcer
24-7
Greater Omentum, Mesentery &
Mesocolon
24-8
Lesser 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
• Lips and cheeks-----contains buccinator muscle that keeps food
between upper & lower teeth
• Vestibule---area between cheeks and teeth
• Oral cavity proper---the roof = hard, soft palate and uvula
– floor = the tongue
24-11
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 –
– duct = Stensen’s duct
– blood supply – external carotid artery
– parasympathetic nerve supply from IX
– sympathetic innervation by superior
cervical ganglion
Submandibular gland –
– duct = Wharton’s duct
– opens lateral to the lingual frenulum
– blood supply from facial and lingual
arteries
– parasympathetic supply – facial nerve
Sublingual gland –
– 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
– facial nerve innervation
minor salivary glands
– 600 to 1000 glands
– small aggregates of secretory tissue
present in the submucosa
– predominantly mucus glands
labial, lingual, palatal, buccal,
glossopalatine and retromolar glands
24-13
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)
Enzyme (lysozyme) ---helps destroy bacteria
Protects mouth from infection with its rinsing action---1 to 1 and 1/2qts/day
•
•
multiple electrolytes: sodium, postassium, chloride, calcium, magnesium
amylase, mucins, peroxidase, lyzozyme, proline-rich proteins
– composition can vary from gland to gland
– parotid gland: watery fluid rich in amylase, proline-rich proteins and glycoproteins
– submandibular gland also contains higher levels of mucins
•
•
•
•
secretory Igs – IgM and IgM and IgA
glucose, amino acids, urea, uric acid and lipids
EGF, insulin, cAMP-binding proteins, albumin
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
24-14
Saliva
• functions:
– 1. buffering – bicarbonate, phosphate
ions
– 2. Pellicle formation
– 3. maintenance of tooth integrity
– 4. antimicrobial action
– 5. tissue repair
– 6. digestion
– 7. taste
24-15
• Cells in acini
(clusters)
• Serous cells
secrete a watery
fluid
• Mucous cells (pale
staining) secrete a
slimy, mucus
secretion
24-16
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
– sight, smell, sounds, memory of food, tongue stimulation---rock in
mouth
– 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
• Stop salivation
– dry mouth when you are afraid
– sympathetic nerves
– also inhibits secretion during dehydration
24-17
Digestion in the Mouth
• Mechanical digestion (mastication or chewing)
• breaks into pieces
• mixes with saliva so it forms a bolus
• Chemical digestion
– salivary amylase
• begins starch digestion at pH of 6.5 or 7.0 found in mouth
• when bolus & enzyme hit the pH 2.5 gastric juices
hydrolysis ceases
– lingual lipase
• secreted by glands in tongue
• begins breakdown of triglycerides into fatty acids and
glycerol
24-18
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
24-19
Teeth:
-grinding, tearing and shearing of food
-two main divisions: crown and root
-crown: above gumline
-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
24-20
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
24-21
Esophagus
•
•
•
•
•
Collapsed muscular tube
In front of vertebrae
Posterior to trachea
Posterior to the heart
Pierces the diaphragm at hiatus
– hiatal hernia or diaphragmatic
hernia
• 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
muscle
24-22
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. Involuntary phase----pharyngeal stage
–
–
–
–
–
breathing stops & airways are closed
soft palate & uvula are lifted to close off nasopharynx
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
24-23
Anatomy of Stomach
• Size when empty
– large sausage
– stretches due to rugae
• Parts of stomach
–
–
–
–
cardia
fundus---air in x-ray
body
pylorus---starts to narrow as
approaches pyloric sphincter
• Empties as small squirts of chyme leave the stomach
through the pyloric valve
24-24
Histology of the Stomach - Mucosa
•
•
•
•
•
•
•
simple columnar epithelium with embedded surface mucus cells
lamina propria layer under the epithelium (areolar connective tissue) +
muscularis mucosae (smooth muscle)
epithelial cells extend down into the LP where they form columns of
secretory cells = gastric glands that line narrow channels called gastric
pits
for the secretion of gastric juice – mix of water, HCl, enzymes and
hormones
food can remain in the fundus of the stomach for up to one hour before
being mixed with gastric juice
during this time – salivary amylase and lipase continue their digestion
after being mixed with gastric juice – these enzymes become inactivated
24-25
•
•
•
•
•
•
•
•
•
•
•
•
Hydrochloric acid (parietal cells) converts pepsinogen (from
chief cells) to the enzyme pepsin = protein digestion
stimulated by Ach release by the parasympathetic neurons,
gastrin secreted by G cells and histamine released by mast
cells in the LP
Gastric Mucosa
HCl from the parietal cells is secreted as H+ and Cl-ions
proton pumps powered by H+/K+ ATPases actively pump H+
into the lumen of the stomach
causes movement of K+ into the parietal cell
opening of K+ and Cl- channels causes the diffusion of these
ions back into the lumen
the H+ comes from the absorption of water into the parietal
cell – combination of water and CO2 by carbonic anhydrase
creates carbonic acid which dissociates into HCO3- and
H+ions
the bicarbonate is pumped into the blood in exchange for Clions
Intrinsic factor (parietal cells)
– absorption of vitamin B12 for RBC production
enzymes: pepsin and gastric lipase
– pepsin is secreted as the inactive form pepsinogen
– converted by contact with HCl
– lipase – splits short-chain triglycerides (milk) into two
fatty acids and a monoglyceride (glycerol + one FA
absorption of water, alcohol, drugs
mucus cells can absorb some short--chain fatty acids
24-26
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
• most effective at pH 5 to 6 (infant stomach)
• HCl kills microbes in food
• Mucous cells protect stomach walls from being
digested with 1-3mm thick layer of mucous
24-27
Absorption of Nutrients by the Stomach
•
•
•
•
Water especially if it is cold
Electrolytes
Some drugs (especially aspirin) & alcohol
Fat content in the stomach slows the passage of alcohol to
the intestine where absorption is more rapid
• Gastric mucosal cells contain alcohol dehydrogenase that
converts some alcohol to acetaldehyde-----more of this
enzyme found in males than females
• Females have less total body fluid that same size male so
end up with higher blood alcohol levels with same intake
of alcohol
24-28
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 receptors
• chemoreceptors monitor pH of the stomach chyme
• impulses travel to the submucosal plexus– activeatethe
parasympathetic and enteric neurons – causes waves of
peristalisis to mix the food and move food into the SI and
stimulates the flow of gastric juice
• as the pH of the stomach chyme decreases (becomes more
acidic) – this inhibits this path (negative feedback)
• as the food leaves the stomach and the stretching of the wall
lessens – this inhibits this path (negative feedback)
– hormonal regulation: digestive hormones
24-29
Digestive Hormones
• Gastrin
– Stomach (increases gastric juice), gastric & ileocecal
sphincters (relaxes)
• Gastric inhibitory peptide--GIP
– stomach & pancreas (inhibits gastrin effects)
• Secretin
– Pancreas (increases pancreatic juice), liver (stimulates
bile production) & stomach (decreases gastric juice)
• Cholecystokinin--CCK
– Pancreas (increases pancreatic enzyme synthesis),
gallbladder (increases production and secretion of bile)
& stomach (slows emptying)
24-30
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
24-31
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
24-32
Small
Intestine
• Structures that increase
surface area
– plica circularis
• permanent ½ inch tall
folds that contain part
of submucosal layer
• not found in lower
ileum
• can not stretch out like
rugae in stomach
– villi
• 1 Millimeter tall
• Core is lamina propria
of mucosal layer
• Contains vascular
capillaries and lacteals
(lymphatic capillaries)
Functions of Microvilli
•
•
•
•
•
– microvilli
• cell surface feature
known as brush border
Absorption and digestion
Digestive enzymes found at cell surface on microvilli
Digestion occurs at cell surfaces
Significant cell division within intestinal glands produces new cells that move up
Once out of the way---rupturing and releasing their digestive enzymes & proteins
24-33
Intestinal
Glands
Small Intestine - Mucosal
layer:
Epithelial layer
• Absorptive cells – digest and
absorb nutrients in the chyme
• Intestinal glands (Crypts of
Lieberkuhn)
• Goblet cells - Unicellular glands
that are part of simple columnar
epithelium
• Enteroendocrine cells
– within the intestinal glands
secretin
– cholecystokinin
– gastric inhibitory peptide
• Paneth cells
– secretes lysozyme
• Submucosal layer has duodenal glands
24-34
– kills bacteria
secretes alkaline mucus
Absorptive cells – production of
intestinal juice
-1 to 2 liters per day
-secreted by the absorptive cells
of the intestinal glands
-enzymes are made and inserted
into the microvilli of the
absorptive cells – enzymatic
digecsion by these enzymes
occurs on the surface of the
intestine rather than the lumen
of the SI
-enzymes for the digestion of
carbohydrates, proteins
(peptidases) and nucleic acids
-as the cells are turned over –
sloughed off and die – releasing
more enzymes that digest food
in the chyme
24-35
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
24-36
Mechanical Digestion in the Small
Intestine
•
1. Weak peristalsis in comparison to the
stomach---chyme remains for 3 to 5 hours
– occurs after most the meal has been absorbed
– also called the migrating motility complex
(MMC)
– 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
– do NOT push the food through the tract
– move chyme back and forth
– done in specific segments that are defined by
the smooth muscle layers
– most rapid in the duodenum and slows at it
reaches the ileum
24-37
Small Intestine-Chemical Digestion
-synthesis of the brush border enzymes by the intestinal
glands
-maltase, sucrase, lactase, a-dextrinase, enterokinase
aminopeptidase, dipeptidase, phosphatases and nucleosidases
-these enzymes are not secreted into the SI lumen but are
expressed on the surface of the absorptive cell
-duodenum is also the site for secretion of the pancreatic juice:
-trypsin, chymotrypsin, elastase, carboxypeptidase
-1-2 qt./day------ at pH 7.6
-Enzymes are made in the pancreas as inactive forms
eg. Chymotrypsinogen, proelastase, procarboxypeptidase
-Trypsin synthesized as trypsinogen - converted to trypsin by the
enzyme enterokinase
-Activated trypsin then converts others into active forms
24-38
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 (120g/hr)
– fructose passes through the apical membrane of the absorptive cells via facilitated
transport
– glucose and galactose are transported into cells via secondary active transport coupled to
the active transport of Na+ (symporter)
– glucose and galactose compete for the saccharide site on this transporter
– monosacharrides leave through the basal surface and enter the blood via facilitated
transport
• proteins – absorbed as amino acids by active transport in the duodenum and jejunum
– amino acids absorbed come from the food (50%) and from the digestive enzymes and the
degrading epithelial cells
– 95-98% of the proteins present in the SI are digested and absorbed
– different transporters carry different amino acids
– some enter absorptive cells through Na+/amino acid symporters
– others through H+.amino acid symporters
– enter into the blood via diffusion
24-39
•
SI: Absorption of digested
fats – absorbed via simple diffusion
– 95% absorbed in the small intestine
nutrients
– bile induced emulsification results in the
breakdown of a fat into 2 fatty acids + a
monoglyceride
– the fatty acids are either short-chain or long
chain
– once inside the cell – reassembled into
triglycerides
– these fats aggregate with phospholipids and
cholesterol to form globules (chylomicrons)
which enter the lacteal from the absorptive
cell via exocytosis
– enter the blood at the subclavian veins
– removed from the blood as they pass
through the liver
– liver expresses lipoprotein lipase (LPL) –
breaksdown the TGs and the cholesterol in
the chylomicron into fatty acids
– hepatocytes and adipose cells recombine
these components to reform the triglyceride
– adipose cells are also capable of breaking
down TGs (lipolysis) by expressing the LPL
enzyme
24-40
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
24-41
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
24-42
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
24-43
Histology of Large Intestine
•
•
Mucosa
– smooth tube -----no villi or plica folds
– mostly contains absorptive cells with microvilli (water) in its epithelial component + goblet cells
– both absorptive and goblet cells are located in long tubular intestinal glands = crypts of
Lieberkuhn)
– lamina propria and muscularis mucosae also
– intestinal glands fill the the mucosa
– simple columnar cells absorb water & goblet cells secrete mucus
Submucosal & mucosa contain lymphatic nodules
24-44
Mechanical Digestion in Large Intestine
• Smooth muscle = 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 –intensifies the peristaltic waves in
the ilium and the older chyme then enters the caecum
– gastrocolic reflex = when stomach fills, a strong peristaltic
wave moves contents of transverse colon into rectum
24-45
Chemical Digestion in Large Intestine
• No enzymes are secreted only mucous – the
goblet cells in the intestinal glands
• chyme is prepared by the action of bacteria
• 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
• Bacteria produce vitamin K and B in colon
24-46
Absorption & Feces Formation in
the Large Intestine
•
•
•
•
•
•
•
•
food has now been in the GI tract for 3 to 10 hours
solid or semisolid due to water reaborption = feces
feces – water, salts, sloughed-off epithelial cells, bateria, 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
also absorbs some electrolytes---Na+ and Cl- and vitamins
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 cholesterol to make more bile salts – takes this cholesterol from
the blood
•
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
24-47
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
24-48
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
24-49
Histology of the Pancreas
• Acini- dark clusters
– 99% of gland
– produce pancreatic
juice
• Islets of Langerhans
– 1% of gland
– pale staining cells
– produce hormones
24-50
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
24-51
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
24-52
Histology of the Liver
•
•
•
•
Hepatocytes arranged in lobules
bile synthesized and drains into bile canals = canaliculi
blood flowing into liver from intestine via hepatic
portal vein
divides into sinusoids – “cleaning” by cells
•
Kupffer cells phagocytize microbes & foreign matter
24-53
Gallbladder
•
•
•
•
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
24-54
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 brush border of the
absorptive cells
– the long-chain fatty acids and monoglycerides then diffuse out into
the absorptive cells cytoplasm – leaving the micelles behind (back
into the chyme)
– micelles act as a lipid “ferry”
– also solubilize other large hydrophobic molecules like the fatsoluble vitamins (A, D, E, K) and cholesterol
24-55
Liver Functions--Carbohydrate
Metabolism
• Turn proteins into glucose
• Turn triglycerides into glucose
• Turn excess glucose into glycogen & store
in the liver
• Turn glycogen back into glucose as needed
Liver Functions --Lipid
Metabolism
• Synthesize cholesterol - Synthesize
lipoproteins----HDL and LDL (used to
transport fatty acids in bloodstream)
• Stores some fat
• Breaks down some fatty acids
Liver Functions--Protein
Metabolism
• Deamination = removes NH2
(amine group) from amino acids
• Converts resulting toxic ammonia
(NH3) into urea for excretion by the
kidney
• Synthesizes plasma proteins utilized
in the clotting mechanism and
immune system
• Convert one amino acid into
another
24-56
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
• Intestines
– synthesis of brush border
enzymes-----aminopeptidase
or dipeptidase------split off
amino acid at amino end of
molecule or split dipeptides
24-58
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
(nucleosidease and phosphatase)
– pentose, phosphate & nitrogenous bases
• Absorbed by active transport
24-59