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Chapter 23The Digestive System
G.R. Pitts, Ph.D., J.R. Schiller, Ph.D. and James F. Thompson,
Ph.D.Use the video clips:CH 23 Digestive System General Anatomy, CH
23 Anatomy of the Mouth and Esophagus, CH 23 Anatomy of the
Stomach, CH 23 Anatomy of the Pancreas, CH 23 Anatomy of the Liver,
CH 23 Anatomy of the Small Intestine and CH 23 Anatomy of the Large
Intestine for a review of digestive system structure
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Digestive ProcessesIngestionMovement of foodDigestionMechanical
digestionChemical digestionAbsorptionDefecation
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General StructureDigestive organs divided into 2 main groups GI
(alimentary) tract Accessory structures cheeks, teeth, tongue,
salivary glandsliver, gallbladder, pancreas
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Salivary Glands3 pairs salivary glands Parotid glands
Submandibular glands Sublingual glands
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Salivary GlandsComposition of Saliva 99.5% water, 0.5% solutes
Na+, K+, Cl-, HCO3-, and PO4-, proteins, waste productslysozyme
salivary amylase digests carbohydrates
Saliva composition differs among the 3 glandsparotid - watery
saliva, amylasesubmandibular - thicker mucous, amylasesublingual -
mostly mucous, a little amylase
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Salivary GlandsFunctions of SalivaWater dissolves food for taste
and digestionMucous moistens and lubricates foodMucous lubricates
oral surfaces for smooth actions in swallowing and speechCl- ions
activate amylase HCO3- and PO4- ions buffer bacterial acidsIgA,
lysozymes, cyanide, defensins: protect against microorganisms
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Salivary GlandsSecretion of Saliva - 1-1.5 L l dayPrimarily
under nervous controlParasympathetic (ANS) normal salivary
secretionssaliva swallowedmost reabsorbedSympathetic (ANS) reduced
flow (dry mouth)Food (mechanically, chemically) stimulates
salivation behavioral memories from cortexstarts digestioncontinues
after ingestion is completeirritating foods or nausea
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Mechanical digestion Chewing = masticationFood mixed with saliva
Shaped into a bolusChemical digestion salivary amylase breaks down
and converts polysaccharides (starches) to disaccharides (maltose)
and monosaccharides (glucose) [no enzymatic action with cellulose
which is also a polymer of glucose]Physiology of Digestion in
Mouth
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Physiology of Deglutition (Swallowing)Moving bolus from mouth to
stomachThree phasesFacilitated by saliva, mucous secretionsInvolves
mouth, pharynx, esophagus
Buccal phaseVoluntaryMoves bolus to oropharynx
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Physiology of DeglutitionPharyngeal phase Involuntary
Receptors in oropharynx stimulate medulla and pons to:Block
mouth with tongueBlock nasopharynx with soft palateRaise larynx to
seal epiglottis, blocking airwaysRelax upper esophageal
sphincter
Bolus is moved through pharynx into esophagus
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Physiology of DeglutitionEsophageal stageUpper esophageal
sphincter closesGastroesopaheal sphincter opensEsophagus controls
involuntary peristaltic movementEpiglottis reopensBolus moves from
esophagus to stomach
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EsophagusPeristalsisInvoluntary, rhythmic contraction of
muscularisControlled by medullary centersA movement activity: inner
circular layer of smooth muscle contracts behind bolus to push it
forward; outer longitudinal muscle contracts to pull esophagus wall
up
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EsophagusPhysiologyUpper esophageal sphincterPeristalsisLower
esophageal (cardiac) sphincterSharp transition from nonkeratinized
stratified squamous epithelium to simple columnar
epithiliumEsophageal epithelium resistant to abrasion but not to
acid and proteolytic enzyme attack acid reflux disease
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StomachPhysiology of digestion - Mechanical digestion
peristaltic movement (mixing waves) back and forth between body and
pylorus3 muscle layers: longitudinal, circular, and
obliquechyme
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StomachPhysiology of digestion - Chemical digestionparietal
cells secrete intrinsic factor for B12 absorptionparietal cells
secrete HCl by active transportkills microbes, denatures
proteinscauses some acid hydrolysis of food moleculesstimulates
secretion of hormones for bile & pancreatic juice flowchief
cells secrete pepsinogen (inactive precursor)activated to pepsin by
HCl acid and by other pepsinsonly an effective protease at acid
pHcleaves proteins into smaller peptides
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Stomach: MucosaGastric gland chief cellsSecrete pepsinogen
(inactive precursor)activated to pepsin by HCl acid and by other
activated pepsin enzymesonly an effective protease at acid
pHcleaves proteins into smaller peptides
Secrete rennin in neonatescurdles milk to increase time for
gastric processing
Secrete gastric lipase in neonatessplits short chain
triglycerides common in milklimited role in digestion since it
works best at pH 5-6
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Stomach: MucosaSimple columnar epithelium with goblet cells and
gastric pitsSecretes 2-3 L l day
Gastric gland parietal cellsSecrete intrinsic factor for B12
absorptionSecrete HCl by active transportkills microbes, denatures
proteinscauses some acid hydrolysis of food moleculesstimulates
secretion of hormones for bile & pancreatic juice flow
Goblet & gastric pit mucous cells secrete mucin1-3 mm mucus
layer in the stomach prevents self-digestion
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Stomach: MucosaGastric gland G cells (enteroendocrine)Secrete
gastrin, histamine, serotonin, somatostatin
Absorption
Impermeable to diffusion of most molecules into the
bloodstream
Absorbs a few lipid soluble compounds:certain drugs (e,g.,
aspirin) alcohol
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Stomach: Regulation of Secretion and MotilityRegulated by
combination of neuronal and hormonal factors
3 phasesCephalicGastricIntestinal
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Stomach: Regulation of Secretion and MotilityCephalic
phaseStimulisightsmelltastethoughts/memories
EffectParasympathetic impulses increase gastric secretion
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Stomach: Regulation of Secretion and MotilityGastric phase
Neural negative feedback mechanisms
Distension activates stretch receptors causing myenteric and
vagovagal reflexes to release AchAch stimulates gastric juice
secretion
Chemoreceptors respond to partially digested proteins, caffeine
and rising pHStimulate gastrin secretion from G cells
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Stomach: Regulation of Secretion and MotilityGastric phase
(cont.)GastrinInhibited at pH < 2 Gastrin transported in the
blood to the gastric glandsGreatly stimulates HCl
secretionStimulates histamine secretionSlightly stimulates
pepsinogen secretionContracts lower esophageal sphincterIncreases
gastric motilityRelaxes pyloric sphincter
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StomachGastric phase (continued)Control of HCl secreting
parietal cells stimulation by three signal
chemicalsgastrinacetylcholinehistamineAll three needed for strong
H+ secretionH+ pumps work in conjunction with carbonic
anhydraseblockage of the histamine H2 receptor decreases HCl
secretionTagametZantacHClKHCO3
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Stomach: Regulation of Secretion and MotilityIntestinal phase
has excitatory and inhibitory components:
Excitatory Very short phase
Initiated by chyme entry into duodenum
Stretch receptors stimulate release of intestinal (enteric)
gastrin
Chemoreceptors detect fatty acids, & glucose in the
duodenumStimulate enteric gastrin release
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Stomach: Regulation of Secretion and MotilityIntestinal phase
(cont.)InhibitoryEnterogastric reflex: stretch receptors,
chemoreceptors trigger 3 reflexes thatInhibit vagoval reflexInhibit
myenteric reflexActivate sympathetic nervous system to close
pyloric sphincterInhibit gastric secretion
Enterogastrone secretionEnteroendocrine cells in the small
intestine release:Cholecystokinin (CCK) Gastric inhibitory peptide
(GIP) SecretinVasoactive intestinal peptide (VIP)Hormones inhibit
gastric secretion
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Stomach: Regulation of Gastric EmptyingFood normally passes
through stomach in 4 hours
Hormonal/neuronal reflexes regulate gastric emptying
Large meals and large amounts of liquid increase stomach
distension increasing rate of emptying
Stomach emptying inhibited by the enterogastric reflex,
enterogastrones, and fat in the duodenum
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StomachSummary
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PancreasPancreatic juice 1.2-1.5 L/day
Mostly water some salts, bicarbonate, enzymesalkaline, pH
7.1-8.2buffers acidic gastric juice, stops pepsin activity, creates
proper alkaline pH for enzymes acting in the intestine
Enzymes include:pancreatic amylasetrypsinogen, chymotrypsinogen,
procarboxypeptidase (inactive zymogens)pancreatic
lipaseribonuclease and deoxyribonuclease
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Regulation of Pancreatic SecretionNeural control from
parasympathetic division of ANS via vagus nerveAutoregulation by
sensing the presence of fatty acids and amino acids in the acidic
chymeHormonal control by the secretion of enteroendocrines from
duodenumSecretin stimulates secretion of water, HCO3- CCK
stimulates secretion of enzymes
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Liver: Blood SupplyTwo sourcesHepatic artery - oxygenated blood
from aorta
Hepatic portal vein -deoxygenated blood: absorbed nutrients and
toxins from the stomach and intestines hormones from the
pancreasbreakdown products of RBCs from the spleenBlood mixes in
the sinusoids
Hepatocytes (liver cells) modify and exchange molecules with the
blood
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LiverNote the portal triads and fenestrated capillary
sinusoidshepatic portal venous blood and arterial blood mix in the
sinusoidsCentral veins return blood to the systemic circulation via
the hepatic vein and inferior vena cava
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LiverSinusoids are patrolled by monocyte-derived stellate
reticuloendothelial (Kupffer's) cells
Phagocytize damaged blood cells, bacteria, and other
microbes
Phagocytize some foreign molecules such as toxins
Hepatocytes can also store or breakdown toxic molecules
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LiverHepatocytes Receive, process and store nutrients from
meals
Process heme and cholesterol breakdown products to make bile
components
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Liver: Bile SecretionBile from the hepatocytes enters bile
capillaries (canaliculi)
Canaliculi empty into small bile ducts
Hepatic ducts join the cystic duct from the gallbladder to form
the common bile duct
Gallbladder stores bile
Common bile duct meets pancreatic duct at the hepatopancreatic
ampulla (of Vater)
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LiverBile800-1000 ml/day
Yellow, brownish, or olive-green liquid
pH 7.6-8.6, mostly water, bile salts, bile acids, cholesterol,
lecithin (phospholipid), bile pigments, ions
Part digestive secretion, part excretory productbile salts help
in emulsification of ingested fatsbilirubin and other bile pigments
are wastes from lipid catabolism
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Liver: Bile SynthesisRegulation of bile
production/secretionnervous control from parasympathetic division
of ANS via vagus nerve
autoregulation by sensing the resence of fatty acids and amino
acids in the acidic chyme
hormonal control by the secretion of the enteroendocrines, CCK
and secretin, from the duodenumCholecystokinin = gall bladder
moves
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LiverPhysiology of the liver processes vital to lifeCarbohydrate
metabolism regulates blood glucose levelsglycogenesis
(insulin)glycogenolysis (glucagon)gluconeogenesis (glucagon)Lipid
metabolism - stores, metabolizes some triglyceridessynthesizes new
cholesterol degrades excess cholesterol for bile salt
productionProtein metabolism -deaminates AAs by removing amino
groups (-NH2) from AAsdeaminated AA's used for ATP production or
changed to carbohydrates or fats as neededdetoxifies ammonia (NH3)
by synthesizing urea (1 CO2 + 2 NH3 = urea)can convert AA's from
one to another (transamination)synthesizes and secretes most plasma
proteins
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LiverPhysiology of the liver - processes vital to life Storage
oil-soluble vitamins, iron, other nutrients and minerals
PhagocytosisRemoval of dietary toxins, hormones, drugsdetoxify or
store or secrete compounds into bile metabolize thyroid, steroid
hormonesSynthesis of bile saltsExcretion of bile -
bilirubinActivation of Vitamin D (?)
Pathologies of the liver hepatitis (viral, toxic), cirrhosis,
cancer
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Gall BladderPear-shaped sac, 7-10 cm longPhysiology stores and
concentrates bile between mealsCCK stimulates bile release for
fatty mealswhen the small intestine is empty, the hepatopancreatic
sphincter closes, forcing bile into the gallbladder for
storagePathology:gallstones
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Summary: Digestive HormonesenteroendocrinesGastrinGastric
Inhibitory Peptide
SecretinCholecystokinin
(There are others.)
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Small Intestine: Segmentationprimary action of small intestine
when food is present
a form of mechanical digestion
a mixing activity
alternate contraction, relaxation of antagonistic smooth
(circular and longitudinal) muscle segments in the intestine
controlled by the autonomic nervous system
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Small Intestine: Peristalsisas absorption continues, distension
decreases and true peristalsis starts
a movement which propels chyme onward
these weak movements which occur only after most nutrients have
been absorbed
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Small Intestine: Motility and SecretionIntestinal secretions 1-2
L/day, pH 7.6 mostly water and mucusbicarbonate buffer neutralizes
gastric acidprovide enzymes for final chemical digestion
Regulation of intestinal secretion and motility stimulated by
distension and acidic chymelocal reflexes increase Ach releaseVIP
stimulates production of intestinal secretionsbasal motility is
controlled by autorhythmic pacemakerslocal hormones and
parasympathetic ANS reflexes increase motility
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Small Intestine: Chemical DigestionIntestinal secretions 1-2
L/day, pH 7.6 mostly water and mucusalong with pancreatic
secretions provide acid neutralization, final chemical digestion,
and more water for absorption Brush border enzymesbrush border
enzymes complete digestion of protein and carbohydrate
molecules
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Small Intestine: Chemical DigestionBrush border
enzymesEnteropeptidase (enterokinase) converts trypsinogen to
trypsinTrypsin activates other zymogens
Various other brush border enzymes complete digestion of protein
and carbohydrate molecules
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Small Intestine: Chemical DigestionComplete digestion is a
function of bile, pancreatic secretions and intestinal
secretionsAlthough produced by different organs, they all function
in the small intestine
Prior to small intestine, only limited activitymouth salivary
amylase stomachpepsin lingual lipase
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Small Intestine: Chemical DigestionChemical digestion in the
small intestine:Carbohydrate digestion pancreatic amylase digests
starchesdisaccharidases liberate monosaccharides
Protein digestion pancreatic proteases (trypsin, chymotrypsin,
carboxypeptidase)finished by brush border proteases in the lining
epithelium
Lipid digestion bile salts for emulsification pancreatic
lipase
Nucleic acid digestion pancreatic ribonuclease and
deoxyribonucleasebrush border enzymes digest nucleotides
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Small Intestine: AbsorptionAbout 90% of all absorption takes
place in small intestine
Nutrient absorption
Monosaccharides: facilitated diffusion and Na+-driven secondary
active transport
Amino acids: primary and Na+-driven secondary active
transport
Di- and tripeptides: H+-driven secondary active
transportNutrients enter capillaries via diffusion, facilitated
diffusion, or active transport
Nutrients are transported in the blood to the liver via the
hepatic portal circulation
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Small Intestine: Nutrient AbsorptionElectrolytes (minerals)Na+:
Primary active transportK+: facilitated diffusionFe: Active
transportCa2+: Active transport, vitamin D is a cofactor
VitaminsWater-soluble vitamins (B complex & C) absorbed by
diffusion - B12 absorbed with intrinsic factor Fat-soluble vitamins
(A, D, E, K) included with other lipids in
micelles/chylomicrons
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Small Intestine: Nutrient AbsorptionLipids (neutral fats,
cholesterol, phospholipids, etc.) are emulsified by bile salts,
forming micellesPancreatic Lipase breaks triglycerides into 2 fatty
acids and 1 monoglyceride
monoglycerides, fatty acids and other lipids diffuse into
cells
SER re-synthesizes triglycerides
all lipids packaged into chylomicrons by Golgi
apparatuschylomicrons leave the cell by exocytosis and enter
lacteals of the lymphatic system
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Small Intestine: Water AbsorptionTotal volume added to the small
intestine/day - 9.3 L ~2.3 L from ingestion~7.0 L from
secretions
Small intestine absorbs ~8.3 L /daypassive absorption following
nutrient moleculesosmosis
The rest of the water (~1.0L/day) passes to large intestine
where most is reabsorbed (~0.9 L/day)
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Large IntestineFunctionsCompletion of absorption, especially
final absorption of H2ONormal flora manufacture certain vitamins (B
complex, K)Formation and expulsion of fecesAnatomy 1.5 m L, 6.5 cm
WDivided into 4 general areas:cecumcolonrectum anal canal
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Large Intestine: DigestionMechanical digestion Chyme passage
regulated by ileocecal sphinctervalve generally closed - slow
passagefollowing a meal gastroileal reflex: ileal motility
increases, sphincter relaxes, chyme moves to the cecumwhen the
cecum is full, the sphincter contractsColon movements start when
chyme passes sphincterhaustral churning haustra relaxed, distended
until full then contract, squeeze contents into next
haustrumperistalsis is slowmass peristalsis (gastrocolic reflex)
during or immediately following a meal, 3-4 times daystrong
peristaltic waves from middle of transverse colon push contents
into the rectum
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Large Intestine: DigestionChemical digestion
much mucus but no enzymes are secreted
some digestion of chyme by bacteria in colon
final breakdown of substances, mostly carbohydrates
bacteria produce some vitamins, B complex and K
some bacterial metabolites are toxic, but the liver normally
deals successfully with them
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Large IntestineAbsorption and feces formation Chymeafter 3-10
hours in the large intestine, chyme becomes solidified (due to
water reabsorption) into feceslarge intestine absorbs water,
electrolytes, some vitamins and any toxinsFeces water, inorganic
salts, sloughed off intestinal epithelial cells, bacteria, products
of bacterial decomposition, undigested parts of foodmost water is
reabsorbed in small intestine, but the large intestine is also
important in water reabsorption
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Large IntestinePhysiology of defecation Mass peristalsispushes
fecal matter into rectumdistension stimulates stretch receptors
initiating reflex for defecation
Parasympathetic ANS stimulated by stretch receptorsstimulates
contraction of rectumshortens and increases pressure in
rectumparasympathetic stimulation relaxes internal sphincter
Conscious stimulation relaxes external sphincterfeces
expelled