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Anatomy and Physiology:
The Digestive System
Overview
1 Gastrointestinal (GI) Tract2 Accessory Organs of the Head3 Swallowing4 Stomach5 Accessory Organs of the Abdomen6 Small Intestine7 Large Intestine8 Phases of Digestion 9 Food Molecules
Essential Termsdigestionprocess of mechanically or chemically
breaking down foodabsorptionpassage of small molecules into blood and
lymphdigestive systemorgans which carry out process of digestion
and absorptionmetabolismall the chemical reactions of the body
Introduction
Digestive System1. Composed of GI tract and accessory
organs2. Breaks down ingested food for use by
the body3. Digestion occurs by mechanical and
chemical mechanisms4. Excretes waste products or feces
through process of defecation
GI Tract / Alimentary CanalContinuous tube from mouth to anusMouthPharynxEsophagusStomachSmall intestineLarge intestine
Accessory Digestive OrgansProvide mechanical and chemical mechanisms to
aid digestionTeethTongueSalivary glandsLiverGallbladderPancreas
Functions of Digestive System1. Ingestion2. Secretion3. Mixing and propulsion
• Motility
4. Digestion• Mechanical and chemical
5. Absorption6. Defecation
Layers of GI TractSame in all areas of GI tract
From deep to superficial:MucosaSubmucosaMuscularisSerosa
Figure 23.2
Layers of GI TractMucosa
EpitheliumType varies
Lamina propria – areolar connective tissueMALT – mucus-associated lymphatic tissue
Muscularis mucosae – smooth muscleSubmucosa
Areolar connective tissueBlood and lymphatic vesselsNeurons – submucosal plexus
Layers of GI TractMuscularis
Skeletal and smooth muscleNeurons – myenteric plexus
SerosaAreolar and simple squamous epitheliumVisceral peritoneum
PeritoneumMesotheliumParietal peritoneumVisceral peritoneumPeritoneal cavityRetroperitoneal
Figure 23.3a
Figure 23.3b
Figure 23.3c
Figure 23.3d
Neural Innervation of GI Tract Regulated by autonomic nervous system
Enteric divisionMyenteric plexus / plexus of AuerbachSubmucosal plexus / plexus of Meissner
Able to function independently from rest of nervous system
Linked to CNS by extrinsic sympathetic and parasympathetic nerves
Sympathetic nerves decrease GI secretions & motility Parasympathetic nerves increase GI secretion and
motility
Mouth Parts of Digestive System
Mouth formed by several parts:
CheeksLips / labiaLabial frenulumOrbicularisVestibuleOral cavity properFaucesHard and soft palateUvulaPalatoglossal and palatopharyngeal arch
Figure 23.4
TongueSkeletal muscle and mucous membraneHelps form floor of oral cavityExtrinsic musclesIntrinsic musclesLingual frenulumPapillae
FungiformFiliformCircumvallateFoliate
Lingual glandsLingual lipase
Salivary GlandsRelease saliva to oral cavity
3 pairs of salivary glandsParotidSubmandibularSublingual
Composition of Saliva99.5 % water0.5% other solutes
IonsMucusImmunoglobulin AEnzymes
Salivation controlled by autonomic nervous system
Stimulated by various mechanisms
Figure 23.5
Teeth External regions
1. Crown2. Root3. Neck
Internal components1. Enamel2. Dentin
Cementum3. Pulp cavity
PulpRoot canals Apical foramen
Figure 23.6
TeethDentitionsDeciduous teeth – first setPermanent teeth – secondary
Carry out mechanical digestion by mastication
Creates bolus
Salivary amylaseBreakdown starchLingual lipaseBreakdown triglycerides
Figure 23.7
PharynxComposed of skeletal muscleLined by mucous membrane
NasopharynxOropharynxLaryngopharynx
EsophagusCollapsible muscular tube through
esophageal hiatus of diaphragm
MucosaSubmucosa contains areolar connective tissue
MuscularisSkeletal muscleUpper and lower esophageal sphincter
AdventitiaAttaches esophagus to nearby structures
Secrets mucus and transports food
Figure 23.8
DeglutitionStages of swallowingVoluntary
Mouth to oropharynxPharyngeal
Deglutition center in medulla oblongata and ponsClosing of epiglottisInvoluntary
EsophagealInvoluntaryPeristaltic contractions
Figure 23.9a,b
Figure 23.9c
Table 23.2
Stomach Serves as mixing chamber and storage
area for ingested food Rugae allow for increased volume 4 main regions1. Cardia2. Fundus3. Body4. Pylorus
Pyloric antrum and canal Pyloric sphincter Lesser and greater curvatures
Figure 23.10a
Stomach Histology1. Mucosa
Surface mucous cells Lamina propria Muscularis mucosae Gastric glands and pits Parietal cells Chief cells G cells
2. Submucosa – areolar connective tissue3. Muscularis
3 layers of smooth muscle
4. Serosa
Figure 23.11a
Figure 23.11b
Mechanical and Chemical Digestion
Mixing waves caused by peristaltic movementChyme released in process of gastric emptyingProton pumps bring H+ into the lumenCarbonic anhydrase forms carbonic acid to
provide H+ and bicarbonate ions (HCO3-)
Figure 23.12
Mechanical and Chemical Digestion
Chemical digestion stimulated by nervous system
Parasympathetic neurons release acetylcholineWorks with gastrinHCl released in presence of histamine
Pepsin begins digestion of proteinsStomach protected by alkaline mucus secretion
Gastric lipase digests triglyceridesFew molecules absorbed by stomach
Water, ions, short-chain fatty acids, alcohol
Table 23.3 pt 1
Table 23.3 pt 2
PancreasProduces secretions to aid digestion HeadBodyTailPancreatic duct /duct of Wirsung
Hepatopancreatic ampullaSphincter of the heatopancreatic ampulla
(sphincter of (Oddi)Regulates passage of pancreatic juice and bile
Accessory duct (duct of Santorini)
Figure 23.13a
Figure 23.13b
Figure 23.13c
Histology of PancreasGlandular epithelial cells
99% exocrine clustersSecrete pancreatic juice
Fluid and enzymesPancreatic islets (islets of Langerhans)
1% endocrine cellsHormones
GlucagonInsulinSomatostatin
Pancreatic polypeptide
Pancreatic Juice 1200-1500 mL/day pH 7.1-8.2 Water Salts Sodium bicarbonate Enzymes
Pancreatic amylaseTrypsin
EntereokinaseChymotrypsinCarboxypeptidaseElastasePancreatic lipaseRibonuclease and deoxyribonuclease
Liver and GallbladderLiverLargest gland at 1.4 kg (~3 lb)
GallbladderClosely associated with liver
Anatomy of LiverRight and left lobe separated by falciform
ligamentQuadrate lobeCaudate lobe
Round ligament (ligamentum teres)Remnant of umbilical vein
coronary ligaments
Histology of LiverLobule
Hepatocytes radiating from central veinSinusoids
Reticuloendothelial (Kupffer) cellsStationary phagocytes
Figure 23.14a
Figure 23.14b
Figure 23.14c
Figure 23.14d
Bile Duct SystemBile secreted by hepatocytesBile canaliculiBile ductsRight and left hepatic ductsCommon hepatic ductCommon bile duct
Gallbladder for temporary storage of bileCystic duct
Blood Supply of LiverHepatic artery provides oxygenated bloodHepatic portal vein provides deoxygenated
bloodNutrients, drugs, toxins, microbes
Hepatic artery and vein carry blood to sinusoidsSubstances exchanged by hepatocytesBlood drains to central vein and eventually hepatic
veinPortal triad
Hepatic portal veinHepatic arteryBile duct
Figure 23.15
Bile800-1000 mL/daypH 7.6 – 8.6 WaterBile acidsBile salts
EmulsificationCholesterolLecithinBile pigments
BilirubinStercobilin
Liver Functions Metabolism of:
CarbohydratesLipidsProteins
Process drugs and hormones Excrete bilirubin Synthesize bile salts Storage
GlycogenVtaminsMinerals
Phagocytosis Activate Vitamin D
Small IntestineAdapted for digestion and absorption3 m (10 ft) living6.5 m (21 ft) without muscle tone
DuodenumJejunumIleum
Ileocecal sphincterConnection to large intestine
Figure 23.16a
Figure 23.16b
Histology of Small IntestineMucosaCell types
AbsorptiveGobletEndocrinePaneth
Lysozyme
Intestinal glands (crypts of Lieberkühn)S cells
Hormone secretinCCK cells
Hormone – cholecystokinin (CCK)
Figure 23.17a
Figure 23.17b
Histology of Small IntestineMALT – mucosa-associated lymphoid tissueSolitary lymphatic nodulesAggregated lymphatic follicles (Peyer’s patches)
SubmucosaDuodenal (Brunner’s glands)Alkaline secretion
MuscularisSerosa
Adaptive Structures Small IntestineCircular folds / plicae circularesVilliLacteal
Lymphatic capillaryMicrovilliBrush border
Brush border enzymesIntestinal juice
1-2 liters / daypH 7.6
Figure 23.18a
Figure 23.18b
Mechanical Digestion in Small Intestine
SegmentationLocalizedMix chyme with digestive juicesImportant for process of absorption
PeristalsisMovement along the length of small intestine
Chemical Digestion in Small Intestine
Completes digestion of food from the stomachCarbohydrates
Pancreatic amylaseGlycogen and starch only
-dextrinaseSucraseLactaseMaltase
Chemical Digestion in Small Intestine
ProteinsTrypsinChymotrypsinElastaseCarboxypeptidasePeptidases
Chemical Digestion in Small Intestine
LipidsPancreatic lipaseEmulsification
Amphipathic bile salts
Nucleic acidsNucleosidasesPhosphatases
Table 23.4 pt 1
Table 23.4 pt 2
Absorption in Small Intestine Passage of digested nutrients from gastointestinal
tract into blood or lymph 90% of nutrients absorbed through small intestine
Monosaccharides Facilitated diffusion
Fructose Secondary active transport
GlucoseGalactose
Enter blood through hepatic portal system
Absorption in Small IntestineAmino acids
Active transportNa+-dependent secondary active transport
Dipeptides and tripeptidesSymporter with H+
Absorption in Small IntestineLipids by simple diffusion
Due to emulsification and digestionMicelles formed due to amphipathic nature
of bile saltsChylomicrons
Triglycerides coated with proteinsLeave cells via exocytosisEnter blood vessels via lymphatic system
Enterohepatic circulation
Absorption in Small IntestineElectrolytes
DiffusionActive transportSecondary active transport
Vitamins
WaterOsmosis
Anatomy of Large Intestine Mesocolon attaches to posterior
abdominal wall
Regions1. Cecum2. Colon3. Rectum4. Anal canal
Ileocecal sphincter (valve) Allows passage into large intestine
Figure 23.21a
Figure 23.21b
Anatomy of Large Intestine1. Cecum
• Pouch• Attached appendix / veriform appendix
2. Colon• Ascending• Transverse• Descending• Sigmoid• Right and Left colic (splenic) flexures
3. Rectum4. Anal canal
• Anal columns• Anus• Internal and external sphincter
Histology of Large Intestine Mucosa
Absorptive cells absorb mainly waterGoblet cells secrete mucusLymphatic nodules
Submucosa Muscularis
HaustraExternal longitudinal smooth muscleTeniae coliInternal circular smooth muscle
SerosaEpiploic appendages
Figure 23.22a
Figure 23.22b
Figure 23.22c
Figure 23.22d
Mechanical digestion in Large Intestine
Gastroileal reflexIntensifies after a mealOccurs 3 or 4 times a day
Haustral churningDistension and contraction of haustra
PeristalsisMass peristalsis
Chemical Digestion in Large Intestine
Bacteria:Ferment carbohydrates
Gases produced are flatus or flatulence when excessive
Break down proteinsDecompose bilirubinFeces formed of dried chyme, inorganic salts,
mucus, bacteria, undigested foods and other substances
Defecation ReflexEmpties the rectumResponse to distention of rectal wallExternal anal sphincter voluntarily relaxed
defecation occurs
Table 23.7
Phases of DigestionCephalicGastricIntestinal
Cephalic PhaseStimulation of the senses activates CNSPrepares mouth and stomach for food
Phases of DigestionGastric PhaseBegins with food in the stomachNeural regulation
Negative feedback systemStretch receptorsChemoreceptors
Hormonal regulationGastrinReleased by G cells of gastric glandsControlled by negative feedback mechanism (pH)
Phases of DigestionIntestinal Phase
Begins with food in the small intestineInhibitory effects to slow exit of chyme
Neural regulationEnterogastric reflexDistension of duodenum
Phases of DigestionIntestinal Phase continued
Hormonal regulationCholecystokinin (CCK)
Stimulates release of pancreatic juiceContraction of gallbladder wallRelaxes sphincter of hepatopancreatic ampulla
SecretinResponse to acidic chymeStimulates flow of pancreatic juice for bufferingInhibits secretion of gastric juice
Table 23.8
Six Main Types of Nutrients1. Water2. Carbohydrates3. Lipids4. Proteins5. Minerals6. Vitamins
Essential nutrients cannot be made in sufficient amounts by the body
Guidelines for Healthy EatingVarietyMaintain healthy weightChoose low fat foodsLots of vegetables, fruits, and grainsSugar in moderationSalt and sodium in moderationAlcohol in moderation
Food Guide Pyramid
Figure 23.24
Nutrients Minerals
Inorganic elements constitute 4% of body massRegulate enzymatic reactionsServe as coenzymes
VitaminsOrganic molecules required in small amountsMost function as coenzymesMost cannot be synthesized by bodyProvitaminsFat-soluble vitamins
A, D, E, and KWater soluble vitamins
B and CAntioxidant vitamins
End