Gastrointestinal Physiology (Digestive System)

Gastrointestinal PhysiologyGastrointestinal Physiology(Digestive System)(Digestive System)

Dr. G. AHMADDr. G. AHMADAssistant Professor PhysiologyAssistant Professor Physiology

Functions of the Digestive SystemFunctions of the Digestive System

Ingest foodIngest food

Break down food into nutrient moleculesBreak down food into nutrient molecules

Absorb molecules into the bloodstreamAbsorb molecules into the bloodstream

Rid the body of indigestible remainsRid the body of indigestible remains

Main Divisions of the Digestive Main Divisions of the Digestive SystemSystem

Alimentary CanalAlimentary Canal Continuous, muscular digestive tube winding Continuous, muscular digestive tube winding

throughout the bodythroughout the body Digests and absorbs food particlesDigests and absorbs food particles Contains the following organs:Contains the following organs:

Mouth, Pharynx, Esophagus, Stomach, Small and Large Mouth, Pharynx, Esophagus, Stomach, Small and Large IntestinesIntestines

Accessory Digestive OrgansAccessory Digestive Organs Contains the following organs:Contains the following organs:

Teeth, Tongue, Gallbladder, Salivary Glands, Liver, and Teeth, Tongue, Gallbladder, Salivary Glands, Liver, and PancreasPancreas

Digestive System DivisionsDigestive System Divisions

Digestive ProcessesDigestive Processes

IngestionIngestion Mechanical digestionMechanical digestion PropulsionPropulsion Chemical digestionChemical digestion AbsorptionAbsorption DefecationDefecation

Actions of Digestive (GI) TractActions of Digestive (GI) Tract IngestionIngestion

Occurs when material Occurs when material enters via the mouthenters via the mouth

Mechanical ProcessingMechanical Processing Crushing / Shearing – Crushing / Shearing –

makes material easier to makes material easier to move through the tractmove through the tract

DigestionDigestion Chemical breakdown of Chemical breakdown of

food into small organic food into small organic compounds for compounds for absorptionabsorption

SecretionSecretion Release of water acids, Release of water acids,

buffers, enzymes & salts buffers, enzymes & salts by epithelium of GI tract by epithelium of GI tract and glandular organsand glandular organs

AbsorptionAbsorption Movement of organic Movement of organic

substrates, electrolytes, substrates, electrolytes, vitamins & water across vitamins & water across digestive epitheliumdigestive epithelium

ExcretionExcretion Removal of waste Removal of waste

products from body products from body fluidsfluids

Features and Functions of the Features and Functions of the MouthMouth

Buccal/oral cavityBuccal/oral cavity Contains stratified squamous epitheliumContains stratified squamous epithelium Vestibule: Vestibule: area bounded by lips and area bounded by lips and

cheeks externally and teeth and gums cheeks externally and teeth and gums internallyinternally

Lips: Lips: posses no sweat or oil glandsposses no sweat or oil glands Palate: Palate: forms roof of the mouth, soft and forms roof of the mouth, soft and

hard palate, uvulahard palate, uvula

Features and Functions of the Features and Functions of the TongueTongue

Helps grind food into a Helps grind food into a bolus bolus which which contains partially digested food and salivacontains partially digested food and saliva

Helps form words and is a sensory organ Helps form words and is a sensory organ for tastefor taste

Three surface features:Three surface features: Filiform papillae (roughness and grip)Filiform papillae (roughness and grip) Fungiform papillae (contains taste buds)Fungiform papillae (contains taste buds) Foliate papilae (contains taste buds)Foliate papilae (contains taste buds) Circumvallate papillae (contains taste buds)Circumvallate papillae (contains taste buds)

Papillae of the TonguePapillae of the Tongue

Features and Functions of the Features and Functions of the Salivary GlandsSalivary Glands

Main functions:Main functions: Produces and secretes salivaProduces and secretes saliva Cleanses the mouthCleanses the mouth Dissolves food chemicals so they can be Dissolves food chemicals so they can be

tastedtasted Moistens food, compacting it into a bolusMoistens food, compacting it into a bolus Begins the chemical breakdown of foodBegins the chemical breakdown of food

Salivary amylase: starchSalivary amylase: starch

Types of Salivary GlandsTypes of Salivary Glands

Submandibular GlandsSubmandibular Glands Found underneath the mandibleFound underneath the mandible

Sublingual GlandsSublingual Glands Found underneath the tongueFound underneath the tongue

Parotid GlandsParotid Glands Found anterior to the ear between masseter and skinFound anterior to the ear between masseter and skin

Saliva travels to oral orifice via ducts from all Saliva travels to oral orifice via ducts from all three glandsthree glands

Anatomy of the Salivary GlandsAnatomy of the Salivary Glands

Composition of Saliva: 97-99.5% waterComposition of Saliva: 97-99.5% water 0.5 % electrolytes0.5 % electrolytes Sodium, potassium, chloride, phosphate, Sodium, potassium, chloride, phosphate,

and bicarbonateand bicarbonate Most important proteins are salivary Most important proteins are salivary

amylase, mucus and lysozymesamylase, mucus and lysozymes

Features and Functions of the Features and Functions of the TeethTeeth

Break food into smaller parts, increasing Break food into smaller parts, increasing surface area for digestionsurface area for digestion

Types of TeethTypes of Teeth Deciduous Teeth (“baby” teeth)Deciduous Teeth (“baby” teeth) Permanent TeethPermanent Teeth

Incisors- cutting and shreddingIncisors- cutting and shredding Canines- piercing and tearingCanines- piercing and tearing Molars- grindingMolars- grinding Premolars- grinding and crushingPremolars- grinding and crushing

Tooth StructureTooth Structure

CrownCrown : exposed : exposed portion of tooth portion of tooth covered by enamel covered by enamel which covers dentinwhich covers dentin

RootRoot : internal portion : internal portion that is beneath the that is beneath the gums (gingiva) and is gums (gingiva) and is anchored by anchored by periodontal ligamentsperiodontal ligaments

PharynxPharynx

Responsible for swallowingResponsible for swallowing Swallowing is divided into oropharyngeal stage and Swallowing is divided into oropharyngeal stage and

esophageal stageesophageal stage

Features and Functions of the Features and Functions of the EsophagusEsophagus

Muscular tube that propels food to stomach; Muscular tube that propels food to stomach; bolus enters stomach through bolus enters stomach through esophageal hiatusesophageal hiatus

Esophageal glands – produce mucus to lubricate Esophageal glands – produce mucus to lubricate bolusbolus

Esophageal sphincter – prevents backflow into Esophageal sphincter – prevents backflow into oral cavityoral cavity

Cardiac sphincter- prevents backflow into Cardiac sphincter- prevents backflow into esophagus esophagus

Features and Functions of the Features and Functions of the EsophagusEsophagus

A hollow muscular tubeA hollow muscular tube About 25 cm (10 in.) long and 2 About 25 cm (10 in.) long and 2

cm (0.80 in.) widecm (0.80 in.) wide Conveys solid food and liquids Conveys solid food and liquids

to the stomachto the stomach Begins posterior to cricoid Begins posterior to cricoid

cartilage cartilage Esophageal glands – produce Esophageal glands – produce

mucus to lubricate bolusmucus to lubricate bolus Esophageal sphincter – Esophageal sphincter –

prevents backflow into oral prevents backflow into oral cavitycavity

Anatomy of the EsophagusAnatomy of the Esophagus

Digestive Processes in the Mouth, Digestive Processes in the Mouth, Pharynx, and EsophagusPharynx, and Esophagus

Mouth processes:Mouth processes: IngestionIngestion Mechanical digestion (e.g. salivary amylase)Mechanical digestion (e.g. salivary amylase) Initiation of PropulsionInitiation of Propulsion Mastication: chewingMastication: chewing

Pharyngeal processes:Pharyngeal processes: Deglutition = swallowingDeglutition = swallowing

Esophageal processes:Esophageal processes: Peristalsis (rhythmic contractions, involuntary)Peristalsis (rhythmic contractions, involuntary)

PeristalsisPeristalsis

Features and Functions of the Features and Functions of the StomachStomach

Temporary storage area for food and allows it to Temporary storage area for food and allows it to mix with gastric juice to produce mix with gastric juice to produce chymechyme

Regions: cardiac, fundus, body, and pyloricRegions: cardiac, fundus, body, and pyloric

Stomach FunctionStomach Function

Major Functions of the StomachMajor Functions of the Stomach Storage of ingested foodStorage of ingested food Mechanical breakdown of ingested foodMechanical breakdown of ingested food Disruption of chemical bonds in food material by acid Disruption of chemical bonds in food material by acid

and enzymesand enzymes Production of Production of intr insic factorintr insic factor , a glycoprotein , a glycoprotein

required for absorption of vitamin Brequired for absorption of vitamin B1212 in small intestine in small intestine

Anatomy of the StomachAnatomy of the Stomach

Gastric Juices (hormones)Gastric Juices (hormones) Simple columnar epithelium – contains Simple columnar epithelium – contains gastric gastric

pits pits that secrete that secrete gastric juicesgastric juices Goblet cells Goblet cells – secrete mucus that coats – secrete mucus that coats

stomach and prevents it from being digested stomach and prevents it from being digested itselfitself

Parietal cells Parietal cells – secrete – secrete hydrochloric acidhydrochloric acid (converts pepsinogen into pepsin) and (converts pepsinogen into pepsin) and intrinsic intrinsic factor factor (necessary for absorption of vitamin B(necessary for absorption of vitamin B12)12)

Chief cells Chief cells – secrete pepsinogen which is – secrete pepsinogen which is converted to pepsin to aid in protein digestionconverted to pepsin to aid in protein digestion

Enteroendocrine cells Enteroendocrine cells – release hormones – release hormones such as:such as: Histamine, Serotonin, Gastrin, Endorphins, and Histamine, Serotonin, Gastrin, Endorphins, and

SomatostatinSomatostatin

HistamineHistamine- activates parietal cells to release HCl- activates parietal cells to release HCl

SerotoninSerotonin- contraction of stomach muscle- contraction of stomach muscle

GastrinGastrin- gastric glands to increase secretion- gastric glands to increase secretion

EndorphinsEndorphins- natural opiates- natural opiates

SomatostatinSomatostatin- inhibits gastric secretion- inhibits gastric secretion- inhibits gastric emptying- inhibits gastric emptying

Digestive Processes in the Digestive Processes in the StomachStomach

Three PhasesThree Phases Cephalic PhaseCephalic Phase

Stimulated by the thought, sight, taste, or aroma or Stimulated by the thought, sight, taste, or aroma or foodfood

Inputs from olfactory receptors and taste buds Inputs from olfactory receptors and taste buds travel to parasympathetic enteric ganglia which travel to parasympathetic enteric ganglia which then stimulate stomach glandsthen stimulate stomach glands

Gastric PhaseGastric Phase Stomach distension activates stretch receptorsStomach distension activates stretch receptors Food chemicals (e.g. peptides, rising pH) activate Food chemicals (e.g. peptides, rising pH) activate

chemoreceptors which activate G cells which chemoreceptors which activate G cells which secrete gastrinsecrete gastrin

Intestinal PhaseIntestinal Phase Presence of low pH and partially digested foods in Presence of low pH and partially digested foods in

duodenum stimulates intestinal gastrin secretionduodenum stimulates intestinal gastrin secretion

Digestive Processes in the Digestive Processes in the StomachStomach

Gastric Motility and EmptyingGastric Motility and Emptying

Peristaltic waves approach stomach and become Peristaltic waves approach stomach and become stronger near pyloric regionstronger near pyloric region

Pyloric sphincter allows ~ 3 mL of chyme to pass to Pyloric sphincter allows ~ 3 mL of chyme to pass to duodenum and the rest to return to stomach for further duodenum and the rest to return to stomach for further mixingmixing

Small IntestineSmall Intestine 90% of absorption occurs in the small intestine90% of absorption occurs in the small intestine

Small IntestineSmall Intestine

The Duodenum The Duodenum The segment of small intestine closest to stomachThe segment of small intestine closest to stomach 25 cm (10 in.) long25 cm (10 in.) long ““Mixing bowl” that receives Mixing bowl” that receives chymechyme from stomach and from stomach and

digestive secretions from pancreas and liverdigestive secretions from pancreas and liver Functions of the duodenum Functions of the duodenum

To receive chyme from stomachTo receive chyme from stomach To neutralize acids before they can damage the absorptive To neutralize acids before they can damage the absorptive

surfaces of the small intestinesurfaces of the small intestine


The Jejunum The Jejunum Is the middle segment of small intestineIs the middle segment of small intestine 2.5 meters (8.2 ft) long2.5 meters (8.2 ft) long Is the location of mostIs the location of most

Chemical digestionChemical digestion Nutrient absorptionNutrient absorption

Small villiSmall villi


The IleumThe Ileum The final segment of small intestineThe final segment of small intestine 3.5 meters (11.48 ft) long 3.5 meters (11.48 ft) long Ends at the Ends at the i leocecal valveileocecal valve , a sphincter that , a sphincter that

controls flow of material from the ileum into controls flow of material from the ileum into the large intestinethe large intestine


Intestinal SecretionsIntestinal Secretions Watery intestinal juiceWatery intestinal juice 1.8 liters per day enter 1.8 liters per day enter

intestinal lumenintestinal lumen Moisten chymeMoisten chyme Assist in buffering acidsAssist in buffering acids Keep digestive enzymes Keep digestive enzymes

and products of digestion and products of digestion in solutionin solution

Intestinal MovementsIntestinal Movements Chyme arrives in Chyme arrives in

duodenumduodenum Weak peristaltic Weak peristaltic

contractions move it slowly contractions move it slowly toward jejunumtoward jejunum

Microscopic Anatomy of Small Microscopic Anatomy of Small IntestineIntestine

Vill iVil l i : fingerlike projections that increase the : fingerlike projections that increase the surface area of the SIsurface area of the SI

Microvil l iMicrovil l i : tiny projections on the plasma : tiny projections on the plasma membranes of columnar cells that appear fuzzy membranes of columnar cells that appear fuzzy (i.e. brush border cells)(i.e. brush border cells)

Crypts of LieberkuhnCrypts of Lieberkuhn : secrete intestinal juice : secrete intestinal juice and special lysozymes that protect against and special lysozymes that protect against bacteriabacteria

Peyer’s PatchesPeyer’s Patches : aggregated lymphoid : aggregated lymphoid tissues containing lymphocytes tissues containing lymphocytes

Microscopic Anatomy of the Small Microscopic Anatomy of the Small IntestineIntestine

Secretions of the Small IntestineSecretions of the Small Intestine

SecretinSecretin : released by enteroendocrine cells when : released by enteroendocrine cells when acidic chyme enters SI; causes release of bicarbonate-acidic chyme enters SI; causes release of bicarbonate-rich pancreatic juicesrich pancreatic juices

SomatostatinSomatostatin : slows gastric motility and emptying and : slows gastric motility and emptying and inhibits production of gastric secretionsinhibits production of gastric secretions

Cholecystokinin (CCK)Cholecystokinin (CCK) : released when fatty, protein-: released when fatty, protein-rich chyme enters SI; causes release of enzyme-rich rich chyme enters SI; causes release of enzyme-rich pancreatic juices and bilepancreatic juices and bile

Brush border enzymesBrush border enzymes : process long peptides, : process long peptides, nucleic acids, and sugars into smaller onesnucleic acids, and sugars into smaller ones

After Digestion: AbsorptionAfter Digestion: Absorption

Protein Fats

Functions of the LiverFunctions of the Liver

Largest internal organLargest internal organ Functions: Functions:

Filters and processes nutrient-rich blood of Filters and processes nutrient-rich blood of carbohydrates, proteins, and lipids from carbohydrates, proteins, and lipids from intestineintestine

Production and regulation of cholesterolProduction and regulation of cholesterol Production of Production of bile bile which emulsifies fatswhich emulsifies fats Removes drugs and hormones from Removes drugs and hormones from

circulationcirculation Storage of vitamins and mineralsStorage of vitamins and minerals

Anatomy of the LiverAnatomy of the Liver

Right and Left LobesRight and Left Lobes : separated by : separated by falciform falciform ligamentligament

Caudate and Quadrate LobesCaudate and Quadrate Lobes : found on : found on posterior sideposterior side

Blood vessels:Blood vessels: Hepatic artery/vein and hepatic portal veinHepatic artery/vein and hepatic portal vein

GallbladderGallbladder : found underneath left lobe, stores : found underneath left lobe, stores bilebile

Gross Anatomy of the LiverGross Anatomy of the Liver

Microscopic Anatomy of the LiverMicroscopic Anatomy of the Liver

Liver LobulesLiver Lobules : structural unit of liver: structural unit of liver HepatocytesHepatocytes : liver cells contained within the lobules: liver cells contained within the lobules Hepatic portal vein & Hepatic ArteryHepatic portal vein & Hepatic Artery : the : the

circulation of the liver, they bring blood into the liver circulation of the liver, they bring blood into the liver where it is filtered through the where it is filtered through the l iver sinusoidal l iver sinusoidal capil lariescapil laries

Kupffer cellsKupffer cells : remove debris: remove debris Filtered blood drains into the central vein, then to the Filtered blood drains into the central vein, then to the

hepatic vein, and eventually to the inferior vena cavahepatic vein, and eventually to the inferior vena cava BileBile (produced by hepatocytes) drains into the (produced by hepatocytes) drains into the bile bile

ductduct after passing through after passing through portal tr iadportal tr iad Bile then shipped to gallbladder for storageBile then shipped to gallbladder for storage

Microscopic Anatomy of the LiverMicroscopic Anatomy of the Liver

Function and Regulation of BileFunction and Regulation of Bile Bile ducts are present at every portal triadBile ducts are present at every portal triad Bile flows down Bile flows down bile canaliculi bile canaliculi (tiny canals) (tiny canals)

between adjacent hepatocytes towards bile duct between adjacent hepatocytes towards bile duct branches at every portal triadbranches at every portal triad

Bile enters the bile ducts which drain into the Bile enters the bile ducts which drain into the common hepatic ductcommon hepatic duct

Bile Bile emulsifies emulsifies fats, separating them into smaller fats, separating them into smaller partsparts

BilirubinBilirubin: the chief bile pigment, a waste product : the chief bile pigment, a waste product of the heme of hemoglobin formed during the of the heme of hemoglobin formed during the breakdown of worn-out erythrocytesbreakdown of worn-out erythrocytes

Regulation of Bile ProductionRegulation of Bile Production

Bile exits Bile exits cystic ductcystic duct upon upon stimulationstimulation

CCK released when CCK released when acidic, fatty chyme enters acidic, fatty chyme enters intestinesintestines

Causes:Causes: Gallbladder ContractionGallbladder Contraction Pancreatic Juice SecretionPancreatic Juice Secretion Relaxation of Relaxation of

hepatopancreatic sphincterhepatopancreatic sphincter

Features and Functions of the Features and Functions of the PancreasPancreas

Pancreatic Juice secreted by Pancreatic Juice secreted by acinar acinar cellscells Islets of Langerhans Islets of Langerhans release release insulin insulin and and

glucagon glucagon (important in glucose metabolism)(important in glucose metabolism) Pancreatic Juice contains:Pancreatic Juice contains:

Sodium Bicarbonate (buffers HCl in stomach)Sodium Bicarbonate (buffers HCl in stomach) Proteases (break down polypeptides)Proteases (break down polypeptides) Pancreatic amylase (digests oligosaccarides and Pancreatic amylase (digests oligosaccarides and

disaccharides into monosaccharides)disaccharides into monosaccharides) Pancreatic lipases (break down lipids into fatty acids Pancreatic lipases (break down lipids into fatty acids

and glycerol)and glycerol) Pancreatic nucleases (break down nucleic acids)Pancreatic nucleases (break down nucleic acids)

Anatomy of the PancreasAnatomy of the Pancreas

Features and Functions of the Features and Functions of the Large IntestineLarge Intestine

Functions:Functions: Reabsorption of remaining water and Reabsorption of remaining water and

electrolyteselectrolytes Production and absorption of Vitamins B and Production and absorption of Vitamins B and

KK Elimination of fecesElimination of feces

Diameter is only 7 cm but is larger than Diameter is only 7 cm but is larger than that of the small intestinethat of the small intestine

Gross Anatomy of the Large Gross Anatomy of the Large IntestineIntestine

Teniae ColiTeniae Coli : bands of smooth muscle that create : bands of smooth muscle that create pocket-like sacs (pocket-like sacs (haustra)haustra)

CecumCecum : sac-like connection between the small and : sac-like connection between the small and large intestineslarge intestines

AppendixAppendix : small structure containing lymphoid tissue; : small structure containing lymphoid tissue; small immune functionsmall immune function

Ascending, Descending, Transverse, and Ascending, Descending, Transverse, and Sigmoid ColonSigmoid Colon

Splenic and hepatic f lexureSplenic and hepatic f lexure RectumRectum : storage area: storage area AnusAnus : regulates defecation with two sphincter muscles; : regulates defecation with two sphincter muscles;

internal and external internal and external

Anatomy of the ColonAnatomy of the Colon

Microscopic Anatomy of the Large Microscopic Anatomy of the Large IntestineIntestine

Simple columnar epithelium for absorption Simple columnar epithelium for absorption except in the anal canal where there is stratified except in the anal canal where there is stratified squamoussquamous

No villi, no digestive-secreting cellsNo villi, no digestive-secreting cells Goblet cells produce mucus for lubrication of Goblet cells produce mucus for lubrication of

fecesfeces Bacterial flora synthesize vitamin B and most of Bacterial flora synthesize vitamin B and most of

the vitamin K needed for blood clottingthe vitamin K needed for blood clotting

The Process of Absorption The Process of Absorption

Clinical CornerClinical Corner

Gastrit isGastrit is - Inflammation of gastric mucosa- Inflammation of gastric mucosa Caused by chronic bacterial infectionCaused by chronic bacterial infection Ingestion of irritating substances e.g. alcohol/asprine Ingestion of irritating substances e.g. alcohol/asprine

AchlorhydriaAchlorhydria :absence of HCL secretion :absence of HCL secretion HypochlorhydriaHypochlorhydria : diminished acid secretions: diminished acid secretions Pernicious AnemiaPernicious Anemia : occurs when intrinsic : occurs when intrinsic

factor is not secreted. Absorption of Vit B12 does factor is not secreted. Absorption of Vit B12 does not occur leading to Pernicious anemia (large not occur leading to Pernicious anemia (large immature red blood ells)immature red blood ells)


Peptic Ulcer: Peptic Ulcer: The damaged are of stomach or The damaged are of stomach or upper part of small intestine mucosa caused by the upper part of small intestine mucosa caused by the digestive action of gastric or small intestinal digestive action of gastric or small intestinal secretions. secretions.

Causes: Causes: i) imbalance between gastric and intestinal i) imbalance between gastric and intestinal

secretion and the protective coating of mucosasecretion and the protective coating of mucosa ii)Helicobacter bacterial infectionii)Helicobacter bacterial infection iii) smoking, alcohol and aspirineiii) smoking, alcohol and aspirine


SprueSprue : Decreased or malabsorption of nutrients: Decreased or malabsorption of nutrients CirrohsisCirrohsis - scarred liver due to chronic - scarred liver due to chronic

inflammationinflammation Hepatitis - A,B,C,D, and E Hepatitis - A,B,C,D, and E Biliary calculi Bil iary calculi - gall stones - crystals of - gall stones - crystals of

cholesterol in bilecholesterol in bile Borborygmus Borborygmus - rumbling noise caused by gas - rumbling noise caused by gas

through intestines through intestines

Cholecystitis - inflammation of gall bladder Cholecystitis - inflammation of gall bladder Colitis - inflammation of colon Colitis - inflammation of colon Dysphagia - difficulty in swallowingDysphagia - difficulty in swallowing Enteritis - inflammation of the intestines Enteritis - inflammation of the intestines ConstipationConstipation Diarrhea Diarrhea

Clinical corner

Gastrointestinal Physiology (Digestive System)

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