Gastrointestinal System

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Gastrointestinal System

Faisal I. Mohammed, MD, PhD

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The Gastrointestinal System- Overviewand Mouth

Objectives:1. Introduce the GITS2. Give anatomical overview3. Describe the functions of the GITS

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Organs of the GITS

Gastrointenstinal (GI) tract or alimentary canal – mouth, most of pharynx, esophagus, stomach, small intestine, and large intestine

Accessory digestive organs – teeth, tongue, salivary glands, liver, gallbladder, and pancreas

Organs of the digestive system

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Functions of the Gastrointestinal system(Gatrointestinal Processes)

1. Ingestion2. Secretion of water, acid, buffers, and enzymes

into lumen3. Mixing and propulsion4. Digestion

Mechanical digestion churns food Chemical digestion – hydrolysis

5. Absorption – passing into blood or lymph6. Defecation – elimination of feces

Digestive Process

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Layers of the GI tract Wall of GI tract from lower esophagus to anal canal has

same basic 4 layers1. Mucosa – inner lining

Epithelium protection, secretion, absorption Lamina propria – connective tissue with blood and

lymphatic vessels and mucosa-associated lymphatic tissue (MALT)

Muscularis mucosae – thin layer of smooth muscle making folds to increase surface area

2. Submucosa Connective tissue binding mucosa to muscularis Contains many blood and lymphatic vessels Submucosal plexus (Meissner’s)

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Layers of the GI tract …cont3. Muscularis

Voluntary skeletal muscle found in mouth, pharynx, upper 2/3 of esophagus, and anal sphincter

Involuntary smooth muscle elsewhere Arranged in inner circular fibers and outer longitudinal

fibers Myenteric plexus between muscle layers (Auerbach’s)

4. Serosa Outermost covering of organs suspended in

abdominopelvic cavity Also called visceral peritoneum Esophagus lacks serosa – has adventitia

Layers of the gastrointestinal tract

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Summary of pathways controlling Gastrointestinal activities

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Neural innervation Enteric nervous system (ENS)

Intrinsic set of nerves - “brain of gut” Neurons extending from esophagus to anus 2 plexuses

Myenteric plexus – GI tract motility Submucosal plexus – controlling secretions

Autonomic nervous system Extrinsic set of nerves Parasympathetic stimulation increases secretion and

activity by stimulating ENS Sympathetic stimulation decreases secretions and

activity by inhibiting ENS

Organization of the enteric nervous system

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Extrinsic innervation: Parasympathetic nervous system:

Vagus and spinal nerves: Stimulate motility and GI secretions.

Sympathetic nervous system: Postganglionic sympathetic fibers that pass through

submucosal and myenteric plexuses and innervate GI tract:

Reduce peristalsis and secretory activity.

Regulation of the GI Tract

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Enteric nervous system: Sites where parasympathetic fibers synapse with

postganglionic neurons that innervate smooth muscle. Submucosal and myenteric plexuses:

Local regulation of the GI tract. Paracrine secretion:

Molecules acting locally. Hormonal secretion:

Secreted by the mucosa.

Regulation of the GI Tract (continued)

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Mouth Oral or buccal cavity Formed by cheeks, hard and sot palates, and tongue Oral cavity proper is a space that extends from gums

and teeth to fauces (opening between oral cavity and oropharynx)

Salivary glands release saliva Ordinarily, just enough is secreted to keep mouth and

pharynx moist and clean When food enters mouth, secretion increases to

lubricate, dissolve and begin chemical digestion 3 pairs of major salivary glands secrete most of the

saliva Parotid, submandibular, and sublingual

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Structures of the mouth (oral cavity)

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The three major salivary glands- parotid, sublingual, and submandibular

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Salivary Glands

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Saliva Saliva

Mostly water 99.5% 0.5% solutes – ions, dissolved gases, urea, uric

acid, mucus, immunoglobulin A, lysozyme, and salivary amylase (acts on starch)

Not all salivary glands produce the same saliva Salivation

Controlled by autonomic nervous system Parasympathetic stimulation promotes secretion of

moderate amount of saliva Sympathetic stimulation decreases salivation

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Saliva: Source and Composition

Secreted from serous and mucous cells of salivary glands

A 97-99.5% water, hypo-osmotic, slightly acidic solution containing Electrolytes – Na+, K+, Cl–, PO4

2–, HCO3–

Digestive enzyme – salivary amylase Proteins – mucin, lysozyme, defensins, and

IgA Metabolic wastes – urea and uric acid

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Salivary Glands Produce and secrete saliva that:

Cleanses the mouth Moistens and dissolves food chemicals Aids in bolus formation Contains enzymes that break down starch

Three pairs of extrinsic glands – parotid, submandibular, and sublingual

Intrinsic salivary glands (buccal glands) – scattered throughout the oral mucosa

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Control of Salivation

Intrinsic glands keep the mouth moist Extrinsic salivary glands secrete serous,

enzyme-rich saliva in response to: Ingested food which stimulates

chemoreceptors and pressoreceptors The thought of food

Strong sympathetic stimulation inhibits salivation and results in dry mouth

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Tongue

Tongue Accessory digestive organ Skeletal muscle covered by mucous

membrane Maneuvers food for chewing, shapes

mass, forces food back for swallowing Lingual glands secrete salivary lipase

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Digestion in the mouth Mechanical digestion in the mouth

Chewing or mastication Food manipulated by tongue, ground by teeth, and mixed

with saliva Forms bolus

Chemical digestion in the mouth Salivary amylase secreted by salivary glands acts on

starches Only monosaccharides can be absorbed Continues to act until inactivated by stomach acid

Lingual lipase secreted by lingual glands of tongue acts on triglycerides

Becomes activated in acidic environment of stomach

Thank YouThank You

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Gastrointestinal System L2


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Saliva and Stomach

Objectives:1. Describe salivary secretion composition,

function and regulation2. Give anatomical overview of the stomach3. Describe gastric Movements, Secretion,

Absorption and Digestion4. Explain Gastric Emptying and its

regulation

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The three major salivary glands- parotid, sublingual, and submandibular

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Salivary Glands

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Saliva: Source and Composition Secreted from serous and mucous cells of salivary glands

around 1.5 liters/day A 97-99.5% water, hypo-osmotic, slightly acidic solution

containing Electrolytes – Na+, K+, Cl–, PO4

2–, HCO3–

Na+ - 0.1 x plasma (15 mEq/L)Cl- - 0.15 x plasma (15 mEq/L)K+ - 7 x plasma (30 mEq/L)HCO-

3 - 3 x plasma (70 mEq/L) Digestive enzyme – salivary amylase Proteins – mucin, lysozyme, defensins, and IgA Metabolic wastes – urea and uric acid

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Salivary Glands Produce and secrete saliva that:

Cleanses the mouth, prevent dental caries Moistens and dissolves food chemicals Facilitates speech Aids in bolus formation Contains enzymes- amylase- that break down starch

Three pairs of extrinsic glands – parotid, submandibular and sublingual

Intrinsic salivary glands (buccal glands) – scattered throughout the oral mucosa

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Control of Salivation Salivation is exclusively under neural control Parasympathetic through glossopharyngeal nerve (IX) is

stimulatory and sympathetic is inhibitory Intrinsic glands keep the mouth moist Extrinsic salivary glands secrete serous, enzyme-rich saliva in

response to: Ingested food which stimulates chemoreceptors and

pressoreceptors salivatory center at medulla oblongata The thought of food – cerebral cortex

Strong sympathetic stimulation inhibits salivation and results in dry mouth

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Tongue

Tongue Accessory digestive organ Skeletal muscle covered by mucous

membrane Maneuvers food for chewing, shapes

mass, forces food back for swallowing Lingual glands secrete salivary lipase

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Digestion in the mouth Mechanical digestion in the mouth

Chewing or mastication Food manipulated by tongue, ground by teeth, and mixed

with saliva Forms bolus

Chemical digestion in the mouth Salivary amylase secreted by salivary glands acts on

starches ( 1-4 glucosidic linkages) Only monosaccharides can be absorbed Continues to act until inactivated by stomach acid

Lingual lipase secreted by lingual glands of tongue acts on triglycerides

Becomes activated in acidic environment of stomach

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Pharynx

Passes from mouth into pharynx 3 parts

Nasopharynx Functions only in respiration

Oropharynx Digestive and respiratory functions

Laryngopharynx Digestive and respiratory functions

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Esophagus Secretes mucous, transports food – no enzymes produced,

no absorption Mucosa – protection against wear and tear Submucosa Muscularis divided in thirds

Superior 1/3 skeletal muscle Middle 1/3 skeletal and smooth muscle Inferior 1/3 smooth muscle 2 sphincters – upper esophageal sphincter (UES)

regulates movement into esophagus, lower esophageal sphincter (LES) regulates movement into stomach

Adventitia – no serosa – attaches to surroundings

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Deglutition (Swallowing) Act of swallowing Facilitated by secretions of saliva and mucus Involves mouth, pharynx, and esophagus 3 stages

Oral phase: Voluntary – bolus passed to oropharynx Pharyngeal phase– involuntary passage through pharynx

into esophagus (reflex action), Fast 1-2 second. controlled by the medulla and lower pons (Deglutition center)

All routes except into the digestive tract are sealed off Esophageal phase – involuntary passage through esophagus

to stomach (depend on gravity ~ 5seconds) Peristalsis pushes bolus forward

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Swallowing (mouth, pharynx and esophagus)

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Swallowing … cont (esophagus)

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Deglutition (Swallowing)

(a) Upper esophageal sphincter contracted

(b) Upper esophageal sphincter relaxed

(c) Upper esophageal sphincter contracted

(e)(d)

Bolus of food

Uvula

Bolus

Relaxed musclesRelaxed muscles

Tongue

PharynxEpiglottis

Glottis

Trachea

Bolus

Epiglottis

Bolus of food

Longitudinal muscles contract, shortening

passageway ahead of bolus

Gastroesophageal sphincter closed

Circular muscles contract, constricting passageway and pushing bolus down

Stomach

Gastroesophageal sphincter open

Esophagus

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Oropharyngeal stages of swallowing

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Stomach

Serves as mixing chamber and holding reservoir 4 main regions

Cardia, fundus, body, pylorus Same 4 layers

Mucosa – gastric glands open into gastric pits 3 types of exocrine gland cells – mucous neck cells

(mucus), parietal cells (intrinsic factor and HCl), and chief cells (pepsinogen and gastric lipase)

G cell – endocrine cell – secretes gastrin Submucosa Muscularis – additional 3rd inner oblique layer Serosa – part of visceral peritoneum

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Stomach

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Gastric secretions

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Gastric emptying (movements)

Stomach factors are stimulatory

Duodenal factors are inhibitory

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Mechanical and Chemical Digestion Mechanical digestion

Mixing waves – gentle, rippling peristaltic movements – creates chyme

Chemical digestion Digestion by salivary amylase continues until inactivated by

acidic gastric juice Acidic gastric juice activates lingual lipase

Digest triglycerides into fatty acids and diglycerides Parietal cells secrete H+ and Cl- separately but net effect is

HCl Kills many microbes, denatures proteins

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Chemical Digestion

Chemical digestion (cont.)Pepsin secreted by chief cells digest proteins

Secreted as pepsinogen activated into pepsin by HClPepsin is an endopeptidase breaks peptide bonds in the inside of the polypeptides

Gastric lipase splits triglycerides into fatty acids and monoglycerides

Small amount of nutrient absorptionSome water, ions, short chain fatty acids, certain lipid soluble drugs (aspirin) and alcohol

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Gastrointestinal SystemL3


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Stomach…Cont Biliary System

Objectives:1. Outline the regulation of gastric function (Emptying,

Secretion and Digestion)2. Describe phases of gastric secretion (Cephalic, Gastric

and Intestinal)3. Give anatomical overview of the biliary system (Liver,

Pancreas and Gall bladder)4. Describe Pancreatic Secretion5. Describe biliary secretion6. Outline the functions of pancreatic secretion and bile7. Describe the regulation of pancreatic and bile secretion

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Regulation of Gastric Secretion

Neural and hormonal mechanisms regulate the release of gastric juice

Stimulatory and inhibitory events occur in three phases Cephalic (reflex) phase: prior to food entry (neural) Gastric phase: once food enters the stomach (neural

and hormonal) Intestinal phase: as partially digested food enters

the duodenum (mostly hormonal)

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Cephalic Phase

Excitatory events include: Sight or thought of food Stimulation of taste or smell receptors

Inhibitory events include: Loss of appetite or depression Decrease in stimulation of the parasympathetic

division

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Gastric Phase Excitatory events include:

Stomach distension Activation of stretch receptors (neural activation) Activation of chemoreceptors by peptides, caffeine,

and rising pH Release of gastrin to the blood

Inhibitory events include: A pH lower than 2 Emotional upset that overrides the parasympathetic

division

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Intestinal Phase

Excitatory phase – low pH; partially digested food enters the duodenum and encourages gastric gland activity

Inhibitory phase – distension of duodenum, presence of fatty, acidic, or hypertonic/hypotonic chyme, and/or irritants in the duodenum Initiates inhibition of local reflexes and vagal nuclei Closes the pyloric sphincter Releases enterogastrones that inhibit gastric secretion

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Release of Gastric Juice

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Gastric SecretionAccounts for ~ 20% of Gastric secretion

Accounts for ~ 70% of Gastric Secretion

Accounts for < 10% of Gastric Secretion

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Regulation and Mechanism of HCl Secretion HCl secretion is stimulated by ACh, histamine,

and gastrin through second-messenger systems Release of hydrochloric acid:

Is low if only one ligand binds to parietal cells Is high if all three ligands bind to parietal cells

Antihistamines block H2 receptors and decrease HCl release

Proton pump inhibitors (prazol group common omeprazol, lansoprazol)

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Regulation and Mechanism of HCl Secretion

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Response of the Stomach to Filling Stomach pressure remains constant until about 1L of

food is ingested Relative unchanging pressure results from reflex-

mediated relaxation and plasticity Reflex-mediated events include:

Receptive relaxation – as food travels in the esophagus, stomach muscles relax

Adaptive relaxation – the stomach dilates in response to gastric filling

Plasticity – intrinsic ability of smooth muscle to exhibit the stress-relaxation response

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Gastric Contractile Activity Peristaltic waves move toward the pylorus at the

rate of 3 per minute This basic electrical rhythm (BER) is initiated by

pacemaker cells (cells of Cajal) Most vigorous peristalsis and mixing occurs near

the pylorus Chyme is either:

Delivered in small amounts to the duodenum or Forced backward into the stomach for further

mixing (retropulsion)

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Gastric Contractile Activity

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Chyme:=A mixture of gastric secretion and food

Gastric mixing

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Regulation of Gastric Emptying

Gastric emptying is regulated by: The neural enterogastric reflex Hormonal (enterogastrone) mechanisms

These mechanisms inhibit gastric secretion and duodenal filling

Carbohydrate-rich chyme quickly moves through the duodenum slower for protein

Fat-laden chyme is digested more slowly causing food to remain in the stomach the longest

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Regulation of Gastric Emptying

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Gastric emptying (movements)

Stomach factors are stimulatory

Duodenal factors are inhibitory

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Biliary System

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Pancreas

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Pancreas

Exocrine function Secretes pancreatic juice which breaks

down all categories of foodstuff Acini (clusters of secretory cells) contain

zymogen granules with digestive enzymes

The pancreas also has an endocrine function – release of insulin and glucagon

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Acinus of the Pancreas

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Composition and Function of Pancreatic Juice

Water solution of enzymes and electrolytes (primarily HCO3

–) around 2 liter/day Neutralizes acid chyme Provides optimal environment for

pancreatic enzymes Enzymes that digest proteins are released in

inactive form and activated in the duodenum

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Composition and Function of Pancreatic Juice

Examples include Trypsinogen is activated to trypsin in the duodenum

by enterokinase at the duodenal epithelial cells then trypsin autoactivates itself and chymotrysinogen

Procarboxypeptidase is activated to carboxypeptidase by trypsin

Active enzymes secreted Amylase, lipases, and nucleases These enzymes require ions or bile for optimal activity

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Regulation of Pancreatic Secretion Secretin and CCK are released when fatty or acidic

chyme enters the duodenum CCK and secretin enter the bloodstream Upon reaching the pancreas:

CCK induces the secretion of enzyme-rich pancreatic juice

Secretin causes secretion of bicarbonate-rich pancreatic juice

Vagal stimulation also causes release of pancreatic juice

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Regulation of Pancreatic Secretion

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Liver and Gall Bladder (Bile)

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Composition of Bile A yellow-green, alkaline solution containing bile salts, bile

pigments, cholesterol, neutral fats, phospholipids, and electrolytes. About 1 liter/day.

Bile salts are cholesterol derivatives (cholic acid, chenodeoxycholic acid, deoxycholic and lithocholic acids are also found in human usually conjugated with either taurine or glycine) that: Emulsify fat to facilitate it digestion Facilitate fat and cholesterol absorption Help solubilize cholesterol

Enterohepatic circulation recycles bile salts The chief bile pigment is bilirubin, a waste product of heme

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Regulation of Bile Release

Acidic, fatty chyme causes the duodenum to release: Cholecystokinin (CCK) and secretin into the

bloodstream Bile salts and secretin transported in blood

stimulate the liver to produce bile Vagal stimulation causes weak contractions of the

gallbladder

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Cholecystokinin causes: The gallbladder to contract The hepatopancreatic sphincter to relax

As a result, bile enters the duodenum

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Biliary system …cont, Small Intestine, Digestion and Absorption

Objectives:1. Describe bile secretion2. Outline the functions and regulation of bile secretion3. Give anatomical overview of the small intestine4. Describe small intestinal secretion its function,

composition and regulation5. Point out the digestive processes of food nutrients6. Compare and contrast absorption of Carbohydrates,

Proteins, Lipids, Electrolytes and Fluids

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Liver and Gall Bladder (Bile)

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Composition of Bile A yellow-green, alkaline solution containing bile salts, bile

pigments, cholesterol, neutral fats, phospholipids, and electrolytes. About 1 liter/day.

Bile salts are cholesterol derivatives (cholic acid, chenodeoxycholic acid, deoxycholic and lithocholic acids are also found in human usually conjugated with either taurine or glycine) that: Emulsify fat to facilitate it digestion Facilitate fat and cholesterol absorption Help solubilize cholesterol

Enterohepatic circulation recycles bile salts The chief bile pigment is bilirubin, a waste product of heme

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Acidic, fatty chyme causes the duodenum to release: Cholecystokinin (CCK) and secretin into the

bloodstream Bile salts and secretin transported in blood

stimulate the liver to produce bile Vagal stimulation causes weak contractions of the

gallbladder

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Cholecystokinin causes: The gallbladder to contract The hepatopancreatic sphincter to relax

As a result, bile enters the duodenum

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Small Intestine

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Small intestine 3 regions – duodenum, jejunum, and ileum Same 4 layers

1. Mucosa Absorptive cells (digest and absorb), goblet cells (mucus),

intesrinal glnds (intestinal juice), Paneth cells (lysozyme), and enteroendocrine cells

Abundance of MALT2. Submucosa

Duodenal glands secrete alkaline mucus3. Muscularis4. Serosa

Completely surrounds except for major portion of duodenum

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Special structural features increase surface area for digestion and absorption

Circular folds (increase by 3 times) Permanent ridges of mucosa and submucosa Cause chyme to spiral

Villi (Increase by 10 times) Fingerlike projections of mucosa Contains arteriole, venule, blood capillary, and lacteal

Microvilli (Increase by 20 times) Projects of apical membrane of absorptive cells Brush border with brush border enzymes Total increase in surface area is 600 time

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Intestinal Juice

Secreted by intestinal glands in response to distension or irritation of the mucosa

Slightly alkaline and isotonic with blood plasma Largely water, enzyme-poor, but contains mucus

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Mechanical Digestion Governed by myenteric plexus Segmentations

Localized, mixing contractions Mix chyme and bring it in contact with mucosa for

absorption Migrating motility complexes (MMC)

Type of peristalsis Begins in lower portion of stomach and pushes food

forward It occurs between meals (interdigestive state)

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. Small Intestinal Motility

1. Peristalsis: movement along the tract

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2. Segmentation: mix contents to promote digestion & absorption

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Chemical digestion

Carbohydrates Pancreatic amylase ( 1-4 glucosidic linkages) α-dextrinase ( 1-6 linkages), sucrase, lactase,

maltase in brush border Ends with monosaccharides which can be

absorbed Proteins

Trypsin, chymotrypsin, carboxypeptidase, and elastase from pancreas

Aminopeptidase and dipeptidase in brush border

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Lipids and Nucleic Acids Lipids

Pancreatic lipase most important in triglyceride digestion

Emulsification by bile salts increases surface area

Amphipathic – hydrophobic and hydrophilic regions Nucleic acids

Ribonuclease and deoxyribonuclease in pancreatic juice

Nucleosidases and phosphatases in brush border

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Chemical Digestion: Carbohydrates Absorption: via cotransport with Na+, and

facilitated diffusion Enter the capillary bed in the villi Transported to the liver via the hepatic portal

vein Enzymes used: salivary amylase, pancreatic

amylase, and brush border enzymes

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Chemical Digestion: Proteins

Absorption: similar to carbohydrates Enzymes used: pepsin in the stomach Enzymes acting in the small intestine

Pancreatic enzymes – trypsin, chymotrypsin, and carboxypeptidase

Brush border enzymes – aminopeptidases, carboxypeptidases, and dipeptidases

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Chemical Digestion: Proteins

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Chemical Digestion: Fats

Absorption: Diffusion into intestinal cells where they: Combine with proteins and extrude

chylomicrons Enter lacteals and are transported to systemic

circulation via lymph Glycerol and short chain fatty acids are:

Absorbed into the capillary blood in villi Transported via the hepatic portal vein

Enzymes/chemicals used: bile salts and pancreatic lipase

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Chemical Digestion: Fats

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Fatty Acid Absorption

Fatty acids and monoglycerides enter intestinal cells via diffusion

They are combined with proteins within the cells Resulting chylomicrons are extruded They enter lacteals and are transported to the

circulation via lymph

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Chemical Digestion: Nucleic Acids

Absorption: active transport via membrane carriers

Absorbed in villi and transported to liver via hepatic portal vein

Enzymes used: pancreatic ribonucleases and deoxyribonuclease in the small intestines

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Electrolyte Absorption

Most ions are actively absorbed along the length of small intestine Na+ is coupled with absorption of glucose and

amino acids Ionic iron is transported into mucosal cells where it

binds to ferritin Anions passively follow the electrical potential

established by Na+

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K+ diffuses across the intestinal mucosa in response to osmotic gradients

Ca2+ absorption: Is related to blood levels of ionic calcium Is regulated by vitamin D and parathyroid hormone

(PTH)

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Water Absorption 95% of water is absorbed in the small intestines by

osmosis Water moves in both directions across intestinal

mucosa Net osmosis occurs whenever a concentration

gradient is established by active transport of solutes into the mucosal cells

Water uptake is coupled with solute uptake, and as water moves into mucosal cells, substances follow along their concentration gradients

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Malabsorption of Nutrients

Results from anything that interferes with delivery of bile or pancreatic juice

Factors that damage the intestinal mucosa (e.g., bacterial infection)

Gluten enteropathy (adult celiac disease) – gluten damages the intestinal villi and reduces the length of microvilli Treated by eliminating gluten from the diet (all

grains but rice and corn)

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Absorption of:

Monosaccharides All dietary carbohydrates digested are absorbed Only indigestible cellulose and fibers left in feces Absorbed by facilitated diffusion or active transport

into blood Amino acids, dipetides and tripeptides

Most absorbed as amino acids via active transport into blood

½ of absorbed amino acids come from proteins in digestive juice and dead mucosal cells

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Absorption of Digested Nutrients

in the Small Intestine

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Lipids All dietary lipids absorbed by simple diffusion Short-chain fatty acids go into blood for transport Long-chain fatty acids and monoglycerides

Large and hydrophobic Bile salts form micelles to ferry them to absorptive cell

surface Reform into triglycerides forming chylomicrons Leave cell by exocytosis Enter lacteals to eventually enter blood with protein coat

of chylomicron keeping them suspended and separate

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Absorption of Lipids

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Absorption of:

Electrolytes From GI secretions or food Sodium ions (Na+) reclaimed by active transport Other ions also absorbed by active transport

Vitamins Fat-soluble vitamins A, D, E, and K absorbed by simple diffusion

and transported with lipids in micelles Most water-soluble vitamins also absorbed by simple diffusion

Water 9.3L comes from ingestion (2.3L) and GI secretions (7.0L) Most absorbed in small intestine, some in large intestine Only 100ml excreted in feces All water absorption by osmosis

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Daily Volumes of Fluid Ingested, Secreted, Absorbed, and Excreted From the GI Tract

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Absorption…cont Large intestine

Objectives:1. Describe Basic nutrients absorption2. Outline the Absorption of fluids3. Give anatomical overview of the large

intestine4. Describe large intestinal secretion its

function, composition and regulation5. Outline the defecation process and its neural

regulation

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Fatty acids and monoglycerides enter intestinal cells via diffusion

They are combined with proteins within the cells Resulting chylomicrons are extruded They enter lacteals and are transported to the

circulation via lymph

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Chemical Digestion: Nucleic Acids

Absorption: active transport via membrane carriers

Absorbed in villi and transported to liver via hepatic portal vein

Enzymes used: pancreatic ribonucleases and deoxyribonuclease in the small intestines

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Most ions are actively absorbed along the length of small intestine Na+ is coupled with absorption of glucose and

amino acids Ionic iron is transported into mucosal cells where it

binds to ferritin Anions passively follow the electrical potential

established by Na+

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K+ diffuses across the intestinal mucosa in response to osmotic gradients

Ca2+ absorption: Is related to blood levels of ionic calcium Is regulated by vitamin D and parathyroid hormone

(PTH)

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Water Absorption 95% of water is absorbed in the small intestines by

osmosis Water moves in both directions across intestinal

mucosa Net osmosis occurs whenever a concentration

gradient is established by active transport of solutes into the mucosal cells

Water uptake is coupled with solute uptake, and as water moves into mucosal cells, substances follow along their concentration gradients

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Malabsorption of Nutrients

Results from anything that interferes with delivery of bile or pancreatic juice

Factors that damage the intestinal mucosa (e.g., bacterial infection)

Gluten enteropathy (adult celiac disease) – gluten damages the intestinal villi and reduces the length of microvilli Treated by eliminating gluten from the diet (all

grains but rice and corn)

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Absorption of:

Monosaccharides All dietary carbohydrates digested are absorbed Only indigestible cellulose and fibers left in feces Absorbed by facilitated diffusion or active transport

into blood Amino acids, dipetides and tripeptides

Most absorbed as amino acids via active transport into blood

½ of absorbed amino acids come from proteins in digestive juice and dead mucosal cells

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Absorption of Digested Nutrients

in the Small Intestine

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Lipids All dietary lipids absorbed by simple diffusion Short-chain fatty acids go into blood for transport Long-chain fatty acids and monoglycerides

Large and hydrophobic Bile salts form micelles to ferry them to absorptive cell

surface Reform into triglycerides forming chylomicrons Leave cell by exocytosis Enter lacteals to eventually enter blood with protein coat

of chylomicron keeping them suspended and separate

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Absorption of Lipids

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Absorption of:

Electrolytes From GI secretions or food Sodium ions (Na+) reclaimed by active transport Other ions also absorbed by active transport

Vitamins Fat-soluble vitamins A, D, E, and K absorbed by simple diffusion

and transported with lipids in micelles Most water-soluble vitamins also absorbed by simple diffusion

Water 9.3L comes from ingestion (2.3L) and GI secretions (7.0L) Most absorbed in small intestine, some in large intestine Only 100ml excreted in feces All water absorption by osmosis

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Daily Volumes of Fluid Ingested, Secreted, Absorbed, and Excreted From the GI Tract

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Anatomy of the large intestine

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Large intestine

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Histology of the large intestine

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Large intestine Overall function to complete absorption, produce

certain vitamins, and form and expel feces 4 major regions – cecum, colon, rectum, and anal

canal Ileocecal sphincter between small and large

intestine Colon divided into ascending, transverse,

descending and sigmoid Opening of anal canal (anus) guarded by internal

anal sphincter of smooth muscle and external anal sphincter of skeletal muscle

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Large Intestine Same 4 layers Mucosa – mostly absorptive and goblet cells

No circular folds or villi Does have microvilli

Submucosa Muscularis

Longitudinal muscle modified to form teniae coli

Forms haustra – pouches Serosa

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Digestion of the Large Intestine Mechanical digestion

Haustral churning Peristalsis Mass peristalsis – drives contents of colon toward rectum

initiated by gastrocolic reflex. Chemical digestion

Final stage of digestion through bacterial action Ferment carbohydrates, produce some B vitamins and vitamin

K Mucus but no enzymes secreted

Remaining water absorbed along with ions and some vitamins

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Defecation Intrinsic defecation reflex set up by gastro-colic reflex Extrinsic defecation reflex set up by increasing the

intra abdominal pressure (contraction of the abdominal muscles, deep inspiration)

Distension of rectal walls caused by feces: Stimulates contraction of the rectal walls Relaxes the internal anal sphincter

Voluntary signals stimulate relaxation of the external anal sphincter and defecation occurs

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Anal sphincters

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Defecation

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Gastrointestinal System

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