Digestive System Introduction

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SHS112The Digestive System

(Based on Marieb & Hoehn 8th Ed, Chapter 23)

Arthur Jenkins PhD41.312

Office Hours: Tue 10:30-12:30, Thu 12:30-2:[email protected]

9/9/10 Copyright © 2010 Pearson Edcation, Inc.

Digestive System

• Two groups of organs

1. Alimentary canal (gastrointestinal or GI tract)

• Digests and absorbs food

• Mouth, pharynx, esophagus, stomach,small intestine, and large intestine

Copyright © 2010 Pearson Edcation, Inc.

Digestive System

2. Accessory digestive organs

• Teeth, tongue, gallbladder

• Digestive glands

• Salivary glands

• Liver

• pancreas

Copyright © 2010 Pearson Education, Inc. Figure 23.1

Mouth (oral cavity)Tongue

Esophagus

LiverGallbladder

Anus

DuodenumJejunumIleum

Small intestine

Parotid glandSublingual glandSubmandibulargland

Salivaryglands

Pharynx

StomachPancreas(Spleen)

Transverse colonDescending colonAscending colonCecumSigmoid colonRectumVermiform appendixAnal canal

Largeintestine

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Digestive Processes

• Six essential activities

1. Ingestion

2. Propulsion

3. Mechanical digestion

4. Chemical digestion

5. Absorption

6. Defecation


FoodIngestion

PropulsionEsophagus

Stomach

PharynxMechanicaldigestion

Chemicaldigestion

• Chewing (mouth)• Churning (stomach)• Segmentation (small intestine)

Smallintestine Largeintestine

Defecation Anus

Feces

Bloodvessel

Lymphvessel

Absorption

• Swallowing (oropharynx)• Peristalsis (esophagus, stomach, small intestine, large intestine)

Mainly H2O


Frommouth

(b) Segmentation: Nonadjacent segments of alimentary tract organs alternately contract and relax, moving the food forward then backward. Food mixing and slow food propulsion occurs.

(a) Peristalsis: Adjacent segments ofalimentary tract organs alternately contractand relax, which moves food along the tractdistally.


GI tract regulatory mechanisms

1. Mechanoreceptors and chemoreceptors

• Respond to stretch, changes in osmolarityand pH, and presence of substrate and endproducts of digestion

• Initiate reflexes that

• Activate or inhibit digestive glands

• Stimulate smooth muscle to mix andmove lumen contents

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GI tract regulatory mechanisms

2. Intrinsic and extrinsic controls

• Enteric nerve plexuses (gut brain) initiateshort reflexes in response to stimuli in the GItract

• Long reflexes in response to stimuli inside oroutside the GI tract involve CNS centers andautonomic nerves

• Hormones from cells in the stomach andsmall intestine stimulate target cells in thesame or different organs


External stimuli(sight, smell, taste,

thought of food)

Central nervous systemand extrinsic autonomic nerves

Afferent impulses Efferent impulses

Long reflexes

Internal(GI tract)stimuli

Chemoreceptors,osmoreceptors, ormechanoreceptors

Local (intrinsic)nerve plexus(“gut brain”)

Effectors:Smooth muscle

or glands

Gastrointestinalwall (site of shortreflexes)

Response:Change in

contractile orsecretory activityLumen of the

alimentary canal

Short reflexes


Blood Supply: Splanchnic Circulation

• Arteries

• Hepatic, splenic, and left gastric

• Inferior and superior mesenteric

• Hepatic portal circulation

• Drains nutrient-rich blood from digestiveorgans

• Delivers it to the liver for processing


Histology of the Alimentary Canal

• Four basic layers (tunics)

• Mucosa

• Submucosa

• Muscularis externa

• Serosa

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Glands in submucosa

Submucosa

LumenMucosa-associatedlymphoid tissue

Duct of gland outsidealimentary canal

Gland in mucosa

NerveArteryVein

Lymphaticvessel Mesentery

Intrinsic nerve plexuses• Myenteric nerve plexus• Submucosal nerve plexus

Mucosa• Epithelium• Lamina propria• Muscularis mucosae

Muscularis externa• Longitudinal muscle • Circular muscleSerosa• Epithelium• Connective tissue


Mucosa

• Lines the lumen

• Functions

• Secretes mucus, digestive enzymes andhormones

• Absorbs end products of digestion

• Protects against infectious disease

• Three sublayers: epithelium, lamina propria,and muscularis mucosae


Mucosa

• Epithelium

• Simple columnar epithelium and mucus-secreting cells

• Mucus

• Protects digestive organs from enzymes

• Eases food passage

• May secrete enzymes and hormones (e.g., instomach and small intestine)


Mucosa

• Lamina propria

• Loose areolar connective tissue

• Capillaries for nourishment and absorption

• Lymphoid follicles (part of MALT)

• Muscularis mucosae: smooth muscle thatproduces local movements of mucosa

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Submucosa and Muscularis Externa

• Submucosa

• Dense connective tissue

• Blood and lymphatic vessels, lymphoidfollicles, and submucosal nerve plexus


Submucosa and Muscularis Externa


• Responsible for segmentation and peristalsis

• Inner circular and outer longitudinal layers

• Myenteric nerve plexus

• Sphincters in some regions


Serosa

• Visceral peritoneum

• Replaced by the fibrous adventitia in theesophagus

• Retroperitoneal organs have both an adventitiaand serosa


Glands in submucosa

Submucosa

LumenMucosa-associatedlymphoid tissue

Duct of gland outsidealimentary canal

Gland in mucosa

NerveArteryVein

Lymphaticvessel Mesentery

Intrinsic nerve plexuses• Myenteric nerve plexus• Submucosal nerve plexus

Mucosa• Epithelium• Lamina propria• Muscularis mucosae

Muscularis externa

• Longitudinal muscle • Circular muscleSerosa• Epithelium• Connective tissue

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Enteric Nervous System

• Intrinsic nerve supply of the alimentary canal

• Submucosal nerve plexus

• Regulates glands and smooth muscle in themucosa

• Myenteric nerve plexus

• Controls GI tract motility


Enteric Nervous System

• Linked to the CNS via afferent visceral fibers

• Long ANS fibers synapse with entericplexuses

• Sympathetic impulses inhibit secretion andmotility

• Parasympathetic impulses stimulate


Mouth

• Oral (buccal) cavity

• Bounded by lips, cheeks, palate, and tongue

• Oral orifice is the anterior opening

• Lined with stratified squamous epithelium

Copyright © 2010 Pearson Education, Inc. Figure 23.7a

UvulaSoft palate Palatoglossal arch

Palatine tonsil

Hard palate

Oral cavity

Tongue

Lingual tonsilOropharynx

EpiglottisHyoid bone

Laryngopharynx

Esophagus

Trachea(a) Sagittal section of the oral cavity and pharynx

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Salivary Glands

• Extrinsic salivary glands (parotid, submandibular,and sublingual)

• Intrinsic (buccal) salivary glands are scattered in theoral mucosa

• Secretion (saliva)

• Cleanses the mouth

• Moistens and dissolves food chemicals

• Aids in bolus formation

• Contains enzymes that begin the breakdown of starch


Composition of Saliva

• Secreted by serous and mucous cells

• 97–99.5% water, slightly acidic solution containing

• Electrolytes—Na+, K+, Cl–, PO4 2–, HCO3–

• Salivary amylase and lingual lipase

• Mucin

• Metabolic wastes—urea and uric acid

• Lysozyme, IgA, defensins, and a cyanide compoundprotect against microorganisms


Control of Salivation

• Intrinsic glands continuously keep the mouth moist

• Extrinsic salivary glands produce secretions when

• Ingested food stimulates chemoreceptors andmechanoreceptors in the mouth

• Salivatory nuclei in the brain stem send impulsesalong parasympathetic fibers in cranial nerves VIIand IX

• Strong sympathetic stimulation inhibits salivation andresults in dry mouth (xerostomia)


Pharynx

• Oropharynx and laryngopharynx

• Allow passage of food, fluids, and air

• Stratified squamous epithelium lining

• Skeletal muscle layers: inner longitudinal,outer pharyngeal constrictors

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Esophagus

• Flat muscular tube from laryngopharynx tostomach

• Pierces diaphragm at esophageal hiatus

• Joins stomach at the cardiac orifice


Digestive Processes: Mouth

• Ingestion

• Mechanical digestion

• Mastication is partly voluntary, partly reflexive

• Chemical digestion (salivary amylase andlingual lipase)

• Propulsion

• Deglutition (swallowing)


Deglutition

• Involves the tongue, soft palate, pharynx,esophagus, and 22 muscle groups

• Buccal phase

• Voluntary contraction of the tongue

• Pharyngeal-esophageal phase

• Involuntary

• Control center in the medulla and lower pons

Copyright © 2010 Pearson Edcation, Inc. Figure 23.13

Tongue

Trachea

Pharynx

Epiglottis

Glottis

Bolus of food

Epiglottis

Esophagus

Uvula

Bolus

Bolus

Relaxed muscles

Circular musclescontract

Bolus of food

Longitudinal musclescontract

Stomach

Relaxedmuscles

Gastroesophagealsphincter opens

Gastroesophagealsphincter closed

Upper esophageal sphincter iscontracted. During the buccal phase, thetongue presses against the hard palate,forcing the food bolus into the oropharynxwhere the involuntary phase begins.

Food is movedthrough the esophagusto the stomach byperistalsis.

The gastroesophagealsphincter opens, and foodenters the stomach.

The uvula and larynx rise to prevent foodfrom entering respiratory passageways. Thetongue blocks off the mouth. The upperesophageal sphincter relaxes, allowing foodto enter the esophagus.

The constrictor muscles of thepharynx contract, forcing foodinto the esophagus inferiorly. Theupper esophageal sphinctercontracts (closes) after entry.

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4

3

5

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Stomach: Gross Anatomy

• Cardiac region (cardia)

• Surrounds the cardiac orifice

• Fundus

• Dome-shaped region beneath the diaphragm

• Body

• Midportion

• Pyloric region: antrum, pyloric canal, and pylorus

• Pylorus is continuous with the duodenum through thepyloric valve (sphincter)

Copyright © 2010 Pearson Edcation, Inc. Figure 23.14a

Cardia

Esophagus

Pyloric sphincter(valve) at pylorus

Pyloriccanal

Pyloricantrum

Rugae ofmucosa

Body

Lumen

Serosa

Fundus

Lessercurvature

Greatercurvature

Muscularisexterna • Longitudinal layer • Circular layer • Oblique layer

(a)

Duodenum


Stomach: Gross Anatomy

• ANS nerve supply

• Sympathetic via splanchnic nerves and celiacplexus

• Parasympathetic via vagus nerve

• Blood supply

• Celiac trunk

• Veins of the hepatic portal system


Stomach: Microscopic Anatomy

• Four tunics

• Muscularis and mucosa are modified


• Three layers of smooth muscle

• Inner oblique layer allows stomach to churn,mix, move, and physically break down food

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Mucosa

Surfaceepithelium

Lamina propria

Muscularismucosae

Oblique layer

Circular layer

Longitudinallayer

Serosa

(a) Layers of the stomach wall (l.s.)Stomach wall

Muscularis externa(contains myentericplexus)

Submucosa(contains submucosalplexus)


Stomach: Microscopic Anatomy

• Mucosa

• Simple columnar epithelium composed ofmucous cells

• Layer of mucus traps bicarbonate-rich fluidbeneath it

• Gastric pits lead into gastric glands

Copyright © 2010 Pearson Edcation, Inc. Figure 23.15b

(b) Enlarged view of gastric pits and gastric glands

Mucous neck cells

Parietal cell

Surface epithelium(mucous cells)

Gastric pits

Chief cell

Enteroendocrine cell

Gastric pit

Gastric gland


Gastric Glands

• Cell types

• Mucous neck cells (secrete thin, acidic mucus)

• Parietal cells

• Chief cells

• Enteroendocrine cells

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Copyright © 2010 Pearson Edcation, Inc. Figure 23.15c

(c) Location of the HCl-producing parietal cells and pepsin-secreting chief cells in a gastric gland

Pepsinogen

Mitochondria

PepsinHCl

Chief cell

Enteroendocrinecell

Parietal cell


Gastric Gland Secretions

• Glands in the fundus and body produce most of the gastricjuice

• Parietal cell secretions

• HCl

• → pH 1.5–3.5 denatures protein in food, activates pepsin, andkills many bacteria

• Intrinsic factor

• Glycoprotein required for absorption of vitamin B12 in smallintestine



• Chief cell secretions

• Inactive enzyme pepsinogen

• Activated to pepsin by HCl and by pepsin itself(a positive feedback mechanism)




• Secrete chemical messengers into the laminapropria

• Paracrines

• Serotonin and histamine

• Hormones

• Somatostatin and gastrin

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Mucosal Barrier

• Layer of bicarbonate-rich mucus

• Tight junctions between epithelial cells

• Damaged epithelial cells are quickly replacedby division of stem cells


Homeostatic Imbalance

• Gastritis: inflammation caused by anythingthat breaches the mucosal barrier

• Peptic or gastric ulcers: erosion of thestomach wall

• Most are caused by Helicobacter pyloribacteria


Bacteria

Mucosalayer ofstomach

(a) A gastric ulcer lesion (b) H. pylori bacteria


Digestive Processes in the Stomach

• Physical digestion

• Denaturation of proteins

• Enzymatic digestion of proteins by pepsin(and rennin in infants)

• Secretes intrinsic factor required forabsorption of vitamin B12

• Lack of intrinsic factor → pernicious anemia

• Delivers chyme to the small intestine

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Regulation of Gastric Secretion

• Neural and hormonal mechanisms

• Stimulatory and inhibitory events occur in threephases:

1. Cephalic (reflex) phase: few minutes prior to foodentry

2. Gastric phase: 3–4 hours after food enters thestomach

3. Intestinal phase: brief stimulatory effect as partiallydigested food enters the duodenum, followed byinhibitory effects (enterogastric reflex andenterogastrones)


Presence of lowpH, partially digested foods, fats, or hypertonic solution in duodenum when stomach begins to empty

Distension;presence offatty, acidic,partiallydigested foodin theduodenum

Briefeffect

Intestinal(enteric)gastrinreleaseto blood

Entero-gastricreflex

Release of intestinalhormones (secretin,cholecystokinin, vasoactiveintestinal peptide)

Localreflexes

Vagalnucleiin medullaPyloricsphincter

Stimulate

Inhibit

1

1

2

Stomachsecretoryactivity

Sight and thoughtof food

Stomachdistensionactivatesstretchreceptors

Stimulation oftaste and smellreceptors

Food chemicals(especially peptides and caffeine) and rising pHactivate chemoreceptors

Loss ofappetite,depression

Emotionalupset

Lack ofstimulatoryimpulses toparasym-patheticcenter

Cerebralcortex

Cerebral cortexConditioned reflex

Vagovagalreflexes

Localreflexes

Medulla

G cells

Hypothalamusand medullaoblongata

Vagusnerve

Vagusnerve

Gastrinreleaseto blood

Gastrinsecretiondeclines

G cells

Overridesparasym-patheticcontrols

Sympatheticnervoussystemactivation

1

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1

2

2

2

Stimulatory events Inhibitory events

Cephalicphase

Gastricphase

Intestinalphase

Excessiveacidity (pH <2) in stomach

Distension of duodenum; presence of fatty, acidic, hypertonic chyme, and/or irritants in the duodenum


Regulation and Mechanism of HCl Secretion

• Three chemicals (ACh, histamine, andgastrin) stimulate parietal cells throughsecond-messenger systems

• All three are necessary for maximum HClsecretion

• Antihistamines block H2 receptors anddecrease HCl release


Stomach lumenChief cell

Parietal cell

Inter-stitialfluid

Carbonicanhydrase

Alkalinetide

HCO3–

Bloodcapillary

CO2

Cl–

CO2 + H2O

H2CO3

HCO3–- Cl–

antiporter

HCO3–

H+

Cl– Cl–l

K+ K+

H+

H+-K+

ATPase

HCI

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Response of the Stomach to Filling

• Stretches to accommodate incoming food

• Reflex-mediated receptive relaxation

• Coordinated by the swallowing center of thebrain stem

• Gastric accommodation

• Plasticity (stress-relaxation response) ofsmooth muscle


Gastric Contractile Activity

• Peristaltic waves move toward the pylorus atthe rate of 3 per minute

• Basic electrical rhythm (BER) initiated bypacemaker cells (cells of Cajal)

• Distension and gastrin increase force ofcontraction


Gastric Contractile Activity

• Most vigorous near the pylorus

• Chyme is either

• Delivered in ~ 3 ml spurts to the duodenum, or

• Forced backward into the stomach


1 Propulsion: Peristaltic waves move from the fundus toward the pylorus.

2 3 Grinding: The most vigorous peristalsis and mixing action occur close to the pylorus.

Retropulsion: The pyloric end of the stomach acts as a pump that delivers small amounts of chyme into the duodenum, simultaneously forcing most of its contained material backward into the stomach.

Pyloricvalveclosed

Pyloricvalveclosed

Pyloricvalveslightlyopened

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Regulation of Gastric Emptying

• As chyme enters the duodenum

• Receptors respond to stretch and chemicalsignals

• Enterogastric reflex and enterogastronesinhibit gastric secretion and duodenal filling

• Carbohydrate-rich chyme moves quicklythrough the duodenum

• Fatty chyme remains in the duodenum6 hours or more


Presence of fatty, hypertonic,acidic chyme in duodenum

Duodenal entero-endocrine cells

Chemoreceptors andstretch receptors

Enterogastrones(secretin, cholecystokinin,vasoactive intestinalpeptide)

Duodenalstimulidecline

Via shortreflexes

Via longreflexes

Entericneurons

Initial stimulus

Physiological response

Result

Contractile force andrate of stomachemptying decline

CNS centers sympathetic activity; parasympathetic activity

Stimulate

Inhibit

Secrete Target


Small Intestine: Gross Anatomy

• Major organ of digestion and absorption

• 2–4 m long; from pyloric sphincter toileocecal valve

• Subdivisions

1. Duodenum (retroperitoneal)

2. Jejunum (attached posteriorly by mesentery)

3. Ileum (attached posteriorly by mesentery)


Mouth (oral cavity)Tongue

Esophagus

LiverGallbladder

Anus

DuodenumJejunumIleum

Small intestine

Parotid glandSublingual glandSubmandibulargland

Salivaryglands

Pharynx

StomachPancreas(Spleen)

Transverse colonDescending colonAscending colonCecumSigmoid colonRectumVermiform appendixAnal canal

Largeintestine

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Duodenum

• The bile duct and main pancreatic duct

• Join at the hepatopancreatic ampulla

• Enter the duodenum at the major duodenalpapilla

• Are controlled by the hepatopancreaticsphincter


Jejunum

Mucosawith folds

Cystic duct

DuodenumHepatopancreaticampulla and sphincter

Gallbladder

Right and lefthepatic ducts of liver

Bile duct and sphincter

Main pancreatic ductand sphincter

PancreasTail of pancreas

Head of pancreas

Common hepatic duct

Major duodenalpapilla

Accessory pancreatic duct


Structural Modifications

• Increase surface area of proximal part fornutrient absorption

• Circular folds (plicae circulares)

• Villi

• Microvilli



• Circular folds

• Permanent (~1 cm deep)

• Force chyme to slowly spiral through lumen

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Vein carrying blood tohepatic portal vessel

MusclelayersCircularfoldsVilli

(a)

Lumen



• Villi

• Motile fingerlike extensions (~1 mm high) ofthe mucosa

• Villus epithelium

• Simple columnar absorptive cells(enterocytes)

• Goblet cells



• Microvilli

• Projections (brush border) of absorptive cells

• Bear brush border enzymes


Intestinal Crypts

• Intestinal crypt epithelium

• Secretory cells that produce intestinal juice


• Intraepithelial lymphocytes (IELs)

• Release cytokines that kill infected cells

• Paneth cells

• Secrete antimicrobial agents (defensins and lysozyme)

• Stem cells

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(b)

Absorptive cells

Lacteal

Intestinal crypt

Mucosaassociatedlymphoid tissue

MuscularismucosaeDuodenal gland Submucosa

EnteroendocrinecellsVenuleLymphatic vessel

Goblet cellBloodcapillaries

Vilus

Microvilli(brush border)


(b)

Microvilli

Absorptivecell


Submucosa

• Peyer’s patches protect distal part againstbacteria

• Duodenal (Brunner’s) glands of theduodenum secrete alkaline mucus


Intestinal Juice

• Secreted in response to distension or irritationof the mucosa

• Slightly alkaline and isotonic with bloodplasma

• Largely water, enzyme-poor, but containsmucus

• Facilitates transport and absorption ofnutrients

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Liver

• Largest gland in the body

• Four lobes—right, left, caudate, and quadrate


Liver: Associated Structures

• Lesser omentum anchors liver to stomach

• Hepatic artery and vein at the porta hepatis

• Bile ducts

• Common hepatic duct leaves the liver

• Cystic duct connects to gallbladder

• Bile duct formed by the union of the above twoducts


Jejunum

Mucosawith folds

Cystic duct

DuodenumHepatopancreaticampulla and sphincter

Gallbladder

Right and lefthepatic ducts of liver

Bile duct and sphincter

Main pancreatic ductand sphincter

PancreasTail of pancreas

Head of pancreas

Common hepatic duct

Major duodenalpapilla

Accessory pancreatic duct


Liver: Microscopic Anatomy

• Liver lobules

• Hexagonal structural and functional units

• Filter and process nutrient-rich blood

• Composed of plates of hepatocytes (livercells)

• Longitudinal central vein

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Copyright © 2010 Pearson Edcation, Inc. Figure 23.25a, b

(a) (b)Lobule Central vein Connectivetissue septum



• Portal triad at each corner of lobule

• Bile duct receives bile from bile canaliculi

• Portal arteriole is a branch of the hepatic artery

• Hepatic venule is a branch of the hepatic portalvein

• Liver sinusoids are leaky capillaries betweenhepatic plates

• Kupffer cells (hepatic macrophages) in liversinusoids


(c)

Interlobular veins(to hepatic vein) Central vein

Sinusoids

Portal triad

Plates ofhepatocytes

Portal vein

Fenestratedlining (endothelial cells) of sinusoids

Bile duct (receivesbile from bile canaliculi)

Bile duct

Portal arteriolePortal venuleHepatic

macrophagesin sinusoid walls

Bile canaliculi



• Hepatocyte functions

• Process bloodborne nutrients

• Store fat-soluble vitamins

• Perform detoxification

• Produce ~900 ml bile per day

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Bile

• Yellow-green, alkaline solution containing

• Bile salts: cholesterol derivatives that functionin fat emulsification and absorption

• Bilirubin: pigment formed from heme

• Cholesterol, neutral fats, phospholipids, andelectrolytes


Bile

• Enterohepatic circulation

• Recycles bile salts

• Bile salts → duodenum → reabsorbed fromileum → hepatic portal blood → liver →secreted into bile

Bile secretion, release & recycling

• Bile salts are major stimulus for bile secretion

• Fat in duodenum -> CCK -> gall bladdercontraction and opens sphincter of Oddi

gall bladder

bile duct

liver

duodenum

terminal ileumbile salts

(via portal vein)

sphincter of Oddi


The Gallbladder

• Thin-walled muscular sac on the ventralsurface of the liver

• Stores and concentrates bile by absorbing itswater and ions

• Releases bile via the cystic duct, which flowsinto the bile duct

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Pancreas

• Location

• Mostly retroperitoneal, deep to the greatercurvature of the stomach

• Head is encircled by the duodenum; tail abutsthe spleen


Pancreas

• Endocrine function

• Pancreatic islets secrete insulin and glucagon

• Exocrine function

• Acini (clusters of secretory cells) secretepancreatic juice

• Zymogen granules of secretory cells containdigestive enzymes


Smallduct

Acinar cells

Basementmembrane

Zymogengranules

Roughendoplasmicreticulum

(a)


Pancreatic Juice

• Watery alkaline solution (pH 8) neutralizeschyme

• Electrolytes (primarily HCO3–)

• Enzymes

• Amylase, lipases, nucleases are secreted inactive form but require ions or bile for optimalactivity

• Proteases secreted in inactive form

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Pancreatic Juice

• Protease activation in duodenum

• Trypsinogen is activated to trypsin by brushborder enzyme enteropeptidase

• Procarboxypeptidase and chymotrypsinogenare activated by trypsin


Stomach

Pancreas

Epithelialcells

Trypsinogen(inactive)Chymotrypsinogen(inactive)Procarboxypeptidase(inactive)

Trypsin

Chymotrypsin

Carboxypeptidase

Membrane-boundenteropeptidase


Regulation of Bile Secretion

• Bile secretion is stimulated by

• Bile salts in enterohepatic circulation

• Secretin from intestinal cells exposed to HCland fatty chyme


Regulation of Bile Secretion

• Gallbladder contraction is stimulated by

• Cholecystokinin (CCK) from intestinal cellsexposed to proteins and fat in chyme

• Vagal stimulation (minor stimulus)

• CKK also causes the hepatopancreaticsphincter to relax

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Regulation of Pancreatic Secretion

• CCK induces the secretion of enzyme-richpancreatic juice by acini

• Secretin causes secretion of bicarbonate-richpancreatic juice by duct cells

• Vagal stimulation also causes release ofpancreatic juice (minor stimulus)


Chyme enter-ing duodenum causes release of cholecystokinin (CCK) and secretin from duodenal enteroendocrine cells.

CCK (red dots) and secretin (yellow dots) enter the bloodstream.

CCK induces secretion of enzyme-rich pancreatic juice. Secretin causes secretion of HCO3

–-rich pancreatic juice.

Bile salts and, to a lesser extent, secretin transported via bloodstream stimulate liver to produce bile more rapidly.

CCK (via bloodstream) causes gallbladder to contract and hepatopancreatic sphincter to relax; bile enters duodenum.

During cephalic and gastric phases, vagal nerve stimulation causes weak contractions of gallbladder.

Slide 1

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Digestion in the Small Intestine

• Chyme from stomach contains

• Partially digested carbohydrates and proteins

• Undigested fats

• Slow delivery of hypertonic chyme

• Delivery of bile, enzymes, and bicarbonate from theliver and pancreas

• Mixing

Requirements for Digestion and Absorptionin the Small Intestine


Motility of the Small Intestine

• Segmentation

• Initiated by intrinsic pacemaker cells

• Mixes and moves contents slowly and steadilytoward the ileocecal valve

• Intensity is altered by long and short reflexes

• Wanes in the late intestinal (fasting) phase

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• Peristalsis

• Initiated by motilin in the late intestinal phase

• Each wave starts distal to the previous (themigrating motility complex)

• Meal remnants, bacteria, and debris aremoved to the large intestine


From mouth

(a) Peristalsis: Adjacent segments of alimentary tract organs alternately contract and relax, which moves food along the tract distally.



• Local enteric neurons coordinate intestinalmotility

• Cholinergic sensory neurons may activate themyenteric plexus

• Causes contraction of the circular muscleproximally and of longitudinal muscle distally

• Forces chyme along the tract



• Ileocecal sphincter relaxes and admits chymeinto the large intestine when

• Gastroileal reflex enhances the force ofsegmentation in the ileum

• Gastrin increases the motility of the ileum

• Ileocecal valve flaps close when chyme exertsbackward pressure

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Large Intestine

• Unique features

• Teniae coli

• Three bands of longitudinal smooth muscle in the muscularis

• Haustra

• Pocketlike sacs caused by the tone of the teniae coli

• Epiploic appendages

• Fat-filled pouches of visceral peritoneum


Large Intestine

• Regions

• Cecum (pouch with attached vermiformappendix)

• Colon

• Rectum

• Anal canal


Left colic(splenic) flexure

Transversemesocolon

Epiploicappendages

Descendingcolon

Teniae coli

Sigmoidcolon

Cut edge ofmesentery

External anal sphincter

Rectum

Anal canal(a)

Right colic(hepatic) flexureTransversecolon SuperiormesentericarteryHaustrum

Ascendingcolon IIeum

IIeocecal valve

Vermiform appendix

Cecum


Colon

• Ascending colon and descending colon areretroperitoneal

• Transverse colon and sigmoid colon areanchored via mesocolons (mesenteries)

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Transverse colon

Greater omentum

Descending colonJejunumMesentery

Transversemesocolon

SigmoidmesocolonSigmoid colon

Ileum

(c)Copyright © 2010 Pearson Edcation, Inc.

Rectum and Anus

• Rectum

• Three rectal valves stop feces from beingpassed with gas

• Anal canal

• The last segment of the large intestine

• Sphincters

• Internal anal sphincter—smooth muscle

• External anal sphincter—skeletal muscle


(b)

Rectal valveRectum

Anal canal

Levator animuscle

Anus

Anal sinuses

Anal columns

Internal analsphincter

External analsphincter

Hemorrhoidalveins

Pectinate line


Bacterial Flora

• Enter from the small intestine or anus

• Colonize the colon

• Ferment indigestible carbohydrates

• Release irritating acids and gases

• Synthesize B complex vitamins and vitamin K

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Functions of the Large Intestine

• Vitamins, water, and electrolytes arereclaimed

• Major function is propulsion of feces towardthe anus

• Colon is not essential for life


Motility of the Large Intestine

• Haustral contractions

• Slow segmenting movements

• Haustra sequentially contract in response todistension


Motility of the Large Intestine

• Gastrocolic reflex

• Initiated by presence of food in the stomach

• Activates three to four slow powerful peristalticwaves per day in the colon (mass movements)


Defecation

• Mass movements force feces into rectum

• Distension initiates spinal defecation reflex

• Parasympathetic signals

• Stimulate contraction of the sigmoid colon andrectum

• Relax the internal anal sphincter

• Conscious control allows relaxation ofexternal anal sphincter

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Impulses fromcerebral cortex(consciouscontrol)

Voluntary motornerve to externalanal sphincter

External analsphincter(skeletal muscle)

Internal anal sphincter(smooth muscle)

Sensorynerve fibers

Involuntary motor nerve(parasympathetic division)

Stretch receptors in wall

Rectum

Sigmoidcolon

3

1

2

Distension, or stretch, of therectal walls due to movement of feces into the rectum stimulates stretch receptors there. The receptors transmit signals along afferent fibers to spinal cord neurons.

A spinal reflex is initiated in which parasympathetic motor (efferent) fibers stimulate contraction of the rectal walls and relaxation of the internal anal sphincter.

If it is convenient to defecate, voluntary motor neurons are inhibited, allowing the external anal sphincter to relax so that feces may pass.


Chemical Digestion and Absorption ofCarbohydrates

• Digestive enzymes

• Salivary amylase, pancreatic amylase, andbrush border enzymes (dextrinase,glucoamylase, lactase, maltase, and sucrase)


Chemical Digestion and Absorption ofCarbohydrates

• Absorption

• Secondary active transport (cotransport) withNa+

• Facilitated diffusion of some monosaccharides

• Enter the capillary beds in the villi

• Transported to the liver via the hepatic portalvein

Copyright © 2010 Pearson Edcation, Inc. Figure 23.32 (1 of 4)

Carbohydrate digestion

• Glucose and galactose are absorbed via cotransport with sodium ions.• Fructose passes via facilitated diffusion.• All monosaccharides leave the epithelial cells via facilitated diffusion, enter the capillary blood in the villi, and are transported to the liver via the hepatic portal vein.

Starch and disaccharides

Oligosaccharidesand disaccharides

Lactose Maltose Sucrose

Glucose Fructose

Salivaryamylase

Mouth

Pancreaticamylase

Brush borderenzymes in small intestine(dextrinase, gluco-amylase, lactase, maltase, and sucrase)

Smallintestine

Smallintestine

Foodstuff

Galactose

Path of absorptionEnzyme(s)and source

Site ofaction

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Chemical Digestion and Absorption ofProteins

• Enzymes: pepsin in the stomach

• Pancreatic proteases

• Trypsin, chymotrypsin, and carboxypeptidase

• Brush border enzymes

• Aminopeptidases, carboxypeptidases, anddipeptidases

• Absorption of amino acids is coupled to activetransport of Na+


Absorptiveepithelialcell

Apical membrane (microvilli)

Aminoacid carrier

Capillary

Lumen of intestine

Pancreaticproteases

Amino acids of protein fragmentsBrush border enzymes

Na+

Na+

1 Proteins and protein fragments are digested to amino acids by pancreatic proteases (trypsin, chymotrypsin, and carboxypeptidase), and by brush border enzymes (carboxypeptidase, aminopeptidase, and dipeptidase)of mucosal cells.

2 The amino acids are then absorbed by active transport into the absorptive cells, and move to their opposite side (transcytosis).

3 The amino acids leave the villus epithelial cell by facilitated diffusion and enter the capillary via intercellular clefts.

Active transport

Passive transport


Protein digestion

• Amino acids are absorbed by cotransport with sodium ions.• Some dipeptides and tripeptides are absorbed via cotransport with H+

and hydrolyzed to amino acids within the cells.

+

• Amino acids leave the epithelial cells by facilitated diffusion, enter the capillary blood in the villi, and are transported to the liver via the hepatic portal vein.

Smallintestine

Smallintestine

Stomach

Foodstuff

Protein

Large polypeptides

Pepsin(stomach glands)in presence of HCl

Small polypeptides,small peptides

Pancreaticenzymes (trypsin, chymotrypsin,carboxypeptidase)

Amino acids(some dipeptidesand tripeptides)

Brush border enzymes(aminopeptidase,carboxypeptidase,and dipeptidase)


Site ofaction


Chemical Digestion and Absorption ofLipids

• Pre-treatment—emulsification by bile salts

• Enzymes—pancreatic lipase

• Absorption of glycerol and short chain fattyacids

• Absorbed into the capillary blood in villi

• Transported via the hepatic portal vein

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Chemical Digestion and Absorption ofLipids

• Absorption of monoglycerides and fatty acids

• Cluster with bile salts and lecithin to formmicelles

• Released by micelles to diffuse into epithelialcells

• Combine with proteins to form chylomicrons

• Enter lacteals and are transported to systemiccirculation


Epithelialcells ofsmallintestine

Fat dropletscoated withbile salts

Fat globule

Lacteal

Bile salts

Micelles made up of fatty acids, monoglycerides,and bile salts

1 Large fat globules are emulsified (physically broken up into smaller fat droplets) by bile salts in the duodenum.

2 Digestion of fat by the pancreatic enzyme lipase yields free fatty acids and monoglycerides. These then associate with bile salts to form micelles which “ferry” them to the intestinal mucosa.

3 Fatty acids and monoglycerides leave micelles and diffuse into epithelial cells. There they are recombined and packaged with other lipoid substances and proteins to form chylomicrons.

4 Chylomicrons are extruded from the epithelial cells by exocytosis. The chylomicrons enter lacteals. They are carried away from the intestine by lymph.


Fat digestion

Small intestine

Small intestine

Foodstuff

Unemulsifiedfats

Emulsification by the detergent action of bile salts ductedin from the liver

Pancreatic lipases

Monoglyceridesand fatty acids

Glyceroland

fatty acids


Site ofaction

• Fatty acids and monoglycerides enter the intestinal cells via diffusion. • Fatty acids and monoglycerides are recombined to form triglycerides and then combined with other lipids and proteins within the cells, and the resulting chylomicrons are extruded by exocytosis.

• The chylomicrons enter the lacteals of the villi and are transported to the systemic circulation via the lymph in the thoracic duct.• Some short-chain fatty acids are absorbed, move into the capillary blood in the villi by diffusion, and are transported to the liver via the hepatic portal vein.


Vitamin Absorption

• In small intestine

• Fat-soluble vitamins (A, D, E, and K) arecarried by micelles and then diffuse intoabsorptive cells

• Water-soluble vitamins (vitamin C and Bvitamins) are absorbed by diffusion or bypassive or active transporters.

• Vitamin B12 binds with intrinsic factor, and isabsorbed by endocytosis

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Vitamin Absorption

• In large intestine

• Vitamin K and B vitamins from bacterialmetabolism are absorbed


Electrolyte Absorption

• Mostly along the length of small intestine

• Iron and calcium are absorbed in duodenum

• Na+ is coupled with absorption of glucose andamino acids

• Ionic iron is stored in mucosal cells with ferritin

• K+ diffuses in response to osmotic gradients

• Ca2+ absorption is regulated by vitamin D andparathyroid hormone (PTH)

Water Absorption

• 95% is absorbed in the small intestine byosmosis

• Net osmosis occurs whenever a concentrationgradient is established by active transport ofsolutes

• Water uptake is coupled with solute uptake


Water fluxes

Fluid drunk

Saliva

Gastric Juice

Pancreatic Juice

Bile

Intestinal Juice

Sources of FluidsTotal = 9.5 l/day

Small Intestine

Large Intestine

AbsorptionTotal = 9.35 l/day

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Malabsorption of Nutrients

• Causes

• Anything that interferes with delivery of bile orpancreatic juice

• Damaged intestinal mucosa (e.g., bacterialinfection)


Malabsorption of Nutrients

• Gluten-sensitive enteropathy (celiac disease)

• Gluten damages the intestinal villi and brushborder

• Treated by eliminating gluten from the diet (allgrains but rice and corn)

Digestive System Introduction

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