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Copyright 2004 Pearson Education, Inc., publishing as Benjamin Cummings

3/20/2013 1diges

tion & absorption

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Human Anatomy & Physiology, Sixth Edition

Elaine N. Marieb

PowerPoint Lecture Slides prepared by Vince Austin, University of Kentucky

23The Digestive System

Part B

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Pharynx

From the mouth, the oro- and laryngopharynx allow

passage of:

Food and fluids to the esophagus

Air to the trachea

Lined with stratified squamous epithelium and

mucus glands

Has two skeletal muscle layers

Inner longitudinal

Outer pharyngeal constrictors3/20/2013 3digestion & absorption

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Esophagus

Muscular tube going from the laryngopharynx to the

stomach

Travels through the mediastinum and pierces the

diaphragm

Joins the stomach at the cardiac orifice


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Esophageal Characteristics

Esophageal mucosanonkeratinized stratifiedsquamous epithelium

The empty esophagus is folded longitudinally and

flattens when food is present

Glands secrete mucus as a bolus moves through the

esophagus

Muscularis changes from skeletal (superiorly) to

smooth muscle (inferiorly)


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Digestive Processes in the Mouth

Food is ingested

Mechanical digestion begins (chewing)

Propulsion is initiated by swallowing

Salivary amylase begins chemical breakdown of

starch

The pharynx and esophagus serve as conduits to

pass food from the mouth to the stomach


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Deglutition (Swallowing)

Involves the coordinated activity of the tongue, soft

palate, pharynx, esophagus and 22 separate musclegroups

Buccal phasebolus is forced into the oropharynx

Pharyngeal-esophageal phasecontrolled by themedulla and lower pons

All routes except into the digestive tract are sealedoff

Peristalsis moves food through the pharynx to theesophagus3/20/2013 7digestion & absorption

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Deglutition (Swallowing)

Figure 23.13

(a) Upper esophagealsphincter contracted

(b) Upper esophagealsphincter relaxed

(c) Upper esophageal

sphincter contracted

(e)(d)

Bolus offood

Uvula

Bolus

Relaxed musclesRelaxedmuscles

Tongue

Pharynx

Epiglottis

Glottis

Trachea

Bolus

Epiglottis

Bolus of food

Longitudinal musclescontract, shortening

passageway ahead of bolus

Gastroesophagealsphincter closed

Circular musclescontract, constrictingpassageway and pushingbolus down

Stomach

Gastroesophagealsphincter open

Esophagus


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Stomach

Chemical breakdown of proteins begins and food is

converted to chyme

Cardiac regionsurrounds the cardiac orifice

Fundusdome-shaped region beneath the

diaphragm

Bodymidportion of the stomach

Pyloric regionmade up of the antrum and canalwhich terminates at the pylorus

The pylorus is continuous with the duodenumthrough the pyloric sphincter3/20/2013 9digestion & absorption

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Stomach

Greater curvatureentire extent of the convex

lateral surface

Lesser curvatureconcave medial surface

Lesser omentumruns from the liver to the lesser

curvature

Greater omentumdrapes inferiorly from the

greater curvature to the small intestine


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Stomach

Nerve supplysympathetic and parasympathetic

fibers of the autonomic nervous system

Blood supplyceliac trunk, and corresponding

veins (part of the hepatic portal system)


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Stomach

Figure 23.14a3/20/2013 12digestion & absorption

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Microscopic Anatomy of the Stomach

Muscularishas an additional oblique layer that:

Allows the stomach to churn, mix, and pummelfood physically

Breaks down food into smaller fragments

Epithelial lining is composed of:

Goblet cells that produce a coat of alkaline mucus

The mucous surface layer traps a bicarbonate-rich fluid beneath it

Gastric pits contain gastric glands that secretegastric juice, mucus, and gastrin3/20/2013 13digestion & absorption

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Microscopic Anatomy of the Stomach

Figure 23.153/20/2013 14digestion & absorption

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Glands of the Stomach Fundus and Body

Gastric glands of the fundus and body have a variety

of secretory cells

Mucous neck cellssecrete acid mucus

Parietal cellssecrete HCl and intrinsic factor


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Glands of the Stomach Fundus and Body

Chief cellsproduce pepsinogen

Pepsinogen is activated to pepsin by:

HCl in the stomach

Pepsin itself via a positive feedback

mechanism

Enteroendocrine cellssecrete gastrin, histamine,endorphins, serotonin, cholecystokinin (CCK), and

somatostatin into the lamina propria


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Stomach Lining

The stomach is exposed to the harshest conditions in

the digestive tract

To keep from digesting itself, the stomach has a

mucosal barrier with:

A thick coat of bicarbonate-rich mucus on the

stomach wall

Epithelial cells that are joined by tight junctions

Gastric glands that have cells impermeable to HCl

Damaged epithelial cells are quickly replaced3/20/2013 17digestion & absorption

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Digestion in the Stomach

The stomach:

Holds ingested food

Degrades this food both physically and chemically

Delivers chyme to the small intestine

Enzymatically digests proteins with pepsin

Secretes intrinsic factor required for absorption of

vitamin B12


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Regulation of Gastric Secretion

Neural and hormonal mechanisms regulate therelease of gastric juice

Stimulatory and inhibitory events occur in three

phases

Cephalic (reflex) phase: prior to food entry

Gastric phase: once food enters the stomach Intestinal phase: as partially digested food enters

the duodenum


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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


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Gastric Phase

Inhibitory events include:

A pH lower than 2

Emotional upset that overrides the parasympathetic

division


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Intestinal Phase

Excitatory phaselow pH; partially digested food

enters the duodenum and encourages gastric glandactivity

Inhibitory phasedistension of duodenum, presence

of fatty, acidic, or hypertonic chyme, and/or irritantsin the duodenum

Initiates inhibition of local reflexes and vagal nuclei

Closes the pyloric sphincter

Releases enterogastrones that inhibit gastricsecretion


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Release of Gastric Juice


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Regulation and Mechanism of HCl Secretion

HCl secretion is stimulated by ACh, histamine, andgastrin 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


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Regulation and Mechanism of HCl Secretion


S

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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 relaxationas food travels in the

esophagus, stomach muscles relax

Adaptive relaxationthe stomach dilates inresponse to gastric filling

Plasticityintrinsic ability of smooth muscle toexhibit the stress-relaxation response3/20/2013 27digestion & absorption

G i C il A i i

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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 bypacemaker cells (cells of Cajal)

Most vigorous peristalsis and mixing occurs near thepylorus

Chyme is either:

Delivered in small amounts to the duodenum or

Forced backward into the stomach for furthermixing


G i C il A i i

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Gastric Contractile Activity


R l ti f G t i E t i

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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 andduodenal filling

Carbohydrate-rich chyme quickly moves through the

duodenum

Fat-laden chyme is digested more slowly causing

food to remain in the stomach longer3/20/2013 30digestion & absorption

R l ti f G t i E t i

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Regulation of Gastric Emptying


S ll I t ti G A t

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Small Intestine: Gross Anatomy

Runs from pyloric sphincter to the ileocecal valve

Has three subdivisions: duodenum, jejunum, andileum

The bile duct and main pancreatic duct:

Join the duodenum at the hepatopancreatic ampulla

Are controlled by the sphincter of Oddi

The jejunum extends from the duodenum to theileum

The ileum joins the large intestine at the ileocecalvalve3/20/2013 32digestion & absorption

S ll I t ti Mi i A t

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Small Intestine: Microscopic Anatomy

Structural modifications of the small intestine wall

increase surface area

Plicae circulares: deep circular folds of the mucosaand submucosa

Villifingerlike extensions of the mucosa

Microvillitiny projections of absorptive mucosal

cells plasma membranes


S ll I t ti Mi i A t

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Small Intestine: Microscopic Anatomy


Small Intestine Histolog of the Wall

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Small Intestine: Histology of the Wall

The epithelium of the mucosa is made up of:

Absorptive cells and goblet cells

Enteroendocrine cells

Interspersed T cells called intraepithelial

lymphocytes (IELs)

IELs immediately release cytokines upon

encountering Ag



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Cells of intestinal crypts secrete intestinal juice

Peyers patches are found in the submucosa

Brunners glands in the duodenum secrete alkaline

mucus


Intestinal Juice

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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


Liver

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Liver

The largest gland in the body

Superficially has four lobesright, left, caudate,

and quadrate

The falciform ligament:

Separates the right and left lobes anteriorly

Suspends the liver from the diaphragm and anterior

abdominal wall


Liver

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Liver

The ligamentum teres:

Is a remnant of the fetal umbilical vein

Runs along the free edge of the falciform ligament


Liver: Associated Structures

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The lesser omentum anchors the liver to the stomach

The hepatic blood vessels enter the liver at the portahepatis

The gallbladder rests in a recess on the inferior

surface of the right lobe



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Bile leaves the liver via:

Bile ducts, which fuse into the common hepatic

duct

The common hepatic duct, which fuses with the

cystic duct

These two ducts form the bile duct


Gallbladder and Associated Ducts

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Gallbladder and Associated Ducts


Liver: Microscopic Anatomy

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Hexagonal-shaped liver lobules are the structural andfunctional units of the liver

Composed of hepatocyte (liver cell) plates radiatingoutward from a central vein

Portal triads are found at each of the six corners of eachliver lobule

Portal triads consist of a bile duct and

Hepatic arterysupplies oxygen-rich blood to the liver

Hepatic portal veincarries venous blood with nutrientsfrom digestive viscera



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Liver sinusoidsenlarged, leaky capillaries located

between hepatic plates

Kupffer cellshepatic macrophages found in liver

sinusoids



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Hepatocytes functions include:

Production of bile

Processing bloodborne nutrients

Storage of fat-soluble vitamins

Detoxification

Secreted bile flows between hepatocytes toward the

bile ducts in the portal triads


Microscopic Anatomy of the Liver

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Microscopic Anatomy of the Liver

Figure 23.24c, d3/20/2013 46digestion & absorption

Composition of Bile

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Composition of Bile

A yellow-green, alkaline solution containing bilesalts, bile pigments, cholesterol, neutral fats,phospholipids, and electrolytes

Bile salts are cholesterol derivatives that:

Emulsify fat Facilitate fat and cholesterol absorption

Help solubilize cholesterol

Enterohepatic circulation recycles bile salts

The chief bile pigment is bilirubin, a waste productof heme3/20/2013 47digestion & absorption

The Gallbladder

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The Gallbladder

Thin-walled, green muscular sac on the ventral

surface of the liver

Stores and concentrates bile by absorbing its water

and ions

Releases bile via the cystic duct, which flows into

the bile duct


Regulation of Bile Release

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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 thegallbladder



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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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