Document23

PowerPoint® Lecture Slides prepared by Janice Meeking, Mount Royal College

C H A P T E R

Copyright © 2010 Pearson Education, Inc.

23

The Digestive System: Part B


Pharynx

• Oropharynx and laryngopharynx

• Allow passage of food, fluids, and air

• Stratified squamous epithelium lining

• Skeletal muscle layers: inner longitudinal, outer pharyngeal constrictors


Esophagus

• Flat muscular tube from laryngopharynx to stomach

• Pierces diaphragm at esophageal hiatus

• Joins stomach at the cardiac orifice


Esophagus

• Esophageal mucosa contains stratified squamous epithelium

• Changes to simple columnar at the stomach

• Esophageal glands in submucosa secrete mucus to aid in bolus movement

• Muscularis: skeletal superiorly; smooth inferiorly

• Adventitia instead of serosa

Copyright © 2010 Pearson Education, Inc. Figure 23.12a

Mucosa(contains a stratifiedsquamous epithelium)

Submucosa (areolarconnective tissue)

LumenMuscularis externa

Adventitia (fibrousconnective tissue)

(a)

• Circular layer • Longitudinal layer

Copyright © 2010 Pearson Education, Inc. Figure 23.12b

Mucosa(contains a stratifiedsquamous epithelium)

(b)


Digestive Processes: Mouth

• Ingestion

• Mechanical digestion

• Mastication is partly voluntary, partly reflexive

• Chemical digestion (salivary amylase and lingual 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 Education, 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.

1 2

4

3

5

Copyright © 2010 Pearson Education, Inc. Figure 23.13, step 1

Tongue

Trachea

Pharynx

Epiglottis

Glottis

Bolus of food

Upper esophageal sphincter is contracted. Duringthe buccal phase, the tongue presses against the hardpalate, forcing the food bolus into the oropharynxwhere the involuntary phase begins.

1


Epiglottis

Esophagus

Uvula

Bolus

The uvula and larynx rise to prevent food fromentering respiratory passageways. The tongue blocksoff the mouth. The upper esophageal sphincterrelaxes, allowing food to enter the esophagus.

2


Bolus

The constrictor muscles of the pharynx contract,forcing food into the esophagus inferiorly. The upperesophageal sphincter contracts (closes) after entry.

3


Relaxed muscles

Bolus of food

Stomach




Food is moved throughthe esophagus to thestomach by peristalsis.

4


Relaxedmuscles



5


Tongue

Trachea

Pharynx

Epiglottis

Glottis

Bolus of food

Epiglottis

Esophagus

Uvula

Bolus

Bolus

Relaxed muscles


Bolus of food


Stomach

Relaxedmuscles



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 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.

1 2

4

3

5


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 the pyloric valve (sphincter)

• Greater curvature

• Convex lateral surface

• Lesser curvature

• Concave medial surface


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



• Lesser omentum

• From the liver to the lesser curvature

• Greater omentum

• Drapes from greater curvature

• Anterior to the small intestine


Falciform ligament

Liver

Gallbladder

Spleen

Stomach

Ligamentum teres

Greater omentum

Small intestine

Cecum

(a)


Liver

Lesser omentumGallbladder

StomachDuodenum

Transverse colon

Small intestine

Cecum

Urinary bladder(b)



• ANS nerve supply

• Sympathetic via splanchnic nerves and celiac plexus

• Parasympathetic via vagus nerve

• Blood supply

• Celiac trunk

• Veins of the hepatic portal system


Stomach: Microscopic Anatomy

• Four tunics

• Muscularis and mucosa are modified

• Muscularis externa

• Three layers of smooth muscle

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


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 of mucous cells

• Layer of mucus traps bicarbonate-rich fluid beneath it

• Gastric pits lead into gastric glands


(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

Copyright © 2010 Pearson Education, 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 gastric juice

• Parietal cell secretions

• HCl

• pH 1.5–3.5 denatures protein in food, activates pepsin, and kills many bacteria

• Intrinsic factor

• Glycoprotein required for absorption of vitamin B12 in small intestine



• Chief cell secretions

• Inactive enzyme pepsinogen

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




• Secrete chemical messengers into the lamina propria

• Paracrines

• Serotonin and histamine

• Hormones

• Somatostatin and gastrin


Mucosal Barrier

• Layer of bicarbonate-rich mucus

• Tight junctions between epithelial cells

• Damaged epithelial cells are quickly replaced by division of stem cells


Homeostatic Imbalance

• Gastritis: inflammation caused by anything that breaches the mucosal barrier

• Peptic or gastric ulcers: erosion of the stomach wall

• Most are caused by Helicobacter pylori bacteria


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 for absorption of vitamin B12

• Lack of intrinsic factor pernicious anemia

• Delivers chyme to the small intestine


Regulation of Gastric Secretion

• Neural and hormonal mechanisms

• Stimulatory and inhibitory events occur in three phases:

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

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


Regulation of Gastric Secretion

3. Intestinal phase: brief stimulatory effect as partially digested food enters the duodenum, followed by inhibitory effects (enterogastric reflex and enterogastrones)


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 medulla

Pyloricsphincter

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

11

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, and gastrin) stimulate parietal cells through second-messenger systems

• All three are necessary for maximum HCl secretion

• Antihistamines block H2 receptors and decrease 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


Response of the Stomach to Filling

• Stretches to accommodate incoming food

• Reflex-mediated receptive relaxation

• Coordinated by the swallowing center of the brain stem

• Gastric accommodation

• Plasticity (stress-relaxation response) of smooth muscle


Gastric Contractile Activity

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

• Basic electrical rhythm (BER) initiated by pacemaker cells (cells of Cajal)

• Distension and gastrin increase force of contraction


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


Regulation of Gastric Emptying

• As chyme enters the duodenum

• Receptors respond to stretch and chemical signals

• Enterogastric reflex and enterogastrones inhibit gastric secretion and duodenal filling

• Carbohydrate-rich chyme moves quickly through the duodenum

• Fatty chyme remains in the duodenum 6 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 to ileocecal 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


Duodenum

• The bile duct and main pancreatic duct

• Join at the hepatopancreatic ampulla

• Enter the duodenum at the major duodenal papilla

• Are controlled by the hepatopancreatic sphincter


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 for nutrient absorption

• Circular folds (plicae circulares)

• Villi

• Microvilli



• Circular folds

• Permanent (~1 cm deep)

• Force chyme to slowly spiral through lumen


Vein carrying blood tohepatic portal vessel

MusclelayersCircularfoldsVilli

(a)

Lumen



• Villi

• Motile fingerlike extensions (~1 mm high) of the 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


(b)

Absorptive cells

Lacteal

Intestinal crypt

Mucosaassociatedlymphoid tissue

MuscularismucosaeDuodenal gland Submucosa

EnteroendocrinecellsVenuleLymphatic vessel

Goblet cellBloodcapillaries

Vilus

Microvilli(brush border)


Submucosa

• Peyer’s patches protect distal part against bacteria

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


Intestinal Juice

• Secreted in response to distension or irritation of the mucosa

• Slightly alkaline and isotonic with blood plasma

• Largely water, enzyme-poor, but contains mucus

• Facilitates transport and absorption of nutrients


Liver

• Largest gland in the body

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


Liver

• Falciform ligament

• Separates the (larger) right and (smaller) left lobes

• Suspends liver from the diaphragm and anterior abdominal wall

• Round ligament (ligamentum teres)

• Remnant of fetal umbilical vein along free edge of falciform ligament


SternumNipple

Liver

Right lobeof liver

Gallbladder

(a)

Bare area

Falciformligament

Left lobe of liver

Round ligament(ligamentum teres)


Lesser omentum(in fissure)

Left lobe of liver

(b)

Porta hepatiscontaining hepaticartery (left) andhepatic portal vein(right)

Quadrate lobeof liverLigamentum teres

Gallbladder

Hepatic vein (cut)

Sulcus forinferiorvena cava

Caudate lobeof liver

Bare area

Bile duct (cut)

Right lobe ofliver

Sternum

Nipple

Liver


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 two ducts


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 (liver cells)

• Longitudinal central vein

Copyright © 2010 Pearson Education, 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 portal vein

• Liver sinusoids are leaky capillaries between hepatic plates

• Kupffer cells (hepatic macrophages) in liver sinusoids

Copyright © 2010 Pearson Education, Inc. Figure 23.25c

(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


Bile

• Yellow-green, alkaline solution containing

• Bile salts: cholesterol derivatives that function in fat emulsification and absorption

• Bilirubin: pigment formed from heme

• Cholesterol, neutral fats, phospholipids, and electrolytes


Bile

• Enterohepatic circulation

• Recycles bile salts

• Bile salts duodenum reabsorbed from ileum hepatic portal blood liver secreted into bile


The Gallbladder

• Thin-walled 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

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