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Digestive System
• Two groups of organs1. Alimentary canal (gastrointestinal or GI
tract)• Mouth to anus• Digests food and absorbs fragments• Mouth, pharynx, esophagus, stomach,
small intestine, and large intestine
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Digestive System
2. Accessory digestive organs• Teeth, tongue, gallbladder• Digestive glands
– Salivary glands– Liver– Pancreas
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Figure 23.1 Alimentary canal and related accessory digestive organs.
Mouth (oral cavity)Tongue*
Esophagus
Liver*
Gallbladder*
Smallintestine
Salivaryglands*
Pharynx
StomachPancreas*
Largeintestine
(Spleen)
Parotid glandSublingual glandSubmandibular gland
DuodenumJejunum
Ileum
Anus
Transverse colon
Descending colon
Ascending colonCecumSigmoid colonRectumAppendix
Anal canal
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Digestive Processes
• Six essential activities1. Ingestion
2. Propulsion
3. Mechanical breakdown
4. Digestion
5. Absorption
6. Defecation
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Figure 23.2 Gastrointestinal tract activities.
Ingestion
Mechanicalbreakdown
Digestion
Propulsion
Absorption
Defecation
Food
PharynxEsophagus• Chewing (mouth)
• Swallowing (oropharynx)• Peristalsis (esophagus, stomach, small intestine, large intestine)
Stomach
Lymphvessel
Small intestineLargeintestine
Bloodvessel
Mainly H2OFeces
Anus
• Churning (stomach)• Segmentation (small intestine)
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Figure 23.3 Peristalsis and segmentation.
Frommouth
Peristalsis: Adjacent segments of alimentary tract organs alternately contract and relax, moving food along the tract distally.
Segmentation: Nonadjacent segments of alimentary tract organs alternately contract and relax, moving food forward then backward.Food mixing and slow food propulsion occur.
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Peritoneum and Peritoneal Cavity
• Peritoneum - serous membrane of abdominal cavity– Visceral peritoneum on external surface of
most digestive organs– Parietal peritoneum lines body wall
• Peritoneal cavity– Between two peritoneums– Fluid lubricates mobile organs
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Figure 23.5a The peritoneum and the peritoneal cavity.
Abdominopelviccavity
Vertebra
Peritonealcavity
Alimentarycanal organ
Liver
Two schematic cross sections of abdominal cavity illustratethe peritoneums and mesenteries.
Ventralmesentery
Parietalperitoneum
Visceralperitoneum
Dorsalmesentery
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Peritoneum and Peritoneal Cavity
• Mesentery - double layer of peritoneum– Routes for blood vessels, lymphatics, and
nerves– Holds organs in place; stores fat
• Retroperitoneal organs posterior to peritoneum
• Intraperitoneal (peritoneal) organs surrounded by peritoneum
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Figure 23.5b The peritoneum and the peritoneal cavity.
Alimentarycanal organ
Alimentary canal organ ina retroperitoneal position
Some organs lose their mesentery and move,becoming retroperitoneal, during development.
Mesenteryresorbedand lost
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Blood Supply: Splanchnic Circulation
• Branches of aorta serving digestive organs– Hepatic, splenic, and left gastric arteries– Inferior and superior mesenteric arteries
• Hepatic portal circulation– Drains nutrient-rich blood from digestive
organs– Delivers it to the liver for processing
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Histology of the Alimentary Canal
• Four basic layers (tunics)– Mucosa– Submucosa– Muscularis externa– Serosa
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Figure 23.6 Basic structure of the alimentary canal.
Intrinsic nerve plexuses
Mucosa
Submucosa
Muscularis externa
Glands insubmucosa
Serosa
LumenMucosa-associatedlymphoid tissue
Duct of gland outside alimentary canal
Gland in mucosa
Lymphatic vesselVein
ArteryNerve
Mesentery
• Myenteric nerve plexus• Submucosal nerve plexus
• Epithelium• Lamina propria• Muscularis mucosae
• Longitudinal muscle• Circular muscle
• Connective tissue
• Epithelium (mesothelium)
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Mucosa
• Lines lumen
• Functions – different layers perform 1 or all 3– Secretes mucus, digestive enzymes, and
hormones– Absorbs end products of digestion– Protects against infectious disease
• Three sublayers: epithelium, lamina propria, and muscularis mucosae
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Mucosa
• Epithelium– Simple columnar epithelium and mucus-
secreting cells (most of tract)• Mucus
– Protects digestive organs from enzymes– Eases food passage
– May secrete enzymes and hormones (e.g., in stomach and small intestine)
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Mucosa
• Lamina propria– Loose areolar connective tissue– Capillaries for nourishment and absorption – Lymphoid follicles (part of MALT)
• Defend against microorganisms
• Muscularis mucosae: smooth muscle local movements of mucosa
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Submucosa
• Submucosa– Areolar connective tissue– Blood and lymphatic vessels, lymphoid
follicles, and submucosal nerve plexus
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Muscularis Externa
• Muscularis externa– Responsible for segmentation and
peristalsis – Inner circular and outer longitudinal layers
• Circular layer thickens in some areas sphincters
• Myenteric nerve plexus between two muscle layers
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Serosa
• Visceral peritoneum– Areolar connective tissue covered with
mesothelium in most organs– Replaced by fibrous adventitia in esophagus – Retroperitoneal organs have both an
adventitia and serosa
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Figure 23.6 Basic structure of the alimentary canal.
Intrinsic nerve plexuses
Mucosa
Submucosa
Muscularis externa
Glands insubmucosa
Serosa
LumenMucosa-associatedlymphoid tissue
Duct of gland outside alimentary canal
Gland in mucosa
Lymphatic vesselVein
ArteryNerve
Mesentery
• Myenteric nerve plexus• Submucosal nerve plexus
• Epithelium• Lamina propria• Muscularis mucosae
• Longitudinal muscle• Circular muscle
• Connective tissue
• Epithelium (mesothelium)
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Enteric Nervous System
• Intrinsic nerve supply of alimentary canal – enteric neurons (more than spinal cord)
• Major nerve supply to GI tract wall; control motility– Submucosal nerve plexus
• Regulates glands and smooth muscle in the mucosa
– Myenteric nerve plexus• Controls GI tract motility
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Enteric Nervous System
• Linked to CNS via afferent visceral fibers
• Long ANS fibers synapse with enteric plexuses– Sympathetic impulses inhibit digestive
activities– Parasympathetic impulses stimulate digestive
activities
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Functional Anatomy: Mouth
• Oral (buccal) cavity– Bounded by lips, cheeks, palate, and tongue – Oral orifice is anterior opening– Lined with stratified squamous epithelium
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Figure 23.7a Anatomy of the oral cavity (mouth).
Palatoglossalarch
Softpalate
HardpalateOral cavityPalatinetonsil
Tongue
Oropharynx
Lingual tonsil
Epiglottis
Hyoid bone
Laryngopharynx
Esophagus
Trachea
Uvula
Sagittal section of the oral cavity and pharynx
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Lips and Cheeks
• Contain orbicularis oris and buccinator muscles
• Oral vestibule - recess internal to lips (labia) and cheeks, external to teeth and gums
• Oral cavity proper lies within teeth and gums
• Labial frenulum - median attachment of each lip to gum
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Figure 23.7b Anatomy of the oral cavity (mouth).
Gingivae (gums)
Palatineraphe
HardpalateSoftpalate
Palatinetonsil
Sublingualfold withopenings ofsublingualducts
Oral vestibule
Lower lip
Uvula
Upper lip
Superiorlabialfrenulum
Palatoglossalarch
Palatopharyngealarch
Posterior wallof oropharynx
Tongue
Lingual frenulum
Opening ofSubmandibularduct
Gingivae (gums)
Inferior labialfrenulum
Anterior view
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Palate
• Hard palate - palatine bones and palatine processes of maxillae– Slightly corrugated to help create friction
against tongue
• Soft palate - fold formed mostly of skeletal muscle– Closes off nasopharynx during swallowing– Uvula projects downward from its free edge
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Tongue
• Skeletal muscle
• Functions include– Repositioning and mixing food during chewing – Formation of bolus– Initiation of swallowing, speech, and taste
• Intrinsic muscles change shape of tongue
• Extrinsic muscles alter tongue's position
• Lingual frenulum: attachment to floor of mouth
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Tongue
• Surface bears papillae– Filiform—whitish, give the tongue roughness
and provide friction; do not contain taste buds – Fungiform—reddish, scattered over tongue;
contain taste buds – Vallate (circumvallate)—V-shaped row in
back of tongue; contain taste buds– Foliate—on lateral aspects of posterior
tongue; contain taste buds that function primarily in infants and children
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Tongue
• Lingual lipase– Secreted by serous cells beneath foliate and
vallate papillae secrete– Fat-digesting enzyme functional in stomach
• Terminal sulcus marks division between– Body - anterior 2/3 residing in oral cavity– Root - posterior third residing in oropharynx– Just posterior to vallate papillae
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Figure 23.8 Dorsal surface of the tongue, and the tonsils.
Epiglottis
Palatopharyngealarch
Palatine tonsil
Lingual tonsil
Palatoglossal arch
Terminal sulcus
Foliate papillae
Vallate papilla
Medial sulcus of the tongue
Dorsum of tongue
Fungiform papilla
Filiform papilla
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Salivary Glands
• Major salivary glands– Produce most saliva; lie outside oral cavity – Parotid– Submandibular– Sublingual
• Minor salivary glands– Scattered throughout oral cavity; augment
slightly
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Salivary Glands
• Function of saliva– Cleanses mouth– Dissolves food chemicals for taste – Moistens food; compacts into bolus – Begins breakdown of starch with enzymes
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Salivary Glands
• Parotid gland– Anterior to ear; external to masseter muscle – Parotid duct opens into oral vestibule next to
second upper molar– Mumps is inflammation of parotid glands
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Salivary Glands
• Submandibular gland – Medial to body of mandible– Duct opens at base of lingual frenulum
• Sublingual gland– Anterior to submandibular gland under tongue– Opens via 10–12 ducts into floor of mouth
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Figure 23.9 The salivary glands.
Tongue
Teeth
Frenulumof tongue
Mylohyoidmuscle (cut)
Anterior belly ofdigastric muscle
Masseter muscle
Body of mandible(cut)
Posterior belly ofdigastric muscle
Serous cellsforming demilunes
Mucous cells
Parotid gland
Submandibularduct
Submandibulargland
Ducts ofsublingualgland
Sublingualgland
Parotid duct
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Salivary Glands
• Two types of secretory cells– Serous cells
• Watery, enzymes, ions, bit of mucin
– Mucous cells• Mucus
• Parotid, submandibular glands mostly serous; sublingual mostly mucous
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• 97–99.5% water, slightly acidic– 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
compound protect against microorganisms
PLAYPLAY Animation: Rotating head
Composition of Saliva
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Control of Salivation
• 1500 ml/day• Intrinsic glands continuously keep mouth moist• Major salivary glands activated by
parasympathetic nervous system when – Ingested food stimulates chemoreceptors and
mechanoreceptors in mouth – Salivatory nuclei in brain stem send impulses along
parasympathetic fibers in cranial nerves VII and IX
• Strong sympathetic stimulation inhibits salivation and results in dry mouth (xerostomia)
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Pharynx
• Food passes from mouth oropharynx laryngopharynx– Allows passage of food, fluids, and air– Stratified squamous epithelium lining; mucus-
producing glands– Skeletal muscle layers: inner longitudinal,
outer pharyngeal constrictors
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Esophagus
• Flat muscular tube from laryngopharynx to stomach
• Pierces diaphragm at esophageal hiatus
• Joins stomach at cardial orifice
• Gastroesophageal (cardiac) sphincter• Surrounds cardial orifice
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Homeostatic Imbalance
• Heartburn– Stomach acid regurgitates into esophagus– Likely with excess food/drink, extreme
obesity, pregnancy, running– Also with hiatal hernia - structural
abnormality• Part of stomach above diaphragm• Can esophagitis, esophageal ulcers,
esophageal cancer
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Esophagus
• Esophageal mucosa contains stratified squamous epithelium– Changes to simple columnar at stomach
• Esophageal glands in submucosa secrete mucus to aid in bolus movement
• Muscularis externa - skeletal superiorly; mixed in middle; smooth inferiorly
• Adventitia instead of serosa
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Figure 23.12a Microscopic structure of the esophagus.
Mucosa(stratifiedsquamousepithelium)
Submucosa(areolarconnectivetissue)LumenMuscularisexterna
• Circular layer • Longitudinal layer
Adventitia(fibrousconnectivetissue)
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Figure 23.12b Microscopic structure of the esophagus.
Esophagus-stomachjunction
Mucosa(stratifiedsquamousepithelium)
Simple columnarepithelium of stomach
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Digestive Processes: Mouth
• Ingestion
• Mechanical breakdown– Chewing
• Propulsion – Deglutition (swallowing)
• Digestion (salivary amylase and lingual lipase)
• ~ No absorption, except for few drugs
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Mastication
• Cheeks and closed lips hold food between teeth
• Tongue mixes food with saliva; compacts food into bolus
• Teeth cut and grind
• Partly voluntary
• Partly reflexive– Stretch reflexes; pressure receptors in
cheeks, gums, tongue
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Deglutition
• Involves tongue, soft palate, pharynx, esophagus
• Requires coordination of 22 muscle groups
• Buccal phase– Voluntary contraction of tongue
• Pharyngeal-esophageal phase– Involuntary – primarily vagus nerve– Control center in the medulla and lower pons
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Figure 23.13 Deglutition (swallowing).Bolus of food
Tongue
Pharynx
Epiglottis
Glottis
Trachea
During the buccal phase, the upper esophageal sphincter is contracted. The tongue presses against the hard palate, forcing the food bolus into the oropharynx.
1
Uvula
Bolus
Epiglottis
Esophagus
The pharyngeal-esophageal phase begins as the uvula and larynx rise to prevent food from entering respiratory passageways. The tongue blocks off the mouth. The upper esophageal sphincter relaxes, allowing food to enter the esophagus.
The constrictor muscles of the pharynx contract, forcing food into the esophagus inferiorly. The upper esophageal sphincter contracts (closes) after food enters.
Peristalsis moves food through the esophagus to the stomach.
The gastroesophageal sphincter surrounding the cardial oriface opens, and food enters the stomach.
Relaxed muscles
Circular musclescontract
Bolus of food
Longitudinal musclescontract
Gastroesophagealsphincter closed
Relaxedmuscles
Circular muscles contract
Gastroesophagealsphincter opens
Upperesophagealsphincter
Bolus
2
4
3
5
Stomach
Slide 1
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Stomach: Gross Anatomy
• In upper left quadrant; temporary storage; digestion of bolus to chyme
• Cardial part (cardia) – Surrounds cardial orifice
• Fundus– Dome-shaped region beneath diaphragm
• Body– Midportion
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Stomach: Gross Anatomy
• Pyloric part – Antrum (superior portion) pyloric canal
pylorus– Pylorus continuous with duodenum through
pyloric valve (sphincter controlling stomach emptying)
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Figure 23.14a Anatomy of the stomach.Cardia
Esophagus
Muscularisexterna
Lessercurvature
DuodenumPyloric sphincter(valve) at pylorus
Pyloriccanal
Pyloricantrum
Greatercurvature
Rugae ofmucosa
Lumen
Body
Serosa
Fundus
• Oblique layer• Circular layer• Longitudinal layer
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Figure 23.14b Anatomy of the stomach.
Liver(cut)
Lessercurvature
Greatercurvature
Spleen
Body
Fundus
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Stomach: Gross Anatomy
• Greater curvature - convex lateral surface
• Lesser curvature - concave medial surface
• Mesenteries tether stomach– Lesser omentum
• From liver to lesser curvature
– Greater omentum – contains fat deposits & lymph nodes
• Greater curvature over small intestine spleen & transverse colon mesocolon
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Figure 23.30a Mesenteries of the abdominal digestive organs.
LiverFalciform ligament
Gallbladder
SpleenStomach
Ligamentum teres
Greater omentum
Small intestine
Cecum
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Figure 23.30b Mesenteries of the abdominal digestive organs.
Liver
Lesser omentum
Small intestine
Cecum
Urinary bladder
Gallbladder
StomachDuodenum
Transverse colon
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Greater omentum
Transverse colon
Transversemesocolon
Descending colon
MesenterySigmoid mesocolon
Jejunum
Sigmoid colon
Ileum
Figure 23.30c Mesenteries of the abdominal digestive organs.
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Stomach: Gross Anatomy
• ANS nerve supply – Sympathetic from thoracic splanchnic nerves
via celiac plexus– Parasympathetic via vagus nerve
• Blood supply – Celiac trunk (gastric and splenic branches)– Veins of hepatic portal system
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Stomach: Microscopic Anatomy
• Four tunics
• Muscularis and mucosa modified– Muscularis externa
• Three layers of smooth muscle• Inner oblique layer allows stomach to churn, mix,
move, and physically break down food
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Surfaceepithelium
Mucosa Laminapropria
Muscularismucosae
ObliquelayerCircularlayerLongitudinallayer
Submucosa(containssubmucosalplexus)Muscularisexterna(containsmyentericplexus)Serosa
Layers of the stomach wallStomach wall
Figure 23.15a Microscopic anatomy of the stomach.
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Stomach: Microscopic Anatomy
• Mucosa– Simple columnar epithelium composed of
mucous cells• Secrete two-layer coat of alkaline mucus
– Surface layer traps bicarbonate-rich fluid beneath it
– Dotted with gastric pits gastric glands• Gastric glands produce gastric juice
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Enteroendocrine cell
Enlarged view of gastric pits andgastric glands
Chief cell
Parietal cell
Mucous neck cells
Surface epithelium (mucous cells)
Gastric pits
Gastricpit
Gastricgland
Figure 23.15b Microscopic anatomy of the stomach.
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Gastric Glands
• Cell types– Mucous neck cells (secrete thin, acidic
mucus of unknown function)– Parietal cells – Chief cells – Enteroendocrine cells
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Figure 23.15c Microscopic anatomy of the stomach.
Mitochondria
Parietal cell
Chief cell
Enteroendocrine cell
Location of the HCl-producing parietal cellsand pepsin-secreting chief cells in a gastricgland
HCIPepsinPepsinogen
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Gastric Gland Secretions
• Glands in fundus and body produce most gastric juice
• Parietal cell secretions– Hydrochloric acid (HCl)
pH 1.5–3.5 denatures protein, activates pepsin, breaks down plant cell walls, kills many bacteria
– Intrinsic factor• Glycoprotein required for absorption of vitamin B12
in small intestine
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Gastric Gland Secretions
• Chief cell secretions– Pepsinogen - inactive enzyme
• Activated to pepsin by HCl and by pepsin itself (a positive feedback mechanism)
– Lipases• Digest ~15% of lipids
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Gastric Gland Secretions
• Enteroendocrine cells– Secrete chemical messengers into lamina
propria• Act as paracrines
– Serotonin and histamine
• Hormones– Somatostatin (also acts as paracrine) and gastrin
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Mucosal Barrier
• Harsh digestive conditions in stomach
• Has mucosal barrier to protect– Thick layer of bicarbonate-rich mucus – Tight junctions between epithelial cells
• Prevent juice seeping underneath tissue
– Damaged epithelial cells quickly replaced by division of stem cells
• Surface cells replaced every 3–6 days
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Digestive Processes in the Stomach
• Mechanical breakdown
• Denaturation of proteins by HCl
• Enzymatic digestion of proteins by pepsin (and milk protein by rennin in infants)
• Delivers chyme to small intestine
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Digestive Processes in the Stomach
• Lipid-soluble alcohol and aspirin absorbed into blood
• Only stomach function essential to life– Secretes intrinsic factor for vitamin B12
absorption • B12 needed mature red blood cells
• Lack of intrinsic factor causes pernicious anemia
• Treated with B12 injections
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Response of the Stomach to Filling
• Stretches to accommodate incoming food– Pressure constant until 1.5 L food ingested
• Reflex-mediated receptive relaxation– Coordinated by swallowing center of brain stem
– Gastric accommodation• Plasticity (stress-relaxation response) of smooth
muscle (see Chapter 9)
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Small Intestine: Gross Anatomy
• Major organ of digestion and absorption
• 2-4 m long; from pyloric sphincter to ileocecal valve
• Subdivisions – Duodenum (retroperitoneal)– Jejunum (attached posteriorly by mesentery)– Ileum (attached posteriorly by mesentery)
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Figure 23.1 Alimentary canal and related accessory digestive organs.
Mouth (oral cavity)Tongue*
Esophagus
Liver*
Gallbladder*
Smallintestine
Salivaryglands*
Pharynx
StomachPancreas*
Largeintestine
(Spleen)
Parotid glandSublingual glandSubmandibular gland
DuodenumJejunum
Ileum
Anus
Transverse colon
Descending colon
Ascending colonCecumSigmoid colonRectumAppendix
Anal canal
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Duodenum
• Curves around head of pancreas; shortest part – 25 cm
• Bile duct (from liver) and main pancreatic duct (from pancreas)– Join at hepatopancreatic ampulla– Enter duodenum at major duodenal papilla – Entry controlled by hepatopancreatic
sphincter
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Figure 23.21 The duodenum of the small intestine, and related organs.
Right and left hepatic ducts of liver
Common hepatic duct
Bile duct and sphincterAccessory pancreatic duct
Tail of pancreasPancreas
Jejunum
Main pancreatic duct and sphincter
Head of pancreasHepatopancreaticampulla and sphincter Duodenum
Mucosawith folds
Gallbladder
Major duodenalpapilla
Cystic duct
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Jejunum and Ileum
• Jejunum– Extends from duodenum to ileum– About 2.5 m long
• Ileum– Joins large intestine at ileocecal valve– About 3.6 m long
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Gross Anatomy of Small Intestine
• Vagus nerve (parasympathetic) and sympathetics from thoracic splanchnic nerves serve small intestine
• Superior mesenteric artery brings blood supply
• Veins (carrying nutrient-rich blood) drain into superior mesenteric veins hepatic portal vein liver
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Structural Modifications
• Increase surface area of proximal part for nutrient absorption– Circular folds (plicae circulares)– Villi– Microvilli
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Structural Modifications
• Circular folds– Permanent folds (~1 cm deep) that force
chyme to slowly spiral through lumen more nutrient absorption
• Villi – Extensions (~1 mm high) of mucosa with
capillary bed and lacteal for absorption
• Microvilli (brush border) – contain enzymes for carbohydrate and protein digestion
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Figure 23.22a Structural modifications of the small intestine that increase its surface area for digestion and absorption.
Vein carryingblood tohepatic portalvessel
Musclelayers
Circularfolds
Villi
Lumen
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Microvilli(brush border)
Absorptivecells
VillusLacteal
GobletcellBloodcapillaries
Mucosa-associatedlymphoidtissue
IntestinalcryptMuscularismucosae
Duodenalgland
Enteroendocrinecells
VenuleLymphatic vessel
Submucosa
Figure 23.22b Structural modifications of the small intestine that increase its surface area for digestion and absorption.
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Figure 23.22c Structural modifications of the small intestine that increase its surface area for digestion and absorption.
Gobletcells
Absorptive cells
Villi
Intestinal crypt
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Figure 23.23 Microvilli of the small intestine.
Mucusgranules
Microvilliforming thebrush border
Absorptive cell
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Intestinal Crypts
• Intestinal crypt epithelium renewed every 2-4 days– Most - secretory cells that produce intestinal
juice– Enteroendocrine cells enterogastrones – Intraepithelial lymphocytes (IELs)
• Release cytokines that kill infected cells
– Paneth cells• Secrete antimicrobial agents (defensins and
lysozyme)
– Stem cells divide to produce crypt cells
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Mucosa
• Peyer's patches protect especially distal part against bacteria– May protrude into submucosa
• B lymphocytes leave intestine, enter blood, protect intestinal lamina propria with their IgA
• Duodenal (Brunner's) glands of the duodenum secrete alkaline mucus to neutralize acidic chyme
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Intestinal Juice
• 1-2 L secreted daily in response to distension or irritation of mucosa
• Slightly alkaline; isotonic with blood plasma
• Largely water; enzyme-poor (enzymes of small intestine only in brush border); contains mucus
• Facilitates transport and absorption of nutrients
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The Liver and Gallbladder
• Accessory organs
• Liver– Many functions; only digestive function bile
production• Bile – fat emulsifier
• Gallbladder– Chief function bile storage
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Liver
• Largest gland in body
• Four lobes—right, left, caudate, and quadrate
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Liver
• Falciform ligament– Separates larger right and smaller left lobes – Suspends liver from diaphragm and anterior
abdominal wall
• Round ligament (ligamentum teres)– Remnant of fetal umbilical vein along free
edge of falciform ligament
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Figure 23.24a Gross anatomy of the human liver.
SternumNipple
Liver
Right lobe of liver
Gallbladder
Bare area
Falciformligament
Left lobe ofliver
Round ligament(ligamentumteres)
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Lesseromentum(in fissure)
Left lobe of liver
Porta hepatiscontaininghepaticartery (left)and hepaticportal vein(right)
Quadratelobe of liver
Ligamentumteres
Bare area
Caudate lobeof liver
Sulcus forinferiorvena cava
Hepatic vein(cut)Bile duct(cut)
Right lobeof liver
Gallbladder
Figure 23.24b Gross anatomy of the human liver.
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Liver: Associated Structures
• Lesser omentum anchors liver to stomach
• Hepatic artery and vein enter at porta hepatis
• Bile ducts– Common hepatic duct leaves liver– Cystic duct connects to gallbladder– Bile duct formed by union of common hepatic
and cystic ducts
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Right and left hepatic ducts of liver
Common hepatic duct
Bile duct and sphincterAccessory pancreatic duct
Tail of pancreasPancreas
Jejunum
Main pancreatic duct and sphincter
Head of pancreasHepatopancreaticampulla and sphincter Duodenum
Mucosawith folds
Gallbladder
Major duodenalpapilla
Cystic duct
Figure 23.21 The duodenum of the small intestine, and related organs.
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Liver: Microscopic Anatomy
• Liver lobules– Hexagonal structural and functional units– Composed of plates of hepatocytes (liver
cells) • Filter and process nutrient-rich blood
– Central vein in longitudinal axis
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Figure 23.25a–b Microscopic anatomy of the liver.
Lobule Centralvein
Connectivetissue septum
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Liver: Microscopic Anatomy
• Portal triad at each corner of lobule– Branch of hepatic artery supplies oxygen– Branch of hepatic portal vein brings nutrient-rich blood– Bile duct receives bile from bile canaliculi
• Liver sinusoids - leaky capillaries between hepatic plates
• Stellate macrophages (hepatic macrophages or Kupffer cells) in liver sinusoids remove debris & old RBCs
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Interlobular veins(to hepatic vein)
Central vein
Sinusoids
Plates ofhepatocytes
Portal vein
Stellate macrophagesin sinusoid walls
Bile canaliculi
Bile duct (receivesbile from bile canaliculi)
Fenestrated lining (endothelial cells) of sinusoids
Bile ductPortal venule
Portal arteriolePortal triad
Figure 23.25c Microscopic anatomy of the liver.
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Liver: Microscopic Anatomy
• Hepatocytes – increased rough & smooth ER, Golgi, peroxisomes, mitochondria
• Hepatocyte functions– Process bloodborne nutrients– Store fat-soluble vitamins– Perform detoxification – Produce ~900 ml bile per day
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Liver
• Regenerative capacity– Restores full size in 6-12 months after 80%
removal– Injury hepatocytes growth factors
endothelial cell proliferation
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Bile
• Yellow-green, alkaline solution containing – Bile salts - cholesterol derivatives that
function in fat emulsification and absorption– Bilirubin - pigment formed from heme
• Bacteria break down in intestine to stercobilin brown color of feces
– Cholesterol, triglycerides, phospholipids, and electrolytes
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Bile
• Enterohepatic circulation– Recycles bile salts– Bile salts duodenum reabsorbed from
ileum hepatic portal blood liver secreted into bile
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The Gallbladder
• Thin-walled muscular sac on ventral surface of liver
• Stores and concentrates bile by absorbing water and ions
• Muscular contractions release bile via cystic duct, which flows into bile duct
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The Gallbladder
• High cholesterol; too few bile salts gallstones (biliary calculi)– Obstruct flow of bile from gallbladder
• May cause obstructive jaundice
– Gallbladder contracts against sharp crystals pain
– Treated with drugs, ultrasound vibrations (lithotripsy), laser vaporization, surgery
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Pancreas
• Location– Mostly retroperitoneal, deep to greater
curvature of stomach– Head encircled by duodenum; tail abuts
spleen
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Pancreas
• Endocrine function– Pancreatic islets secrete insulin and glucagon
• Exocrine function– Acini (clusters of secretory cells) secrete
pancreatic juice• To duodenum via main pancreatic duct • Zymogen granules of acini cells contain
proenzymes
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Smallduct
Acinar cell
Basementmembrane
Zymogengranules
Roughendoplasmicreticulum
One acinus
Duct cell
Figure 23.26a Structure of the enzyme-producing tissue of the pancreas.
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Acinar cells
Pancreaticduct
Figure 23.26b Structure of the enzyme-producing tissue of the pancreas.
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Pancreatic Juice
• 1200 – 1500 ml/day
• Watery alkaline solution (pH 8) neutralizes chyme
• Electrolytes (primarily HCO3–)
• Enzymes– Amylase, lipases, nucleases secreted in
active form but require ions or bile for optimal activity
– Proteases secreted in inactive form
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Pancreatic Juice
• Protease activation in duodenum– Trypsinogen activated to trypsin by brush
border enzyme enteropeptidase– Procarboxypeptidase and chymotrypsinogen
activated by trypsin
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Figure 23.27 Activation of pancreatic proteases in the small intestine.
Stomach
Pancreas
Epithelialcells
Membrane-boundenteropeptidase
Trypsinogen(inactive)
Chymotrypsinogen(inactive)
Procarboxypeptidase(inactive)
Trypsin
Chymotrypsin
Carboxypeptidase
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Figure 23.23 Microvilli of the small intestine.
Mucusgranules
Microvilliforming thebrush border
Absorptive cell
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Large Intestine
• Unique features– Teniae coli
• Three bands of longitudinal smooth muscle in muscularis
– Haustra• Pocketlike sacs caused by tone of teniae coli
– Epiploic appendages• Fat-filled pouches of visceral peritoneum
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Large Intestine
• Regions– Cecum– Appendix– Colon– Rectum– Anal canal
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Figure 23.29a Gross anatomy of the large intestine.
Right colic(hepatic) flexure
Transverse colon
Superiormesenteric artery
Ascending colonIIeum
IIeocecal valve
Cecum
Appendix
Left colic(splenic) flexure
Transversemesocolon
Epiploicappendages
Descending colon
Cut edge ofmesentery
Tenia coli
Sigmoid colon
RectumAnal canal External anal sphincter
Haustrum
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Subdivisions of the Large Intestine
• Cecum – first part of large intestine
• Appendix – masses of lymphoid tissue– Part of MALT of immune system– Bacterial storehouse recolonizes gut when
necessary– Twisted enteric bacteria accumulate and
multiply
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Colon
• Retroperitoneal except for transverse and sigmoid regions
• Ascending colon (right side – to level of right kidney) right colic (hepatic) flexure
• Transverse colon left colic (splenic) flexure
• Descending colon (left side) • Sigmoid colon in pelvis rectum
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Greater omentum
Transverse colon
Transversemesocolon
Descending colon
MesenterySigmoid mesocolon
Jejunum
Sigmoid colon
Ileum
Figure 23.30c Mesenteries of the abdominal digestive organs.
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Figure 23.30d Mesenteries of the abdominal digestive organs.
Liver
Lesser omentum
PancreasStomachDuodenum
Transverse mesocolon
Transverse colon
Mesentery
Greater omentumJejunumIleum
Visceral peritoneum
Parietal peritoneum
Urinary bladderRectum
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Rectum and Anus
• Rectum– Three rectal valves stop feces from being
passed with gas (flatus)
• Anal canal– Last segment of large intestine– Opens to body exterior at anus
• Sphincters– Internal anal sphincter—smooth muscle– External anal sphincter—skeletal muscle
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Figure 23.29b Gross anatomy of the large intestine.
Rectal valveRectumHemorrhoidalveinsLevator ani muscle
Anal canal
External analsphincterInternal analsphincterAnal columns
Pectinate lineAnal sinuses
Anus
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Large Intestine: Microscopic Anatomy
• Thicker mucosa of simple columnar epithelium except in anal canal (stratified squamous to withstand abrasion)
• No circular folds, villi, digestive secretions
• Abundant deep crypts with goblet cells
• Superficial venous plexuses of anal canal form hemorrhoids if inflamed
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Bacterial Flora
• Enter from small intestine or anus – Colonize colon– Synthesize B complex vitamins and vitamin K– Metabolize some host-derived molecules
(mucin, heparin, hyaluronic acid)– Ferment indigestible carbohydrates– Release irritating acids and gases (~500
ml/day)
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Intestinal Flora
• Viruses and protozoans
• Bacteria prevented from breaching mucosal barrier– Epithelial cells recruit dendritic cells to
mucosa sample microbial antigens present to T cells of MALT IgA antibody-mediated response restricts microbes
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Digestive Processes in the Large Intestine
• Residue remains in large intestine 12–24 hours
• No food breakdown except by enteric bacteria
• Vitamins (made by bacterial flora), water, and electrolytes (especially Na+ and Cl–) reclaimed
• Major functions - propulsion of feces to anus; defecation
• Colon not essential for life
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