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Chapter 23A Digestive System
Slides by Barbara Heard and W. Rose.figures from Marieb & Hoehn 9th ed.
Portions copyright Pearson Education
Ch. 23: Digestive System
Introduction/Overview
Functional Anatomy
Physiology of Digestion & Absorption
Digestive SystemIntroduction/Overview
Digestive System Organs
Digestive Processes
Basic Concepts
Relations & Common Features of Digestive Organs
Functional Anatomy
Physiology of Digestion & Absorption
Digestive System Organs (Fig 23.1)Alimentary Canal = path through
Mouth to anusDigests food and absorbs fragmentsMouth, pharynx, esophagus, stomach, small intestine, and large intestine
Accessory organs = "off-to-the-side" helpersTeeth, tongue, gallbladderDigestive glandsSalivary glandsLiverPancreas
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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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Basic Concepts•Gut lumen is "outside"•Digestion control mechanisms try to control chemical environment in lumen to optimize absorption•Digestion is provoked by mechanical & chemical stimuli (sensors in walls): stretch, pH, osmolarity, which can cause secretion and propulsion via reflexes (Fig. 23.4)•Control by local as well as remote reflexes (Fig 23.4)
• Short reflexes: via enteric nerve plexus• Long: via CNS
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GI Tract Regulatory Mechanisms
1. Mechanoreceptors and chemoreceptors – Respond to stretch, changes in osmolarity
and pH, and presence of substrate and end products of digestion
– Initiate reflexes that• Activate or inhibit digestive glands • Stimulate smooth muscle to mix and move lumen
contents
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GI Tract Regulatory Mechanisms
2. Intrinsic and extrinsic controls– Short reflexes - enteric nerve plexuses (gut
brain) respond to stimuli in GI tract– Long reflexes respond to stimuli inside or
outside GI tract; involve CNS centers and autonomic nerves
– Hormones from cells in stomach and small intestine stimulate target cells in same or different organs to secrete or contract
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Figure 23.4 Neural reflex pathways initiated by stimuli inside or outside the gastrointestinal tract.
External stimuli(sight, smell, taste,
thought of food)
Visceral afferents
Internal(GI tract)stimuli
Chemoreceptors,osmoreceptors, ormechanoreceptors
Long reflexes
Central nervous system
Local (intrinsic)nerve plexus("gut brain")
Effectors:Smooth muscle
or glands
Extrinsic visceral (autonomic)efferents
Short reflexes
Lumen of thealimentary canal
Gastrointestinalwall (site of shortreflexes)
Response:Change in
contractile orsecretory activity
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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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Homeostatic Imbalance
• Peritonitis– Inflammation of peritoneum– Causes by e.g., piercing abdominal wound,
perforating ulcer, ruptured appendix– Peritoneal coverings stick together, localizing
infection– Dangerous and lethal if widespread– Treated with debris removal and antibiotics
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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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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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Histology: all alimentary canal organs have same basic 4 layersFrom lumen out (Fig 23.6):•Mucosa = mucous membrane = epithelial layer of cells
Secrete: mucus, digestive enz, hormonesAbsorb broken down compounds into bloodProtect against infection
•SubmucosaConnective tissie with blood vessels & lymphatics for absorption, lymph nodes, elastic fibers (shape retention)
•Muscularis externaInner circular musclesOuter longitudinal muscles
•SerosaThe peritoneum already mentioned
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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•Submucosal plexus
In submucosaControls glandular secretions & thin submucosal muscular layer
•Myenteric nerve plexusBetween circular & longitudinal muscle layersControls GI tract motility
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Enteric Nervous System•Linked to CNS via afferent visceral fibers
•Long ANS fibers synapse with enteric plexusesSympathetic impulses inhibit digestive activities
Parasympathetic impulses stimulate digestive activities