gastrointestinal system chapter 23. gi: overview: organ systems gastrointestinal (gi) tract...
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Gastrointestinal SystemChapter 23
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GI: Overview: Organ systems
Gastrointestinal (GI) tract [Alimentary canal] a continuous muscular digestive tube Digests:
breaks food into smaller fragments
Absorbs: digested material is moved through mucosa into the blood
Eliminates: unabsorbed & secreted wastes.
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Organ systems
Includes: Mouth, pharynx &
esophagus Stomach Small intestine Large intestine
Accessory digestive organs: teeth, tongue, gall bladder, salivary glands, liver & pancreas
Figure 23.1
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Processes
Ingestion Propulsion Mechanical digestion Chemical digestion Absorption Defecation
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Processes
Ingestion: obtaining food Propulsion: moves food along the GI tract by
peristalsis (wave-like muscular contraction) Mechanical digestion :
chewing & mixing with saliva mixing in stomach segmentation (local constriction in intestine to mix food
& digestive juices)
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Processes
Chemical digestion: breaks down food to molecular fragments (monomers) (Hydrolysis). Begins in the mouth with saliva & continues into the
small intestine. Absorption: movement of nutrients across the
mucosal membrane into blood/lymph Defecation: eliminates unused/indigestible &
secreted substances from the body
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Functional Considerations :
Substances in the GI tract lumen are outside of the body.
Multiple sensors & receptors line the GI tract to monitor contents & respond to conditions.
Controls: intrinsic (local control) & extrinsic (CNS)
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Peritoneum : serous membrane
Visceral peritoneum: covers the external surfaces of most digestive organs
Parietal Peritoneum: lines the body wall Peritoneal Space: potential space containing fluid
that separates the visceral & parietal peritoneum
Figure 23.5a
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Peritoneum
Mesentery: double layer of peritoneum fused together that extends to the organs from the posterior body wall. Provides support for the organs Provides support for vessels & nerves supplying the organs
Figure 23.5a
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Peritoneum
Retroperitoneal organs Organs that adhere to the posterior abdominal wall &
lose their peritoneum by resorption Parts of the large & small intestine & most of the
pancreas; (also kidneys)
Figure 23.5b
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GI blood supply
Blood supply: about 25% of cardiac output Arterial: Abdominal aorta celiac trunk Celiac trunk Hepatic, splenic & gastric branches
which serve the liver, spleen & stomach Celiac trunk superior & inferior mesenteric
branches serve small & large intestine
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Histology
GI tract wall has 4 layers: Mucosa Submucosa Muscularis Externa Serosa or Adventitia
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Histology of the Alimentary Canal
Figure 23.6
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Histology
Mucosa: The epithelial membrane that lines the GI tract from mouth anus. Secretes mucous, digestive enzymes & hormones Absorbs nutrients Protects from disease & from the GI contents
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Histology
Mucosa; 3 layers: Epidermis Lamina propria (loose ct : contain capillaries & some
elements of MALT) Muscularis mucosa
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Histology
Submucosa: moderately dense CT with blood, nerve, lymph vessels & lymphoid follicles; rich in elastic fibers
Muscularis externa: smooth muscle Responsible for peristalsis &
segmentation Circular layer Longitudinal layer Sphincters: in some areas
the circular layer thickens; act as valves
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Histology
Serosa of intraperitoneal organs = visceral peritoneum
Esophagus has an outer covering of fibrous connective tissue = adventitia
Retroperitoneal organs: visceral serosa on the surface facing the peritoneal cavity & adventitia on the surface facing the body wall.
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Nerves
Intrinsic: (Local): Short reflex Submucosal nerve plexus:
regulates glands & mucosal muscle Myenteric plexus: controls GI wall & GI motility
Extrinsic: (CNS): Long reflex Parasympathetic NS: enhances gut motility &
secretion SNS: inhibits gut motility & secretion
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Nerves
Intrinsic: (Local): Short reflex Submucosal nerve plexus: regulates glands & mucosal muscle Myenteric plexus: controls GI wall & GI motility
Extrinsic: (CNS): Long reflex Parasympathetic NS: enhances gut motility & secretion SNS: inhibits gut motility & secretion
Figure 23.4
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Functional Anatomy: Mouth
Mouth: lips, palate, & tongue
Mouth cavity = Buccal cavity
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Functional Anatomy: Mouth
Lips: extend from inferior margin of the nose to the superior margin of the chin. Red area = red margin, is poorly keratinized & lacks sweat or sebaceous glands.
Palate: Hard palate: rigid surface against which food is
forced in chewing Soft palate: muscular structure that rises &
blocks off the nasopharynx during swallowing
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Functional Anatomy: Mouth
Tongue: muscular tentacle composed of interlaced muscle fibers that grips & repositions food, mixes food with saliva & compresses food to form a food bolus, prior to swallowing.
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Functional Anatomy: Mouth
Filiform papillae: rough surface Fungiform papillae: house taste buds Circumvallate papillae: house taste buds, Foliate papillae: posterolateral; taste buds
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Functional Anatomy: Mouth Salivary Glands: intrinsic & extrinsic
Intrinsic glands: scattered throughout the buccal cavity mucosa
Extrinsic glands: supply most of the saliva; outside buccal cavity & supply secretions via ducts:
Parotid Submandibular Sublingual
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Functional Anatomy: Mouth Composition of saliva:
97-99.5% H2O Electrolytes: pH 6.75-7.0 Amylase: (digestive enzyme) Proteins: mucin, lysozyme, & IgA
Protection from microbes by saliva: IgA: immunglobulins in secretions Lysozyme: bacteriostatic (inhibits bacterial growth) Cyanide Defensins: local antibiotic activity & when activated
promote chemotaxis by WBCs Normal flora: convert salivary components to nitrates
then to NO. NO is toxic & bacteriocidal
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Functional Anatomy: Mouth
Control of Salivation: Continuous baseline secretory activity
With food ingestion, salivation increases dramatically Parasympathetic NS: chemoreceptors & pressoreceptors
stimulate salivatory nuclei to increase salivation
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Functional Anatomy: Mouth, Pharynx
Teeth: Primary: 2I 1C 2M x 2 = 20
2I 1C 2M Permanent: 2I 1C 2PM 3M x 2 = 32
2I 1C 2PM 3M Structures
Crown: exposed above gingiva (gum) Root: anchored by periodontal ligament to the bone by a
fibrous joint (gomphosis)
Figure 23.07
Figure 23.11
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Functional Anatomy: Throat & Esophagus
Pharynx: oropharynx & laryngopharynx; muscular wall propels food to the esophagus
Esophagus: Muscular 25cm tube from laryngopharynx to
stomach Passes through the diaphragm at the
esophageal hiatus Gastroesophageal (cardiac) sphincter: A
physiologic sphincter that helps keep esophagus closed when empty
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Functional Anatomy: Esophagus
Esophagus (continued) Wall has all 4 GI tract tunics:
Epithelial layer changes at the junction with the stomach from stratified squamous epithelium to simple columnar epithelium
Esophageal mucous glands lubricate food bolus Muscularis externa
Superior 1/3 of muscularis externa is skeletal muscle Middle 1/3 is mixed skeletal & smooth Lower 1/3 is smooth muscle
Adventitia: external covering
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Digestive Processes: Mouth, Pharynx & Esophagus Ingestion Mechanical digestion: chewing Chemical digestion: mixing food with saliva Propulsion: swallowing & initiating peristalsis
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Functional Anatomy: Stomach
Cardiac region: narrow, receives food bolus Fundus: bulge that extends supero-laterally to the cardia,
reaches the diaphragm Body: mid-portion Pyloric antrum : funnel shaped portion narrows to form the; Pyloric canal Pylorous Pyloric sphincter small intestine Rugae
longitudinal mucosalfolds
volume about 4L
Figure 23.14a
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Rugae
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Microscopic Anatomy : Stomach
Stomach: has the 4 tunics of the GI tract. Epithelium: Simple columnar epithelium
(goblet cells-mucous); Muscularis externa has an additional oblique
layer of muscle (allows another dimension of contraction).
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Figure 23.15
Gastric glands secrete gastric juices
MicroscopicAnatomy : Stomach
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Microscopic Anatomy : Stomach
Mucous neck cells: in the duct portion
Figure 23.15
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Microscopic Anatomy : Stomach
Gastric glands secrete gastric juices Mucous neck cells: in
the duct portion Parietal cells: mid
portion secrete HCl & intrinsic factor for B12 absorption
Figure 23.15
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Microscopic Anatomy : Stomach
Gastric glands secrete gastric juices Mucous neck cells: in
the duct portion Parietal cells: mid
portion of glands secrete HCl & intrinsic factor
Chief cells: base of gland; secretes pepsinogen a precursor molecule to pepsin (an enzyme that digests protein)
Figure 23.15
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Microscopic Anatomy : Stomach
Enteroendocrine cells: secrete multiple hormonal products; Gastrin, histamine,
endorphins, serotonin, cholecystokinin, & somatostatin, which influence several digestive system organs
Figure 23.15
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Microscopic Anatomy : Stomach
Mucosal barrier: protects the stomach from its own secretions Viscous mucous overlies a thick coating of HCO3
- rich mucous
Tight junctions between epithelial cellPM of glandular cells are impermeable to HCl
Epithelium is replaced every 3-6 days
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Digestive Processes (Stomach)
Acts as a holding vessel for ingested food
Participates in mechanical & chemical digestion Propulsion: Delivers its product (chyme) to the small
intestine
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Digestive Processes (Stomach)
Protein digestion: HCl denatures protein HCl activates pepsinogen to pepsin Pepsin breaks peptide bonds of proteins Rennin: an enzyme that breaks down casein
(milk protein) secreted in infants Intrinsic factor: required for Vit. B12 absorption
(needed to mature RBC); Absence of B12 results in pernicious anemia
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Regulation of gastric secretion (3 phases)
Cephalic Phase Gastric Phase Intestinal Phase
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Cephalic phase: Stimulation
Cephalic phase: CNS response to presentation of food; enhances gastric gland secretion
Loss of appetite; satiety / depression
Cephalic phase: Inhibition
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Gastric phase: Stimulation
Gastric phase: food entering stomach; Stretch Change in pH (increase) Peptides All cause increased gastric gland secretion
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Gastric phase: Stimulation
Stretch: reflex arc causes increased Acetylcholine release which then causes increased gastric gland secretions
Increased pH / polypeptides / caffeine All enhance Gastrin secretion by enteroendocrine G cells
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Gastric phase: Stimulation
3 chemicals: cause enhanced HCl secretion through 2nd messenger systems Gastrin Acetylcholine Histamine
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Gastric phase: Inhibition
pH <2.0 inhibits Gastrin secretion
SNS inhibits Gastrin (G cell) activity
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Intestinal phase: Stimulation
Excitatory: As chyme enters the duodenum the mucosa secretes enteric gastrin which stimulates secretion by gastric glands
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Intestinal phase: Inhibition
Inhibitory: As more chyme fills the small intestine, the enterogastric reflex is triggered Inhibits CNS stimulation Inhibits local reflexes Controls gastric emptying Activates sympathetic fibers that tighten the
pyloric sphincter
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Regulation of Gastric Activity
Figure 23.16
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Small Intestine: Gross Anatomy
6-7m long: from pyloric sphincter to the ileocecal valve
3 subdivisions: Duodenum Jejunum Ileum
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Small Intestine: Gross Anatomy
Duodenum : Curves around the pancreatic head (~25cm long) Contains the hepatopancreatic ampulla : formed
by the merger of the bile duct & the pancreatic duct.
Hepatopancreatic sphincter controls admission of bile & pancreatic enzymes to the duodenum
Duodenum is retroperitoneal
Fig 23.20
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Small Intestine; Regulatory Function
Duodenenal regulation of gastric emptying: Feedback mechanisms monitor the contents being delivered from the stomach High fat content Low pH (high acidity) Hypertonicity (high osmolality) All result in decreased stomach emptying.
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Regulation of Gastric Emptying
Figure 23.19
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Small Intestine: Gross Anatomy
Jejunum: extends from duodenum to ileum (~2.5m long)
Ileum: from jejunum to ileocecal valve (~3.6m long)
Figure 23.1
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Small Intestine: Gross Anatomy
Jejunum & Ileum function in absorption; Intraperitoneal Suspended from mesentery whose veins &
lymph vessels carry nutrients away from small intestine
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Small Intestine: Microanatomy
Figure 23.21
Plicae circularis: mucosal folds cause chyme to spiral slowly through the lumen
Villi
Villi: velvety mucosa of absorptive epithelium containing capillaries & lymph lacteals
Microvilli: PM projections forming “brush border”; Hold enzymes that complete protein & carbohydrate digestion
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Small intestine Wall
Cell types: Mostly absorptive cells Goblet (mucous) cells increase in number as
the small intestine progresses Enteroendocrine cells T-Lymphocytes
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Small intestine Wall
Intestinal crypts with cells that secrete intestinal juice & contain Paneth cells that secrete protective lysozyme (antibacterial)
Peyer’s Patches: lymphoid follicle in submucosa
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Small intestine Wall
Brunners glands (duodenum) secrete HCO3- rich
mucous to increase the pH of chyme Villus epithelium is replaced every 3-6 days Intestinal Juice : isotonic with blood plasma,
slightly alkaline, low enzyme content
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Liver & Gall Bladder
Liver produces bile (fat emulsifier) that is stored in & concentrated by the gall bladder. (metabolic role of the liver; Ch. 24)
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Liver: Gross Anatomy
Largest gland 4 Lobes Falciform ligament
mesentery supports liver from diaphragm & anterior body wall
separates R & L lobes
Round ligament fibrous remnant of umbilical vein
Fig 23.23
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Blood supply hepatic artery & hepatic
portal vein Bile
drains from biliary ducts to common hepatic duct which fuses with cystic duct from gallbladder to form the bile duct
Fig 23.23
Liver: Gross Anatomy
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Liver: Microscopic Anatomy
Liver lobule: hexagonal (Fig 23.24) Central vein: drains the lobule Hepatocytes form plates that radiate from the
central vein
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Microanatomy of the Liver
Figure 23.24c, d
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Liver: Anatomy
Portal triad at each corner of the hexagonal system. Consists of branches of: Hepatic Artery delivers O2 Hepatic portal vein delivers
nutrients from small intestine Bile duct receives bile from
the bile canaliculi that lie between layers of hepatocytes
Figure 23.24c, d
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Liver: Anatomy
Liver (cont) Liver sinusoids Large
leaky capillaries conduct blood from the artery & portal vein to the central vein
Hepatic macrophages Kupffer cells lie in sinusoid walls
Central veins flow into hepatic veins then to the inferior vena cava
Figure 23.24c, d
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Liver & Gall Bladder
Composition of bile: Alkaline solution: Bile salts, bile pigments, cholesterol, fats & phospholipids Bile salts & phospholipids participate in fat
absorption
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Liver & Gall Bladder
Bile salts are conserved by enterohepatic circulation Reabsorbed in the ileum Return to Liver in hepatic portal blood Re-secreted by the Liver
Bile pigments & bilirubin break down to urobilin then stercobilin
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Gall Bladder
Gall Bladder = a muscular pouch that stores bile & expels bile when needed via the cystic duct & the bile duct.
Figure 23.20
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Regulation of Bile Release
Cholecystokinin (CCK) & secretin released by the small intestine in response to increased fats in chyme
CCK: Stimulates both Gall bladder &
pancreatic secretion Relaxes hepatopancreatic sphincter
Secretin: stimulates bile secretion
Figure 23.25
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Pancreas: Gross Anatomy
Head encircled by duodenum Tail abuts the spleen Mostly retroperitoneal
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Pancreatic Acini
Large numbers of Acinar cells in clusters around ducts; exocrine (Acini) Acinar cells: Secrete
pancreatic digestive enzymes Endocrine cell clusters form the
pancreatic islets that produce insulin & glucagon
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Pancreatic Juice
Pancreatic Juice: Alkaline, watery, contains enzymes & electrolytes
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Pancreatic Juice
Proteolytic enzymes are released as inactive forms then are activated in the duodenum Trypsinogen Trypsin by the brush border
enzyme enterokinase Trypsin activates precursors to form
carboxypeptidase & chymotrypsin Amylase, lipase, & nuclease are released in their
active forms
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Regulation of pancreatic secretion
Neural via PSNS Hormonal: Intestinal Hormones
CCK: released in response to fats & protein. Stimulates pancreatic secretion of enzymes
Secretin: released in response to HCl. Stimulates pancreatic duct cells to release HCO3
-
Figure 23.28
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Digestive Processes : Small Intestine
Optimal digestion requires adequate motility & control of chemical composition
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Digestive Processes : Small Intestine
pH: acidic chyme must be buffered to allow proper enzyme activity
Osmolality: chyme is hypertonic & would pull H2O out of circulation; thus chyme is released in small amounts
Liver & pancreatic function are required for appropriate delivery of bile salts & enzymes to the small intestine
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Digestive Processes : Small Intestine
Segmentation: moves intestinal contents back & forth to mix
Duodenal rhythm for segmentation is greater than the rhythm in the ileum. Contents move toward the ileum.
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Digestive Processes : Small Intestine
After most digestion has occurred, Peristalsis begins sweeping from duodenum distally
Occurs in series with each peristaltic wave, originating more distally (migrating mobility complex)
Ileocecal valve (sphincter) is relaxed by neural (gastroileal reflex) impulses from the stomach & hormonal (gastrin) secretion by the stomach
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Large Intestine: Gross Anatomy
Ileocecal valve to anus (~1.5m) Teniae coli: 3 ribbons of
longitudinal smooth muscle Haustra: pocket-like segments
of large intestine Epiploic appendages: fat filled
pouches of visceral peritoneum
Figure 23.29a
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Large Intestine: Gross Anatomy
Cecum: blind pouch (below ileocecal valve) Appendix: attached to cecum (lymphoid) Colon: ascending, transverse, descending, sigmoid
Transverse colon & sigmoid colon are intraperitoneal; anchored by mesentery. The rest of the colon is retroperitoneal
Rectum Anal canal
Figure 23.29a
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Large Intestine: Microscopic Anatomy
All 4 layers present Mucosa: simple columnar epithelium until anal
canal (stratified squamous) No folds or villi. No significant enzyme secretions Crypts: invaginations of mucosa contain large numbers
of goblet cells that secrete mucous for protection & lubrication
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Large Intestine: Microscopic Anatomy
Bacterial flora Ferment indigestible CHO: produces about 500ml of gas
per day Synthesize B complex vitamins & most vitamin K
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Large Intestine: Digestive Processes
Propulsion: Haustra contractions: stretch stimulate haustra to
contract moving (& mixing) contents to next haustra Mass peristalsis: long, slow contractile waves moving
contents toward rectum (3-4 per day) Gastrocolic reflex: food intake causes mass peristalsis
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Large Intestine: Digestive Processes
H2O reabsorption & vitamin absorption Defecation
Empty rectum receives waste, causing stretch Stretch initiates reflex contraction of the rectum &
relaxation of both anal sphincters Voluntary control of the external anal sphincter can
postpone defecation If suppressed the contraction stops & is reinitiated later With defecation rectal muscle contractions are aided by
increased abdominal pressure (valsalva maneuver)
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Structure of the Anal Canal
Figure 23.29b Figure 23.32
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Chemical Digestion of Specific Food Groups
Chemical Digestion: Mechanism Catabolic process: break down of food molecules to
monomeric form that can be absorbed Enzymes break molecules by hydrolysis (Adding H2O into
a molecular bond)
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Chemical Digestion: Carbohydrates
Carbohydrates (CHO): complex sugars are broken down to simple sugars (monomers) Simple sugars (monosaccharides): glucose,
fructose, & galactose can be absorbed Disaccharides (not absorbed): sucrose,
maltose, & lactose are hydrolyzed by sucrase, maltase, & lactase (brush border enzymes) into monosaccharides
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Chemical Digestion: Carbohydrates
Polysaccharides: broken down to smaller & smaller oligosaccharides Salivary amylase: breaks starch into 2-8 linked
monosaccharide Salivary amylase is denatured by HCl in the stomach
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Chemical Digestion: Carbohydrates
Polysaccharides (cont) Pancreatic amylase: continues breakdown (mostly to
maltose) Brush border enzymes act on oligosaccharides of more than
3 simple sugars (brush border) Sucrose, maltose & lactose are broken down by specific
brush border enzymes (sucrase, maltase, & lactase) Monosaccharides are absorbed.
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Chemical Digestion: Proteins
Proteins: broken down to amino acid monomers. Begins in the stomach. Pepsinogen is activated to pepsin by HCL Pepsin: cleaves peptide bonds associated with tyrosine &
phenylalanine forming polypeptides (+ a few amino acids). Pepsin is inactivated by increased pH in the duodenum
Trypsin & chymotrypsin: pancreatic enzymes that further breakdown polypeptides.
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Chemical Digestion: Proteins
Proteins (cont): Carboxypeptidase (pancreatic & brush border
enzyme): Splits off single amino acid from the end of
polypeptide chain that contains the carboxyl group
Aminopeptidase & dipeptidase (brush border enzymes) release final amino acid monomers
Figure 23.34
Figure 23.34
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Chemical Digestion: Lipids
Lipids: require emulsification to disperse fats into microdroplets which enhances enzyme activity Bile salts: surround fat droplets & keep them
in suspension in the aqueous digestive juices Pancreatic lipase: cleaves off two fatty acid
chains from triglyceride molecules Yields a monoglyceride & 2 free fatty acids
Figure 23.35
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Chemical Digestion: Nucleic acids
Nucleic acids: small amounts occur in the diet Pancreatic nucleases: Hydrolyze nucleic acids to
nucleotide monomers Nucleosidases & phosphatases (brush border) break
down nucleotides
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Absorption: Small Intestine
Most nutrients are absorbed from chyme prior to the ileum (ileum primarily reabsorbs bile salts)
Most nutrients cross the mucosa by active transport. Some products of lipid digestion are absorbed by diffusion. CHO, amino acids & nucleic acid remnants are actively
transported into mucosal cells & then enter capillary blood
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Absorption: Small Intestine (Fats)
Monoglycerides & free fatty acids are associated with bile salts & lecithin (a phospholipid) to form micelles Micelles diffuse between microvilli The lipid substances leave the micelles & diffuse across the
PM of microvilli Inside the intestinal epithelial cell, the components are
reassembled into triglycerides
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Absorption: Small Intestine (Fats)
Monoglycerides (cont) Triglycerides are then packaged with cholesterol &
phospholipids into chylomicrons Chylomicrons are exocytosed & diffuse to lymphatic lacteals
to eventually be placed into blood by the lymphatic system Chylomicrons in the blood are hydrolyzed by lipoprotein lipase
to free fatty acids & glycerol prior to tissue absorption Fig. 23.36
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Fatty Acid Absorption
Figure 23.36
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Absorption: Small Intestine
Vitamins Some Vit. K & B complex vitamins are absorbed in the
large intestine Dietary vitamins are absorbed in the small intestine Fat soluble vitamins (A, D, E, K) dissolve in dietary fats &
are absorbed after being incorporated into micelles
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Absorption: Small Intestine
Electrolytes: most ions are actively absorbed throughout small intestine
Na+ is coupled to absorption of glucose & amino acids (cotransport)
Cl- actively transported into cells; exchange for HCO3-
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Absorption: Small Intestine
K diffuses into the cells Iron is transported into cells & bound to ferritin Ca2+ absorption regulated by blood Ca2+ levels &
Vit D ( Vit D is a required cofactor for Ca2+ absorption)
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Absorption: Small Intestine
Water moves freely in both directions across the mucosa Active transport of solutes create an osmotic gradient &
H2O follows H2O movement changes solute concentrations & effects
absorption of substances that move by diffusion
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Absorption: Small Intestine
Malabsorption: can result from Disruption of secretion Mucosal damage Reduction of absorptive surface area