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Chapter 25 - 3
The Digestive System
The Gall bladder
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Slide #2
• Hollow, pear-shaped organ
• Stores, modifies and concentrates bile
• Contraction of the gall bladder and release of bile is
controlled by CCK and vagal stimulation
• Empties into the Cystic duct
• Gall stones
The Gallbladder
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Slide #3
Bile
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• Made of water, billirubin, ions, cholesterol, other lipids
• Acts as an emulsifier to aid lipid digestion
• 90% is reabsorbed in the ileum - enterohepatic
circulation of bile
Functions of the large intestine
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Slide #5
• Reabsorb water and compact material into feces
• 1500 mL chyme enter the cecum, 90% of volume reabsorbed
yielding 80-150 mL of feces
• Absorb vitamins produced by bacteria
• Store fecal matter prior to defecation
General anatomy of the large intestine
• Lies inferior to the stomach
• Frames the small intestine
• Parts of the large intestine
• Cecum
• Colon
• Ascending
• Transverse
• Descending
• Sigmoid
• Rectum
• Anal canal
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Slide #6
The Large Intestine
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Cecum and Rectum
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• Cecum
• opening protected by ileocecal valve
• veriform appendix
• Rectum
• Last portion of the digestive tract
• Terminates at the anal canal
• Internal anal sphincter - involuntary smooth muscle
• External anal sphincter - voluntary muscle
• Hemorrhoids -inflamed veins
Rectum
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Slide #9
Histology of the large intestine
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• Columnar cells except the rectum and anal canal
(stratified squamous)
• Muscularis externa
• circular muscle
• only 3 bands longitudinal smooth muscle - taeniae
coli
• Absence of villi
• Presence of goblet cells
• Deep intestinal glands
Physiology of the large intestine
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Slide #11
• Absorption in the large intestine includes:
• Water
• Vitamins – K, biotin, and B5
• Organic wastes – urobilinogens and sterobilinogens
• Bile salts
• Some ammonia and other toxins
• Mass movements of material through colon and rectum
• Defecation reflex triggered by distention of rectal
walls
Movement in Large Intestine
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Slide #12
• Mass movements
• Common after meals
• Integrated by the enteric plexus
• Local reflexes
• instigated by the presence of food in the stomach and
duodenum
• Gastrocolic reflex: initiated by stomach
• Duodenocolic reflex: initiated by duodenum
Long reflexes of defecation
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Coordination secretion and absorption
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• Neural and hormonal mechanisms coordinate glands
• GI activity stimulated by parasympathetic innervation
• Inhibited by sympathetic innervation
• Enterogastric, gastroenteric and gastroileal reflexes
coordinate stomach and intestines
• Several hormones alter motility and secretion
Summary Hormone Table
GIP
duodenum
fats, carbohydrates
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stim pancreas insulin secretion, inhibits stomach,
stim adipose glucose and FA uptake & lipogenesis
Marieb Table 23.1.1
Summary Hormone Table (continued)
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Marieb Table 23.1.1
Activities of Major Digestive Tract Hormones
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From Martini Figure 24.22
Digestion, Absorption, Transport
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• Digestion
• Disassembles organic food into smaller fragments
• Mechanical: breaks large food particles to small
• Chemical: breaking of covalent bonds by digestive
enzymes
• Hydrolyzes carbohydrates, proteins, lipids and
nucleic acids for absorption
• Absorption and transport
• Molecules are moved out of digestive tract and into
circulation for distribution throughout body
Carbohydrate digestion and absorption
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Slide #19
• Begins in the mouth (salivary amylase) completed in small
intestine
• Salivary and pancreatic amylase
• Convert starches to disaccharides, trisaccharides,
monosaccharides
• Brush border enzymes
• Make monosaccharides from disaccharides
• maltase (glucose / glucose)
• sucrase (glucose / fructose)
• lactase (glucose / galactose)
• Absorption of monosaccharides occurs across the intestinal
epithelia by facilitaed diffusion or Na+ linked cotransporters
Lipid digestion and absorption
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Slide #20
• Begins in stomach (minor) completed in small intestine
• Lipid digestion utilizes lingual and pancreatic lipases
converts triglycerides to monoglycerides
• Bile salts improve chemical digestion by emulsifying
lipid drops
• Lipid-bile salt complexes called micelles are formed
• Micelles diffuse into intestinal epithelia which resynthesis triglycerides and then release lipids into the
circulation as chylomicrons - mixtures of
phopholipids and proteins
• carried by lymphatic system
Lipid Absorption also (see Saladin fig 25.30)
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from Seeley, Stephens and Tate
Protein digestion and absorption
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(See Saladin Fig 25.29)
• In the stomach, the low pH destroys tertiary and quaternary
structure allows pepsin to digest specific peptide bonds
• In the small intestine, pancreatic enzymes and intestinal
brush border peptidases work at pH 7-8
• Trypsin - breaks bonds next to arginine or lysine
• Chymotrypsin- breaks bonds next to phenylalanine or
tyrosine
• Liberated amino acids, and some dipeptides are
absorbed through by several different carrier proteins
via by facilitaed diffusion or Na+ linked co-transporters
Absorption of other molecules
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Slide #23
• Water
• Nearly all that is ingested is reabsorbed via osmosis
• Ions
• Absorbed via diffusion, cotransport, and active transport
• Vitamins
• Water soluble vitamins (C and B vitamins) are absorbed
by diffusion
• Vitamin B12 requires intrinsic factor
• Fat soluble vitamins (A,D, E, K) are absorbed as part of
micelles
Water Intake, Secretion and Absorption
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Diarrhea
Constipation
from Seeley, Stephens and Tate
Great Summary slide (a)
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Marieb Figure 23.33a
Great Summary slide (b)
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Marieb Figure 23.33b