Chapter 25 - 3
The Digestive System
LaPointe Spring ‘12Slide #2The Gall bladder
• 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
LaPointe Spring ‘12Slide #3The Gallbladder
LaPointe Spring ‘12Slide #4Bile
• 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
LaPointe Spring ‘12Slide #5Functions of the large intestine
• 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
LaPointe Spring ‘12Slide #6
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
LaPointe Spring ‘12Slide #7
The Large Intestine
LaPointe Spring ‘12Slide #8Cecum and Rectum
• 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
LaPointe Spring ‘12Slide #9
Rectum
LaPointe Spring ‘12Slide #10Histology of the large intestine
• 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
LaPointe Spring ‘12Slide #11Physiology of the large intestine
• 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
LaPointe Spring ‘12Slide #12Movement in Large Intestine
• 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
LaPointe Spring ‘12Slide #13
Long reflexes of defecation
LaPointe Spring ‘12Slide #14Coordination secretion and absorption
• 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
LaPointe Spring ‘12Slide #15Summary Hormone Table
Marieb Table 23.1.1
GIP duodenum fats, carbohydrates stim pancreas insulin secretion, inhibits stomach, stim adipose glucose and FA uptake & lipogenesis
LaPointe Spring ‘12Slide #16Summary Hormone Table (continued)
Marieb Table 23.1.1
LaPointe Spring ‘12Slide #17Activities of Major Digestive Tract Hormones
From Martini Figure 24.22
LaPointe Spring ‘12Slide #18Digestion, Absorption, Transport
• 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
LaPointe Spring ‘12Slide #19Carbohydrate digestion and absorption
• 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 co-transporters
LaPointe Spring ‘12Slide #20Lipid digestion and absorption
• 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 re-synthesis triglycerides and then release lipids into the circulation as chylomicrons - mixtures of phopholipids and proteins
• carried by lymphatic system
LaPointe Spring ‘12Slide #21Lipid Absorption also (see Saladin fig 25.30)
from Seeley, Stephens and Tate
LaPointe Spring ‘12Slide #22Protein digestion and absorption
• 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
(See Saladin Fig 25.29)
LaPointe Spring ‘12Slide #23Absorption of other molecules
• 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
LaPointe Spring ‘12Slide #24Water Intake, Secretion and Absorption
Diarrhea
Constipation
from Seeley, Stephens and Tate
LaPointe Spring ‘12Slide #25
Marieb Figure 23.33a
Great Summary slide (a)
LaPointe Spring ‘12Slide #26
Marieb Figure 23.33b
Great Summary slide (b)
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