The Digestive System
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Unit 1
Chapter 19
• Tube that includes: mouth, Pharynx,
Esophagus, Stomach, Small
intestine, Large intestine
• Accessory organs: teeth, tongue,
salivary glands, liver, gallbladder, and
pancreas
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Unit 1
Gastrointestinal (GI) Tract
Figure 19.1
Overview- Operations
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• Ingestion: eating
• Secretion: release of water, enzymes &
buffers
• Mixing & propulsion: movement along
GI tract
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•
breakdown of foods
Absorption: getting it into the body
Defecation: dumping waste products =
defecation
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• Digestion: mechanical and chemical
Wall Layers- Everywhere
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• 4 layers
• Mucosa- epithelium, connective layer, glands,
muscularis mucosae
• Submucosa- connective tissue, blood vessels,
• Muscularis- circular layer, longitudinal layer
In mouth, pharynx & upper esophagus –skeletal muscle
Also in external anal sphincter
• Serosa or Visceral peritoneum
Unit 1
lymphatic vessels, enteric nervous system
Figure 19.2
Figure 19.3a
Figure 19.3b
Mouth
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Unit 1
• Formed by cheeks, hard & soft palate &
tongue
• Soft palate at back includes a “hangy
down” part = uvula
• Tongue- muscular accessory organ
Salivary Glands
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• 3 pairs of salivary glands
• Parotid• Submandibular-
• Saliva contains 99.5% water, salivary
amylase, mucus and other solutes
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• Sublingual
Figure 19.4
Teeth
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• Accessory organs in bony sockets of
mandible & maxilla
• 3 external regions:
• 3 layers of material
Enamel- covers crown
Dentin- majority of interior of tooth
Pulp cavity - nerve, blood vessel & lymphatics
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Crown- above gums
Root- 1 or more parts embedded in socket
Neck – between crown and root near gum line
Figure 19.5
Digestion in the Mouth
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maltose and larger fragments
Continues in the stomach until acidified
• Rounds up food into a soft bolus for
swallowing
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• Mechanical breakdown- chewing
• Mixed with saliva by tongue
• Salivary amylase chemically breaks
down polysaccharides (starch)
Pharynx & Esophagus
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• On swallowing:
• Bolus of food  oropharynx
• Laryngopharynx esophagus
Muscular contractions in pharynx help
Skeletal muscle –controls entry to esophagus
• Lower esophageal sphincter (LES)
Smooth muscle- regulates entry to stomach
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• Upper esophageal sphincter (UES)
Figure 19.6a,b
Swallowing
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Involuntary & breathing interrupted
Soft palate move up-close nasopharynx
Epiglottis seals off larynx
Bolus moves into esophagus through UES
• Esophageal stage peristalsis moves it
toward stomach
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• Voluntary: bolus forced into oropharynx
• Triggers oropharyngeal stage
Figure 19.6c
Stomach
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Cardia- surrounds upper opening
Fundus- superior & to left of cardia
Body – large central portion
Pylorus- lower part leading to pyloric sphincter
& duodenum
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• J- shaped enlargement of tract
• Serves as mixing chamber and holding
reservoir
• Very elastic & muscular
• 4 regions
Figure 19.7
Stomach Wall
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• Mucosa:
Folds called rugae
Epithelium- simple columnar mucous
Form gastric glands lining gastric pits
Chief cells inactive enzyme pepsinogen
Parietal cells HCl & intrinsic factor
Collectively = gastric juice
• Muscularis- 3 Layers: longitudinal,
circular & oblique
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• Secretory cells: mucous neck cells
Figure 19.8
Figure 19.9
Digestion & Absorption
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• Food entry stretch & rise in pH
• Pepsin digests protein peptides
• Little absorption- water, ions & some drugs
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Nerve impulses  secretion & mixing waves
Food mixed with juice Chyme
Small amount pushed through pyloric sphincter
= gastric emptying- Carb. foods fastest, Entry
in duodenum feedback inhibition of stomach
activity
Pancreas
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• Behind stomach-
Produces pancreatic juice
to duodenum via pancreatic duct
• NaHCO3 solution (pH 7.1-8.2)–
1000ml/day
Neutralize stomach acid and dilutes chyme
Proteases: chymotrypsinogen, trypsinogen, et. al.
Activated by entreokinase from intestine
Starch digesting- pancreatic amylase
Pancreatic lipase
Nucleotidases – RNAase & DNAase
Unit 1
• Pancreas- digestive enzymes
• Largest organ after the skin
• right below diaphragm
Bile   hepatic duct
• Gall bladder =Pear-shaped organ
on front (stores bile)
• cystic duct common bile duct
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Unit 1
Liver & Gall Bladder
Bile
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• Bicarbonate, bile salts & waste. – 1000
ml/day
• Important for emulsifying fats
• Pigment is bilirubin- from broken-down
heme during RBC recycling
• Bile salts reabsorbed at end of small
intestine- ileum
• recycle to liver in portal circulation
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Increases surface area for digestion
Figure 19.10
Figure 19.11a
Figure 19.11b
Liver Function
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• Maintains blood glucose
Stores as glycogen
Uses absorbed sugars & Converts amino
acids glucose
• Lipid metabolism
Excretion of bilirubin
Processes drugs and other chemicals
Store fat soluble vitamins
Make active vitamin D
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Produces cholesterol & triglycerides, makes bile
Makes lipoproteins for lipid transport
• 3 parts: duodenum, jejunum, ileum
• Where most of the digestion occurs
• Essentially all of the nutrient
absorption
• Ends in ileocecal sphincter
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Unit 1
Small Intestine
Figure 19.12a
Figure 19.12b
Wall Structure
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• Same 4 layers
• Epithelial- simple columnar
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Absorptive cells with microvilli
Goblet cells- secrete mucus
Wall Structure (Cont.)
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Increase surface area for absorption
Include lacteals for lipid absorption
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• Circular folds- increase surface area
• Villi- finger like projections of
mucosa
Figure 19.13
Motility & Secretions
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• Secretions: alkaline, some enzymes
• Segmentation activity- for mixing
• Peristalsis for movement after most
absorption completed- slow waves
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Water and salt to balance osmolality
~2000 ml/day
• Chyme enters with partially digested
carbohydrates & proteins
• Bile + pancreatic juice + intestinal
juice completes the job
• Absorption is of monosaccharides;
amino acids; phosphate sugar &
bases of DNA & RNA; fatty acids &
monoglycerides
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Unit 1
Digestion & Absorption
Absorption
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• By diffusion, facilitated diffusion, osmosis
& active transport
• Carbohydrates  monosaccharides
Via portal system to liver
Via portal system to liver
• Lipids reformed to triglycerides
Packaged in chlyomicrons with protein
Via lacteals  lymphatics
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• Proteins (jejunum & ileum) amino
acids
Absorption (Cont.)
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• Water & salt
Primarily osmotic movement along with
other nutrients
Fat soluble absorbed with fat
Water soluble with simple diffusion
B12 combines with intrinsic factor &
absorbed by active transport in ileum
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• Vitamins:
Figure 19.14a
Figure 19.14b
Large Intestine
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• Cecum, colon, rectum, anal canal
• Ileocecal canal large intestine
• Colon- ascending, transverse,
descending & sigmoid
•  rectum anal canal
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Below is cecum with appendix
Figure 19.15a
Figure 19.15b
Figure 19.16
Digestion & Absorption
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• Slow emptying of ileum
• Slow peristalsis
• Mass peristalsis with food in stomach
Moves from middle of colon  rectum
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• Bacterial digestion