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Digestive System
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What is Digestion?? Processes that changes food into simpler
forms that can be absorbed thru cell membrane (mechanical means and chemical)
What are these processes?1. Ingestion2. Peristalsis3. Digestion4. Absorption5. Defecation
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Two Categories of Organs
Alimentary canal Mouth Pharynx Esophagus Stomach Small Large intestines
Accessory Organs Salivary glands Liver Gallbladder Pancreas
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Alimentary Canal
9 meters long!! Large intestines: 1.5 m (~5 feet) Small intestines: 6 m (~20 feet)
Originates from the endoderm of the embryo
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FXN: moves food. HOW?
1. Mixing movements Smooth muscle contract rhythmically (every 20
sec or so) in the circular muscle layer Food + digestive juices + mucus
2. peristalsis Wavelike movements propelling movements A ring of contractions begin when food expands the
tube in the longitudinal layer
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Where does digestion begin?
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Mastication Mechanical digestion mixing with
saliva
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Teeth Incisors: 8
bicuspids (premolars): 8
Cuspids (canines): 4
Molars: 12
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Pharynx
Passageway of food into esophagus and air into larynx/trachea
swallowing mechanism (deglutition) InvoluntaryMade of muscles called constrictor muscles
(pull walls inward during swallowing)
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Swallowing Steps1. Chew food and mix with saliva into a
bolus: voluntary2. Food reaches the pharynx and stimulates
sensory receptors: involuntary reflex3. Soft palate rises inhibits food from
entering…4. Epiglottis closes over larynx (no breathing)5. Muscles in lower pharynx relax6. The constrictor muscles contract and
stimulates peristaltic waves7. Esophagus opens
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Esophagus
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Esophagus
Straight tube: 25 cm long Penetrates through an opening: esophageal
hiatus (continuous with stomach) Passageway for food from pharynx to
stomach Contains many mucus glands Movement of food:
Gravity Peristaltic waves meet the esophageal
sphincter which regulates food into the stomach
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Barrette’s esophagus the abnormal growth of intestinal-type cells
from the stomach border, into the esophagus.
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How do we know physiology of the Stomach?
1822: Alexis Martin Shot himself in the stomach Left a fistula Army surgeon studied 8 years
1984: Barry Marshall from Australia Hypothesized that bacteria caused gastritis and ulcers Drank swamp water Heliobacter pylori
IN conclusion: Discoveries are made through observations and experimentation of bizarre phenomenon
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Stomach characteristics J shaped, 25-30 cm long Under diaphragm on left side Hold up to 1 liter of contents Internal Characteristics:
Rugae: mucosal folds What is the benefit of these folds?
Gastric villi aid in secretion and absorption of gastric juices
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Parts of Stomach Cardiac region
Around esophagus Fundic
Large ballooned area Pyloric
Near duodenum Contains pyloric sphincter muscle
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Contents of Gastric Juices Mucous
Lubricates and protects stomach wall Prevents pepsin from digesting the protein in
stomach wall Pepsin-most active in acidic environment
Digests proteins into peptides
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Contents of Gastric Juices HCl
Denatures proteins and kills microbes Intrinsic factors
Aids in absorption of Vit B 12 Gastrin
Regulatory hormone: regulates contents of stomach to the small intestines
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Gastric Cells
Mucus cells: secrete mucus Chief cells: secrete pepsinogen
Pepsinogen vs. pepsin Parietal cells: secrete HCl and
intrinsic factor G-cells: secrete gastrin
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Regulation of Gastric Secretions
Gastric juice produced continuously
Rate controlled Neurally- parasympathetic
Acetylcholine- inhibits stomatostatin Hormonally
Stomatostatin- inhibits acid secretion Gastrin- increases gastric gland secretion
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Phases of Gastric Secretion Cephalic Phase
Sight, taste, smell, or thought of food Gastric juice secreted
Gastric Phase Food enters stomach Stimulates release of gastrin
Intestinal Phase Food enters small intestine Intestinal gastrin released
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Gastric Absorption
Stomach Some water, certain salts, lipid-
soluble drugs Small Intestine
Most nutrients absorbed Large Intestine
Water, salt
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Mixing and Emptying Actions
Chyme Semifluid paste of food particles and
gastric juice Peristaltic waves- mixes food Pyloric Sphincter relaxes
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Mixing and Emptying Actions
Liquids- rapidly Solids- remain until well mixed Fatty foods- 3-6 hours Proteins- move quickly Carbohydrates- more rapidly than
proteins or fats
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Digestion Animation http://highered.mcgraw-hill.com/sites/00
72495855/student_view0/chapter26/animation__organs_of_digestion.html
http://health.howstuffworks.com/adam-200142.htm
http://www.constipationadvice.co.uk/constipation/constipated-digestive-system.html
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Gastric Bypass
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