Digestive System

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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?
• Ingestion
• Peristalsis
• Digestion
• Absorption
• 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
Cuspids (canines) 4
bicuspids (premolars) 8
Molars 12
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Pharynx

• Passageway of food into esophagus and air into
  larynx/trachea
• swallowing mechanism (deglutition)
• Involuntary
• Made of muscles called constrictor muscles (pull
  walls inward during swallowing)

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Swallowing Steps

1. Chew food and mix with saliva into a bolus
   voluntary
2. Food reaches the pharynx and stimulates sensory
   receptors involuntary reflex
3. Soft palate rises inhibits food from entering
4. Epiglottis closes over larynx (no breathing)
5. Muscles in lower pharynx relax
6. The constrictor muscles contract and stimulates
   peristaltic waves
7. 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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• Barrettes 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/0072495855/s
  tudent_view0/chapter26/animation__organs_of_digest
  ion.html
• http//health.howstuffworks.com/adam-200142.htm
• http//www.constipationadvice.co.uk/constipation/c
  onstipated-digestive-system.html

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Gastric Bypass
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