Digestive System
What is Digestion??
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Processes that changes food into simpler forms that
can be absorbed thru cell membrane (mechanical
means and chemical)
What are these processes?
1.
Ingestion
2.
Peristalsis
3.
Digestion
4.
Absorption
5.
Defecation
Two Categories of Organs
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Alimentary canal
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Mouth
Pharynx
Esophagus
Stomach
Small
Large intestines
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Accessory Organs
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Salivary glands
Liver
Gallbladder
Pancreas
Alimentary Canal
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9 meters long!!
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Large intestines: 1.5 m (~5 feet)
Small intestines: 6 m (~20 feet)
Originates from the endoderm of the
embryo
FXN: moves food. HOW?
1. Mixing movements
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Smooth muscle contract rhythmically (every 20 sec or
so) in the circular muscle layer
Food + digestive juices + mucus
2. peristalsis
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Wavelike movements propelling movements
A ring of contractions begin when food expands the
tube in the longitudinal layer
Where does digestion begin?
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Mastication
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Mechanical digestion mixing with saliva
Teeth
Incisors: 8
Cuspids (canines): 4
bicuspids
(premolars): 8
Molars: 12
Pharynx
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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.
2.
3.
4.
5.
6.
7.
Chew food and mix with saliva into a bolus:
voluntary
Food reaches the pharynx and stimulates
sensory receptors: involuntary reflex
Soft palate rises inhibits food from entering…
Epiglottis closes over larynx (no breathing)
Muscles in lower pharynx relax
The constrictor muscles contract and
stimulates peristaltic waves
Esophagus opens
Esophagus
Esophagus
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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:
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Gravity
Peristaltic waves meet the esophageal sphincter
which regulates food into the stomach
Barrette’s esophagus
 the abnormal growth of intestinal-type cells from
the stomach border, into the esophagus.
How do we know physiology
of the Stomach?
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1822: Alexis Martin
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Shot himself in the stomach
Left a fistula
Army surgeon studied 8 years
1984: Barry Marshall from Australia
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Hypothesized that bacteria caused gastritis and ulcers
Drank swamp water
Heliobacter pylori
IN conclusion: Discoveries are made through observations
and experimentation of bizarre phenomenon
Stomach characteristics
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J shaped, 25-30 cm long
Under diaphragm on left side
Hold up to 1 liter of contents
Internal Characteristics:
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Rugae: mucosal folds
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What is the benefit of these folds?
Gastric villi aid in secretion and absorption of
gastric juices
Parts of Stomach
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Cardiac region
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Fundic
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Around esophagus
Large ballooned area
Pyloric
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Near duodenum
Contains pyloric sphincter muscle
Contents of Gastric Juices
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Mucous
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Lubricates and protects stomach wall
Prevents pepsin from digesting the protein in stomach
wall
Pepsin-most active in acidic environment
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Digests proteins into peptides
Contents of Gastric Juices
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HCl
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Intrinsic factors
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Denatures proteins and kills microbes
Aids in absorption of Vit B 12
Gastrin
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Regulatory hormone: regulates contents of
stomach to the small intestines
Gastric Cells
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Mucus cells: secrete mucus
Chief cells: secrete pepsinogen
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Pepsinogen vs. pepsin
Parietal cells: secrete HCl and intrinsic
factor
G-cells: secrete gastrin
Regulation of Gastric
Secretions
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Gastric juice produced continuously
Rate controlled
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Neurally- parasympathetic
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Acetylcholine- inhibits stomatostatin
Hormonally
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Stomatostatin- inhibits acid secretion
Gastrin- increases gastric gland secretion
Phases of Gastric Secretion
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Cephalic Phase
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Gastric Phase
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Sight, taste, smell, or thought of food
Gastric juice secreted
Food enters stomach
Stimulates release of gastrin
Intestinal Phase
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Food enters small intestine
Intestinal gastrin released
Gastric Absorption
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Stomach
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Small Intestine
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Some water, certain salts, lipid-soluble
drugs
Most nutrients absorbed
Large Intestine
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Water, salt
Mixing and Emptying Actions
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Chyme
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Semifluid paste of food particles and
gastric juice
Peristaltic waves- mixes food
Pyloric Sphincter relaxes
Mixing and Emptying Actions
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Liquids- rapidly
Solids- remain until well mixed
Fatty foods- 3-6 hours
Proteins- move quickly
Carbohydrates- more rapidly than
proteins or fats
Digestion Animation
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http://highered.mcgrawhill.com/sites/0072495855/student_view0/cha
pter26/animation__organs_of_digestion.html
http://health.howstuffworks.com/adam200142.htm
http://www.constipationadvice.co.uk/constipa
tion/constipated-digestive-system.html
Gastric Bypass