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THE DIGESTIVE SYSTEM
D. C. Mikulecky
Faculty Mentoring Program
Virginia Commonwealth
University
FUNCTIONS OF THE
DIGESTIVE SYSTEM
Motility
Secretion
Digestion
Absorption
Motility
The digestive tract is surrounded by layers
of smooth muscle
These muscles enable mixing and
propulsive movement to be carried out by
the digestive tract
Secretion
Digestion requires that enzymes be
secreted by the pancreas and other
organs
Mucous secretions protect the digestive
tract
Acid is secreted in the stomach
There are other secretions of importance
DIGESTION
Breaking down complex foodstuffs into
absorbable units by enzymes produced in
the digestive system
Involves the breakdown of carbohydrates,
proteins fats, and other foods
Absorption
All other functions support this one
This is how we obtain the necessary fuel
for our cells
THE DIGESTIVE TRACT
Mouth and pharynx
Esophagus
Stomach
Small Intestine
Large Intestine
Anus
THE DIGESTIVE TRACT
Mucosa
Logitudinal
Muscle
Submucosal
Plexus
Circular
Muscle
Myenteric
Plexus
Lumen
Extrinsic
Nerves
ACCESSORY DIGESTIVE
ORGANS
Salivary Glands
Exocrine Pancreas
Billiary System: (Liver and Gallbladder)
CONTROL OF DIGESTIVE
SYSTEM
External
Influences
Local
Influences
Receptors in Digestive Tract
Intrinsic
Nerve
Plexuses
Extrinsic Autonomic
Nerves
Smooth muscle
Exocrine Gland Cells
Endocrine Gland Cells
GI
Hormones
THE MOUTH
Ingestion of food
Chewing and swallowing
Salivary secretion
Chewing
Chewing is the first step in the digestive
process
Food is mixed with saliva and broken up
into smaller pieces
Taste buds are stimulated
Salivary secretion
Salivary amylase: digests carbohydrate
Moistening and lubricating agent
Antibacterial action: lysozyme and rinsing
Solvent for taste stimuli
Speech facilitation
Oral hygene
Bicarbonate buffers neutralize acids
Swallowing
A programmed all-or-none reflex
Chewing and moving the bolus of food back is manly
voluntary (striated muscle)
Pressure of bolus on pharynx triggers involuntary reflex
(smooth muscle)
Tongue prevents food from moving back
Uvula elevated, sealing nasal passage
Larynx elevates and closure of glottis
Respiration briefly inhibited
Pharyngeal muscles force bolus back
Peristaltic waves move bolus through esophagus
PERISTALSIS
Ringlike
contraction
sweeps down
the esophagus
THE STOMACH
Gastroesophageal
Sphincter
Esophagus
Fundus
Body
Oxyntic
Mucosa
Pyloric
Antrum
Sphincter
Pyloric
Gland
Area
THE STOMACH
Mixing and storage organ
Has its own pacemaker: sets frequency of
rhythmic peristaltic contractions
Peristalsis becomes stronger in antrum
Motility is highly controlled
No absorption of food, only aspirin and
alcohol
FACTORS INFLUENCING
GASTRIC MOTILITY
Distension of stomach: increases
Feedback from the small intestine:
decreases
Control from CNS
Gastrin: increases
MIXING MOTIONS IN THE
STOMACH
Pyloric Sphincter
Closed
Peristalsis
Begins
MIXING MOTIONS IN THE
STOMACH
Pyloric Sphincter
Closed
Peristaltic Wave
Travels Downward
STOMACH EMPTYING
Pyloric Sphincter
Opens
Chyme
Squirted
into
Duodenum
Peristaltic Wave
Travels Downward
FACTORS CONTROLLING
STOMAC EMPTYING
Gastric Motility
Enterogastric reflex: Via intrinsic and
autonomic nerves
Enterogastrones: secretin, cholecystokinin
(CCK), gastric inhibitory peptide
FACTORS IN SMALL INTESTINE
CONTROLLING STOMAC EMPTYING
Fat
Acid
Hypertonicity
Distension
GASTRIC SECRETIONS
Mucous: Surface Cells
Mucous: Mucous Neck Cells
Hydrochloric Acid: Parietal
cells
Intrinsic Factor: Parietal Cells
Pepsinogen: Chief Cells
Gastrin: Only in the Pyloric
Gland Area
Activation of Pepsin
Chief
Cell
PEPSIN-OGEN
PEPSIN
HCl
Parietal
Cell
Digests
Protein
STIMULATION OF GASTRIC
SECRETION: Cephalic Phase
Seeing,
Smelling,
Tasting
Food
Vagus
Intrinsic Nerves
Pyloric Area
Gastrin
Parietal
& Chief
Cells
Increased
Gastric
Secretion
STIMULATION OF GASTRIC
SECRETION: Gastric Phase
Stimuli
Vagus
in Stomach:
protein,
distension,
caffeine,
alcohol
Intrinsic Nerves
Pyloric Area
Gastrin
Parietal
& Chief
Cells
Increased
Gastric
Secretion
THE GASTRIC MUCOSAL
BARRIER
Protects the cells from contents of
stomach
Luminal membranes of cells are
impermeable to protons
Cells are tightly adjoined
Rapid turnover
If broken, peptic ulcer may result: positive
feedback involving histamine
Gastrin secretion
inhibition
Acid in antrum
removal of protein as stomach empties