THE DIGESTIVE SYSTEM

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THE DIGESTIVE SYSTEM

• D. C. Mikulecky
• Faculty Mentoring Program
• Virginia Commonwealth University

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FUNCTIONS OF THE DIGESTIVE SYSTEM

• Motility
• Secretion
• Digestion
• Absorption

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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

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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

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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

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Absorption

• All other functions support this one
• This is how we obtain the necessary fuel for our
  cells

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THE DIGESTIVE TRACT

• Mouth and pharynx
• Esophagus
• Stomach
• Small Intestine
• Large Intestine
• Anus

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THE DIGESTIVE TRACT
Mucosa
Logitudinal Muscle
Submucosal Plexus
Lumen
Circular Muscle
Extrinsic Nerves
Myenteric Plexus
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ACCESSORY DIGESTIVE ORGANS

• Salivary Glands
• Exocrine Pancreas
• Billiary System (Liver and Gallbladder)

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CONTROL OF DIGESTIVE SYSTEM
External Influences
Local Influences
Receptors in Digestive Tract
Intrinsic Nerve Plexuses
Extrinsic Autonomic Nerves
GI Hormones
Smooth muscle Exocrine Gland Cells Endocrine
Gland Cells
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THE MOUTH

• Ingestion of food
• Chewing and swallowing
• Salivary secretion

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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

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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

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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

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PERISTALSIS
Ringlike contraction sweeps down the esophagus
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THE STOMACH
Esophagus
Gastroesophageal Sphincter
Fundus
Body
Oxyntic Mucosa
Antrum
Pyloric Sphincter
Pyloric Gland Area
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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

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FACTORS INFLUENCING GASTRIC MOTILITY

• Distension of stomach increases
• Feedback from the small intestine decreases
• Control from CNS
• Gastrin increases

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MIXING MOTIONS IN THE STOMACH
Peristalsis Begins
Pyloric Sphincter Closed
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MIXING MOTIONS IN THE STOMACH
Peristaltic Wave Travels Downward
Pyloric Sphincter Closed
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STOMACH EMPTYING
Peristaltic Wave Travels Downward
Pyloric Sphincter Opens
Chyme Squirted into Duodenum
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FACTORS CONTROLLING STOMAC EMPTYING

• Gastric Motility
• Enterogastric reflex Via intrinsic and autonomic
  nerves
• Enterogastrones secretin, cholecystokinin (CCK),
  gastric inhibitory peptide

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FACTORS IN SMALL INTESTINE CONTROLLING STOMAC
EMPTYING

• Fat
• Acid
• Hypertonicity
• Distension

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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

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Activation of Pepsin
Chief Cell
PEPSIN-OGEN
PEPSIN
HCl
Digests Protein
Parietal Cell
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STIMULATION OF GASTRIC SECRETION Cephalic Phase
Seeing, Smelling, Tasting Food
Intrinsic Nerves Pyloric Area
Vagus
Parietal Chief Cells
Increased Gastric Secretion
Gastrin
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STIMULATION OF GASTRIC SECRETION Gastric Phase
Stimuli in Stomach protein, distension, caffeine
, alcohol
Intrinsic Nerves Pyloric Area
Parietal Chief Cells
Vagus
Increased Gastric Secretion
Gastrin
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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

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Gastrin secretion inhibition

• Acid in antrum
• removal of protein as stomach empties