Download The Digestive System

The Digestive System
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Digestive System
• Digestive tract
– Continuous from mouth
to anus
– Consists of
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Mouth
Pharynx
Esophagus
Stomach
Small intestine
• Accessory digestive
organs
– Salivary glands
– Exocrine pancreas
– Bilary system
• Liver
• Gallbladder
– Duodenum
– Jejunum
– Ileum
• Large intestine
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Cecum
Appendix
Colon
Rectum
• Anus
Saliva
Salivary secretion is stimulated by cholinergic parasympathetic nerves
Rate of secretion typically around 1,500 ml per day
Functions of saliva are:
prevention of tooth decay by flushing wastes out of mouth and creating
antibacterial environment
lubrication of mouth
preliminary digestion of starches
In absence of saliva:
thirst
impaired speech and swallowing
rapid tooth decay
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Swallowing
Pharyngoesophageal sphincter
Pharynx and Esophagus
• Esophagus
– Fairly straight muscular tube
– Extends between pharynx and stomach
– Sphincters at each end
• Pharyngoesophageal sphincter
– Keeps entrance closed to prevent large volumes of air
from entering esophagus and stomach during breathing
• Gastroesophageal sphincter
– Prevents reflux of gastric contents
– Peristaltic waves push food through
esophagus
– Secretions (mucus) are entirely protective
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Peristalsis in the Esophagus
Gastroesophageal reflux
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Digestive System
• Motility
– Muscular contractions that mix and move
forward the contents of the digestive tract
– Two types of digestive motility
• Propulsive movements
– Push contents forward through the digestive tract
• Mixing movements
– Serve two functions
» Mixing food with digestive juices promotes digestion
of foods
» Facilitates absorption by exposing all parts of
intestinal contents to absorbing surfaces of digestive
tract
Layers of Digestive Tract Wall
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Stomach
Functions of the stomach
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Storage of ingested food
Beginning of protein digestion
Bactericidal activity
Blending of food into chyme
Intermittent release of chyme into
intestine
• Vitamin B12 absorption
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Stomach
• J-shaped sac-like chamber lying between
esophagus and small intestine
• Divided into three sections
– Fundus
– Body
– Antrum
• Three main functions
– Store ingested food until it can be emptied into small
intestine
– Secretes hydrochloric acid (HCl) and enzymes that
begin protein digestion
– Mixing movements convert pulverized food to chyme
• Pyloric sphincter
– Serves as barrier between stomach and upper part of
small intestine
Gastric Motility
• Four aspects
– Filling
• Involves receptive relaxation
– Enhances stomach’s ability to accommodate the extra
volume of food with little rise in stomach pressure
– Triggered by act of eating
– Mediated by vagus nerve
– Storage
• Takes place in body of stomach
– Mixing
• Takes place in antrum of stomach
– Emptying
• Largely controlled by factors in duodenum
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Gastric Emptying and Mixing as a Result of
Antral Peristaltic Contractions
Gastric Emptying
• Factors trigger either
– Neural response
• Mediated through both intrinsic nerve plexuses (short
reflex) and autonomic nerves (long reflex)
• Collectively called enterogastric reflex
– Hormonal response
• Involves release of hormones from duodenal mucosa
collectively known as enterogastrones
– Secretin
– Cholecystokinin (CCK)
• Additional factors that that influence gastric
motility
– Emotions
• Sadness and fear – tend to decrease motility
• Anger and aggression – tend to increase motility
– Intense pain – tends to inhibit motility
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Ulcers
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Protective mucus layer can be lost
Acid --> damage to gut wall
Mucus loss most commonly caused by H. pylori
Caffeine, alcohol increase acid secretion
Treatment: reduce acid, give antibiotic
Liver
• Bile
– Actively secreted by liver and actively diverted to
gallbladder between meals
– Stored and concentrated in gallbladder
– Consists of
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Bile salts
Cholesterol
Lecithin
Bilirubin
– After meal, bile enters duodenum
• Bile salts
– Derivatives of cholesterol
– Convert large fat globules into a liquid emulsion
– After participation in fat digestion and absorption,
most are reabsorbed into the blood
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Absorption
• Nutrients not absorbed in stomach
– Insufficient digestion
– Lack of epithelial transporters
• Absorption of small fat-soluble
molecules
– Aspirin
– Alcohol
Small Intestine
• Site where most digestion and absorption
take place
• Three segments
– Duodenum
– Jejunum
– Ileum
• Motility
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Need for the small intestine
• Carbohydrates
– Salivary amylase
• Fats
– No salivary or gastric
breakdown
• Protein
– Gastric pepsin
– BUT BREAKDOWN
PRODUCTS TOO
LARGE TO BE
ABSORBED
Small Intestine
• Absorption
– Absorbs almost everything presented to it
– Most occurs in duodenum and jejunum
– Adaptations that increase small intestine’s surface area
• Inner surface has permanent circular folds
• Microscopic finger-like projections called villi
• Brush border (microvilli) arise from luminal surface of
epithelial cells
– Lining is replaced about every three days
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Large Intestine
• Primarily a drying and storage organ
• Consists of
– Colon
– Cecum
– Appendix
– Rectum
• Contents received from small intestine consists of
indigestible food residues, unabsorbed biliary
components, and remaining fluid
• Colon
– Extracts more water and salt from contents
– Feces – what remains to be eliminated
LARGE INTESTINE
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Large Intestine
• Mass movements
– Massive contractions
– Moves colonic contents into distal part of large intestine
• Gastrocolic reflex
– Mediated from stomach to colon by gastrin and by autonomic
nerves
– Most evident after first meal of the day
– Often followed by urge to defecate
• Defecation reflex
– Initiated when stretch receptors in rectal wall are stimulated by
distension
– Causes internal anal sphincter to relax and rectum and sigmoid
colon to contract more vigorously
– If external anal sphincter (skeletal muscle under voluntary
control) is also relaxed, defecation occurs
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