Download 16.Digestive

Chapter 16
The Digestive System
Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning
Digestive System
• Primary function
– Transfer nutrients, water, and electrolytes from
ingested food into body’s internal environment
• Four functions
– Motility
– Secretion
– Digestion
– Absorption
Chapter 16 The Digestive System
Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning
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
Chapter 16 The Digestive System
Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning
Digestive System
• Digestive secretions
– Consist of water, electrolytes, and specific
organic constituents
– Secretions are released into digestive tract lumen
on appropriate neural or hormonal stimulation
– Normally reabsorbed in one form or another back
into blood after their participation in digestion
Chapter 16 The Digestive System
Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning
Digestive System
• Digestion
– Biochemical breakdown of structurally complex
foodstuffs into smaller, absorbable units
– Accomplished by enzymatic hydrolysis
– Complex foodstuffs and their absorbable units
• Carbohydrates → monosaccharides
• Proteins → amino acids
• Fats → glycerol and fatty acids
Chapter 16 The Digestive System
Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning
Digestive System
• Absorption
– Small units resulting from digestion, along with
water, vitamins, and electrolytes are transferred
from digestive tract lumen into blood or lymph
Chapter 16 The Digestive System
Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning
Digestive System
• Digestive tract
– Continuous from mouth to
anus
– Consists of
•
•
•
•
•
Mouth
Pharynx
Esophagus
Stomach
Small intestine
• Accessory digestive organs
– Salivary glands
– Exocrine pancreas
– Bilary system
• Liver
• Gallbladder
– Duodenum
– Jejunum
– Ileum
• Large intestine
–
–
–
–
Cecum
Appendix
Colon
Rectum
• Anus
Chapter 16 The Digestive System
Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning
Digestive System
Chapter 16 The Digestive System
Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning
Digestive Tract
• Wall has same general structure throughout length
from esophagus to anus
• Four major tissue layers
– Mucosa
• Innermost layer
– Submucosa
– Muscularis externa
– Serosa
• Outer layer
Chapter 16 The Digestive System
Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning
Layers of Digestive Tract Wall
Chapter 16 The Digestive System
Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning
Mucosa
• Lines luminal surface of digestive tract
• Highly folded surface greatly increases absorptive area
• Three layers
– Mucous membrane
• Serves as protective surface
• Modified for secretion and absorption
• Contains
– Exocrine gland cells – secrete digestive juices
– Endocrine gland cells – secrete blood-borne gastrointestinal
hormones
– Epithelial cells – specialized for absorbing digestive nutrients
– Lamina propria
• Houses gut-associated lymphoid tissue (GALT)
– Important in defense against disease-causing intestinal bacteria
– Muscularis mucosa
• Sparse layer of smooth muscle
Chapter 16 The Digestive System
Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning
Submucosa
• Thick layer of connective tissue
• Provides digestive tract with distensibility and
elasticity
• Contains larger blood and lymph vessels
• Contains nerve network known as submucosal
plexus
Chapter 16 The Digestive System
Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning
Muscularis Externa
• Major smooth muscle coat of digestive tube
• In most areas consists of two layers
– Circular layer
• Inner layer
• Contraction decreases diameter of lumen
– Longitudinal layer
• Outer layer
• Contraction shortens the tube
• Contractile activity produces propulsive and mixing
movements
• Myenteric plexus
– Lies between the two muscle layers
Chapter 16 The Digestive System
Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning
Serosa
• Secretes serous fluid
– Lubricates and prevents friction between
digestive organs and surrounding viscera
• Continuous with mesentery throughout much of the
tract
– Attachment provides relative fixation
– Supports digestive organs in proper place while
allowing them freedom for mixing and propulsive
movements
Chapter 16 The Digestive System
Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning
Digestive System Function
• Digestive motility and secretion are regulated by
– Autonomous smooth muscle function
– Intrinsic nerve plexuses
– Extrinsic nerves
– Gastrointestinal hormones
Chapter 16 The Digestive System
Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning
Summary of
Pathways
Controlling
Digestive
System
Activities
Chapter 16 The Digestive System
Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning
Mouth (Oral Cavity)
• Lips
– Form opening
– Help procure, guide, and contain food in the mouth
– Important in speech
– Well-developed tactile sensation
• Palate
– Forms roof of oral cavity (separates mouth from nasal
passages)
– Uvula (seals off nasal passages during swallowing)
• Tongue
– Forms floor of oral cavity
– Composed of skeletal muscle
– Movements aid in chewing and swallowing
– Plays important role in speech
– Taste buds
Chapter 16 The Digestive System
Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning
Mouth (Oral Cavity)
• Pharynx
– Cavity at rear of throat
– Common passageway for digestive and
respiratory systems
– Tonsils
• Within side walls of pharynx
• Lymphoid tissue
• Teeth
– Responsible for chewing (mastication)
– First step in digestive process
Chapter 16 The Digestive System
Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning
Mouth (Oral Cavity)
• Teeth
– Functions of chewing
• Grind and break food into smaller pieces to make
swallowing easier and increase food surface area on
which salivary enzymes can act
• Mix food with saliva
• Stimulate taste buds
Chapter 16 The Digestive System
Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning
Mouth (Oral Cavity)
• Saliva
– Produced largely by three major pairs of salivary glands
– Composition
• 99.5% H2O
• 0.5% electrolytes and protein (amylase, mucus, lysozyme)
– Functions
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Salivary amylase begins digestion of carbohydrates
Facilitates swallowing by moistening food
Mucus provides lubrication
Antibacterial action
– Lysozyme destroys bacteria
– Saliva rinses away material that could serve as food source for
bacteria
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•
•
•
Solvent for molecules that stimulate taste buds
Aids speech by facilitating movements of lips and tongue
Helps keep mouth and teeth clean
Rich in bicarbonate buffers
Chapter 16 The Digestive System
Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning
Control of Salivary Secretion
Chapter 16 The Digestive System
Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning
Pharynx and Esophagus
• Swallowing
– Motility associated with pharynx and esophagus
– Sequentially programmed all-or-none reflex
– Initiated when bolus is voluntarily forced by
tongue to rear of mouth into pharynx
– Most complex reflex in body
– Can be initiated voluntarily but cannot be stopped
once it has begun
– Process divided into two stages
• Oropharyngeal stage
• Esophageal stage (moves bolus from mouth through
pharynx and into esophagus)
Chapter 16 The Digestive System
Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning
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
Chapter 16 The Digestive System
Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning
Peristalsis in the Esophagus
Chapter 16 The Digestive System
Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning
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
Chapter 16 The Digestive System
Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning
Stomach
Chapter 16 The Digestive System
Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning
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
Chapter 16 The Digestive System
Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning
Gastric Emptying and Mixing as a Result of
Antral Peristaltic Contractions
Chapter 16 The Digestive System
Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning
Gastric Emptying
• Factors in stomach
– Amount of chyme in stomach is main factor that influences
strength of contraction
• Factors in duodenum
– Fat
• Fat digestion and absorption takes place only within lumen of small
intestine
• When fat is already in duodenum, further gastric emptying of
additional fatty stomach contents is prevented
– Acid
• Unneutralized acid in duodenum inhibits further emptying of acidic
gastric contents until neutralization can be accomplished
– Hypertonicity
• Gastric emptying is reflexly inhibited when osmolarity of duodenal
contents starts to rise
– Distension
• Too much chyme in duodenum inhibits emptying of even more
gastric contents
Chapter 16 The Digestive System
Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning
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
Chapter 16 The Digestive System
Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning
Gastric Secretions
• Two distinct areas of gastric mucosa that secrete gastric juice
– Oxyntic mucosa
• Lines body and fundus
– Pyloric gland area (PGA)
• Lines the antrum
• Gastric pits at base of gastric glands
• Three types of gastric exocrine secretory cells
– Mucous cells
• Line gastric pits and entrance of glands
• Secrete thin, watery mucus
– Chief cells
• Secrete enzyme precursor, pepsinogen
– Parietal (oxyntic) cells
• Secrete HCl and intrinsic factor
Chapter 16 The Digestive System
Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning
Gastrointestinal
Secretions
Chapter 16 The Digestive System
Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning
HCl Secretion
• Functions of HCl
– Activates pepsinogen to
active enzyme pepsin
and provides acid
medium for optimal
pepsin activity
– Aids in breakdown of
connective tissue and
muscle fibers
– Denatures protein
– Along with salivary
lysozyme, kills most of
the microorganisms
ingested with food
Chapter 16 The Digestive System
Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning
Phases of Gastric Secretion
• Cephalic phase
– Refers to increased secretion of HCl and
pepsinogen that occurs in response to stimuli
acting in the head before food reaches stomach
• Gastric phase
– Begins when food actually reaches the stomach
– Presence of protein increases gastric secretions
• Intestinal phase
– Inhibitory phase
– Helps shut off flow of gastric juices as chyme
begins to empty into small intestine
Chapter 16 The Digestive System
Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning
Gastric Mucosal Barrier
• Enables stomach to contain acid without injuring
itself
Chapter 16 The Digestive System
Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning
Pancreas
• Mixture of exocrine and endocrine tissue
• Elongated gland located behind and below the
stomach
• Endocrine function
– Islets of Langerhans
• Found throughout pancreas
• Secrete insulin and glucagon
• Exocrine function
– Secretes pancreatic juice consisting of
• Pancreatic enzymes actively secreted by acinar cells
that form the acini
• Aqueous alkaline solution actively secreted by duct
cells that line pancreatic ducts
Chapter 16 The Digestive System
Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning
Pancreatic Enzymes
• Exocrine secretion is regulated by
– Secretin
– CCK
• Proteolytic enzymes
– Digest protein
• Trypsinogen - converted to active form trypsin
• Chymotrypsinogen – converted to active form chymotrysin
• Procarboxypeptidase – converted to active form
carboxypeptidase
• Pancreatic amylase
– Converts polysaccharides into the disaccharide amylase
• Pancreatic lipase
– Only enzyme secreted throughout entire digestive system
that can digest fat
Chapter 16 The Digestive System
Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning
Liver
• Largest and most important metabolic organ in the
body
• Body’s major biochemical factory
• Importance to digestive system – secretion of bile
salts
Chapter 16 The Digestive System
Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning
Liver
• Functions not related to digestion
– Metabolic processing of the major categories of
nutrients
– Detoxifying or degrading body wastes and
hormones, drugs, and other foreign compounds
– Synthesizes plasma proteins
– Stores glycogen, fats, iron, copper, and many
vitamins
– Activates vitamin D
– Removes bacteria and worn-out red blood cells
– Excretes cholesterol and bilirubin
Chapter 16 The Digestive System
Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning
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
Chapter 16 The Digestive System
Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning
Small Intestine
• Site where most digestion and absorption take place
• Three segments
– Duodenum
– Jejunum
– Ileum
• Motility includes
– Segmentation
– Migrating motility complex
Chapter 16 The Digestive System
Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning
Small Intestine
• Segmentation
– Primary method of motility in small intestine
– Consists of ringlike contractions along length of
small intestine
– Within seconds, contracted segments relax and
previously relaxed areas contract
– Action mixes chyme throughout small intestine
lumen
Chapter 16 The Digestive System
Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning
Segmentation
Chapter 16 The Digestive System
Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning
Small Intestine
• Segmentation
– initiated by pacemaker cells in small intestine
which produce basic electrical rhythm (BER)
– Circular smooth muscle responsiveness is
influenced by distension of intestine, gastrin, and
extrinsic nerve activity
– Functions
• Mixing chyme with digestive juices secreted into small
intestine lumen
• Exposing all chyme to absorptive surfaces of small
intestine mucosa
• Migrating motility complex
– Sweeps intestines clean between meals
Chapter 16 The Digestive System
Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning
Small Intestine
• Secretion
– Juice secreted by small intestine does not contain
any digestive enzymes
– Synthesized enzymes act within brush-border
membrane of epithelial cells
• Enterokinase
• Disaccharidases
• aminopeptidases
Chapter 16 The Digestive System
Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning
Small Intestine
• Digestion
– Pancreatic enzymes continue carbohydrate and
protein
– Brush-border enzymes complete digestion of
carbohydrates and protein
– Fat is digested entirely within small intestine
lumen by pancreatic lipase
Chapter 16 The Digestive System
Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning
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
– Products of fat digestion undergo transformations
that enable them to be passively absorbed
• Eventually enter lymph
Chapter 16 The Digestive System
Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning
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
Chapter 16 The Digestive System
Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning
Large Intestine
Chapter 16 The Digestive System
Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning
Large Intestine
• Taeniae coli
– Longitudinal bands of muscle
• Haustra
– Pouches or sacs
– Actively change location as result of contraction
of circular smooth muscle layer
• Haustral contractions
– Main motility
– Initiated by autonomous rhythmicity of colonic
smooth muscle cells
Chapter 16 The Digestive System
Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning
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
Chapter 16 The Digestive System
Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning
Gastrointestinal Hormones
• Gastrin
– Release is stimulated by presence of protein in
stomach
– Secretion inhibited by accumulation of acid in
stomach
– Functions
• Acts in several ways to increase secretion of HCl and
pepsinogen
• Enhances gastric motility, stimulates ileal motility,
relaxes ileocecal sphincter, induces mass movements
in colon
• Helps maintain well-developed, functionally viable
digestive tract lining
Chapter 16 The Digestive System
Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning
Gastrointestinal Hormones
• Secretin
– Presence of acid in duodenum stimulates release
– Functions
• Inhibits gastric emptying in order to prevent further acid
from entering duodenum until acid already present is
neutralized
• Inhibits gastric secretion to reduce amount of acid
being produced
• Stimulates pancreatic duct cells to produce large
volume of aqueous NaHCO3 secretion
• Stimulates liver to secrete NaCO3 rich bile which
assists in neutralization process
• Along with CCK, is trophic to exocrine pancreas
Chapter 16 The Digestive System
Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning
Gastrointestinal Hormones
• CCK
– Functions
• Inhibits gastric motility and secretion
• Stimulates pancreatic acinar cells to increase secretion
of pancreatic enzymes
• Causes contraction of gallbladder and relaxation of
sphincter of Oddi
• Along with secretin, is trophic to exocrine pancreas
• Implicated in long-term adaptive changes in proportion
of pancreatic enzymes in response to prolonged diet
changes
• Important regulator of food intake
Chapter 16 The Digestive System
Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning
Gastrointestinal Hormones
• GIP
– Glucose-dependent insulinotrophic peptide
– Stimulates insulin release by pancreas
Chapter 16 The Digestive System
Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning
Esophageal Phase
Gastric Mixing
Oropharyngeal Stage
Segmentation