Ch. 16: The Digestive System
Breakdown
• Understand that digestion is a process and how
that process works
• Know the general anatomy of the digestive tract
and it organs and accessory organs
• Know the physiology of digestion and absorption
for the macromolecules as well as vitamins and
minerals
Digestion
• What is digestion and why is it necessary?
• How is digestion accomplish?
Stages of Digestion
• Ingestion
• Digestion
– Mechanical and chemical
• Absorption
• Compaction
• Defecation
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Digestive System
General Anatomy
• Structure: common layers through out the system
• Mucosa
• Submucosa
• Muscularis externa
• Serosa
Tissue Layers of GI Tract
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Diaphragm
Esophageal hiatus
Enteric nervous system:
Mucosa:
Stratified squamous
epithelium
Lamina propria
Muscularis mucosae
Myenteric plexus
Submucosal plexus
Parasympathetic ganglion of
myenteric plexus
Submucosa:
Esophageal gland
Lumen
Muscularis externa:
Inner circular layer
Outer longitudinal layer
Blood vessels
Serosa
Figure 25.2
Enteric Nervous System
• Regulates motility, secretion, blood flow
• Independent of CNS
– CNS greatly influences
• Two networks
– Submucosal plexus
– Myenteric plexus
Regulation of Digestive Tract
• Motility and secretion of the digestive tract are controlled by
neural, hormonal, and paracrine mechanisms
• Neural control
– short (myenteric) reflexes – stretch or chemical stimulation acts through
myenteric plexus
• stimulates parastaltic contractions of swallowing
– long (vagovagal) reflexes - parasympathetic stimulation of digestive
motility and secretion
• Hormones
– chemical messengers secreted into bloodstream
– gastrin and secretin
• Paracrine secretions
– chemical messengers that stimulate nearby target cells
Mouth
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• Oral or buccal cavity
• Site of ingestion
• Teeth, tongue, saliva
Figure 25.4
• Mechanical and chemical digestion
The Teeth
• Dentition – the teeth
• Masticate food into smaller pieces
– makes food easier to swallow
– exposes more surface area
• 32 adult teeth – 20 deciduous (baby) teeth
– from midline to the rear of each jaw
•
•
•
•
2 incisors – chisel-like cutting teeth used to bite off a piece of food
1 canine – pointed and act to puncture and shred food
2 premolars – broad surface for crushing and grinding
3 molars – even broader surface for crushing and grinding
Saliva
• Saliva
–
–
–
–
–
–
moisten mouth
begin starch and fat digestion
cleanse teeth
inhibit bacterial growth
dissolves molecules so they can stimulate the taste buds
moistens food and bind it together into bolus to aid in swallowing
• Hypotonic solution of 97.0% to 99.5% water and the following
solutes:
–
–
–
–
–
–
salivary amylase – begins starch digestion in the mouth
lingual lipase – activated by stomach acid and digests fats
mucus – binds and lubricates the mass of food and aids in swallowing
lysozyme – enzyme that kills bacteria
immunoglobulin A (IgA) – an antibody that inhibits bacterial growth
electrolytes - Na+, K+, Cl-, phosphate and bicarbonate
• pH of 6.8 to 7.0
Esophagus
• Esophagus – a straight muscular tube 25-30
cm long
– extends from pharynx to cardiac orifice of
stomach
– lower esophageal sphincter
• prevents stomach content regurgitation
• heartburn – burning sensation produced by acid
reflux into the esophagus
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– nonkeratinized stratified squamous epithelium
– esophageal glands in submucosa secrete mucus
– skeletal muscle in upper one-third, mixture in
middle one-third, and only smooth muscle in the
bottom one-third
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Stomach
• Stomach – muscular sac in upper left abdominal cavity
immediately inferior to the diaphragm
– primarily functions as a food storage organ
• internal volume of about 50 mL when empty
• 1.0 – 1.5 L after a typical meal
• up to 4 L when extremely full and extend nearly as far as the pelvis
• Mechanically breaks up food particles, liquefies the
food, and begins chemical digestion of protein and fat
– chyme – soupy or pasty mixture of semi-digested food in the
stomach
Gross Anatomy of Stomach
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Microscopic Anatomy
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• Mucosa – simple
columnar epithelium
• Gastric pits – depressions
in mucosa
– Cardiac, pyloric and gastric
glands
– Produce gastric juice
Figure 25.13 b-c
Cells of Gastric Glands
• Mucous cells – secrete mucus
– predominate in cardiac and pyloric glands
• Regenerative (stem) cells – found in the base of the
pit and in the neck of the gland
– divide rapidly and produce a continual supply of new
cells
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Mucous neck cell
Parietal cell
• Parietal cells – found mostly in the upper half of the
gland
– secrete hydrochloric acid (HCl), intrinsic factor, hunger
hormone ghrelin
Chief cell
• Chief cells – most numerous
– secrete gastric lipase and pepsinogen
– absent in pyloric and cardiac glands
G cell
• Enteroendocrine cells – concentrated in lower end
of gland
– secrete hormones and paracrine messengers that
regulate digestion
(c) Gastric gland
Figure 25.13c
Hydrochloric Acid
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Blood
Parietal cell
Lumen of gastric gland
Alkaline
tide
Cl–
Cl–
Stomach
acid
HCO3
–
K+
HCO3
–
H+
Figure 25.14
H+–K+ ATPase
CO2
CO2 + H2O
H2CO3
• gastric juice has a high concentration of hydrochloric acid
– pH as low as 0.8
• parietal cells produce HCl and contain carbonic anhydrase (CAH)
CAH
– CO2 + H2O  H2CO3  HCO3- + H+
– H+ is pumped into gastric gland lumen by H+- K+ ATPase pump
• antiporter uses ATP to pump H+ out and K+ in
– HCO3- exchanged for Cl- (chloride shift) from blood plasma
• Cl- (chloride ion) pumped into the lumen of gastric gland to join H+ forming HCl
• elevated HCO3- (bicarbonate ion) in blood causes alkaline tide increasing
blood pH
Functions of Hydrochloric Acid
• activates pepsin and lingual lipase
• breaks up connective tissues and plant cell walls
– helps liquefy food to form chyme
• converts ingested ferric ions (Fe3+) to ferrous ions
(Fe2+)
– Fe2+ absorbed and used for hemoglobin synthesis
• contributes to nonspecific disease resistance by
destroying most ingested pathogens
Production and Action of Pepsin
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Parietal cell
Removed
peptide
Dietary
proteins
HCl
Pepsin
(active enzyme)
Chief cell
Pepsinogen
(zymogen)
Partially digested
protein
Gastric gland
Figure 25.15
Gastric Secretions
• Gastric lipase
– Combines with lingual lipase
– Begin to digest small amount of fats
• Intrinsic factor
– Secreted by the parietal cells
– Required for vitamin B12 absorption
– Only essential function of stomach
Stomach Rumbles
• Digestion and Absorption
– Partial digestion of proteins and lipids
– Absorption of aspirin and lipid-soluble drugs
• Protection
– Mucus
– Tight junctions
– Cell replacement
Regulation of Gastric Function
• Cephalic phase
– Stomach responds to sensory and mental info
• Gastric phase
– Occurs after ingestion and deglutition
– Stimulate gastric activity (secretion)
• Ach, Histamine, Gastrin
– Act on parietal, chief, enteroendocrine cells
– Positive and negative feedback loops
• Intestinal phase
– Initially stimulates gastric secrestions
– Enterogastric reflex- inhibitory signals
– Secretion of secretin and cholecystokinin
• Suppress gastric secretion and motility
Accessory Organs
• Liver
– Produces and secretes bile
• Gall bladder
– Stores and concentrates bile
– Bile
• Contains cholesterol
• Bile acids and lecithin help digest fat
• 80% of bile acids reabsorbed in ilieum
• Pancreas
–
–
–
–
–
Exocrine tissue secretes pancreatic juice
Mixture of zymogens, enzymes, NaCO3, and water
Zymogens include: trypsinogen, chymotrypsinogen, procarboxypeptidase
Enzyms include: lipase, amylase, ribonuclease, deoxyribonuclease
Regulation
• Ach, CCK, Secretin
Small Intestine
• Digestion and absorption completed
• Duodenum
– Pancreatic enzyme digestion
• Jejunum
– Highly vascularized
– Absorption
• Ileum
– Less vascularized
– Absorption
– Ileocecal valve
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Intestinal Motility
• Segmentation
• Peristalsis
– Migrating motor
complex
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Large Intestine
• Begins with cecum, 1.5m
long
• Ascending, transverse,
descending, sigmoid
• Rectum
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• Anus
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Carbohydrate Digestion and
Absorption
• Digestible dietary form is starch
• Primary enzyme – amylase
– Digests starch into
oligosaccharides
• Secondary enzymes –
dextrinase, glucoamylase,
maltase, sucrase, lactase
Protein Digestion and Absorption
• Three sources
– Dietary
– Digested enzymes
– Sloughed epithelial cells
• Digested by proteases
– Pepsin (cleaves between tyrosine and phenylalanine)
• Stomach
– Trypsin, chymotrypsin, carboxypeptidase
• Pancreatic secretions
– aminopeptidase, dipeptidase
• Brush border enzymes
• Absorbed like carbohydrates
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Protein Digestion
and Absorption
Mouth
No chemical digestion occurs.
Protein
N
H
O
C
H
HO
Polypeptides
Stomach
N
H
C
C
H
HO
O
OH
N
C
H
O
H
O
N
OH
H
N
H
H
O
C
Protein
Pepsin ( ) hydrolyzes certain
peptide bonds, breaking protein down
into smaller polypeptides.
H
HO
N
H
O
H
C
HO
Small intestine
Actions of pancreatic enzymes
O
C
H
N
O
OH
O
OH
C
OH
C
H
H
N
N
H
O
C
C
O
H
N
H
O
C
H
N
C
HO
N
C
H
OH
OH
O
Trypsin ( ) and chymotrypsin ( )
hydrolyze other peptide bonds, breaking
polypeptides down into smaller
oligopeptides.
OH
O
OH
Oligopeptides
H
O
C
H
C
N
C
H
O
H
H
N
N
O
H
H
HO
Polypeptides
OH
C
H
H
H
C
N
O
N
H
OH
H
H
N
H
N
N
H
H
OH
HO
H
O
C
H
O
H
OH
H
N
C
O
H
OH
H
N
O
C
OH
Carboxypeptidase ( ) removes one
amino acid at a time from the carboxyl
(–COOH) end of an oligopeptide.
Small intestine
Actions of brush border enzymes (contact digestion)
Carboxypeptidase
H
N
H
C
O
OH
Aminopeptidase
H
N
H
C
Dipeptidase
O
H
OH
H
N
O
C
OH
Carboxypeptidase ( ) of the brush
border continues to remove amino acids
from the carboxyl (–C OOH) end.
Aminopeptidase ( ) of the brush border
removes one amino acid at a time from
the amino (–N H ) end.
Blood capillary
of intestinal villus
Dipeptidase ( ) splits dipeptides
into separate amino acids ( ).
(
)
Figure 25.29
Lipid Digestion and Absorption
• Digested by lipases
• Emulsification
• Micelles
– Transport to brush border
• FFAs, monoglycerides, fat-soluable vitamins, cholesterol
• Resynthesized
– Chylomicrons and exocytosis
• Enter lacteals
Emulsification
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Emulsification
Hydrophilic region
Hydrophobic region
Lecithin
Bile acid
Fat globule is broken up and coated by
lecithin and bile acids.
Fat globule
Emulsification
droplets
Figure 25.30
Fat Hydrolysis and Micelles
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Fat hydrolysis
Pancreatic lipase
Pancreatic lipase
Free fatty acid
Lecithin
Monoglyceride
Bile acid
Dietary lipid
Triglyceride
Emulsification droplets are acted upon by
pancreatic lipase, which hydrolyzes the
first and third fatty acids from triglycerides,
usually leaving the middle fatty acid.
Free fatty acid
Lipid uptake by micelles
Bile acid
Monoglycerides
Cholesterol
Fatty acids
Fat-soluble vitamins
Figure 25.30
Micelles in the bile pass to the small
intestine and pick up several types of
dietary and semidigested lipids.
Lipid core
Micelles
Nucleic Acids and Vitamins
• Nucleic acid
– nucleases (deoxyribonuclease and ribonuclease) hydrolyze
DNA and RNA to nucleotides
– nucleosidases and phosphatases of brush border split
them into phosphate ions, ribose or deoxyribose sugar,
and nitrogenous bases
• Vitamins
– are absorbed unchanged
– fat-soluble vitamins - A, D, E and K absorbed with other
lipids
• if they are ingested without fat-containing food, they are not absorbed at
all, but are passed in the feces and wasted
– water-soluble vitamins, B complex and C, absorbed by
simple diffusion and B12 if bound to intrinsic factor from
the stomach
Minerals
• minerals (electrolytes)
– are absorbed all along small intestine
– Na+ cotransported with sugars and amino acids
– Cl- exchanged for bicarbonate reversing chloride-bicarbonate
exchange that occurs in the stomach
– iron and calcium absorbed as needed
• iron absorption is stimulated by liver hormone hepcidin
• absorptive cells bind ferrous ions (Fe2+) and internalize by active
transport
• unable to absorb ferric ions (Fe3+) but stomach acid reduces ferric
ions to absorbable ferrous ions
• transferrin (extracellular protein) transports iron in blood to bone
marrow, muscle, and liver
Water Balance
• Digestive system is one of several systems involved in water
balance
• Digestive tract receives about 9 L of water/day
– 0.7 L in food, 1.6 L in drink, 6.7 L in gastrointestinal secretions
– 8 L is absorbed by small intestine and 0.8 L by large intestine
– 0.2 L voided in daily fecal output
• Water is absorbed by osmosis following the absorption of salts
and organic nutrients
• Diarrhea - occurs when large intestine absorbs too little water
– feces pass through too quickly if intestine is irritated
– feces contains high concentrations of a solute (lactose)
• Constipation – occurs when fecal movement is slow, too much
water gets reabsorbed, and feces becomes hardened