The Digestive System
Breakdown
• Process and general anatomy
• The alimentary canal: From mouth to anus
• Physiology of digestion and absorption
Digestion
• What is digestion and why is it necessary?
• How is digestion accomplish?
Stages of Digestion
• Ingestion
• Digestion
– Mechanical and chemical
• Absorption
• Compaction
• Defecation
http://www.ootonline.com/OOTnews/?p=1507
Digestive System
General Anatomy
• Structure: common layers through out the system
• Mucosa
• Submucosa
• Muscularis externa
• Serosa
Variations in Layers of GI Tract
• Mucosa (mucous membrane) – lines the lumen and consists of:
– inner epithelium
• simple columnar in most of digestive tract
• stratified squamous from mouth through esophagus, and in lower anal canal
– lamina propria – loose connective tissue layer
– muscularis mucosa – thin layer of smooth muscle
• tenses mucosa creating grooves and ridges that enhance surface area and contact
with food
• improves efficiency of digestion and nutrient absorption
– mucosa-associated lymphatic tissue (MALT) – abundance of lymphocytes
and lymphatic nodules
• Submucosa – thicker layer of loose connective tissue
– contains blood vessels, lymphatic vessels, a nerve plexus, local mucus
secreting glands that dump lubricating mucus into the lumen
Variations in Layers of GI Tract
• Muscularis externa – Two layers of muscle near outer surface
– inner circular layer
• in some places, thickens to form valves (sphincters)
– outer longitudinal layer
• responsible for the motility that propels food
• Serosa – composed of a thin layer of areolar tissue topped by
simple squamous mesothelium
– begins in the lower 3 to 4 cm of the esophagus, ends just before rectum
– adventitia – a fibrous connective tissue layer that binds and blends the
pharynx, most of the esophagus, and the rectum into the adjacent
connective tissue of other organs
Tissue Layers of GI Tract
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
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
– Myentric 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
Lesser and Greater Omentum
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Liver
Gallbladder
Stomach
Lesser
omentum
Greater
omentum
Ascending
colon
Small
intestine
Figure 25.3a
(a)
• lesser omentum- attaches stomach to liver
• greater omentum- covers small intestines like an apron
Mesentery and Mesocolon
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Greater
omentum
(retracted)
Transverse
colon
Mesocolon
Descending
colon
Mesentery
Jejunum
Sigmoid
colon
Figure 25.3b
(b)
• mesentery of small intestines holds many blood vessels
• mesocolon anchors colon to posterior body wall
Mouth
Copyright © The McGraw-Hill Companies, Inc.
• 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
The Teeth
• Alveolus – tooth socket in bone
– gomphosis joint formed between tooth and bone
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Names of teeth
Age at eruption
(months)
6–9
Central incisor
Lateral incisor
7– 11
16–20
Canine
• Periodontal ligament – modified periosteum
anchors tooth firmly in alveolus
– allows slight movement under pressure of chewing
12–16
1st molar
20–26
2nd molar
(a) Deciduous (baby) teeth
• Gingiva (gum) – covers the alveolar bone
Names of teeth
Central incisor
Lateral incisor
• Regions of a tooth
– crown – portion above the gum
– root – the portion below the gum, embedded in
alveolar bone
– neck – the point where crown, root, and gum
meet
– gingival sulcus – space between the tooth and the
gum
• hygiene in the sulcus in important to dental health
Age at eruption
(years)
6–8
7–9
Canine 9–12
1st premolar 10–12
2nd premolar
1st molar
2nd molar
3rd molar
(wisdom tooth)
(b) Permanent teeth
10–12
6–7
1 –13
17–25
Figure 25.6
Tooth Structure
• Dentin – hard yellowish tissue
• Enamel – covers crown and neck
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Enamel
Crown
Dentine
• Cementum – covers root
Pulp in pulp cavity
Gingival sulcus
Neck
Gingiva
Alveolar bone
• Root canal in the roots leading to pulp
cavity in the crown
Periodontal
ligament
– nerves and blood vessels
– apical foramen – pore at the basal end
of each root canal
Cementum
• Occlusion – meeting of the teeth with
the mouth closed
Root canal
Root
Apical
foramen
Artery ,
nerve, vein
Figure 25.7
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
Salivary Glands
• Intrinsic salivary glands – small glands dispersed amid other oral
tissues
–
–
–
–
lingual glands – in the tongue – produce lingual lipase
labial glands – inside of the lips
buccal glands – inside of the cheek
all secrete saliva at a fairly constant rate
• Extrinsic salivary glands – three pair connected to oral cavity by
ducts
– parotid – located beneath the skin anterior to the earlobe
– submandibular gland – located halfway along the body of the mandible
• its duct empties at the side of the lingual frenulum, near the lower central
incisors
– sublingual glands – located in the floor of the mouth
• has multiple ducts that empty posterior to the papilla of the submandibular
duct
Pharynx
• Pharynx – muscular funnel that connects oral cavity to
esophagus
– digestive and respiratory tracts intersect
• Pharyngeal constrictors (superior, middle, and inferior) circular muscles that force food downward during
swallowing
– when not swallowing, the inferior constrictor remains contracted to
exclude air from the esophagus
• Considered upper esophageal sphincter
Esophagus
• Esophagus – a straight muscular tube 25-30
cm long
– begins at level between C6 and the cricoid
cartilage
– extends from pharynx to cardiac orifice of
stomach (T7)
– lower esophageal sphincter
• prevents stomach contents regurgitation
• heartburn – burning sensation produced by acid
reflux into the esophagus
– 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
– covered with adventitia
sciencequiz.net
cytochemistry.net
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
Copyright © The McGraw-Hill Companies, Inc.
© The McGraw-Hill Companies, Inc./Rebecca Gray, photographer/Don Kincaid, dissections
Microscopic Anatomy
Copyright © The McGraw-Hill Companies, Inc.
• 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
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
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
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
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
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
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
Copyright © The McGraw-Hill Companies, Inc.
Copyright © The McGraw-Hill Companies, Inc.
Intestinal Motility
• Segmentation
• Peristalsis
– Migrating motor
complex
Copyright © The McGraw-Hill Companies, Inc.
Large Intestine
• Begins with cecum, 1.5m
long
• Ascending, transverse,
descending, sigmoid
• Rectum
Copyright © The McGraw-Hill Companies, Inc.
• Anus
Copyright © The McGraw-Hill Companies, Inc.
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
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
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
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
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
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
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