Saladin Ch. 25
The Digestive System
Digestion
Digestion
breakdown of larger food molecules
into particles small enough for
absorption
Digestive System
organs that collectively perform
digestion
Digestive System: Overview
Figure 23.1
Digestive Process
• The GI tract is a “disassembly” line
– Nutrients become more available to the
body in each step
• There are six essential activities:
– Ingestion, propulsion, and mechanical
digestion
– Chemical digestion, absorption, and
defecation
Digestive Process
Figure 23.2
Gastrointestinal Tract Activities
• Ingestion – eating
• Propulsion – swallowing & peristalsis
– Peristalsis – waves of contraction &
relaxation of muscles in organ walls
• Mechanical digestion – chewing,
mixing, & churning food
Peristalsis and Segmentation
Figure 23.3
Gastrointestinal Tract Activities
• Chemical Digestion – Secretion of water,
acid, buffers, enzymes. Large molecules
are split into smaller pieces.
• Absorption – material enters epithelial
cells lining GI tract and is passed on to
the lymph or blood.
• Defecation – elimination of indigestible
solid wastes.
Digestive & Accessory Organs
Digestive
• Gastrointestinal (GI) Tract = Alimentary
Canal - continuous tube running from
mouth to anus. Digests and absorbs.
Mouth, pharynx, esophagus, stomach,
small intestine, large intestine
Accessory Digestive Organs
• Aid in processing - food doesn't go into
these organs. Teeth, tongue, salivary
glands, liver, gallbladder, pancreas.
Histology of the Alimentary Canal
• All the walls of the GI tract have the
same four tunics:
– From the lumen outward they are the
mucosa, submucosa, muscularis
externa, & serosa.
Histology of the Alimentary Canal
Figure 23.6
Mucosa
• Moist epithelial layer that lines the
lumen of the alimentary canal.
• Three layers: a lining epithelium,
lamina propria, & muscularis
mucosae.
Mucosa: Epithelial Lining
• Mouth, pharynx, esophagus, anal canal
=non-keratinized stratified squamous.
• Stomach, intestines= simple columnar
epithelium.
• Exocrine glands: Put mucus & fluid in
• Enteroendocrine glands - secrete
hormones into blood stream.
Mucosa: Lamina Propria and Muscularis
Mucosae
• Lamina Propria: layer of areolar tissue
deep to lining - Connects to
muscularis mucosae.
• Muscularis mucosae - thin smooth muscle
fibers - helps increase surface area by
moving folds of mucosa so they are fully
exposed to nutrients.
Submucosa
• Submucosa - binds mucosa to
muscularis externa;
• Areolar CT
• Contains submucosal plexus - part of the
enteric nervous system
• Also contains - glands, blood, lymph
Muscularis Externa & Serosa
• Muscularis Externa – Smooth muscle
layer.
– Contains the myenteric plexus - the other
enteric nervous system component.
• Serosa - superficial layer of GI organs in
the abdominal cavity CT and epithelium;
– Visceral layer of peritoneum - serous
membrane.
Enteric Nervous System
• Composed of two major intrinsic
nerve plexuses:
– Submucosal nerve plexus – regulates
glands and smooth muscle in the
mucosa.
– Myenteric nerve plexus – Major nerve
supply that controls GI tract mobility.
Enteric Nervous System
• Segmentation & peristalsis are
largely automatic involving local
reflex arcs.
• Linked to the CNS via long
autonomic reflex arc.
Peritoneum and Peritoneal Cavity
• Peritoneum – serous membrane of
the abdominal cavity.
– Parietal – lines the body wall.
– Visceral – covers external surface of
most digestive organs.
Peritoneum and Peritoneal Cavity
• Peritoneal cavity:
– Narrow space between parietal and
visceral layers.
– Lubricates digestive organs.
– Allows them to slide across one
another.
Peritoneum and Peritoneal Cavity
Figure 23.5a
Peritoneum and Peritoneal Cavity
• Lesser Omentum - from stomach and
duodenum - attaches these to the liver.
• Greater Omentum - fatty apron hanging
down over colon and small intestine on
the anterior aspect.
• Mesocolon - fold that binds the large
intestine to the posterior abdominal wall
Peritoneum and Peritoneal Cavity
• Retroperitoneal organs – organs
outside the peritoneum.
– Only have peritoneum on anterior
surface.
• Peritoneal organs (intraperitoneal)
– organs surrounded by
peritoneum.
Regulation of GI Tract
Regulation of digestion involves:
– Mechanical & chemical stimuli –
stretch receptors, osmolarity, &
presence of substrate in the lumen
– Extrinsic control
– Intrinsic control
Nervous Control of the GI Tract
• Intrinsic controls:
– Nerve plexuses near the GI tract
initiate short reflexes.
– Short reflexes are mediated by local
enteric plexuses (gut brain).
Nervous Control of the GI Tract
• Extrinsic controls:
– Long reflexes arising within or
outside the GI tract.
– Involve CNS centers and extrinsic
autonomic nerves.
Regulation of GI Tract
• Hormones:
– Gastrin, secretin, CCK, histamine,
paracrines [see hormone chart]
Mouth
• Oral or buccal cavity:
– Is bounded by lips, cheeks, palate, &
tongue.
– Vestibule – bounded by the lips &
cheeks externally, & teeth and gums
internally.
Anatomy of the Oral Cavity: Mouth
Figure 23.7a
Tongue
• Skeletal muscle covered with mucous
membrane.
– Intrinsic muscles – originate and insert in
tongue - For swallowing and speech
– Extrinsic muscles – originate outside the
tongue & insert in it. Move side to side &in &
out. Aid in movement of food into bolus.
Palate
• Palate:
– Hard [palatine bones + palatine
processes of maxilla;
– Soft – skeletal muscle
Oral Cavity and Pharynx: Anterior View
Figure 23.7b
Teeth
• Primary and permanent dentitions
have formed by age 21.
• Primary – 20 deciduous “milk”
teeth.
• Permanent – 32 secondary teeth,
Classification of Teeth
• Teeth are classified according to
their shape and function.
– Incisors – chisel-shaped - adapted for
cutting or nipping.
– Canines – conical or fanglike - tear or
pierce
– Premolars (bicuspids) & molars –
broad crowns with rounded tips - for
grinding or crushing
Deciduous Teeth
Figure 23.10.1
Permanent Teeth
Figure 23.10.2
Tooth Structure
• Two main regions – crown & root.
– Crown – exposed part above the
gingiva (gum).
– Root – portion embedded in the
jawbone.
Tooth Structure
• Enamel – acellular, brittle material
composed mostly of calcium
phosphate.
– Enamel is the hardest substance
produced by the body.
– Encapsulates the crown of the tooth.
Tooth Structure
Anatomy of a tooth:
• Crown, neck, root, apical foramen.
• Enamel – hardest substance produced,
covers crown; Mostly calcium phosphate
• Dentin – bone-like, secreted by cells of
pulp
Tooth Structure
• Cementum – covering of root, similar to
bone
• Periodontal ligament – anchors tooth to
alveolar socket, which is surrounded by
gingiva (gums)
Tooth Structure
Figure 23.11
Salivary Glands
• Parotid – anterior to ear over the
masseter.
– Parotid duct – opens into vestibule next
to the second upper molar.
– Secretes salivary amylase.
– Swollen when infected by mumps virus
Salivary Glands
• Submandibular – beneath posterior
tongue.
– Secretes saliva with salivary amylase.
• Sublingual – anterior to
submandibular gland under the
tongue.
– Opens into the floor of the mouth.
Salivary Glands
Figure 23.9a
Saliva
Saliva
• 1-1.5 L/day
• Blood filtrate + salivary amylase, mucin
& lysozyme
Saliva
• Control – ANS – food stimulates tactile,
pressure and taste sensor  salivatory
nuclei in medulla
– Parasympathetic – normal salivation – facial
and glossopharyngeal nerves
– Sympathetic – decreased salivation
Pharynx
• From the mouth, the oro- &
laryngopharynx allow passage of:
– Food and fluids to the esophagus.
– Air to the trachea.
• See Ch. 22
Esophagus
• Muscular [skeletal to smooth] – a
collapsible tube. All 4 layers, modified.
• Mucosa – stratified squamous
epithelium; esophageal glands in
submucosa; musc. Ext. has both
smooth and skeletal muscle; serosa
replaced with adventitia
Esophagus
• Passes through mediastinum and
diaphragm [esophageal hiatus]
“hiatal hernias”
• Functions: secretes mucus, transports
bolus to stomach [cardiac orifice]
Swallowing
• Deglutition – swallowing controlled by
medulla & pons swallow center – involves
trigeminal, facial, glossopharyngeal, &
hypoglossal nerves.
• 2 phases –
– Buccal phase – bolus formation,
pharyngealesophageal phase –moves to
stomach
– Peristalsis – waves of muscular contraction –
move bolus
Deglutition (Swallowing)
Bolus of
food
Tongue
Uvula
Pharynx
Bolus
Epiglottis
Epiglottis
Glottis
Esophagus
Trachea
(a) Upper esophageal
sphincter contracted
Bolus
(c) Upper esophageal
sphincter contracted
(b) Upper esophageal
sphincter relaxed
Relaxed
muscles
Relaxed muscles
Circular muscles
contract, constricting
passageway and pushing
bolus down
Bolus of food
Gastroesophageal
sphincter open
Longitudinal muscles
contract, shortening
passageway ahead of bolus
Gastroesophageal
sphincter closed
Stomach
(d)
(e)
Figure 23.13
Stomach
• Functions
– Storage
– Mechanical breakdown of food
– Enzymatic digestion of food
– Absorption of simple compounds
– Produces gastrin
– Bacterial protection
– Produces intrinsic factor
Stomach
• Cardiac region – surrounds the cardiac
orifice.
• Fundus – dome-shaped region beneath
the diaphragm.
• Body – midportion of the stomach.
• Pyloric region – made up of the antrum
and canal, through the pyloric sphincter.
Stomach
Figure 23.14a
Stomach
Innervation & circulation
• Para = vagus, sym = celiac ganglion
• Arterial – celiac artery, venous – hepatic
portal system
Stomach Wall
• Muscularis – 3 layers instead of two –
longitudinal, circular and oblique smooth
layers
• Submucosa & mucosa - Ruggae – large folds
in mucosa
• Mucosa – simple columnar epithelium
Glands of the Stomach Fundus and Body
• Gastric Pits of the fundus & body have a
variety of secretory cells:
– Mucous cells –> mucus
– Parietal cells –> HCl & intrinsic factor
– Chief cells –> pepsinogen
• Pepsinogen is activated to pepsin by:
Glands of the Stomach Fundus and Body
– Enteroendocrine cells –> gastrin,
histamine, serotonin, & somatostatin into
the lamina propria
Microscopic Anatomy of the Stomach
Figure 23.15
Gastric Secretions
• Produces 2-3L of gastric juice/day
• HCl – get H from CO2 – carbonic
anhydrase reaction [parietal cells]
pumped out by active transport.
• Pepsin – digests protein; also get
chymosin & gastric lipase in infants
Gastric Secretions
• Intrinsic factor - also from parietal
cells – needed to absorb B12 [only
indispensable function of stomach]
• Chemical messengers enteroendocrine
cells make up to 20 – include gut-brain
peptides like substance P, vasoactive
intestinal peptide, secretin, etc.
•
Stomach Lining
• To keep from digesting itself, the
stomach has a mucosal barrier with:
– A thick coat of bicarbonate-rich mucus
on the stomach wall.
– Epithelial cells that are joined by tight
junctions.
– Gastric juices released only in
presence of food which dilutes its
effects
Gastric Motility
• Receptive-relaxation response – stomach
relaxes to receive food [stim. by medulla]
• Peristaltic contractions – increasing in
strength.
• Squirts 3 mL of chime into duodenum/ wave
Vomiting
• Emetic center of medulla
• Usually preceded by nausea & retching
• Projectile vomiting – no prior nausea, etc.
– common in infants.
• Bulimia – eating disorder – erodes tooth
enamel, aspiration of acid injures
respiratory tree, acid reflux damages
esophagus
Regulation of Gastric Secretion
• Neural & hormonal mechanisms regulate
release of gastric juice.
• 3 phases of events:
– Cephalic (reflex) phase: prior to food entry.
– Gastric phase: once food enters the stomach.
– Intestinal phase: as partially digested food
enters the duodenum.
Cephalic Phase
• Events include:
– Sight or thought of food; Stimulation
of taste or smell receptors.
– Stimulate secretions from parietal
cells, Chief cells, and mucus cells.
Gastric Phase
• Events include:
– Release of gastrin to the blood.
– Stomach distension & increase in pH
due to food.
Intestinal Phase
• Mostly inhibitory – small intestine
receptors are activated by influx of
chyme – enterogastric reflex.
• Slows exit of chyme and prevents
duodenal overload.
Intestinal Phase
• Chyme also stimulates enteroendocrine
cells to release secretin, CCK and gastric
inhibitory protein.
• Secretin & CCK stim. pancreas and
gallbladder. All 3 suppress gastric
secretion & motility
•
Release of Gastric Juice
Figure 23.16
Liver
• The largest gland in the body
• Functions:
– Synthesis of bile salts
– Excretion of bile
Liver
• 2 Lobes [R,L] separated by falciform
running from diaphragm between lobes.
– The right lobe is further subdivided into two
lobes - the quadrate and caudate.
• Falciform Ligament - binds liver to
anterior wall.
• The round ligament exits the falciform
ligament from the liver - former
umbilical vein.
Liver: Microscopic Anatomy
• Lobes are divided into working units
called lobules
– Plates of hepatocytes [specialized
epithelium] that secrete bile
– Central vein
Liver: Microscopic Anatomy
• Liver sinusoids – enlarged, leaky
capillaries located between hepatic plates.
– Blood from stomach & SI is filtered and
glucose, amino acids, iron, vitamins, etc are
removed, as are toxins, drugs, etc.
• Kupffer cells – hepatic macrophages
found in liver sinusoids.
Microscopic Anatomy of the Liver
Figure 23.24c, d
Liver: Bile
• Produced by hepatocytes.
• Bile leaves the liver via:
– Bile canalculi to bile ducts
– Bile ducts fuse into the common hepatic
duct
Liver: Bile
– The common hepatic duct fuses with the
cystic duct the bile duct
– Bile duct [joins pancreatic duct 
hepatopancreatic ampulla  SI
Gallbladder
• Thin-walled, green muscular sac on the
ventral surface of the liver.
• Releases bile via the cystic duct, which
flows into the bile duct.
• Stores and concentrates bile by absorbing
its water and ions.
Composition of Bile
• Bile - Yellow, brownish to olive green 500-1000mL/day
• Water, bile acids, bile salts, cholesterol,
ions, phospholipids [lecithin] bile
pigments [bilirubin etc.]
• Bile Salts [acids] = steroids made from
cholesterol–emulsify fats
Composition of Bile
• Recycled – 80% reabsorbed by SI and
re-secreted by liver
• Wastes – include from RBC breakdown
 bilirubin from heme -- intestine 
broken into stercobilin,
Gallbladder and Associated Ducts
Figure 23.20
Pancreas
• Size – 5 inches long and 1 inch thick
• Regions – head, body, tail
• Ducts –
– 1 – pancreatic duct fuses with common bile
duct forming the hepatopancreatic ampulla
- enters duodenum about 4 inches below
pylorus
– 2 – accessory duct – enters 1 inch above
ampulla
Pancreas
• Exocrine function - Secretes
pancreatic juice which breaks down
all categories of foodstuff.
• Endocrine function – release of
insulin & glucagon.
Acinus of the Pancreas
Figure 23.26a
Pancreatic Juice
• Pancreatic Juice – clear, colorless, 1.21.5L/day, pH about 8
• Mostly water
• Sodium bicarbonate – increase pH, buffers
acidic chyme, stops pepsin action
• Salts
Composition & Function of Pancreatic Juice
• Enzymes secreted:
– Trypsinogen is activated to trypsin
– Carboxypeptidase
– Chymotrypsinogen
– Amylase, lipases, & nucleases
– These enzymes require ions or bile for optimal
activity
Regulation of Secretion
• Increased with parasympathetic, decreased
with sympathetic.
• Bile & pancreatic juice stim. By CCK, gastrin
& secretin
• CCK released from duodenum response to
acid & fat  contraction of gallbladder,
secretion of pancreatic enzymes, relaxation
of hepatopancreatic sphincter
Regulation of Secretion
• Secretin – stimulates secretion of
bicarbonate to neutralize stomach acid in
duodenum.
Small Intestine: Gross Anatomy
• Runs from pyloric sphincter to the
ileocecal valve.
• 3 subdivisions: duodenum, jejunum, &
ileum.
• Ileocecal valve = sphincter to the large
intestines.
• Mesentery – peritoneum supporting the
small intestines.
Small Intestine: Microscopic Anatomy
• Structural modifications of the small
intestine wall increase surface area.
– Circular folds of the mucosa
• Villus – structure with a lamina
propria core containing arterioles,
venules, capillary nets and lymphatic
lacteals, covered with simple columnar
epithelium with microvilli , goblet cells
and absorptive cells
Small Intestine: Microscopic Anatomy
– Lacteals-absorption of fats
– Goblet cells - secrete mucus
– Absorptive cells – simple columnar
epithelium - absorb and make brush
border enzymes
– Intestinal Crypts - tubular glands in
floor of SI between villi bases –
secrete intestinal juice [enzymes]
Small Intestine: Microscopic Anatomy
– Paneth Cells - phagocytic - produce
lysozyme
– Enteroendocrine glands - secrete
secretin and CCK
– Duodenal [Brunner's] in submucosa
-secrete alkaline mucus –
neutralizes stomach acid
Small Intestine: Microscopic Anatomy
Figure 23.21
Intestinal Juice
• Clear yellow - 1-2 L/day, pH 7.6
• Contains water, mucus, enzymes - aid
absorption of nutrients from chyme
• Resorbed by villi
Motility in the Small Intestine
• Segmentation– Localized mixing motions bring
contents in contact with mucosa.
– Regulated by pacemaker cells – 12
contractions/ min – duod.; 8-9 ileum.
• After nutrients have been absorbed:
– Peristalsis begins.
– Meal remnants, bacteria, mucosal cells,
& debris are moved into large intestine.
Control of Motility and Secretion
• Local enteric neurons of the GI tract
coordinate intestinal motility:
– Distension
– Presence of Chyme
– Vasoactive intestinal polypeptide [VIP]
increases secretions
– Gastrin
Control of Motility and Secretion
• ANS:
– Parasympathetic – increases motility.
– Sympathetic-decreases motility.
Chemical Digestion: Carbohydrates
• Enzymes: salivary amylase, pancreatic
amylase, & brush border enzymes
• Absorption - glucose and galactose enter
epithelial cells via symporters,
accompanied by Na+ ions. Fructose
enters by facilitated diffusion.
• All exit to the blood by facilitated
diffusion.
• Lactose intolerance – lack enzyme lactase
[large proportion of world population]
Chemical Digestion: Proteins
• Enzymes used: pepsin in the stomach.
• Enzymes acting in the small intestine:
– Pancreatic enzymes – trypsin,
chymotrypsin
– Brush border enzymes – peptidases.
Chemical Digestion: Proteins
Absorption
• Amino acids, di and tripeptides - aa by simple active transport or in
symporters with Na+
• Di and tripeptides - by symporters with
H+. All exit to blood by diffusion.
Chemical Digestion: Proteins
Figure 23.34
Chemical Digestion: Fats
• Lipids - digested by lipases
• Fats emulsified by bile salts first
– Products are fatty acids[FFA’s]and
monoglycerides
– Micelles in bile take up phospholipids &
cholesterol, fat soluble vitamins, FFA’s and
monoglycerides and transport to intestinal cells
Chemical Digestion: Fats
Figure 23.35
Chemical Digestion: Fats
– FFA’s and monoglycerides enter epithelial cells
and are re-assembled into triglycerides,
combine with cholesterol, phospholipids and
protein to form chylomicrons and exit by
exocytosis into lacteals
Fatty Acid Absorption
Figure 23.36
Chemical Digestion: Nucleic Acids
• Enzymes used: pancreatic nucleases in
the small intestines.
• Brush Border enzymes: nucleosidases,
phosphatases.
• Sugars, bases and phosphate ions
absorbed by active transport  across
epithelium and into blood
Vitamins & Minerals
• Vitamins - water soluble diffuse, lipid
soluble enter with lipids [A,D,E,K]
• Minerals [Electrolytes] - absorbed
through entire length of SI by diffusion
or active transport - Na+/K+ and
chloride/bicarbonate pumps.
• Fe & Ca absorbed as needed, rest
absorbed at constant rate regardless of
need.
Water
• Water - absorbed by osmosis - 9L into
system - about 8L absorbed - into blood
• Diarrhea occurs when the large intestine
absorbs too little
• Constipation occurs when passage
through the LI is too slow and too much
water is absorbed
Large Intestine
• Functions:
– Absorption, manufacture of some vitamins.
– Formation & expulsion of feces.
LI Anatomy
• 1.5 M long, runs from cecum to rectum
• Ileocecal sphincter - joins SI to LI.
• Cecum - region of LI inferior to the ileocecal
sphincter.
• Appendix - next to the cecum.
LI Anatomy
• Colon - above sphincter - ascending,
transverse, descending and Sigmoid
regions
• Rectum & valves- region inferior to
sigmoid colon
• Anal Canal - below rectum
LI Anatomy
• Taenia coli - longitudinal bands of
muscles running the length of the LI
• Haustra - pouches produced in the LI by
tonal contraction of taenia coli.
Large Intestine
Figure 23.29a
Large Intestine: Microscopic Anatomy
• Mucosa – Simple columnar epithelium –
no villi or enzyme secreting cells goblet
cells
Bacterial Flora
• These bacteria:
– Colonize the colon
– Ferment indigestible carbohydrates
– Release irritating acids and gases (flatus)
– Synthesize B complex vitamins and vitamin K
Motility of the Large Intestine
• Haustral contractions:
– Slow segmenting movements that move the
contents of the colon.
– Haustra sequentially contract as they are
stimulated by distension.
• Presence of food in the stomach:
– Activates the gastrocolic reflex.
– Initiates peristalsis that forces contents
toward the rectum.
Defecation
Defecation Reflexes – initiated by rectal
stretching
• Intrinsic – entirely within enteric system
– weak peristalsis
• Sympathetic = Spinal cord reflex - Sends
message to CNS – to decide whether to
relax external sphincter.
Defecation
Figure 23.32