The Digestive System
Functions of the Digestive System
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Ingest food
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Break down food into nutrient molecules
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Absorb molecules into the bloodstream
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Rid the body of indigestible remains
Main Divisions of the Digestive
System
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Alimentary Canal
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Continuous, muscular digestive tube winding
throughout the body
Digests and absorbs food particles
Contains the following organs:
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Mouth, Pharynx, Esophagus, Stomach, Small and Large
Intestines
Accessory Digestive Organs
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Contains the following organs:
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Teeth, Tongue, Gallbladder, Salivary Glands, Liver, and
Pancreas
Digestive System Divisions
Digestive Processes
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Ingestion
Propulsion
Mechanical digestion
Chemical digestion
Absorption
Defecation
Circulation of the Alimentary
Canal/GI Tract
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Splanchnic Circulation
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Includes all arteries branching off the
abdominal aorta and the hepatic portal
circulation
Arterial supply:
Celiac Trunk (hepatic, splenic, and left gastric)
 Mesenteric Arteries: sup. and inf.
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Receives ¼ of the blood volume (CO);
increases after a meal
Histology of the Alimentary
Canal/GI Tract
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From esophagus to anus, the walls of the
alimentary canal have the same 4 layers:
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Mucosa (secretes hormones and mucus, absorbs end
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Submucosa (contains lymphoid follicles and elastic
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Muscularis externa (segmentation and peristalsis,
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products of digestion, and protects against infection
tissue)
contains inner circular layer and outer longitudinal
layer, area where valves are found)
Serosa (same as visceral peritoneum, made of areolar
connective tissue)
Histology of the GI Tract
Peritoneum
Peritoneum is the serous membrane lining the
abdominopelvic cavity
Visceral peritoneum covers the external surfaces of most
digestive organs and is continuous with the parietal
peritoneum that lines the body wall
Between the two peritoneums is the peritoneal cavity
Mesentery is a double layer peritoneum; provides routes
for BV, lymphatics, nerves
Alimentary canal organs are classified as
Retroperitoneal - no mesentery and organs lies
posterior to the peritoneum (SADPUCKER)
Intraperitoneal - mesentery and organs lies within the
peritoneal cavity
Features and Functions of the
Mouth
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Buccal/oral cavity
Contains stratified squamous epithelium
Vestibule: area bounded by lips and
cheeks externally and teeth and gums
internally
Lips: posses no sweat or oil glands
Palate: forms roof of the mouth, soft and
hard palate, uvula
Anatomy of the Mouth
Features and Functions of the
Tongue
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Helps grind food into a bolus which
contains partially digested food and saliva
Helps form words and is a sensory organ
for taste
Three surface features:
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Filiform papillae (roughness and grip)
Fungiform papillae (contains taste buds)
Circumvallate papillae (contains taste buds)
Anatomy of the Tongue
Features and Functions of the
Salivary Glands
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Main functions:
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Produces and secretes saliva
Cleanses the mouth
Dissolves food chemicals so they can be
tasted
Moistens food, compacting it into a bolus
Begins the chemical breakdown of food

Salivary amylase: starch
Types of Salivary Glands
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Submandibular Glands
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Sublingual Glands
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Found underneath the tongue
Parotid Glands
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Found underneath the mandible
Found anterior to the ear between masseter and skin
Saliva travels to oral orifice via ducts from all
three glands
Anatomy of the Salivary Glands
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Composition of Saliva: 97-99.5% water
pH 6.75-7.0
Sodium, potassium, chloride, phosphate,
and bicarbonate
Mucin
Salivary amylase
Features and Functions of the
Teeth
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Break food into smaller parts, increasing
surface area for digestion
Types of Teeth
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Deciduous Teeth (“baby” teeth)
Permanent Teeth
Incisors- cutting and shredding
 Canines- piercing and tearing
 Molars- grinding
 Premolars- grinding and crushing
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Tooth Structure
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Crown: exposed
portion of tooth
covered by enamel
which covers dentin
Root: internal portion
that is beneath the
gums (gingiva) and is
anchored by
periodontal ligaments
Deglutition and the Pharynx
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Deglutition = swallowing
Oropharynx and Laryngopharynx are common
passageways for food and air
Pharynx contains stratified squamous epithelium
(friction-resistant)
Features and Functions of the
Esophagus
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Muscular tube that propels food to stomach;
bolus enters stomach through esophageal hiatus
Skeletal muscle (upper third for swallowing) and
smooth muscle (lower third) for peristalsis
Esophageal glands – produce mucus to lubricate
bolus
Esophageal sphincter – prevents backflow into
oral cavity
Cardiac sphincter- prevents backflow into
esophagus
Anatomy of the Esophagus
Microscopic Anatomy of the
Esophagus
Digestive Processes in the Mouth,
Pharynx, and Esophagus
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Mouth processes:
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Ingestion
Mechanical digestion (e.g. salivary amylase)
Initiation of Propulsion
Mastication: chewing
Pharyngeal processes:
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Deglutition = swallowing
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Voluntary Buccal phase
Involuntary Pharyngeal-Esophageal Phase
Esophageal processes:
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Peristalsis (rhythmic contractions, involuntary)
Deglutition and the Pharynx
Peristalsis
Features and Functions of the
Stomach
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Temporary storage area for food and allows it to
mix with gastric juice to produce chyme
Regions: cardiac, fundus, body, and pyloric
Greater and Lesser Curvatures: connected
to greater and lesser omentums
Rugae folds: longitudinal folds in stomach wall
- mucous b/w folds
Muscle layers arranged circularly, longitudinally,
AND obliquely (aids in digestion)
Anatomy of the Stomach
Microscopic Anatomy of the
Stomach
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Simple columnar epithelium – contains gastric
pits that secrete gastric juices
Goblet cells – secrete mucus that coats stomach
and prevents it from being digested itself
Parietal cells – secrete hydrochloric acid
(converts pepsinogen into pepsin) and intrinsic
factor (necessary for absorption of vitamin B12)
Chief cells – secrete pepsinogen which is
converted to pepsin to aid in protein digestion
Enteroendocrine cells – release hormones such
as:
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Histamine, Serotonin, Gastrin, Endorphins, and
Somatostatin
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Histamine
- activates parietal cells to release HCl
Serotonin
- contraction of stomach muscle
Gastrin
- gastric glands to increase secretion
Endorphins
- natural opiates
Somatostatin
- sympathetic n.s.
- inhibits gastric secretion
- inhibits gastric emptying
Microscopic Anatomy of the
Digestive System
Digestive Processes in the Stomach
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Gastric Secretion
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Cephalic Phase
 Stimulated by the thought, sight, taste, or aroma
or food
 Inputs from olfactory receptors and taste buds
travel to parasympathetic enteric ganglia which
then stimulate stomach glands
Gastric Phase
 Stomach distension activates stretch receptors
 Food chemicals (e.g. peptides, rising pH) activate
chemoreceptors which activate G cells which
secrete gastrin
Intestinal Phase
 Presence of low pH and partially digested foods in
duodenum stimulates intestinal gastrin secretion
Digestive Processes in the Stomach
Gastric Motility and Emptying
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Peristaltic waves approach stomach and become
stronger near pyloric region
Pyloric sphincter allows ~ 3 mL of chyme to pass to
duodenum and the rest to return to stomach for further
mixing
Features and Functions of the
Small Intestine
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Receives chyme from stomach; performs
majority of digestion and absorption of nutrients
Regions:
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Duodenum (upper region receiving chyme from
stomach and digestive enzymes from pancreas and
bile from liver and gallbladder)
Jejunum/Ileum (lower regions where absorption
occurs)
Plicae circulares (permanent folds in mucosa and
submucosa that slow movement of chyme)
Anatomy of the Small Intestine
Microscopic Anatomy of Small
Intestine
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Villi: fingerlike projections that increase the
surface area of the SI
Microvilli: tiny projections on the plasma
membranes of columnar cells that appear fuzzy
(i.e. brush border cells)
Crypts of Lieberkuhn: secrete intestinal juice
and special lysozymes that protect against
bacteria
Peyer’s Patches: aggregated lymphoid tissues
containing lymphocytes
Microscopic Anatomy of the Small
Intestine
Secretions of the Small Intestine
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Secretin: released by enteroendocrine cells when acidic
chyme enters SI; causes release of bicarbonate-rich
pancreatic juices
Somatostatin: slows gastric motility and emptying and
inhibits production of gastric secretions
Cholecystokinin (CCK): released when fatty, proteinrich chyme enters SI; causes release of enzyme-rich
pancreatic juices and bile
Brush border enzymes: process long peptides, nucleic
acids, and sugars into smaller ones
After Digestion: Absorption
Protein
Fats
Functions of the Liver
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Largest internal organ
Functions:
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Filters and processes nutrient-rich blood of
carbohydrates, proteins, and lipids from
intestine
Production and regulation of cholesterol
Production of bile which emulsifies fats
Removes drugs and hormones from
circulation
Storage of vitamins and minerals
Anatomy of the Liver
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Right and Left Lobes: separated by falciform
ligament
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Caudate and Quadrate Lobes: found on
posterior side
Blood vessels:
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Hepatic artery/vein and hepatic portal vein
Gallbladder: found underneath left lobe, stores
bile
Gross Anatomy of the Liver
Microscopic Anatomy of the Liver
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Liver Lobules: structural unit of liver
Hepatocytes: liver cells contained within the lobules
Hepatic portal vein & Hepatic Artery: the circulation
of the liver, they bring blood into the liver where it is
filtered through the liver sinusoidal capillaries
Kupffer cells: remove debris
Filtered blood drains into the central vein, then to the
hepatic vein, and eventually to the inferior vena cava
Bile (produced by hepatocytes) drains into the bile
duct after passing through portal triad
Bile then shipped to gallbladder for storage
Microscopic Anatomy of the Liver
Microscopic Anatomy of the Liver
Blood supply
- receives fresh O2 blood from hepatic
artery (off of aorta)
- receives deoxygenated blood with
nutrients (from small intestine)
- hepatic portal vein
- From liver- hepatic vein- inferior vena
cava
Hepatic portal vein, hepatic artery and
bile ducts make up the portal triad.
Function and Regulation of Bile
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Bile ducts are present at every portal triad
Bile flows down bile canaliculi (tiny canals)
between adjacent hepatocytes towards bile duct
branches at every portal triad
Bile enters the bile ducts which drain into the
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common hepatic duct
Bile emulsifies fats, separating them into smaller
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Bilirubin: the chief bile pigment, a waste product
parts
of the heme of hemoglobin formed during the
breakdown of worn-out erythrocytes
Regulation of Bile Production
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Bile exits cystic duct upon
stimulation
CCK released when
acidic, fatty chyme enters
intestines
Causes:
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Gallbladder Contraction
Pancreatic Juice Secretion
Relaxation of
hepatopancreatic sphincter
Features and Functions of the
Pancreas
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Pancreatic Juice secreted by acinar cells
Islets of Langerhans release insulin and
glucagon (important in glucose metabolism)
Pancreatic Juice contains:
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Sodium Bicarbonate (buffers HCl in stomach)
Proteases (break down polypeptides)
Pancreatic amylase (digests oligosaccarides and
disaccharides into monosaccharides)
Pancreatic lipases (break down lipids into fatty acids
and glycerol)
Pancreatic nucleases (break down nucleic acids)
Anatomy of the Pancreas
Regulation of Pancreatic Secretion
Features and Functions of the
Large Intestine
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Functions:
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Reabsorption of remaining water and
electrolytes
Production and absorption of Vitamins B and
K
Elimination of feces
Diameter is only 7 cm but is larger than
that of the small intestine
Gross Anatomy of the Large
Intestine
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Teniae Coli: bands of smooth muscle that create
pocket-like sacs (haustra)
Cecum: sac-like connection between the small and large
intestines
Appendix: small structure containing lymphoid tissue;
small immune function
Ascending, Descending, Transverse, and Sigmoid
Colon
Splenic and hepatic flexure
Rectum: storage area
Anus: regulates defecation with two sphincter muscles;
internal and external
Anatomy of the Colon
Microscopic Anatomy of the Large
Intestine
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Simple columnar epithelium for absorption
except in the anal canal where there is stratified
squamous
No villi, no digestive-secreting cells
Goblet cells produce mucus for lubrication of
feces
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Bacterial flora synthesize vitamin B and most of
the vitamin K needed for blood clotting
Microscopic Anatomy of the Large
Intestine
Summary of Digestion
Summary of Digestion
The Process of Absorption
Clinical corner
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Peptic ulcers - gastric and duodenal,
caused by Helicobacter pylori, NSAIDS, Hcl
hypersecretion
Cirrohsis - scarred liver due to chronic
inflammation
Hepatitis - A,B,C,D, and E
Biliary calculi - gall stones - crystals of
cholesterol in bile
Borborygmus - rumbling noise caused by
gas through intestines
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Cholecystitis - inflammation of gall bladder
Colitis - inflammation of colon
Dysphagia - difficulty in swallowing
Enteritis - inflammation of the intestines
Flatuation/erucation