Transcript Major Concepts of Anatomy and Physiology

The Digestive
System
Part 4: Regulation &
Maintenance
The Digestive System

Digestive System: The system whose
function it is to break down foods into
molecules small enough to enter body
cells.
 Allows
the body to ingest & digest proteins,
fats & carbohydrates & absorb them into the
bloodstream & lymph to be taken to body cells
for metabolism & conversion to ATP.
The Digestive System

Gastrointestinal (GI) Tract aka Alimentary
Canal: A continuous tube that extends from the
mouth to the anus.
 Tonus:
The sustained muscular contraction of the GI
tract walls that helps to move food along.

Food travels through…
 The
oral cavity (mouth)
 Part of the pharynx
 The esophagus
 The stomach
 The small intestine
 The large intestine
The Digestive System

Accessory Organs: Organs not a part of the GI tract
that aid in the digestive process.

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Produce or store secretions that flow into the GI tract through
ducts to aid in the chemical breakdown of food.
Aside from the teeth & tongue, they do not come in direct contact
with food.
Accessory organs are…
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The teeth
The tongue
The salivary glands
The pancreas
The liver
The gallbladder
GERD

Gastroesophageal Reflux Disease (GERD):
Occurs when the lower esophageal sphincter
fails to close adequately after food has entered
the stomach.
 Allows
the stomach contents to back up into the
inferior portion of the esophagus.
 Hydrochloric acid from the stomach irritates the
esophageal wall & causes “heartburn”.
 Can be controlled with medication & avoiding certain
foods that can strongly stimulate stomach acid
production.
6 Digestive System Functions
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Ingestion (Eating): Taking food or liquids into the body.
Secretion: Cells within the walls of the GI tract & accessory organs secrete
roughly 7L of water, acid, buffers, & enzymes into the lumen of the tract
daily.
Propulsion & Mixing (Motility): Moving food through the GI tract by
peristalsis.
Digestion: Occurs mechanically or chemically.

Mechanical digestion - chewing to break food into smaller pieces, and churning
by the stomach & small intestine to dissolve & mix the food with digestive
enzymes.
 Chemical digestion – hydrolysis splits the large carbohydrates, lipids, proteins,
and nucleic acid molecules into smaller molecules, while water, vitamins, ions,
minerals, amino acids, glucose, & cholesterol can be absorbed without chemical
digestion.
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Absorption: Epithelial lining of the lumen of the GI tract absorbs end
products of digestion & moves them into the blood or lymph – most
absorption occurs in the small intestine.
Defecation: The elimination of feces (composed of indigestible materials,
bacteria, sloughed off cells, & digested materials that were not absorbed)
via the anus.
GI Tract

Made up of 4 basic tissue layers:

Mucosa (Mucous Membrane): The innermost layer closest to
the lumen. Protects against pathogens, absorb nutrients, &
secretes mucous & hormones.
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Epithelium: Comes in direct contact with the GI tract contents –
made up of nonkeratinized stratified squamous epithelium that
serves a protective function, with simple columnar epithelium
functions in secretion & absorption & lines the stomach & intestines.
Lamina Propia: Layer of connective tissue underlying the mucosa
& containing the blood vessels, lymph vessels, & lymph nodes –
made of areolar connective tissue & supplies the GI tract with a
route to export nutrients.


Mucosa-Associated Lymphatic Tissue (MALT): Lymphatic nodules
containing immune system cells located along the entire GI tract –
provide defense against disease.
Muscularis Mucosae: Smooth muscle layer that creates folds in
the mucous membrane to increase surface area for digestion &
absorption – movement ensures that the absorptive cells are fully
exposed to the GI contents.
GI Tract
 Submucosa:
Consists of areolar connective
tissue that binds the mucosa to the
muscularis.
Contains many of the blood & lymphatic vessels
that receive absorbed food molecules – may also
contain glands & lymphatic tissue.
 Submucosal Plexus: Network of neurons that
form the enteric nervous system (ENS) located in
the submucosa – controls the motility & secretions
of the GI tract.

GI Tract
 Muscularis:
The muscular layer outside the
submucousa.
Inner layer is circular & outer layer is longitudinal.
 Segmentation: The mixing of food via muscular
contractions.
 Peristalsis: The propulsion of food through the GI
tract via muscular contractions.
 Thickens in some areas to form sphincters, which
act as valves to control food passage.


Contains the myenteric (Auerbach’s) plexus, which
controls GI motility.
GI Tract
 Serosa:
The outermost layer of the portions
suspended in the abdominopelvic cavity.
Inferior to the diaphragm.
 Known as the visceral peritoneum.
 Adventitia: The external covering of the
esophagus that binds the esophagus to
surrounding structures.

Neural Innervation of the GI Tract

Enteric Nervous System (ENS): Around 100
million neurons extending from the esophagus to
the anus. Arranged in 2 plexuses:
 Myenteric
Plexus aka Plexus of Auerbach: Found
between the longitudinal and circular smooth muscle
layers of the muscularis.

Mostly controls the GI tract motility, particularly the frequency
& strength of the muscle contractions of the musclaris.
 Submucosal
Plexus aka Plexus of Meissner:
Found within the submucosa.

Motor neurons in this plexus supply the secretory cells of the
mucosal epithelium, controlling the secretions of the GI
accessory organs.
Neural Innervation of the GI Tract
Motor Neurons: Supply the muscularis
layers via the myenteric plexus.
 Interneurons: Connect the myenteric &
submucosal plexus neurons.
 Sensory Neurons: Act as
chemoreceptors and/or stretch receptors.

 Chemoreceptors:
Activated by the presence
of certain chemicals in food.
 Stretch Receptors: Activated when food
distends the wall of a GI organ.
Neural Innervation of the GI Tract

Autonomic Nervous System: The portion of
the nervous system that controls the ENS.
 Vagus
Nerve (Cranial Nerve X): Supplies
parasympathetic fibers to most parts of the GI tract,
except the last half of the large intestine.
 Stimulation of the parasympathetic nerves causes an
increase in the GI secretion & motility by increasing
the activity of the ENS neurons.
 Stimulation of the sympathetic nerves causes a
decrease in GI secretion & motility by inhibiting the
ENS neurons.
 Strong emotions can slow down digestion due to
stimulation of the sympathetic nervous system.
Mouth (Oral or Buccal Cavity)

Mouth: The cheeks (forming the lateral
walls), soft and hard palates, & the tongue.
 Externally
covered by skin & internally
covered by a mucous membrane.
 Buccinator Muscles & connective tissue lie
between the skin & mucous membranes of
the cheeks.
Mouth (Oral or Buccal Cavity)

Lips aka Labia: Fleshy folds that surround the
opening of the mouth. Contain the..
 Obrbicularis
Oris: Muscles covered externally by
skin & internally by mucous membranes.
 Labia Frenulum: The medial fold of mucous
membrane between the inner surface of the lips & the
gums.
 “Red Area” gets its color from the tall dermal papillae
& the proximity of blood vessels to the surface.
 Cutaneous area appears as roughly the same color
as the skin tone.

Buccinator & orbicularis oris muscles
contract during chewing to keep food between
the teeth & aid in speech.
Mouth (Oral or Buccal Cavity)
Vestibule: The mouth area bounded
externally by the cheeks & lips and
internally by the gums & teeth.
 Oral Cavity Proper: The space extending
from the gums & teeth to the fauces.
 Fauces: The opening between the oral
cavity & the pharynx.
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Mouth (Oral or Buccal Cavity)

Palate: Separates the oral cavity from the
nasal cavity and makes it possible to
breath while chewing.
 Hard
Palate: The anterior portion of the roof
of the mouth – supported by the palatine
process of the maxilla & the palatine bones &
is covered by a mucous membrane.
 Soft Palate: The posterior portion of the roof
of the mouth – arch-shaped muscular portion
between the oropharynx & nasopharynx that
is lined with a mucous membrane.
Mouth (Oral or Buccal Cavity)

Uvula: The soft, fleshy mass that descends from
the soft palate at the back of the mouth.
 When
swallowing, the soft palate & uvula draw
superiorly to close off the nasopharynx and prevent
swallowed foods & liquids from entering the nasal
cavity.
 Palatoglossal Arch: Extends to the side of the base
of the tongue.
 Palatopharyngeal Arch: Extends to the side of the
pharynx.
 Palatine Tonsils: Located between the two arches.
 Lingual Tonsils: Located at the base of the tongue.
The Pharynx

Pharynx: The tube extending from the internal
nerves to the esophagus posteriorly & to the
larynx anteriorly.
 Composed
of skeletal muscles lined by mucous
membrane.
 Divided into 3 parts:
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Nasopharynx: Only functions in respiration.
Oropharynx: Has digestive & respiratory functions –
swallowed food passes from the mouth into the oropharynx.
Laryngopharynx: Has digestive & respiratory functions –
food passes from the oropharynx to the laryngopharynx.
 Muscular
contractions in the oropharynx &
laryngopharynx help propel food into the esophagus.
Saliva

Salivary Gland: Any cell or organ that
secretes saliva into the mouth.
 The
mucous membrane of the mouth &
tongue contain many small salivary glands
that open to the mouth via short ducts.
 Labial, buccal & palatal glands are located in
the lips, cheeks, & palate respoectively.
 Lingual glands are located in the tongue.
Saliva

Major Salivary Glands: Located beyond the oral
mucosa & empty their secretions into ducts that lead into
the mouth – 3 pairs.

Parotid Glands: Inferior & anterior to the ears, between the skin
& the masseter muscle.
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Submandibular Glands: Located in the floor of the mouth,
medial & partially inferior to the body of the mandible.
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Secretes saliva into the mouth via the parotid duct.
Secretes serous liquid that contains salivary amylase (enzymes).
Secrete a mucousy fluid that contains amylase via the
submandibular ducts entering lateral to the lingual freulum.
Sublingual Glands: Located beneath the tongue & superior to
the submandibular glands.

Secrete an even thicker fluid containing a small amount of salivary
amylase.
Saliva

Saliva: Fluid produced by the salivary glands
that lubricates, dissolves, and begins chemical
digestion of food.
water – allows for particles to be dissolved so
they can be tasted by the gustatory receptors.
 0.5% solutes:
 99.5%
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Chloride ions (activate salivary amylase to start the
breakdown of starch).
Bicarbonate & phosphate ions that buffer acidic foods
Organic substances such as urea & uric acid
Mucous that lubricates food so it can be swallowed.
Immunoglobin A, which prevents the attachment of microbes
to the epithelium.
Lysozyme (enzymes that kill some bacteria)
Salivary amylase – a digestive enxyme that acts on starch.
Saliva
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Salivation: The secretion of saliva into the mouth increases when food enters the mouth.
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Chemicals in food stimulate the taste bud receptors which send
impulses to the superior & inferior salivary nuclei in the brain
stem.
The sight, sound, smell, taste, or thought of food can increase
salivation.
Controlled by the autonomic nervous system.
Average daily saliva secretion is 1,000 – 1,500 mL.
Parasympathetic stimulation maintains moderate secretion to
keep membranes moist & lubricate the movement of the tongue
during speech.
When swallowed, aids in moistening the esophagus.
Most saliva components are reabsorbed to prevent fluid loss.
Salivation ceases in dehydration to conserve water & create the
sensation of thirst.
The Tongue

The Tongue: The accessory digestive organ
composed of skeletal muscle & covered in a
mucous membrane.
 Forms
the floor of the oral cavity in conjunction with
its associated muscles.
 Performs the sensory task of tasting our food.
 Median Septum: Divides the tongue in half.
 Lingual Frenulum: Connects the tongue to the floor
of the mouth.
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Extrinsic Muscles: Originate outside the tongue
& function to change the tongue’s position.
Intrinsic Muscles: Originate within the tongue &
function to change the tongue’s shape.
The Tongue
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Bolus: A compact mass of food & saliva mixed
by movements of the tongue.
Papillae: The projections along the superior
surface of the tongue that provide friction, helps
to manipulate soft foods, & contain taste buds.
Lingual Glands: Secrete mucus & a watery
serous fluid containing lingual lipase (an enzyme
that acts on triglycerides).
 Located
in the lamina propria of the tongue.
Teeth
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Dentes aka Teeth: Function in mastication
(chewing) of food. Part of mechanical digestion.
Primary Dentition aka Deciduous Teeth:
“Baby teeth” that first appear around 6 months,
with all 20 present by 24 months.
Permanent Teeth: “Adult teeth” that replace the
baby teeth with a full set of 32:
8
incisors for cutting
 4 canines for puncturing & tearing
 8 premolars for crushing & grinding
 12 molars for crushing & grinding
Teeth

Two regions to each tooth:
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Crown: The exposed portion above the gum/gingiva – covered
with enamel (the hardest substance in the body!).
Root: The portion of the tooth embedded in the bone – covered
with cementum.
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Periodontal Ligaments aka Periodontal Membranes: sAttach the
cementum of the tooth to the bony socket.
Dentin: The material underneath the cementum &
enamel forming the bulk of the tooth. The secondhardest material in the body.

Pulp Cavity: Located within the dentin & containing the pulp.

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Pulp: Connective tissue containing the blood vessels, nerves, &
lymphatic vessels supplying each tooth.
Root Canal: The cavity extending into the root portion.

Apical Foramen: The place where vessels & nerves enter & exit –
located at the proximal end of the root canal.
Teeth

Dental Plaque: The accumulation of
bacteria, sugar, & debris upon the teeth.
 Can
cause the enamel & dentin to decay,
causing cavities aka dental caries.

Gingivitis: An inflammation or infection of
the gums due to the presence of dental
plaque.
The Esophagus

Esophagus: The collapsible “food tube:
connecting the pharynx to the stomach.
 Extends
from the larynx to the stomach at the
cardiac orafice.
 Gastroesophageal (Cardiac) Sphincter aka
Lower Esophageal Sphincter: Regulates the
movement of food from the esophagus into
the stomach.
 Secretes mucus & transports food into the
stomach.
The Esophagus
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4 Basic Tissue Layers:
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Mucosa: Nonkeratinized stratified squamous epithelium, lamina
propria, & muscularis mucosae.
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Also contains mucous glands near the stomach.
Submucosa: Contains the mucous glands & consists of areolar
connective tissue & blood vessels.
Muscularis: The upper 1/3 is skeletal muscle, the lower 1/3 is
smooth muscle, & the middle is a mixture of both.

Becomes slightly more prominent & forms sphincters at either end
of the esophagus.
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Upper Esophageal Sphincter (UES): Skeletal muscle – regulates the
movement of food from the pharynx into the esophagus.
Lower Esophageal Sphincter (LES): Smooth muscle – regulates the
movement of food from the esophagus into the stomach.
Adventitia: The superficial layer that attaches the esophagus to
the surrounding structures.
Deglutition (Swallowing)
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Deglutition aka Swallowing: Follows the ingestion of food & mastication –
facilitated by the secretion of saliva & mucous.
3 phases of Swallowing:
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Voluntary Phase: Food bolus is passed into the oropharynx, causing…
Pharyngoesophageal Phase: The involuntary passage of the bolus through the
pharynx into the esophagus.
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Esophageal Phase: The involuntary passage of the bolus through the
esophagus & into the stomach.
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This stimulates receptors in the oropharynx which send impulses to the deglutition
center of the medulla oblongata & lower pons of the brain stem.
Returning impulses cause the soft palate & uvula to elevate & close of the nasopharynx
(preventing the bolus from entering the nasal cavity).
Epiglottis closes off the opening to the larynx to prevent the bolus from entering the
respiratory tract.
The UES relaxes & the bolus moves through the oropharynx & laryngopharynx to the
esophagus.
Peristalsis: The wavelike progression of contractions & relaxations of the circular &
longitudinal layers of the muscularis – pushes the bolus toward the stomach.
The LES relaxes & the bolus moves into the stomach.
Liquid reaches the stomach in 1-2 seconds.
A food bolus reaches the stomach in 4-8 seconds.
The Peritoneum

Peritoneum: The largest serous membrane of the body.
Divided into…
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Peritoneal Cavity: The slim space containing serous
fluid & located between the parietal & visceral
peritoneum.
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Parietal Peritoneum: The portion of the peritoneum lining the
wall of the abdominopelvic cavity.
Visceral Peritoneum: The portion of the peritoneum covering
some of the organs in the abdominopelvic cavity & serving as
their serosa.
Ascites: A condition where the peritoneal cavity becomes
distended by the accumulation of fluid.
Retroperitoneal: The term for organs who only have
peritoneal covering on the anterior surface, e.g. the
kidneys & pancreas.
The Peritoneum

Two folds of peritoneum hold the
intestines loosely in place. Allows for
movement while muscular contractions
mix & move their contents along the GI
tract.
 Mesentary:
The fan-shaped fold of
peritoneum that binds the small intestine to
the posterior abdominal wall.
 Mesocolon: The fold of the peritoneum that
binds the large intestine to the posterior
abdominal wall.
The Stomach

Stomach: A muscular sac that stores the ingested food
while being mechanically & chemically digested.
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Located in the upper left quadrant of the abdominal cavity, nearly
hidden by the liver.
J-shaped and collapsible with temporary rugae folds & a volume
of 50 mL when empty & 4 L when full.
Mixing Waves: Muscular contractions that pass through the
stomach every 15-20 seconds, causing the stomach to
mechanically break up food particles, liquefy food, & begin
chemical digestion of proteins & a small amount of fat.

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Chyme: A semidigested mixture of food produced by mixing waves
in the stomach.
Gastric Emptying: The movement of around 3 mL of chyme into
the duodenum through the pyloric sphincter – caused by
increasingly strong mixing waves as the food moves down the GI
tract.
The Stomach
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Greater Curvature: The convex lateral surface of the stomach.

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Lesser Curvature: The concave medial surface of the stomach.
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Greater Omentum: The peritoneal fold attached to the stomach & colon
& hanging over the small intestine – anchors at the greater curvature &
hangs from the stomach to interconnect the stomach, duodenum, &
large intestine.
Lesser Omentum: Connects the stomach to the liver – attached to the
stomach, part of the duodenum, & part of the liver & supporting hepatic
vessels.
4 Major Regions:
Cardia: Where the food enters from the esophagus – surrounds the
cardiac orifice.
 Fundis: The dome shaped portion adjacent to the cardia.
 Body: The large midportion.
 Pyloris: The funnel-shaped bottom portion that connects with the
duodenum via the pyloric sphincter. Divides into the..
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Pyloric Antrum: Connects to the body of the stomach.
Pyloric Canal: Leads to the duodenum – terminates at the pyloris.
Stomach
The Stomach
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4 Layers of the Stomach:
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Mucosa:
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Surface Mucous Cells: Simple columnar epithelial cells along the mucosa’s
surface.
Gastric Glands: Epithelial cells that extend down into the lamina propria &
forming columns of secretary cells.
Gastric Pits: Channels lined by gastric glands & containing secretions from
those glands.
Exocrine Gland Cells: 3 types found within the gastric glands; form gastric
juice.
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Mucous Neck Cells & Mucous Cells: Secrete mucous.
Parietal Cells: Produce intrinsic factor & hydrochloric acid.
Chief Cells: Secrete pepsinogen & gastic lipase, which split short-chain
triglycerides in fat molecules into fatty acids & monoglycerides.
Submucosa: Composed of areolar connective tissue.
Muscularis: Composed of an outer longitudinal layer, a middle circular
layer, and an inner oblique layer.
Serosa: Simple squamous epithelium & areolar connective tissue.
Phases of Gastric Digestion

Cephalic Phase: Triggered by the smell,
taste, thought & sight of food – prepares
the mouth & stomach for food.
 Facial
& glossopharyngeal nerves stimulate
salivary glands to secrete saliva.
 Vagus nerve stimulates the secretion of
gastric juices.
Phases of Gastric Digestion

Gastric Phase: Stimulated by the presence of
food in the stomach – accounts for 2/3 of gastric
juice secretion.
 Stomach distension activates stretch receptors.
 Stretch receptors initiate reflexes that increase the
neural stimulation of gastric glands.
 Partially digested proteins, caffeine, & increased pH
stimulate enteroendocrine cells (G cells) to secrete
gastrin into the bloodstream.
 Gastrin causes mucous cells to secrete mucous,
chief cells to secrete pepsinogen, and parietal cells to
secrete HCl & intrinsic factor.

Also increases gastric motility (the muscle activity & gastric
emptying).
Phases of Gastric Digestion
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Intestinal Phase: Divides into two aspects.
 Excitatory
Aspect: Chyme reaches the duodenum &
stimulates the release of gastrin.
 Inhibitory Aspect (Enterogastric Reflex): Caused
by the intestinal distension & an accumulation of
chyme within the duodenum.
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Causes an increase in sympathetic stimulation to the
stomach.
Secretions decrease, motility decreases, & the pyloric
sphincter tightens to limit the movement of more chyme – this
gives the duodenum time to work on the chyme already
present.
Arrival of chyme spurs the release of intestinal hormones,
which decrease gastric activity.

Cholecystokinin & secretin hormones are released.
Vomiting
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Vomiting: Reflexive contraction of the
diaphragm & abdominal wall muscles, relaxation
of the cardiac sphincter & rise of the soft palate,
which results in the movement of the stomach
contents back into the oral cavity.
Can be triggered by…
 Extreme
stretching in the stomach or intestines
 Irritants present will trigger the emetic center of the
medulla, which sends signals to the diaphragm & the
abdominal wall muscles.
Small Intestine
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Small Intestine: The major organ of digestion & absorption of
nutrients – a convoluted tube extending from the pyloric sphincter to
the ileocecal valve.
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Receives chyme from the stomach & secretions from the liver &
pancreas.
Divides into the…

Duodenum: The 10” long section that curves around the head of the
pancreas – starts at the pyloric sphincter of the stomach & extends ~25
cm until it merges with the jejunum.
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Common Bile Duct: Delivers bile from the liver & the gallbladder.
Main Pancreatic Duct: Delivers pancreatic juice from the pancreas.
Hepatopancreatic Ampulla: The uniting of the common bile & main
pancreatic ducts in the wall of the duodenum.
Hepatopancreatic Sphincter: Muscle that controls the entry of bile &
pancreatic juice into the duodenum.
Jejunum: Connects the duodenum & ileum.
Ileum: Extends from the jejunum to the ileocecal valve, joining the large
intestine via the ileocecal sphincter.
Small Intestine
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4 Tissue Layers in the Small Intestine:

Mucosa: Simple columnar absorptive cells interspersed with goblet cells
(secrete mucous) make up the epithelium.
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Absorptive Cells: Digest & absorb nutrients in the small intestinal chyme.
T lymphocytes: Interspersed among the absorptive cells to provide immune defense.
Deep crevices lined with glandular epithelium…

Intestinal Glands aka Crypts of Lieberkuhn: Pits located between the villi that secrete
intestinal juice.
 Paneth Cells: Located in the intestinal glands & secrete lysozyme – capable of
phagocytosis.
 Enteroendocrine Cells: Include S cells, CCK cells, & K cells – secrete the hormones
secretin, cholecystokinin (CCK) & glucose-dependant insulinotropic peptide (GIP).

Lamina Propria: Contains areolar connective tissue & an abundance of
mucosa-associated lymphoid tissue (MALT).
 Submucosa: Contains the proximal duodenum (houses Brunner’s glands which
secrete alkaline mucus to help neutralize acidic chyme) & terminal ileum.
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Peyer;s Patches: Clisters of lymphatic nodules that help prevent colonic bacteria from
entering the small intestine.
Muscularis: The muscular layer composed of inner circular & outer longitudinal
layers of smooth muscle.
Except for a major part of the duodenum, most of the small intestine is also
covered by a serosa.
Small Intestine
Small Intestine
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The small intestine has lots of surface area to aid
digestion & absorption.
3 Main surface-area enhancing adaptations:
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Circular Folds: Large, deep, permanent folds of the mucosa &
submucosa to increase the surface area.
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Villi: Finger-like projections of the mucosa that increase surface
area for digestion & absorption.
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Slow the movement of chyme to allow more time for digestion &
absorption.
Core contains a capillary bed & lacteal (lymphatic capillary) for the
transport of absorbed nutrients.
Microvilli: Tiny projections of the apical membrane of the
absorptive cells.
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

Known as the brush border due to its appearance.
Further increases the surface area.
Secretes membrane-bound enzymes that are involved in digestion.
Small Intestine

Intestinal Juice: A clear-yellow fluid containing
water & mucous, which is slightly alkaline.
 1-2
liters secreted daily.
 Pancreatic & intestinal juice aid in the absorption of
nutrients from the chyme.

Absorptive Cells: Cells in the small intestine
that synthesize several digestive enzymes
(brush border enzymes) & insert them into the
plasma membrane of the microvilli.
 Release
enzymes that help digest nutrients in the
chyme as they slough off & break apart.
Mechanical Digestion

Myenteric Nerve Plexus controls the two types
of movements in the small intestine.
 Segmentation:
Common movement of the small
intestine, where ring-like constrictions form at several
places along the intestine then relax while
constrictions occur elsewhere.


Results in churning of the intestinal contents.
When most nutrients have been absorbed, only residue
remains, & segmentation tapers off and peristalsis begins.
 Migrating
Motility Complex (MMC): The type of
peristalsis that occurs in the small intestine.


Begins in the lower part of the stomach & pushes chyme
forward along a short stretch of small intestine before
stopping.
Slowly migrates down the small intestine until it reaches the
end of the ileum.
Mechanical Digestion

Contractions in the small intestine….
 Mix
chyme with intestinal juice, bile, &
pancreatic juice.
 Churn chyme & bring it in contact with the
mucosa for digestion & absorption.
 Moves residue toward the large intestine.
The Liver

Liver: An accessory organ of digestion.
 Has
both digestive & nondigestive functions.
 Primary digestive function is the synthesis &
secretion of bile (emulsifies fats).
 Largest gland in the body!
 Located beneath the diaphragm & runs
across the entire body.
The Liver

4 Lobes of the Liver:
 Right



& Left Lobes: The larger lobes in the liver.
Falciform Ligament: A fold of ligament that attaches the
liver to the abdominal wall & diaphragm & separates the right
& left lobes.
Ligamentum Teres (Round Ligament): The free border of
the falciform ligament – a remnant of the umbilical vein of the
fetus, which extends from the liver to the umbilicus.
Right & Left Coronary Ligaments: Suspend the liver fro the
diaphragm.
 Inferior
Quadrate Lobe & Posterior Cuadate Lobe:
Smaller lobes associated with the right lobe of the
liver.
Functions of the Liver



Carbohydrate Metabolism: Glycogen is broken down into glucose & released into
the blood stream if blood glucose is low, & glucose is converted into glycogen &
triglycerides for storage if blood glucose is high.
Lipid Metabolism: Hepatocytes store some triglycerides, break down fatty acids for
ATP generation, synthesizes lipoproteins, synthesize cholesterol, & uses cholesterol
to make bile salts.
Protein Metabolism: Hepatocytes deaminate (remove amino group NH2) amino
acids to be used for ATP production or converted to carbohydrates or fats.








Results in toxic ammonia (NH3) which is converted into urea & excreted via the urine.
Also synthesize most plasma proteins.
Processing of Drugs & Hormones: Detoxifies alcohol & drugs, chemically alters &
excretes hormones.
Excretion of Bilirubin into Bile: Bilirubin (derived from the heme in aged RBCs) in
bile is metabolized by the bacteria in the small intestine & is eliminated in feces.
Synthesis of Bile Salts: Used for the emulsification & absorption of lipids in the
small intestine.
Storage: Stores glycogen, certain vitamins & minerals which are released from the
liver when they are needed elsewhere in the body.
Phagocytosis: Stellate reticuloendothelial (Kupffer) cells of the liver phagocytize
aged RBCs, WBCs, & some bacteria.
Activation of Vitamin D: Skin, liver, & kidneys synthesize the active form of Vitamin
D.
Histology of the Liver

Lobule: The structural & functional units of the liver – a hexagonal (6-sided)
structure made up of hepatocytes (liver cells) that surround a central vein.

Hepatocytes: Liver cells that exchange oxygen for carbon dioxide & take up
nutrients for processing & storage.



Also take up fat-soluble vitamins (A, D, E, K) for storage & toxins for detoxification.
Produce bile, which flows through the lobule in the opposite direction of the blood.
Bile Canaliculi: Small channels that hepatocytes secrete bile into – run into bile
ductules which pass into bile ducts at the periphery of the lobules.

Portal Triads: Found at each of the 6 corners of the lobule. Includes a branch of
the hepatic artery, hepatic portal vein, & bile duct.
 Liver Sinusoids: Connect the hepatic artery (where blood enters the lobule) &
the hepatic vein with the central vein.




Stellate Reticuloendothelial (Kupffer) Cells: Fixed phagocytes contained within the
sinusoids.
Hepatic Vein: The collection of all the central veins from all the lobules.
Right & Left Hepatic Ducts: Merging of the bile ducts which unit & exit the
liver as the common hepatic duct.
Common Bile Duct: Fusing of the common hepatic duct & the cystic duct
from the gallbladder, which then empties into the duodenum.
The Liver




Oxygenated blood enters the liver from the
hepatic artery.
Deoxygenated blood enters the liver from the
hepatic portal vein.
The veins branch into the liver sinusoids where
oxygen, nutrients, & certain toxic substances are
taken up by hepatocytes.
Products produced by the hepatocytes &
nutrients needed by other cells are secreted
back into the blood, which drains into the central
vein & eventually enters the hepatic vein.
The Liver

Bile: A yellow-green, alkaline solution that contains bile salts, bile
pigments, cholesterol, neutral fats, phospholipids, & electrolytes.

Phospholipids & bile acids aid in fat digestion & absorption - are
reabsorbed in the ileum & returned to the liver.
 Bile is continuously made by the liver & stored & modified in the
gallbladder until it is needed.
 Phagocytosis of aged RBCs release iron, globin, & bilirubin.



Iron & globin are recycled while bilirubin is secreted into the bile to be broken
down in the intestine.
Bilirubin: The main bile pigment, derived from the decomposition of
hemoglobin in the RBCs.
Bile Salts: Cholesterol derivatives whose chief function is the
emulsification of fats.


Nonpolar – causes ingested fats to clump together in the watery environs of
the digestive tract which decreases the surface area for lipases (fatdigesting enzymes).
Emulsify fats by separating them into smaller pieces to increase the surface
area for digestive enzymes.
The Gallbladder

Gallbladder: The thin-walled, pear-shaped
muscular sac responsible for storing &
concentrating bile until it is needed by the small
intestine.
 Located
on the inferior surface of the right lobe of the
liver, within a shallow fossa on the ventral surface of
the liver.
 Consists of…



A fundus projecting inferiorly beyond the inferior border of the
liver.
A body, which is the center portion.
A neck, which is the tapered portion.
 Covered
by visceral peritoneum.
 Concentrates the bile by absorbing water & ions.
The Gallbladder
Chyme, consisting of fat & protein, enters
the duodenum, causing enteroendocrine
cells to secrete cholecystokinin (CCK) into
the bloodstream.
 CCK causes the gallbladder to contract &
the hepatopancreatic sphincter to relax &
the pancreas to secrete pancreatic juice.
 Muscular contractions cause bile to be
expelled into the cystic duct, common bile
duct, & the duodenum.

The Gallbladder

Gallstones: Caused by cholesterol & bile
salts crystalizing within the gallbladder.
 Obstruct
the flow of bile, causing severe pain.
The Pancreas

Pancreas: A soft, tadpole-shaped organ
that functions as both an endocrine &
exocrine gland.
 Head:
The expanded portion located near the
curve of the duodenum & is encircled by the
duodenum.
 Body: Superior to and to the left of the head.
 Tail: Tapers off & abuts the spleen.
The Pancreas

The Endocrine Portion of the Pancreas:
 Islets
of Langerhans: Patches of hormonesecreting cells responsible for the secretion of
insulin & glucagon (regulate blood glucose
levels via negative feedback).
 Beta Cells: Release insulin in response to
high blood glucose levels.
 Alpha Cells: Release glucagon in response
to low blood glucose levels.
The Pancreas

The Exocrine Portion of the Pancreas:

Main Pancreatic Duct (Duct of Wirsung): Empties into the duodenum
& receives secretions form multiple, smaller ducts from within the
pancreas.
 Pancreatic Acini: Secretory cells that secrete a mixture of fluid &
digestive enzymes called pancreatic juice - located at the end of the
smaller ducts.






Cells lining the ducts secrete an alkaline solution.
Secrete proteases, pancreatic amylase, lipase, & nuclease enzymes.
Zymogens: Inactive proteins that are activated by brush border enzymes
within the duodenum – many enzymes secreted this way, particularly
pepsinogen.
Enzyme release triggered by the CCK produced in the duodenum in
response to chyme.
Secretin: A hormone that stimulates pancreatic duct cells to release alkaline
fluid in response to an acidic chyme.
Functions to secrete digestive enzymes and bicarbonate-rich fluid.
Large Intestine

Large Intestine: Receives indigestible material
from the small intestine, reabsorbs water from
the food residue, & eliminates the rest as feces.
 0.5-1.0
liters of water enter the large intestine daily,
and all but 100-200 mL is reabsorbed through
osmosis.
 Some vitamins & ions (including sodium & chloride)
are reabsorbed in the large intestine as well.
 Frames the small intestine on 3 sides:
 Extends from the ileocecal valve to the anal canal.
 Larger in diameter than the small intestine, but much
shorter.
Histology of the Large Intestine

Mucosa: Simple columnar epithelium, lamina propria, &
musclaris mucosae.


Contains mostly absorptive cells that function in water absorption
& globlet cells that secrete mucous to lubricate the passage of
the contents.
Muscularis: The muscle layer – once again, there is an
external longitudinal layer & internal circular layer.

Taenia Coli: 3 bands of smooth muscle in the longitudinal layer.




Tonic contractions in these bands gather the colon together into
haustra (pocket-like sacs or pouches).
Epiploic Appendages: Fat-filled bags of visceral peritoneum
attached to the taenia coli.
Submucosa: Similar to that in the rest of the GI tract.
Serosa: In the large intestine, this is part of the visceral
peritoneum.
Large Intestine
Large Intestine

Divided into 4 regions:

Cecum: The sac-like area located below the iliocecal valve – the
first part of the large intestine.



Ileocecal Sphincter (Valve): The opening from the ileum to the
large intestine – allows material from the small intestine to pass into
the large intestine.
Appendix (Vermiform Appendix): The twisted, coiled tube
attached to the cecum – bacteria often accumulates here and
induces appendicitis.
Colon: The long tube merging from the open end of the cecum.




Ascending Colon: The portion that travels up the right side of the
abdominal cavity & turns medially.
Transverse Colon: The portion that travels across the abdominal
cavity.
Descending Colon: The portion that turns downward at the spleen
& descends on the left side of the abdominal cavity.
Sigmoid Colon: The final portion of the colon that enters the pelvis
inferiorly & merges with the rectum.
Large Intestine
Anterior to the sacrum & coccyx – connects
the colon & anal canal.
 Anal Canal: The last segment of the large intestine –
begins where the rectum penetrates the muscles of
the pelvic floor & continues to the outside of the body.
 Rectum:

Anus: The external opening controlled by the sphincter
muscles.



Internal Anal Sphincter: Smooth & involuntary muscle band.
External Anal Sphincter: Skeletal muscle that allows control
for keeping the anus closed except during elimination of feces.
Anal Columns: Longitudinal folds in the mucous membranes
of the anal canal containing a network of arteries & veins.
Types of Digestion

Mechanical Digestion in the Large Intestine: The
breakdown of food through physical means.







Gastroileal Reflex: Intensifies peristalsis after a meal to force
chyme into the cecum.
Gastrin relaxes the sphincter allow movement of the chyme.
Contraction of the ileocecal sphincter intesifieds when the cecum
is distended.
When chyme passes the ileocecal sphincter & accumulate in the
ascending colon, colon movements begin.
Distension of the haustra triggers haustric churning to squeeze
contents forward.
Peristalsis slows down temporarily.
Gastrocolic Reflex: The presence of food in the stomach
initiates mass peristalsis 3-4 times a day.

Mass peristalsis: The final movement of a strong peristaltic wave
to push the contents of the colon into the rectum.
Types of Digestion

Chemical Digestion: The breakdown of food
through enzymes.
final stage of digestion occurs in the colon – no
enzymes are secreted, but bacteria will ferment the
remaining carbohydrates to release hydrogen, carbon
dioxide, & methane gases (causes flatulence when
excessive).
 Bacteria convert proteins into simpler substances,
including indole & skatole (eliminated in feces &
contribute to the odor).
 Bacteria also decompose bilirubin to simpler
pigments, including stercobilin (gives feces its brown
color).
 Bacteria may produce vitamins B & K to be absorbed
in the colon.
 The
Types of Digestion

Feces: Chyme that has remained in the large
intestine for 3-10 hours and has become solid or
semisolid.
 Composed
of 75% water & 25% solid matter
(inorganic salts, sloughed off epithelial cells from the
GI tact, bacteria (30% of the solid matter), products
from bacterial decomposition, unabsorbed digested
materials, & indigestible parts of food (30% of solid
matter)).

Defecation Reflex: Initiated when feces enter &
stretch the rectal wall, resulting in sigmoid colon
& rectal contractions, & relaxation of the anal
sphincter muscles.
Types of Digestion

Diarrhea: An increase in frequency, volume, &
fluid content of the feces.
 Caused
by increase motility of & decreased
absorption by the intestines.
 Prolonged diarrhea may lead to dehydration, acidosis,
& electrolyte imbalances.

Constipation: Infrequent or difficult defecation.
 Caused
by decreased motility of the intestines.
 The prolonging of the time in the colon leads to
excessive water absorption & drying out of the fecal
matter.
Nutrient Digestion & Absorption

Absorption: The passage of digested
nutrients from the gastrointestinal tract into
the blood or lymph.
 Occurs
via diffusion, facilitated diffusion,
osmosis, and active transport.
 90% of absorption occurs in the small
intestine, the other 10% in the stomach &
large intestine.
 Undigested & unabsorbed material remaining
in the small intestine passes on to the large
intestine.
Carbohydrate Digestion &
Absorption

Most ingested carbohydrates:
 Monosaccharides:
Simple sugars, including glucose,
fructose, & galactose.

Readily absorbed & need no further digestion.
 Disaccharides:
Formed by 2 monosaccharides
chemically bonded together, including sucrose &
lactose.

Must be broken down into monosaccharides to be absorbed.
 Polysaccharides:
Large polymers of
monosaccharides, including plant starch or glycogen
(animal starch).

Must be broken down into monosaccharides to be absorbed.
Carbohydrate Digestion &
Absorption
Salivary Amylase: Secreted into the
mouth to begin the breakdown of startch
into oligosaccharides (2-8
monosaccharides bonded together).
 Pancreatic Amylase: Continues the
chemical digestion of starch in the small
intestine.
 Brush Border Enzymes: Include sucrase
& lactase break down oligosaccharides &
disaccharides.

Carbohydrate Digestion &
Absorption

Absorption of Monosaccharides:
 Cotransport
with Sodium: Allows glucose &
galactose to be absorbed in the intestinal
epithelial cells along with sodium ions.
 Facilitated Diffusion: Diffusion using a
facilitating molecule.

All monosaccharides use this to move out of the
epithelial cells through their basolateral surfaces &
enter the villus capillaries to be transported to the
liver via the hepatic portal vein.
Protein Digestion & Absorption

Protein: Polymers of amino acids.
 Digestion
begins in the stomach with pepsin.
 Digestion continues in the small intestine with trypsin,
chymotrypsin, carboxypeptidase, & elastase
(enzymes in pancreatic juice).
 Digestion is completed in the intestinal brush border
by two peptidases..


Aminopeptidase: Cleaves off the terminal amino acid at the
amino end of the peptide.
Dipeptidase: Splits dipeptides into single amino acids.
 Free
amino acids are the end result of protein
digestion.
Protein Digestion & Absorption

Protein Absorption:
 Cotransport
with Sodium: Helps amino
acids to enter the intestinal epithelial cells to
move into the villus capillaries & travel to the
liver via the hepatic portal vein.
 Most proteins absorbed as amino acids via
active transport (the use of ATP to move
molecules across the membrane).
Lipid Digestion & Absorption


Lipid: Fatty acid molecules – most abundant type is the
triglyceride (a molecule of glycerol bonded to 3 fatty acid
molecules).
Lipases: Enzymes that split triglycerides &
phospholipids. 3 types participate in lipid digestion…




Lingual Lipase: Aid in digestion in the stomach.
Gastric Lipase: Aid in digestion in the stomach.
Pancreatic Lipase: Aid in digestion in the small intestine –
break down the lipids into fatty acids & monoglycerides.
Emulsification: The process of breaking down large
lipid globules into several small globules.

Bile salts are ideally suited for emulsification due to their
amphipathic nature (have hydrophobic & hydrophilic regions).
Lipid Digestion & Absorption

Lipid Absorption: All lipids are absorbed via simple diffusion – molecules
move from an area of low concentration to areas of high concentration.

Micelles: Tiny spheres formed by the bile salts around long-chain fatty acids &
monoglycerides in the intestinal chyme.




Long-chain fatty acids & monoglycerides recombine to form triglycerides
inside the absorptive cells.
Chylomicrons: Masses of triglycerides coated with proteins – leave the
absorptive cells by exocytosis & enter the lymph vessels & then the blood
through the left subclavian vein.
Lipoprotein Lipase: An enzyme attached to the apical surface of the
capillary endothelial cells – helps to move chylomicrons from the blood.


Move from the interior of the small intestinal lumen to the brush border of absorptive
cells, allowing long-chain fatty acids & monoglycerides to diffuse into the absorptive
cells leaving the micelles behind in the chyme.
Breaks down triglycerides in the chylomicorns & other lipoproteins into fatty acids
& glycerol.
Enterohepatic Circulation: The cycle of bile salt secretion by hepatocytes
in bile, reabsorption by the ileum (small intestine), & resecretion into the
bile.
Nucleic Acid Digestion &
Absorption

Nucleic Acids:
 Pancreatic
juices contain two nucleases to
break down nucleic acid Ribonuclease: Digests RNA
 Deoxyribonuclease: Digests DNA.

 Nucletides
resulting from nucleic acid
digestion are further digested by the brush
border enzymes.
 The final products of digestion are absorbed
via active transport.
Absorption Of Electrolytes


Many of the electrolytes absorbed by the small
intestine are from gastrointestinal secretions –
others are part of ingested foods & liquids.
Sodium ions move into absorptive cells via
diffusion & secondary active transport.
 Are

Negatively charged ions can be actively
transported.
 E.g,

actively transported out of the absorptive cells.
bicarbonate, chloride, iodide, & nitrate.
Calcium Ions are absorbed actively, along with
iron, potassium, magnesium, & phosphate.
Absorption of Vitamins

Water-Soluble Vitamins: Vitamins that
are soluble in water and are absorbed via
simple diffusion.
 E.g.
B vitamins, Vitamin C.
 Exception: B12 must bind with an intrinsic
factor produced by the stomach lining & the
combination is absorbed into the ileum via
active transport.
Absorption by Diffusion
Absorption of Water




All water absorption in the GI tract occurs via osmosis
from the lumen of the intestines through the absorptive
cells & into blood capillaries.
Absorption of water depends on the absorption of
electrolytes & nutrients to maintain the osmotic balance
of the blood
Absorbed electrolytes, monosaccharides, & amino acids
establish a concentration gradient for water that
promotes water absorption.
Approximately 9.3 liters per day enters the small
intestine – 2.3 liters from ingestion, and 7.0 liters from GI
secretions.


Around 8.3 liters is absorbed by the small intestine.
Only 0.1 liters (100mL) is secreted via feces each day.
Absorption by Osmosis