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Digestive & Absorption
Digestion & Absorption
• Digestion is the process of breaking down
food into molecules that are small enough to
enter the body cells.
• Absorption is the passage of these smaller
molecules through the plasma membrane of
cells lining the stomach and intestines into the
blood and lymph.
Digestive System
• The organs that perform the functions of digestion
and absorption are collectively referred to as the
digestive system.
• Gastroenterology deals with the structure, function,
diagnosis and treatment of diseases of the stomach
and intestines.
• Proctology deals with the diagnosis and treatment of
disorders of the rectum and anus.
Components Of The Digestive System
• The gastrointestinal (GI) tract or alimentary canal is
a continuous tube that extends from the mouth to
the anus through the ventral body cavity.
– Organs of the GI tract include the mouth, most of the
pharynx, esophagus, stomach, small intestine, and large
intestine.
– The length of the GI tract in a cadaver is about 9m (30 ft).
It is shorter in a living person due to clonus.
Components Of The Digestive System
• Accessory digestive organs include the teeth,
tongue, salivary glands, liver, gallbladder, and
pancreas.
– The teeth aid in the physical breakdown of food and the
tongue assists in chewing and swallowing.
– The other accessory digestive organs never come into
direct contact with the food. They produce or store
secretions that flow into the GI tract and aid in the
chemical breakdown of food.
Functions Of The Digestive System
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Ingestion
Secretion
Mixing and propulsion
Digestion
Absorption
Defecation
Ingestion
• This process involves taking foods and liquids
into the mouth (eating).
Secretion
• Cells within the walls of the GI tract secrete
about 7 liters of water, acid, buffers, and
enzymes into the lumen of the GI tract daily.
Mixing & Propulsion
• Alternating contraction and relaxation of
smooth muscle in the walls of the GI tract mix
food and secretions and propel them toward
the anus.
• This is referred to as motility.
Digestion
• Mechanical digestion.
– The teeth cut and grind food.
– The smooth muscles of the stomach and small
intestine churn the food to help it dissolve and
mix with enzymes.
Digestion
• Chemical digestion.
– The large carbohydrate, lipid, protein, and nucleic acid
molecules in food are split into smaller molecules by
hydrolysis.
– Digestive enzymes produced by the salivary glands,
tongue, stomach, pancreas, and small intestines catalyze
these catabolic reactions.
– Amino acids, cholesterol, glucose, vitamins, minerals, and
water can be absorbed without chemical digestion.
Absorption
• Absorption is the entrance of ingested and
secreted fluids, ions, and small molecules that
are products of digestion into the epithelial
cells lining the lumen of the GI tract.
• The absorbed substances pass into the blood
or lymph and circulate to all cells of the body.
Defecation
• Substances that were not absorbed leave the
body through the anus in a process called
defecation.
• These substances include wastes, indigestible
substances, bacteria, cells sloughed from the
GI tract, and digested materials that were not
absorbed.
• The eliminated material is called feces.
Layers Of The GI Tract
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Mucosa
Submucosa
Muscularis
Serosa
Mucosa
• The mucosa (inner lining) is a mucous
membrane.
• It is composed of a layer of epithelium in
direct contact with the contents of the GI
tract, areolar connective tissue, and a thin
layer of smooth muscle (muscularis mucosae).
Mucosa
• Epithelium.
– Epithelium in the mouth, phaynx, esophagus, and
anal canal is nonkeratinized stratified squamous
epithelium and serves a protective function.
– Epithelium in the stomach and intestines is simple
columnar epithelium and functions in secretion
and absorption.
Mucosa
• Lamina propria.
– Areolar connective tissue containing many blood and
lymphatic vessels, which are routes through which
nutrients are absorbed.
– Mucosa-associated lymphatic tissue is also present to
protect against microbes.
• Muscularis mucosa.
– A thin layer of smooth muscle fibers which creates folds in
the stomach and small intestine to increase surface area.
Submucosa
• The submucosa consists of areolar connective
tissue that binds the mucosa to the
muscularis.
Submucosa
• It contains blood and lymphatic vessels that
receive absorbed food molecules.
Submucosa
• It also contains the submucosal plexus (plexus of
Meissner) which is an extensive network of neurons.
– These neurons are part of the enteric nervous system or
“brain of the gut”.
– They regulate movements of the mucosa and
vasoconstriction of the blood vessels.
– The nerves innervate secretory cells of the mucosal and
submucosal glands.
Muscularis
• The muscularis of the mouth, pharynx, and
superior and middle parts of the esophagus
contains skeletal muscle that produces
voluntary swallowing.
• Skeletal muscle also forms the external anal
sphincter, which permits voluntary control of
defecation.
Muscularis
• The rest of the GI tract muscularis contains
smooth muscle.
• The myenteric plexus (plexus of Auerbach) is
within the layers of smooth muscle. It is also
part of the enteric nervous system and
controls GI motility (i.E. GI frequency and
strength of contraction).
Serosa
• The serosa is the superficial layer of the
portions of the GI tract that are suspended in
the abdominopelvic cavity.
• Inferior to the diaphragm it is called the
visceral peritoneum.
Peritoneum
• The peritoneum is divided into parietal peritoneum
which lines the wall of the abdominopelvic cavity and
visceral peritoneum which lines some of the organs
in the cavity.
• The space between the parietal and visceral
peritoneum is called the peritoneal cavity.
• In some diseases, the peritoneal cavity becomes
distended by the accumulation of fluid in a condition
called ascites.
Retroperitoneal
• Some organs lie on the posterior abdominal
wall and are only covered by peritoneum on
their anterior surface.
• These organs are said to be retroperitoneal
and include the kidneys and pancreas.
Peritoneum Functions
• The peritoneum contains large folds that
weave between the viscera.
• These folds bind the organs to each other and
to the walls of the abdominal cavity.
• They also contain blood vessels, lymphatic
vessels, and nerves that supply the abdominal
organs.
Peritoneal Folds
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Greater omentum.
Falciform ligament.
Lesser omentum.
Mesentery.
Mesocolon.
Greater Omentum
• The largest peritoneal fold.
• It drapes over the transverse colon and coils of the
small intestine like a “fatty apron”.
• It contains a considerable amount of fatty tissue.
• It can greatly expand with weight gain, giving rise to
the characteristic “beer belly”.
• There are many lymph nodes in the greater
momentum.
Falciform Ligament
• The falciform ligament attaches the liver to
the anterior abdominal wall and diaphragm.
• The liver is the only digestive organ that is
attached to the anterior abdominal wall.
Lesser Omentum
• The lesser omentum suspends the stomach
and duodenum from the liver.
• It contains some lymph nodes.
Mesentery
• The mesentery is fan-shaped and binds the
small intestine to the posterior abdominal
wall.
• Blood vessels, lymphatic vessels, and lymph
nodes lie between the two layers of
mesentery.
Mesocolon
• The mesocolon binds the large intestine to the
posterior abdominal wall.
• It carries blood vessels and lymphatic vessels.
• The mesentary and mesocolon work together
to loosely hold the intestines in place. This
allows for great movement to allow them to
mix food and propel food along the GI tract.
Peritonitis
• Peritonitis is an acute inflammation of the
peritoneum.
• Contamination of the peritoneum by
infectious microbes causes it.
Peritonitis
• This is the result of accidental or surgical
wounds in the abdominal wall.
• Perforation or rupture of abdominal organs
also causes this.
• When inflamed peritoneal surfaces rub
together, peritonitis can result.
Mouth
• The mouth is also referred to as the oral or
buccal cavity.
• It is formed by the cheeks, hard and soft
palates, and tongue.
Mouth
• The lips (labia) are fleshy folds surrounding
the opening of the mouth.
• The labial frenulum is a midline fold of
mucous membrane that attaches the inner
surface of each lip to its corresponding gum.
• The orbicularis oris and buccinator muscles
keep food between the upper and lower teeth
to assist in chewing.
Mouth
• The vestibule of the oral cavity is the space
bounded by the cheeks and lips externally and
the teeth and gums internally.
• The oral cavity proper is the space that
extends between the teeth and gums to the
fauces (opening between the oral cavity and
throat).
Mouth
• The hard palate is the anterior portion of the
roof of the mouth and is formed by the
maxillae and palatine bones.
Mouth
• The soft palate is the posterior portion of the roof of
the mouth. It is an arch-shaped muscular partition
that is lined by mucous membrane.
• The uvula is a conical muscular process hanging from
the free border of the soft palate. During
swallowing, the uvula and soft palate are drawn
superiorly and closing off the nasopharynx to
prevent foods from entering the nasal cavity.
Salivary Glands
• A salivary gland is any cell or organ that
releases saliva into the oral cavity.
• Saliva cleanses the mouth and teeth.
• When food enters the mouth, secretion of
saliva increases.
• Saliva lubricates, dissolves, and begins the
chemical breakdown of food.
Minor Salivary Glands
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Labial glands in the lips.
Buccal glands in the cheeks.
Palatal glands in the palate.
Lingual glands in the tongue.
Major Salivary Glands
• These glands lie beyond the oral mucosa and empty
their secretions into ducts that lead to the oral cavity.
• Parotid glands and parotid duct.
• Submandibular glands and submandibular ducts.
• Sublingual glands and lesser sublingual ducts.
Composition & Functions Of Saliva
• Saliva is 95/5% water and 0.5% solutes.
• Lysozyme – a bacteriolytic enzyme.
• Salivary amylase – a digestive enzyme that
acts on starch.
Salivation
• Salivation is the secretion of saliva.
• It is controlled by the autonomic nervous
system.
• The feel and taste of food are potent
stimulators of salivary gland secretions.
Salivation
• Chemicals in food stimulate taste receptors on the
tongue and impulses are propagated to the salivary
nuclei in the brain stem.
• Impulses from the facial nerve (CN VII) and the
glossopharyngeal nerve (CN IX) stimulate the
secretion of saliva.
• Saliva continues to be secreted heavily for some time
after food is swallowing. This washes out the mouth.
Mumps
• Mumps is an inflammation and enlargement of the
parotid glands accompanied by moderate fever,
malaise (general discomfort), and extreme pain in
the throat, especially when swallowing sour foods or
acidic juices.
• Swelling also occurs on one or both sides of the face.
• In about 30% of males past puberty, the testes may
also become inflamed (orchitis).
Tongue
• The tongue is an accessory digestive organ composed
of skeletal muscle covered with a mucous
membrane.
• Extrinsic muscles of the tongue move the tongue
from side to side and in and out to maneuver food
for chewing and push food to the back of the mouth.
• The intrinsic muscles of the tongue alter the shape
and size of the tongue for speech and swallowing.
Tongue
• The lingual frenulum is a fold of mucous
membrane in the midline of the undersurface
of the tongue. It limits movement of the
tongue posteriorly.
• Ankyloglossia is a condition in which the
lingual frenulum is abnormally short impairing
eating and speaking (“tongue-tied”).
Tongue
• Papillae cover the dorsum and lateral surfaces of the
tongue.
– Fungiform papillae – mushroom like elevations near the tip
of the tongue that contain taste buds.
– Vallate (circumvallate) papillae – contain taste buds and
are located in a V shape on the posterior surface.
– Foliate papillae – located in small trenches on the lateral
margins of the tongue. Most of the taste buds degenerate
during childhood.
– Filiform papillae – distributed in parallel rows. They lack
taste buds, but contain receptors for touch.
Tongue
• Lingual glands secrete both mucus and a
watery serous fluid that contain the enzyme
lingual lipase.
Teeth
• Teeth or dentes are accessory digestive organs
located in the sockets of the alveolar processes.
• The processes are covered by gingivae (gums).
• The sockets are lined by the periodontal ligament or
membrane.
• The teeth are composed primarily of dentin, a
calcified connective tissue.
Teeth
• Teeth are harder than bone because of the
higher content of calcium salts.
• The dentin encloses a pulp cavity. The pulp is
a connective tissue containing blood vessels,
nerves, and lymphatic vessels.
Teeth
• Root canals are narrow extensions of the pulp
cavity.
• A hard substance called enamel covers the
dentin of the crown. Enamel is the hardest
substance in the body.
Branches Of Dentistry
• Endodontics – deals with prevention, diagnosis, and
treatment of diseases that affect the pulp, root,
periodontal ligament, and alveolar bone.
• Orthodontics – deals with prevention and correction
of abnormally aligned teeth.
• Periodontics – deals with treatment of abnormal
conditions of the tissues immediately surrounding
the teeth.
Dentitions
• Humans have two dentitions (sets of teeth).
– Deciduous teeth.
– Permanent teeth.
Dentitions
• Deciduous teeth – also called primary teeth, milk
teeth, or baby teeth.
– Begin to erupt at about 6 months or age and one pair of
teeth appears at about each month thereafter until all 20
are present.
• Permanent teeth – also called secondary teeth.
– The deciduous teeth are lost between the ages of 6 and 12
years and replaced by permanent teeth.
– The permanent dentition contains 32 teeth that erupt
between age 6 and adulthood.
Types Of Teeth
• Incisors – chisel-shaped to cut into food.
• Cuspids (canines) – have a pointed surface
called a cusp. They are used to tear or shred
food.
• Molars – crush and grind food.
Root Canal Therapy
• All traces of pulp tissue are removed from the pulp
cavity and root canals of a badly diseased tooth.
• A hole is made in the tooth and the root canals are
filed out and irrigated to remove bacteria.
• The canals are treated with medication and sealed
tightly.
• The damaged crown is then repaired.
Mechanical & Chemical Digestion
In The Mouth
• Mechanical digestion results from mastication
(chewing) in which the food is manipulated by
the tongue, ground by the teeth, and mixed
with saliva.
Mechanical & Chemical Digestion
In The Mouth
• The food becomes a soft, flexible mass called
a bolus (lump) that is easily swallowed.
Mechanical & Chemical Digestion
In The Mouth
• Chemical digestion is assisted by two enzymes in the
mouth.
– Salivary amylase initiates the breakdown of starch. It
reduces the long-chain polysaccharides to disaccharides
and trisaccharides. It is deactivated by the stomach acid in
about an hour.
– Lingual lipase is secreted by glands in the tongue and
begins to work in the acidic environment of the stomach.
It breaks down triglycerides into fatty acids and
diglycerides.
Summary Of Digestive Activities Of
The Mouth
• Cheeks and lips – keep food between teeth.
Food is uniformly chewed during mastication.
• Salivary glands – secrete saliva which softens,
moistens, and dissolves food. Saliva cleanses
the mouth and teeth. Salivary amylase splits
starch into smaller fragments.
Summary Of Digestive Activities Of
The Mouth
• Tongue – maneuvers food for mastication and
swallowing. Receptors for gustation (taste)
which stimulates salivary glands. Secretes
lingual lipase which breaks down triglycerides.
• Teeth – cut, tear, and pulverize food to create
smaller particles for swallowing and increase
surface area for enzymatic reactions to occur.
Pharynx
• When food is first swallowed, it passes from
the mouth into the pharynx.
• The pharynx is composed of skeletal muscle
tissue lined by a mucous membrane.
Pharynx
• The nasopharynx functions only in respiration;
However, the oropharynx and laryngopharynx
function in both respiration and digestion.
Pharynx
• Swallowing (deglutition) moves food from the mouth
to the stomach.
– Voluntary stage – the bolus is passed into the oropharynx.
– Pharyngeal stage – involuntary passage of the bolus
through the pharynx into the esophagus.
– Esophageal stage – involuntary passage of the bolus from
the esophagus to the stomach.
– The bolus stimulates receptors in the oropharynx, which
sends signals to the deglutition center in the medulla
oblongata and lower pons of the brain stem.
Esophagus
• The esophagus is a collapsible muscular tube
that lies superior to the trachea.
• It lies posterior to the trachea and is about 25
cm (10 in.) Long.
Esophagus
• It pierces the diaphragm at an opening called
the esophageal hiatus and ends in the
superior portion of the stomach.
• Sometimes part of the stomach protrudes
above the diaphragm through the esophageal
hiatus (hiatal hernia).
Physiology Of The Esophagus
• The esophagus secretes mucus and transports
food into the stomach.
• The esophagus does NOT secrete digestive
enzymes and does NOT participate in
absorption.
Physiology Of The Esophagus
• The upper esophageal sphincter regulates the
entrance of food into the esophagus from the
laryngopharynx.
Physiology Of The Esophagus
• During the esophageal stage of swallowing,
peristalsis occurs. Peristalsis is a progression
of coordinated contractions and relaxations
that push the food bolus onward.
• The lower esophageal sphincter relaxes during
swallowing and allows the bolus to pass
through to the stomach.
Gastroesophageal Reflux Disease
• If the lower esophageal sphincter fails to close
adequately after food has entered the
stomach, the stomach contents can reflux, or
back up, into the inferior portion of the
esophagus.
• This is known as gastroesophageal reflux
disease (GERD).
Gastroesophageal Reflux Disease
• Hydrochloric acid (HCl) from the stomach
contents can irritate the esophageal wall,
resulting in a burning sensation called
heartburn.
• Drinking alcohol and smoking can cause the
sphincter to relax, worsening the problem.
• GERD may be associated with cancer of the
esophagus.
Stomach
• The stomach is a j-shaped enlargement of the
GI tract.
• It lies inferior to the diaphragm in the
epigastric, umbilical, and left hypochondriac
regions of the abdomen.
Stomach
• The stomach connects the esophagus to the
duodenum.
• A meal can be eaten much faster than the
intestines can digest and absorb it.
• The stomach is the most distensible part of
the GI tract.
Functions Of The Stomach
• Reservoir for holding food before release to SI.
• Mixes the saliva, food and gastric juice to form
chyme.
• The semisolid bolus of food is converted into a liquid.
• Secretes gastric juice, which contains HCL, pepsin,
intrinsic factor, and gastric lipase.
• Digestion of starch continues.
Functions Of The Stomach
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HCL kills bacteria and denatures proteins.
Pepsin begins digestion of proteins.
Intrinsic factor aids absorption of vitamin B12.
Gastric lipase aids in digestion of triglycerides.
Secretes gastrin into blood.
Certain substances are absorbed.
Anatomy Of The Stomach
• Four main regions:
– Cardia – surrounds the superior opening of the stomach.
– Fundus – the rounded portion superior and to the left of
the cardia.
– Body – the large central portion of the stomach inferior to
the fundus.
– Pylorus – the region of the stomach that connects to the
duodenum.
Pylorus
• Pyloric antrum – connects to the body of the
stomach.
• Pyloric canal – leads to the duodenum.
• Pyloric sphincter – connects the stomach to
the duodenum and regulates passage of food.
Anatomy Of The Stomach
• Rugae – large folds in the mucosa of the
stomach when it is empty.
• Lesser curvature – the concave medial border
of the stomach.
• Greater curvature – the convex lateral border
of the stomach.
Abnormalities Of The Pyloric
Sphincter In Infants
• Pylorospasm – the muscle fibers of the sphincter fail
to relax normally and fails to allow passage of food.
The stomach becomes overly full and the infant
vomits. Drugs to relax the muscles are used.
• Pyloric Stenosis – narrowing of the pyloric sphincter.
This must be corrected surgically. Projectile vomiting
is the hallmark symptom of this condition.
Histology Of The Stomach
• The stomach wall is composed of the same
four basic layers of the rest of the GI tract with
minor modifications.
Histology Of The Stomach
• The surface of the mucosa contains simple
columnar epithelial cells called surface
mucous cells.
• Epithelial cells extend into the lamina propria,
where they form columns of secretory cells
called gastric glands that line chambers called
gastric pits.
Gastric Glands
• The gastric glands contain 3 types of exocrine gland
cells that secrete their products into the lumen of
the stomach.
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Mucous neck cells – secrete mucous.
Parietal cells – produce intrinsic factor.
Chief cells – secrete pepsinogen and gastric lipase.
These secretions are called gastric juice (approximately
2000 – 300 ml per day).
Mechanical Digestion In The
Stomach
• Mixing waves occur several minutes after food
enters the stomach – these are gentle,
rippling, peristaltic movements. They occur
every 15 to 25 seconds.
• Few mixing waves occur in the fundus. It
serves primarily a storage function.
Mechanical Digestion In The
Stomach
• These waves macerate food, mix it with the
secretions of the gastric glands, and reduce it
to a soupy liquid called chyme.
• The pylorus remains almost, but not
completely closed. Each mixing wave forces
some food through the pyloric sphincter.
Chemical Digestion In The Stomach
• Foods may remain in the fundus for up to an hour
without becoming mixed with gastric juice.
• During this time, digestion by salivary amylase
continues.
• Once the food becomes mixed with gastric juice, the
salivary amylase is inactivated and the lingual lipase
is activated.
• The parietal cells secrete HCl.
Chemical Digestion In The Stomach
• The chief cells secrete pepsin, which is a
proteolytic enzyme. Pepsin is activate in the
acidic environment of the stomach.
• Pepsin is secreted in an inactive form called
pepsinogen and therefore does not digest the
proteins of the chief cells that secrete it.
Chemical Digestion In The Stomach
• Pepsinogen does not become activate until it
comes into contact with active pepsin
molecules or HCl.
• The stomach epithelial cells are protected by
mucous secreted from the mucous neck cells.
• Gastric lipase begins breakdown of
triglycerides; However, it does not work well
in the acidic environment.
Gastric Emptying
• Gastric emptying is the periodic release of
chyme from the stomach into the duodenum.
• Stimuli such as distention of the stomach and
the presence of partially digested proteins,
alcohol, and caffeine initiate gastric emptying.
Gastric Emptying
• The enterogastric reflex ensures that the
stomach does not release more chyme than
the small intestine can handle.
• Stimuli such as distention of the duodenum
and the presence of fatty acids, glucose, and
partially digested proteins in the duodenal
chyme inhibit gastric emptying.
Gastric Emptying
• Within 2 – 4 hours after eating a meal, the
stomach has emptied its contents into the
duodenum.
• Foods rich in carbohydrates spend the least
time in the stomach, proteins longer, and fatladen meals the longest.
Vomiting
• Vomiting or emesis is the forcible expulsion of the
contents of the upper GI tract (stomach and
sometimes duodenum) through the mouth.
• Stimuli include the following:
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Irritation and distention of the stomach.
Unpleasant sights.
General anesthesia.
Dizziness.
Certain drugs (morphine, derivatives of digitalis).
Vomiting
• Vomiting involves squeezing the stomach
between the diaphragm and abdominal
muscles and expelling the contents through
open esophageal sphincters.
• Prolonged vomiting can lead to alkalosis
(higher than normal blood pH).
Pancreas
• Stomach chyme passes into the duodenum for
chemical digestion.
• Activities of the pancreas, liver, and
gallbladder are necessary for this chemical
digestion to occur.
Anatomy Of The Pancreas
• The pancreas is a retroperitoneal gland.
• It is about 12-15 cm long and 2.5 cm thick.
• It lies posterior to the greater curvature of the
stomach.
Anatomy Of The Pancreas
• The pancreas consists of a head, body and tail.
• It is connected to the duodenum by 2 ducts.
– Pancreatic duct (duct of Wirsung) – larger. In most
people, the pancreatic duct joins the common bile duct
from the liver and gallbladder and enters the duodenum as
the hepatopancreatic ampulla (ampulla of Vater). The
ampulla opens at the major duodenal papilla.
– Accessory duct (duct of Santorini) – smaller.
Histology Of The Pancreas
• 99% of the pancreas is made up of small
clusters of glandular epithelial cells called
acini, which make up the exocrine portion of
the gland.
• The acini secrete the pancreatic juice, which is
a mixture of fluid and digestive enzymes.
Histology Of The Pancreas
• The remaining 1% of the pancreas consists of
the pancreatic islets (islets of Langerhans),
which make up the endocrine portion of the
pancreas.
• The islets secrete the hormones glucagon,
insulin, somatostatin, and pancreatic
polypeptide.
Pancreatic Juice:
Composition & Function
• The pancreatic juice is a clear, colorless liquid
consisting mostly of water, some salts, sodium
bicarbonate, and several enzymes.
Pancreatic Juice:
Composition & Function
• Sodium bicarbonate buffers acidic juice in
chyme, stops the action of stomach pepsin,
and creates the proper pH for digestive
enzymes of the small intestine.
Pancreatic Juice:
Composition & Function
• Enzymes include:
– Pancreatic amylase.
– Protein digesting enzymes.
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Trypsin.
Chymotrypsin.
Carboxypeptidase.
Elastase.
– Pancreatic lipase.
– Nucleic acid digesting enzymes.
• Ribonuclease.
• Deoxyribonuclease.
Pancreatitis
• Pancreatitis is inflammation of the pancreas.
• It can occur with alcohol abuse or with chronic
gallstones.
• Acute pancreatitis is a more severe condition
associated with heavy alcohol intake or biliary
tract obstruction.
– Trypsin begins to digest the pancreatic cells.
– Recurrent attacks are common.
Liver
• The liver is the heaviest gland of the body,
weighing about 1.4 kg.
• It is the 2nd largest organ in the body after the
skin.
• It is inferior to the diaphragm and occupies
most of the right hypochondriac region and
part of the epigastric region.
Gallbladder
• The gallbladder is a pear-shaped sac that is
located in a depression of the posterior
surface of the liver.
• It is about 7-10 cm long and typically hangs
from the anterior inferior margin of the liver.
Anatomy Of The Liver
• The liver is divided into a large right lobe and
a smaller left lobe by the falciform ligament.
• The falciform ligament also suspends the liver.
• The liver is almost completely covered by
visceral peritoneum.
Anatomy Of The Gallbladder
• The parts of the gallbladder are the broad
fundus, the body (central portion), and the
neck (tapered portion).
Functions Of Gallbladder
• The gallbladder stores and concentrates bile
until it is needed in the small intestine.
• In the concentration process, water and ions
are absorbed by the gallbladder mucosa.
Histology Of The Liver &
Gallbladder
• The lobes of the liver are made up of many
functional units called lobules.
• The lobules contain hepatocytes arranged in
irregular, branching, interconnected plates
around a central vein.
Histology Of The Liver &
Gallbladder
• The liver has large endothelial lined spaces
called sinusoids instead of capillaries.
• Fixed phagocytes called stellate
reticuloendothelial (Kuppfer) cells destroy
worn out WBCs, RBCs, bacteria, and any other
foreign material in venous blood draining from
the GI tract.
Histology Of The Liver &
Gallbladder
• Bile is secreted from the hepatocytes and
travels through the right and left hepatic
ducts.
• These ducts merge to form the common
hepatic duct, which later joins the cystic duct
from the gallbladder.
• Bile is stored in the gallbladder for later
release.
Jaundice
• Jaundice is a yellowish coloration of the sclera,
skin, and mucous membranes due to buildup
of a yellow compound called bilirubin.
• As RBCs break down they release bilirubin.
Jaundice
• Bilirubin is processed by the liver and excreted into
bile.
• 3 categories of jaundice:
– Prehepatic jaundice – excess production of bilirubin.
– Hepatic jaundice – congenital liver disease, cirrhosis of the
liver, or hepatitis.
– Extrahepatic jaundice – blockage of bile drainage by
gallstones, cancer of the bowel or pancreas.
Bile
• Bile is a yellow, brownish, or olive-green liquid.
• Bile salts play a role in emulsification, the breakdown
of large lipid globules into a suspension of droplets.
• This also aids in the absorption of lipids following
digestion.
• These droplets increase the surface area allowing
pancreatic lipase to function more efficiently.
Functions Of The Liver
•
•
•
•
Carbohydrate metabolism.
Lipid metabolism.
Protein metabolism.
Processing of drugs and hormones.
Functions Of The Liver
•
•
•
•
•
Excretion of bilirubin.
Synthesis of bile salts.
Storage.
Phagocytosis.
Activation of vitamin D.
Gallstones
• If bile contains insufficient bile salts,
insufficient lecithin, or excessive cholesterol,
the cholesterol may crystallize to form
gallstones.
• As the gallstones grow in size or number, they
may cause minimal, intermittent, or complete
obstruction to the flow of bile from the
gallbladder to the duodenum.
Gallstones
• Treatment consists of using gallstonedissolving drugs, lithotripsy (shock-wave
therapy), or surgery.
• Recurrent gallstones, failure of drugs, or
contraindication to lithotripsy may warrant
cholecystectomy (removal of the gallbladder).
Digestive Hormones
• Gastrin promotes secretion of gastric juice, increases
gastric motility, and promotes growth of the gastric
mucosa.
• Secretin stimulates the secretion of pancreatic juice
and bile. Inhibits secretion of gastric juice.
• Cholecystokinin stimulates secretion of pancreatic
juice and causes ejection of bile from the gallbladder.
Enhances the effects of secretin.
Small Intestine (SI)
• The major events of digestion and absorption occur
in the small intestine.
• The length of the SI provides great surface area for
this to occur.
• Circular folds, villi, and microvilli also serve to
increase the surface area of the SI.
• The SI begins at the pyloric sphincter of the stomach,
coils through the central and inferior parts of the
abdomen, and eventually opens into the LI.
Functions Of The SI
• Segmentations mix chyme with digestive
juices and bring food into contact with the
mucosa for absorption.
• Peristalsis propels food through the SI.
Functions Of The SI
• Completes the digestion of carbohydrates,
proteins, and lipids.
• Begins and completes the digestion of nucleic
acids.
• Absorption of 90% of nutrients and water.
Anatomy Of The SI
• The SI is divided into 3 regions:
– Duodenum – shortest region, retroperitoneal,
starts at the pyloric sphincter of the stomach.
– Jejunum – between the duodenum and ileum.
– Ileum – the longest region, joins the large
intestine at the ileocecal sphincter.
Anatomy Of The SI
• Circular folds – permanent ridges in the
mucosa. They enhance absorption by
increasing the surface area of the SI and by
causes the chyme to spiral, rather than move
in a straight line.
Histology Of The SI
• The same 4 basic layers that make up the rest
of the GI tract exist in the SI as well, with
some basic exceptions.
• The mucosa forms a series of fingerlike villi
(tufts of hair) projections, which increase the
surface area available for absorption.
Histology Of The SI
• Each villus contains an arteriole, a venule, a
blood capillary network, and a lacteal through
which nutrients are absorbed.
Histology Of The SI
• The mucosa is simple columnar epithelium,
which contains absorptive cells, goblet cells,
enteroendocrine cells, and Paneth cells.
• The apical surface of the absorptive cells
contains microvilli (bundles of actin filaments).
Collectively, they are referred to as the brush
border. They increase the surface area.
Histology Of The SI
• The mucosa contains deep crevices lined with
glandular epithelium.
• Cells lining the crevices form the intestinal
glands (crypts of Lieberkuhn), which secrete
intestinal juice.
Histology Of The SI
• Paneth cells secrete lysozyme, a bactericidal
enzyme.
• The lamina propria of the SI has an abundance of
mucosa-associated lymphatic tissue (MALT).
• Solitary lymphatic nodules are present as well as
groups of aggregated lymphatic nodules (Peyer’s
patches).
• Duodenal (Brunner’s) glands of the submucosa
secrete an alkaline mucus.
Intestinal Juice
• Intestinal juice is a clear yellow fluid that
contains water and mucus.
• It is slightly alkaline (pH 7.6).
• It provides a liquid medium to assist in the
absorption of substances from chyme.
Brush Border Enzymes
• The absorptive epithelial cells synthesize
several digestive enzymes, called brush border
enzymes, and insert them into the plasma
membrane of the microvilli.
• Consequently, some digestion occurs at the
surface of the epithelial cells and not
exclusively in the lumen of the SI.
Mechanical Digestion In The Small
Intestine
• Segmentations – localized mixing contractions
that occur in portions of the intestines
distended by a large volume of chyme.
Sloshes chyme back and forth.
• Migrating motility complexes – a type of
peristalsis that moves the chyme down the
length of the SI after segmentation has
occurred.
Chemical Digestion In The SI
• Chyme entering the small intestines contains
partially digested carbohydrates, proteins, and
lipids.
• Pancreatic juice, bile, and intestinal juice
complete the effort of digestion.
Digestion Of Carbohydrates
• Pancreatic amylase, sucrase, lactase, and maltase
complete the digestion of carbohydrates.
• These enzymes break complex carbohydrates into
monosaccharides, which can be absorbed.
• Lactose intolerance occurs in people whose mucosal
cells fail to produce enough of the enzyme lactase.
– Symptoms include siarrhea, gas, bloating, and abdominal
cramps after the consumption of dairy products.
Digestion Of Proteins
• Pepsin, trypsin, chymotrypsin,
carboxypeptidase, elastase, and peptidases
complete the process of protein digestion.
• Proteins are broken down into single amino
acids, which can be absorbed.
Digestion Of Lipids
• Lipases complete the process of lipid digestion
in the SI.
• Bile salts increase the surface area of
triglycerides through the process of
emulsification. The globules are converted
into droplets.
• Lipids are broken down into monoglycerides,
which can then be absorbed.
Digestion Of Nucleic Acids
• Pancreatic juice contains two nucleases:
ribonuclease (breaks down RNA) and
deoxyribonuclease (breaks down DNA).
• Brush border enzymes further break these
down into pentoses, phosphates, and
nitrogenous bases, which can be absorbed.
Absorption In The SI
• Forms that can be absorbed:
– Monosaccharides (glucose, fructose, and
galactose) from carbohydrates.
– Single amino acids, dipeptides, and tripeptides
from proteins.
– Fatty acids, glycerol, and monoglycerides from
triglycerides.
Absorption In The SI
• Mechanisms of absorption:
– Diffusion.
– Facilitated diffusion.
– Osmosis.
– Active transport.
Absorption In The SI
• Passage of digested nutrients from the
gastrointestinal tract into the blood or lymph is called
absorption.
• About 90% of the nutrients are absorbed in the SI.
• The other 10% occurs in the stomach and the large
intestine.
• Any undigested or unabsorbed material passes
through to the LI.
Absorption Of Monosaccharides
• All carbohydrates are absorbed as
monosaccharides.
• They are absorbed via facilitated diffusion and
active transport.
• The SI can absorb up to 120 grams of
carbohydrates per hour.
Absorption Of Amino Acids,
Dipeptides, & Tripeptides
• Most proteins are absorbed as amino acids via
active transport processes.
• About half of the amino acids come from
food.
• The other half of the amino acids come from
proteins in digestive juices and dead cells that
slough off the mucosal surface.
Absorption Of Lipids
• All dietary lipids are absorbed via simple diffusion.
• Adults absorb about 95% of the lipids present in the
SI.
• Most dietary fatty acids require bile for adequate
absorption.
• When lipids are not absorbed properly, the fatsoluble vitamins A, D, E, & K are not absorbed
properly.
Absorption Of Electrolytes
• Most of the electrolytes absorbed by the SI
come from gastrointestinal secretions, and
some come from ingested foods and liquids.
• Active transport mechanisms are utilized to
absorb Na+ ions.
Absorption Of Electrolytes
• Negatively charged bicarbonate, chloride,
iodide, and nitrate ions can passively follow
Na+ or be actively transported.
• Iron, potassium, magnesium, and phosphate
ions are absorbed via active transport.
Absorption Of Vitamins
• The fat-soluble vitamins A, D, E, & K are
included with dietary lipids and absorbed via
simple diffusion.
• Most water-soluble vitamins are absorbed via
simple diffusion.
• Vitamin B12 combines with intrinsic factor and
the combination is absorbed via active
transport.
Absorption Of Water
• The volume of water in the SI (about 9.3 liters
daily) comes from ingested liquids and gastric
secretions.
• The SI absorbs about 8.3 liters of it.
• 90% of the remaining water (about 0.9 liters)
is absorbed in the large intestine.
• Water absorption occurs via osmosis.
Absorption Of Alcohol
• Alcohol is lipid soluble and begins to be absorbed in
the stomach.
• There is greater surface area for absorption in the SI;
therefore, the longer alcohol remains in the stomach,
the more slowly blood alcohol rises.
• Fatty acids in the chyme slow gastric emptying;
therefore, eating fatty foods with alcohol will cause a
slower rise in blood alcohol.
Large Intestine (LI)
• The large intestine is the terminal portion of
the GI tract.
• It is divided into four principal regions.
Large Intestine Functions
• Completion of absorption (water, ions, and vitamins).
• Production of some B vitamins and vitamin K by
bacteria in the LI.
• Formation of feces.
• Expulsion of feces (defecation) from the body
through haustral churning and peristalsis.
LI Anatomy
• The LI extends from the ileum to the anus.
• It is about 1.5 m long and 6.5 cm in diameter.
• It is attached to the posterior abdominal wall
by mesocolon.
Major Regions Of The LI
•
•
•
•
Cecum.
Colon.
Rectum.
Anal canal.
LI Anatomy Continued…
• The iliocecal valve guards the opening to the LI from
the ileum.
• The cecum hangs inferior to the iliocecal valve.
• The appendix is a twisted, coiled tube hanging on the
cecum.
• The open end of the cecum merges with the colon,
which is divided into ascending, transverse,
descending, and sigmoid portions.
LI Anatomy Continued…
• The last 20 cm of the GI tract make up the rectum.
• The anal canal is the termination of the rectum.
• The anus is the opening of the anal canal to the
exterior.
• This opening is guarded by an internal anal sphincter
(involuntary) and an external anal sphincter
(voluntary).
Appendicitis
• Inflammation of the appendix is termed appendicitis.
• Obstruction of the lumen of the appendix by chyme,
inflammation, a foreign body, carcinoma, stenosis, or
kinking of the organ precedes the appendicitis.
• It is characterized by high fever, elevated WBC count,
and a neutrophil count higher than 75%.
• Subsequent infection can produce edema and
ischemia.
• Perforation can occur within 24 hours.
Appendicitis
• An appendicitis typically begins with referred
pain to the umbilical region of the abdomen,
followed by anorexia, nausea, and vomiting.
• After several hours, pain localizes in the right
lower quadrant.
Appendicitis
• The pain is continuous, dull or severe.
• Coughing, sneezing, or body movements can
exacerbate the pain.
• Early appendectomy (removal of the
appendix) is recommended, because it is safer
to undergo surgery than to risk rupture and
peritonitis.
Histology Of The Large Intestine
• No villi or permanent circular folds are found
in the mucosa of the large intestine.
• The epithelium contains mostly absorptive
and goblet cells. The absorptive cells
participate mainly in water absorption.
Histology Of The Large Intestine
• The submucosa is similar to that found in the rest of
the GI tract.
• The muscularis consists of an external layer of
longitudinal smooth muscle and an internal layer of
circular smooth muscle. This forms three
conspicuous longitudinal bands called the teniae
coli.
• Tonic contractions of the bands draw the LI together
into pouches called haustra.
Mechanical Digestion In LI
• The iliocecal sphincter regulates the passage of
chyme from the ileum into the cecum.
• Normally, the valve remains partially closed to limit
passage of chyme.
• After a meal, the gastroileal reflex intensifies ileal
peristalsis.
• The hormone gastrin also relaxes the sphincter.
Mechanical Digestion In LI
• Haustral churning – The haustra remain relaxed and
become distended while they fill up. When the
distention reaches a certain point, the walls contract
and squeeze the contents into the next haustrum.
• Peristalsis occurs.
• Mass peristalsis is a movement that occurs at the
middle of the transverse colon and quickly drives
contents into the rectum.
• Food in the stomach initiates the gastrocolic reflex.
Chemical Digestion LI
• Mucous is secreted by the glands of the LI, but
no enzymes are secreted.
• The bacteria of the LI perform the final stages
of digestion.
Chemical Digestion LI
• Bacteria ferment any remaining carbohydrates,
which releases hydrogen, carbon dioxide, and
methane gases. These gases constitute the flatus
(gas) in the colon. When the gas is excessive it is
termed flatulence.
• Bacteria convert any remaining proteins into amino
acids.
• Some B vitamins and vitamin K are produced by the
bacteria in the colon.
Absorption & Feces Formation In
The LI
• Water is absorbed from the chyme over a period of
3-10 hours. As the water becomes absorbed it
becomes solid or semi-solid and is termed feces.
• The feces consists of water, inorganic salts, sloughed
off epithelial cells, bacteria, products of bacterial
decomposition, unabsorbed digested materials, and
indigestible parts of food.
• The LI absorbs water, vitamins and ions (I.e. sodium
and chloride).
Occult Blood
• Occult Blood refers to blood that is hidden
and not detectable by the human eye.
• Urine and feces are often examined for occult
blood.
• Occult blood testing is utilized to screen for
colorectal cancer.
Defecation Reflex
• Mass peristalsis movements push fecal material from
the sigmoid colon into the rectum.
• This causes distention of the rectal wall, which
stimulates stretch receptors and initiates a
defecation reflex that empties the rectum.
• This reflex opens the internal anal sphincter.
• The external anal sphincter is voluntarily controlled.
Diarrhea
• Diarrhea is an increase in the frequency, volume, and
fluid content of the feces caused by increased
motility and decreased absorption by the intestines.
• Frequent diarrhea can result in dehydration and
electrolyte imbalances.
• Excessive motility can be caused by lactose
intolerance, stress, and microbes that irritate the
gastrointestinal mucosa.
Constipation
• Constipation refers to infrequent or difficult
defecation caused by decreased motility of
the intestines.
• Feces remain in the LI for prolonged periods of
time. This causes increased water absorption
and the feces become dry and hard.
Constipation
• Constipation can be caused by poor habits
(delaying defecation), spasms, insufficient
fiber in the diet, inadequate fluid intake, lack
of exercise, emotional stress, and certain
drugs.
Constipation
• Treatment often involves laxatives to induce
defecation.
• Laxatives can be habit forming; therefore,
adding fiber to the diet, increasing the
amount of exercise, and increasing fluid intake
are safer ways to control the problem.
Dietary Fiber
• Dietary fiber consists of indigestible plant
carbohydrates such as cellulose, lignin, and
pectin. These are found in fruits, vegetables,
grains, and beans.
Dietary Fiber
• Insoluble fiber does not dissolve in water.
Insoluble fiber speeds up passage of materials
through the track.
– Woody or structural parts of plants include the
skins of fruits and vegetables and the bran coating
around wheat and corn kernels.
Dietary Fiber
• Soluble fiber dissolves in water and forms a
gel that slows passage of material through the
tract.
– Soluble fiber is found in beans, oats, barley,
broccoli, prunes, apples, and citrus fruits.