Download Digestive System

Digestive System
Dr. Michael P. Gillespie
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
 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
 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
 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
 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 (“tonguetied”).
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 and pushed 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
 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.
 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 fat-laden 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:





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.
 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 gallstone-dissolving 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 fat-soluble
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.