Digestive

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Digestive Absorption
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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.

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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.

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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.

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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.

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Functions Of The Digestive System

• Ingestion
• Secretion
• Mixing and propulsion
• Digestion
• Absorption
• Defecation

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Ingestion

• This process involves taking foods and liquids
  into the mouth (eating).

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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.

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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.

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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.

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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.

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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.

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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.

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Layers Of The GI Tract

• Mucosa
• Submucosa
• Muscularis
• Serosa

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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).

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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.

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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.

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Submucosa

• The submucosa consists of areolar connective
  tissue that binds the mucosa to the muscularis.

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Submucosa

• It contains blood and lymphatic vessels that
  receive absorbed food molecules.

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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.

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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.

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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).

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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.

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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.

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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.

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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.

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Peritoneal Folds

• Greater omentum.
• Falciform ligament.
• Lesser omentum.
• Mesentery.
• Mesocolon.

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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.

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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.

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Lesser Omentum

• The lesser omentum suspends the stomach and
  duodenum from the liver.
• It contains some lymph nodes.

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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.

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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.

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Peritonitis

• Peritonitis is an acute inflammation of the
  peritoneum.
• Contamination of the peritoneum by infectious
  microbes causes it.

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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.

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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.

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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.

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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).

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Mouth

• The hard palate is the anterior portion of the
  roof of the mouth and is formed by the maxillae
  and palatine bones.

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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.

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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.

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Minor Salivary Glands

• Labial glands in the lips.
• Buccal glands in the cheeks.
• Palatal glands in the palate.
• Lingual glands in the tongue.

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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.

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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.

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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.

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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.

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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).

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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.

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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).

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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.

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Tongue

• Lingual glands secrete both mucus and a watery
  serous fluid that contain the enzyme lingual
  lipase.

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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.

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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.

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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.

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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.

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Dentitions

• Humans have two dentitions (sets of teeth).
• Deciduous teeth.
• Permanent teeth.

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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.

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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.

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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.

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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.

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Mechanical Chemical Digestion In The Mouth

• The food becomes a soft, flexible mass called a
  bolus (lump) that is easily swallowed.

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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.

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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.

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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.

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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.

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Pharynx

• The nasopharynx functions only in respiration
  However, the oropharynx and laryngopharynx
  function in both respiration and digestion.

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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.

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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.

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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).

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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.

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Physiology Of The Esophagus

• The upper esophageal sphincter regulates the
  entrance of food into the esophagus from the
  laryngopharynx.

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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.

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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).

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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).

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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).

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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).

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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.

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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.

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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.

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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.

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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.

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Pancreatic JuiceComposition Function

• The pancreatic juice is a clear, colorless liquid
  consisting mostly of water, some salts, sodium
  bicarbonate, and several enzymes.

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Pancreatic JuiceComposition 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.

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Pancreatic JuiceComposition Function

• Enzymes include
• Pancreatic amylase.
• Protein digesting enzymes.
• Trypsin.
• Chymotrypsin.
• Carboxypeptidase.
• Elastase.
• Pancreatic lipase.
• Nucleic acid digesting enzymes.
• Ribonuclease.
• Deoxyribonuclease.

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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.

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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.

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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.

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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.

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Anatomy Of The Gallbladder

• The parts of the gallbladder are the broad
  fundus, the body (central portion), and the neck
  (tapered portion).

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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.

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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.

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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.

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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.

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129
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.

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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.

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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.

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Functions Of The Liver

• Carbohydrate metabolism.
• Lipid metabolism.
• Protein metabolism.
• Processing of drugs and hormones.

133
Functions Of The Liver

• Excretion of bilirubin.
• Synthesis of bile salts.
• Storage.
• Phagocytosis.
• Activation of vitamin D.

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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.

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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).

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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Histology Of The SI

• Each villus contains an arteriole, a venule, a
  blood capillary network, and a lacteal through
  which nutrients are absorbed.

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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.

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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.

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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 (Peyers
  patches).
• Duodenal (Brunners) glands of the submucosa
  secrete an alkaline mucus.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

157
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.

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Absorption In The SI

• Mechanisms of absorption
• Diffusion.
• Facilitated diffusion.
• Osmosis.
• Active transport.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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Large Intestine (LI)

• The large intestine is the terminal portion of
  the GI tract.
• It is divided into four principal regions.

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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.

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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.

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Major Regions Of The LI

• Cecum.
• Colon.
• Rectum.
• Anal canal.

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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.

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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).

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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).

190
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.

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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.

192
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.

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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.

194
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.

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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.

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Dietary Fiber

• Dietary fiber consists of indigestible plant
  carbohydrates such as cellulose, lignin, and
  pectin. These are found in fruits, vegetables,
  grains, and beans.

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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.

198
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.