Chapter 25 Anatomy of the Digestive System

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Chapter 25 Anatomy of the Digestive System
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Organization of the Digestive System

• Digestion is the complete process of altering the
  physical and chemical composition of ingested
  food material so that it can be absorbed and used
  by the body cells
• Organs of digestion (Box 25-1)
• Alimentary canal the tube of digestive organs
  that moves from the mouth to the anus
• It is an extension of the external environment so
  food passing through is not within the internal
  environment of the body

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Organization of the Digestive System
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Organization of the Digestive System

• Walls of the GI Tract
• Refers to the stomach and intestines
• Consists of 4 layers of tissue
• Mucosa
• Innermost layer made up of the mucous epithelium,
  lamina propria and the muscularis mucosa
• Submucosa
• Contains many glands, blood vessels and
  parasympathetic nerves that forms the submucosal
  (Meissners) plexus
• Muscularis
• Inner layer of circular muscle and outer layer of
  longitudinal smooth muscle
• Contains Auerbach ( myenteric) plexus between the
  2 muscle layers
• Serosa
• Outermost layer, same as the visceral peritoneum
• Connected to the parietal peritoneum via the
  mesentery

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Organization of the Digestive System
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Mouth

• Structure of the Oral Cavity
• Lips
• Externally covered by skin, internally by a
  mucous membrane
• Keep food in the mouth
• Sense temperature and texture of food
• Needed to form speech sounds
• Cheeks
• Lateral boundaries of the oral cavity
• Formed largely by the buccinator muscle lined
  with mucous membrane

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Mouth

• Hard and Soft Palate
• Hard palate palatine and maxillary bones
• soft palate muscular arch separating the mouth
  from the nose
• Uvula projects off the soft palate
• Tongue
• Skeletal muscle covered by mucous membrane
• Covered with papillae which contain taste buds
• Lingual frenulum helps anchor the tongue to the
  floor of the mouth
• Rich supply of blood vessels allows for quick
  absorption (sublingual medications)
• Intrinsic muscles originate and insert in the
  mouth, used for mastication and speech
• Extrinsic muscles insert into the tongue, but
  originate from the hyoid or skull bones, used for
  swallowing (deglutition) and speech

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Mouth

• Salivary Glands
• Parotid glands
• Anterior and inferior to the ear
• Produce watery saliva containing enzymes open
  into the mouth via the Stenson ducts
• Inflammation mumps
• Submandibular glands
• At the mandibular angle
• Produce saliva containing enzymes and mucus
• Wharton ducts open into the mouth on either side
  of the frenulum
• Sublingual glands
• In front of the submandibular glands
• Drained by ducts of Rivinus
• Produce mucus saliva

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Teeth

• Involved in mastication and speech
• Crown Exposed portion of the tooth
• Neck enameled part of tooth below gum line
• Root anchors the tooth into the periodontal
  membrane
• Enamel Hard, protective outer covering
• Dentin living, cellular, calcified tissue within
  the root, dentin is covered by cellular bone-like
  structure that helps hold tooth in the socket.
• Pulp cavity within the dentin, filled with blood
  vessels, nerves, and connective tissue
• Periodontal ligaments hold tooth in socket.
• Cementum Anchors the root
• Gingiva dense, fibrous C.T. covered by
  stratified squamous epithelium.

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Teeth

• Two sets
• Primary, deciduous, milk Childhood
• Permanent or secondary Adult (32)
• Types
• Incisors, canines, premolars and molars

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Pharynx and Esophagus

• Swallowing moves the food mass, or bolus, from
  the mouth into the pahrynx
• Esophagus a muscular tube which connects the
  pharynx to the stomach
• Lies posterior to the trachea
• Pierces through the diaphragm
• 1st segment of the digestive tube where the 4
  layers of tissue can be observed
• Upper 1/3 voluntary, striated muscle
• Middle1/3 involuntary, mixed
• Lower 1/3 Involuntary, smooth
• Upper and lower esophageal sphincters
• Lower AKA cardiac sphincter
• Stretching of the esophageal hiatus may allow
  upward bulging of the stomach and lower end of
  the esophagus hiatal hernia
• GERD stomach acid flows back up through the LES

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Stomach

• Size and Position
• Enlarged after a meal and collapses as the food
  is pushed through
• Holds 1- 1.5 L
• Under the liver and the diaphragm, most left of
  midline
• Divisions
• Fundus, body and pyloris
• Cardiac portion where esophagus meets the
  stomach
• LES or cardiac sphincter controls movement of
  food from the esophagus into the stomach
• Pyloric sphincter controls movement from the
  stomach into the small intestine

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Stomach Wall

• Gastric mucosa
• Epithelial lining of the stomach made up of rugae
  folds and gastric pits
• Gastric glands, which secrete HCl and digestive
  enzymes, are found within the pits
• Contain 3 major cell types
• Chief cells secrete pepsin
• Parietal cells secrete HCl and Intrinsic Factor
• Endocrine cells produce ghrelin and gastrin
• Gastric muscle
• 3 layers of muscle longitudinal, circular and
  oblique
• Increases contraction and mixing ability

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Functions of the Stomach

• Food reservoir food is stored until it can be
  digested
• Secrete gastric juice for digestion of food
• Churns and mixes food
• Secretes intrinsic factor important in B12 uptake
• Small amount of absorption drugs, water,
  alcohol, some fats
• Secretes gastrin ( regulates digestive functions)
  and ghrelin (increases apetite)
• HCl destroys pathogens

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Small Intestine

• 3 divisions duodenum, jejunum, ileum
• Wall of the Small Intestine
• Made up of circular plicae containing villi
  projections
• Each villus contains an arteriole, a venule and
  lacteal
• Microvilli line each villus creating a brush
  border
• Goblet cells mucus
• Endocrine cells hormones
• Intestinal crypts of Lieberkuhn are sites of
  mitosis where new enterocytes are formed
• New cells are pushed up and the old ones shed
• Paneth cells are located at the base of each
  crypt and produce enzymes to inhibit bacterial
  growth

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Large Intestine

• Diameter is greater than the small intestine, but
  it is much shorter (5 ft vs 20 ft)
• Divisions
• Cecum blind pouch off the first part of the LI,
  located in the LRQ
• Ileocecal valve allowsmaterial to move from SI to
  LI
• Colon
• Ascending colon moves up the right side of the
  abdomen
• Transverse colon moves horizontally across the
  abdomen from the hepatic flexure to the splenic
  flexure
• Descending colon moves down the left side of the
  abdomen
• Sigmoid colon extends below the colon and
  connects to the rectum
• Rectum
• Last part of the colon which terminates at the
  anus contains the internal (involuntary) and
  external (voluntary) anal sphincters

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Large Intestine

• Modifications of the walls of the LI
• Intestinal mucus glands lubricate feces
• Longitudinal muscle group produce taeniae coli
• Circular muscle group produces haustra

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Veniform Appendix

• Accessory organ whose function is not fully
  understood
• Possible breeding ground for nonpathogenic
  intestinal bacteria
• Contains lymph tissue
• Inflammation is known as appendicitis
• McBurneys point area in the RLQ which is
  extremely tender to touch with appendicitis

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Peritoneum and Mesenteries

• Peritoneum large sheet of serous membrane
• Visceral Covers organs
• Parietal Covers interior surface of body wall
• Retroperitoneal Certain organs covered by
  peritoneum on only one surface and are considered
  behind the peritoneum e.g., kidneys, pancreas,
  duodenum
• Mesenteries two layers of peritoneum with thin
  layer of loose C.T. between
• Routes by which vessels and nerves pass from body
  wall to organs
• Greater omentum connects greater curvature of
  the stomach to the transverse colon.
• Lesser omentum connects lesser curvature of the
  stomach and the proximal part of the duodenum to
  the liver and diaphragm.
• Transverse mesocolon, sigmoid mesocolon,
  mesoappendix.

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Liver

• Lies immediately beneath the diaphragm, within
  the right hypochondrium
• 2 lobes separated by the falciform ligament
• Right lobe
• Right lobe proper, caudate lobe and quadrate lobe
• Left lobe
• Each lobe is separated into lobules and supported
  by a capsule of Glisson
• A central vein extends through each lobule
• Hepatic cells, sinusoids, bile canaliculi,
  arteries and veins also make up the lobules

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Liver

• Hepatic lobule function
• Blood enters lobule from hepatic artery
• Blood oxygenates hepatocytes
• Sinusoids contain phagocytic Kupffer cells
• Blood continues along the sinusoids to the
  central vein
• Central veins lead to the main hepatic veins
  which drain into the inferior vena cava
• Bile formed by hepatocyes passes through the
  canaliculi to join bile ducts

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Liver

• Bile Ducts
• Small bile ducts join to form the right and left
  hepatic ducts which join to form the common
  hepatic duct
• The common hepatic duct merges with the cystic
  duct from the gallbladder to form the common bile
  duct
• Bile is emptied into the SI at the duodenum via
  the major duodenal papilla

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Liver functions

• Liver cells detoxify substances
• Liver cells secrete bile which aids in fat
  digestion
• Liver cells help metabolize proteins, fats and
  carbohydrates
• Liver cells store iron, vitamins A, B12 and D
• Produces plasma proteins and serves as a site of
  hematopoeisis during fetal development

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Gallbladder

• Lies underneath the liver
• Cholecystitis GB inflammation
• Cholelithiasis gallstone formation
• Cholecytectomy GB removal

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Gallbladder Functions

• The GB stores bile and concentrates it
• During fat digestion, the GB contracts and ejects
  bile into the duodenum
• Jaundice results when an obstruction of bile flow
  occurs
• Bile cannot be lost through the feces and enters
  the blood, creating a yellowish skin hue

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Pancreas

• Pancreas both endocrine and exocrine
• Head, body and tail
• Endocrine pancreatic islets. Produce insulin,
  glucose, and somatostatin
• Exocrine groups acini (grape-like cluster) form
  lobules separated by septa, produce digestive
  enzymes
• Intercalated ducts lead to intralobular ducts
  lead to interlobular ducts lead to the pancreatic
  duct.
• Pancreatic duct joins common bile duct and enters
  duodenum at the hepatopancreatic ampulla
  controlled by the hepatopancreatic ampullar
  sphincter