Digestive System

• Accessory Digestive Organs include the salivary glands, liver, pancreas, teeth, tongue, and gallbladder
• Accessory digestive organs assist in the breakdown of food, some produces secretions that empty into the GI tract
• Gastrointestinal (GI) tract includes the oral cavity, pharynx, esophagus, stomach, small intestine, large intestine, and anus
• Six main functions of the Digestive System:

  1. Ingestion
  2. Motility
  3. Secretion
  4. Digestion
  5. Absorption
  6. Elimination
• Ingestion is the process of placing solids and liquids in the oral cavity
• Motility is the muscular contraction that mix and move materials through the GI
• Secretion is the producing and releasing of fluid products to facilitate digestion
• Digestion is the breakdown of ingested food into smaller structures

  • mechanical digestion: physical breakdown w/o change in chemical structure
  • chemical digestion: an enzymatic breakdown that changes the chemical structure
• Absorption: the movement/transport of substance from the GI tract to blood or lymph
• Elimination is the excretion of waste
• GI tract is a hollow tube of four consecutive layers called tunics
• Tunics of the GI Tract (Innermost to Outermost):

  1. Mucosa
  2. Submucosa
  3. Muscularis
  4. Adventitia
• Mucosa: is the inner lining mucous membrane

  • comes in contact w/ substances w/n the lumen
  • simple columnar epithelium and non-keratinized simple squamous
• Submucosa: contains blood vessels, lymph vessels, and nerves

  • Areolar and Dense Irregular CT
  • Has a submucosal nerve plexus
  • Houses MALT
  • peyer patches: large aggregates of lymphatic nodules in the small intestine
• Muscularis: has a circular inner layer of muscles and a longitudinal outer layer of muscles

  • Myenteric Nerve plexus integrates the muscle
  • propulsion (directional movement) and mixing (backward and forward motion) are the two types of movement
• Enteric nervous system: consist of the myenteric nerve plexus and submucosal nerve plexus

  • it is thickened at several points to create a sphincter, that controls movement to the next section of the GI tract
• Adventitia: outermost layer

  • areolar CT located outside of the peritoneal cavity
• Autonomic Nervous System:

  • parasympathetic promotes GI activity
  • sympathetic opposes GI activity
• Regulation of the Digestive System:

  • reflexes
  • hormonal control
  • receptors
• Reflexes (ANS or ENS)

  • short reflex: only involves ENS; coordinates small segments of GI tract
  • long reflex: involves input to CNS and autonomic motor output, coordinates GI tract motility, secretions, and accessory digestive organs
• Hormonal Control

  • gastrin, secretin, cholecystokinin, motilin are hormones that regulate digestion
• Receptors: initiate reflexes

  • baroreceptors detect stretch of the GI tract wall
  • chemoreceptors monitor chemical contents in the lumen
• Peritoneum: serous membrane associated with the abdominopelvic cavity

  • parietal peritoneum: lines the inner surface of the abdominal wall
  • visceral peritoneum: covers the surfaces of internal organs
  • peritoneal cavity: filled with peritoneal fluid for lubrication
• Intraperitoneal organs: organs surrounded by the visceral peritoneum

  • stomach, majority of small intestine, some of large intestine
• Retroperitoneal organs: organs that lie outside of the parietal peritoneum

  • most of the duodenum, pancreas, ascending/descending colon, rectum
• Mesenteries: double layer of peritoneum that supports, suspends, and stabilizes intraperitoneal GI organs

  • greater omentum: covers most of the abdominal organs, serves to insulate organs and stores fat
  • lesser omentum: connects stomach and small intestine to liver
• falciform ligament attaches the liver to the abdominal wall
• Upper GI tract organs and accessory structures

  • oral cavity, salivary glands, pharynx, esophagus, stomach, duodenum
• oral cavity and salivary glands

  • mechanical digestion begins here, involves the formation of bolus
  • salivary glands secrete saliva, salivary amylase digest starch
• Pharynx:

  • bolus is is moved to pharynx (throat) during swallowing, mucus is secreted to facilitate swallowing
  • funnel shaped and lined w/ nonkeratinized simple squamous epithelium to protect against abrasion
• Esophagus:

  • bolus is transported from the pharynx into the stomach, lubricated by mucus
  • normally a collapsed tubular passageway
  • has thick elastic fibers and nonkeratinized squamous epithelium
• Stomach:

  • bolus mixes w/ gastric secretions, chyme is formed when mixing
• Oral Cavity regions

  1. Vestibule: space b/w gums, lips, cheeks
  2. Oral Cavity proper: leads to oropharynx
• Oral Cavity structure

  • lips, palate, uvula, tongue
  • tongue has projections called papillae, these are involved in taste
• Oral Cavity Histology:

  • stratified squamous epithelium protects against abrasion
• Salivary glands: produce saliva

  • Intrinsic salivary glands: w/n oral cavity, unicellular glands, contains lingual lipase (enzyme that begins digestion)
  • Extrinsic salivary glands: Outside the oral cavity, produces most saliva, parotid, submandibular, sublingual glands
• saliva is conducted through the parotid duct to the oral cavity
• submandibular salivary glands produce 60-70% if saliva
• sublingual salivary gland produces 3-5% of saliva
• Types of secretory cells

  1. Mucous cells: secrete mucin, forming mucus upon hydration
  2. Serous cells: secrete watery fluid containing electrolytes and salivary amylase