Digestive System

• Digestive system

  – Acquires nutrients from environment

  • Used to synthesize essential compounds (anabolism)

  • Broken down to provide energy to cells (catabolism)
• Digestive system

  • Digestive tract
  • Gastrointestinal (GI) tract or alimentary canal
  • Muscular tube
  • Extends from oral cavity to anus
  • Accessory organs
  • Teeth, tongue, and various glandular organs
• Major organs of digestive tract

  • Oral Cavity (Mouth): ingestion, mechanical digestion with accessory organs, moistening, mixing with salivary secretions
  • Pharynx: Muscular propulsion of materials into the esophagus
  • Esophagus: Transport of materials to the stomach
  • Stomach: Chemical digestion; mechanical digestion through muscular contractions
  • Small Intestine: Enzymatic digestion and absorption of water, organic substrates, vitamins, and ions
  • Large Intestine: Dehydration and compaction of indigestible materials in preparation for elimination
• Accessory Organs of the Digestive System

  • Teeth: Mechanical digestion by chewing (mastication)
  • Tongue: Assists mechanical digestion with teeth, sensory analysis
  • Salivary Glands: Secretion of lubricating fluid containing enzymes that break down carbohydrates
  • Liver: Secretion of bile (important for lipid digestion), storage of nutrients, many other vital functions
  • Gallbladder: Storage and concentration of bile
  • Pancreas: Exocrine cells secrete buffers and digestive enzymes; endocrine cells secrete hormones
• Integrated processes of digestive system

  • Ingestion
  • Mechanical digestion and propulsion
  • Chemical digestion
  • Secretion
  • Absorption
  • Defecation
• § Ingestion

  – Occurs when food enters oral cavity

  § Mechanical digestion and propulsion 

  – Crushing and shearing of food

  – Propelling food along digestive tract

  § Chemical digestion

  – Chemical breakdown of food into small organic fragments for absorption by digestive epithelium

  § Secretion

  – Release of water, acids, enzymes, buffers, and salts

  – By epithelium of digestive tract, glandular organs, and gallbladder
• Absorption

  • Movement of organic molecules, electrolytes, vitamins,minerals, and water
  • Across digestive epithelium

  Defecation

  • Elimination of wastes from body
  • Compacted, dehydrated wastes are called feces
  • Into interstitial fluid of digestive tract
• Lining of digestive tract

  • Safeguards surrounding tissues against
  • Corrosive effects of digestive acids and enzymes, so we don’t auto ingest ourselves
  • Mechanical stresses, such as abrasion
  • Bacteria either ingested with food or that reside in digestive tract
• Peritoneum

  • Serous membrane lining peritoneal cavity
  • Superficial mesothelium covering a layer of areolar tissue
  • Visceral peritoneum (serosa)
  • Covers organs within peritoneal cavity
  • Parietal peritoneum
  • Lines inner surfaces of body wall
• Peritoneal fluid

  • Produced by serous membrane lining
  • Allows sliding of parietal and visceral surfaces without friction or irritation
  • About 7 liters produced and absorbed daily, but very little in peritoneal cavity at one time
  • Ascites—abdominal swelling due to buildup of peritoneal fluid
• Mesenteries

  • Double sheets of peritoneal membrane
  • Suspend portions of digestive tract within peritoneal cavity
  • Connect parietal peritoneum with visceral peritoneum
  • Provide a route to and from digestive tract for bloodvessels, nerves, and lymphatic vessels
  • Stabilize positions of attached organs
  • Prevent intestines from becoming entangled
• Lesser omentum

  – Stabilizes position of stomach

  – Provides access route for blood vessels and other structures entering or leaving liver

  Falciform ligament

  – Helps stabilize position of liver relative to diaphragm and abdominal wall

  Dorsal mesentery

  – Enlarges to form an enormous pouch, the greater omentum

  • Extends inferiorly between body wall and anterior surface of small intestine

  • Hangs like an apron from lateral and inferior borders of stomach
• Adipose tissue in greater omentum

  • Conforms to shapes of surrounding organs
  • Pads and protects surfaces of abdomen
  • Provides insulation to reduce heat loss
  • Stores lipid energy reserves
  • Contributes to “beer belly"
• § Mesentery proper

  – Thick mesenterial sheet

  – Provides stability

  – Permits some independent movement

  – Suspends all but first 25 cm of small intestine

  § Mesentery associated with duodenum and pancreas

  – Fuses with abdominal wall, locking organs in place 

  – Posterior to peritoneal cavity — retroperitoneal
• Mesocolon

  • Mesentery associated with part of large intestine
  • During development, mesocolon of ascending colon, descending colon, and rectum
  • Fuse to posterior body wall
  • Lock regions in place
• Histology of digestive tract

  • Major layers of digestive tract
  • Mucosa
  • Submucosa
  • Muscular layer 
  • Serosa
• Histological Organization of the Digestive Tract 

  • throughout GI tract, epithelium is either stratified or simple, depends on location, function and stresses it encounters
  • in oral cavity, pharynx, esophagus and anal canal, it is stratified squamous epithelium more friction and potential for damage so more layers
  • stomach, small intestine and most of large intestine we have simple columnar epithelium and neuroendocrine cells that secrete hormones that coordinate activity of GI tract and muscles along the way
  • Mucosa: inner lining of digestive tract, also moistened by glandular secretions
• Submucosa

  • Layer of dense irregular connective tissue
  • Binds mucosa to muscular layer
  • Numerous blood vessels and lymphatic vessels
  • May contain exocrine glands
  • Secrete buffers and enzymes into digestive tract
  • Submucosal neural plexus
  • Innervates mucosa and submucosa
  • Sensory neurons, parasympathetic ganglionic neurons, and sympathetic postganglionic fibers
• Muscular layer

  • Dominated by smooth muscle cells• Inner circular layer and outer longitudinal layer
  • Involved in mechanical digestion and moving materialsalong digestive tract
  • Movements coordinated by enteric nervous system (ENS)
  • Innervated primarily by parasympathetic division
  • Also by sympathetic postganglionic fibers
• Muscular layer

  • Myenteric plexus
  • Network of parasympathetic ganglia, sensory neurons, interneurons, and sympathetic postganglionic fibers
  • Between circular and longitudinal muscle layers
• Serosa

  • Serous membrane covering muscular layer
  • Along most portions of digestive tract enclosed by peritoneal cavity
  • In areas where serosa is lacking
  • Adventitia (dense network of collagen fibers) firmly attaches digestive tract to adjacent structures
• Motility of digestive tract

  • Visceral smooth muscle tissue
  • Rhythmic cycles of activity
  • Controlled by pacesetter cells that undergo spontaneous depolarization
  • Wave of contraction spreads throughout entire muscular sheet
• Peristalsis

  - Waves of muscular contractions that move a bolus along length of digestive tract

  1. Circular muscles behind bolus contract

  • While circular muscles ahead of bolus relax 

  2. Longitudinal muscles ahead of bolus contract

  • Shortening adjacent segments

  3. Wave of contraction in circular muscle layer

  • Forces bolus forward
• Segmentation

  – Cycles of contraction that churn and fragment the bolus

  • Mixing contents with intestinal secretions 

  – Does not follow a set pattern

  • Does not push materials in any one direction

  § Regulation of digestive functions

  – Local factors

  – Neural mechanisms

  – Hormonal mechanisms
• Local factors

  • pH, volume, or chemical composition of intestinal contents
  • Can have direct, localized effects on digestive activity
  • Stretching of intestinal wall
  • Can stimulate localized contractions
  • Local factors may stimulate release of chemicals
  • Prostaglandins, histamine, and other chemicals may affect adjacent cells
• Neural mechanisms

  • Visceral motor neurons
  • Control smooth muscle contraction and glandular secretion
  • Located in myenteric plexus
• Neural mechanisms

  • Short reflexes (local reflexes)
  • Control small segments of digestive tract
  • Operate entirely outside of CNS control
  • Long reflexes
  • Involve interneurons and motor neurons in CNS
  • Provide higher level control
  • Stimulate large-scale peristalsis
  • Parasympathetic motor fibers synapse in myenteric plexus
• Hormonal mechanisms

  • Enteroendocrine cells in digestive tract produce many peptide hormones
  • Affect almost every aspect of digestion
  • Some also affect other systems
  • Travel through bloodstream to reach target organs
• Functions of oral cavity

  • Sensory analysis
  • Of food before swallowing
  • Mechanical digestion
  • Through actions of teeth, tongue, and palatal surfaces
  • Lubrication
  • By mixing with mucus and saliva
  • Limited chemical digestion
  • Of carbohydrates and lipids
• Oral mucosa

  • Lining of oral cavity
  • Stratified squamous epithelium
  • Relatively thin and nonkeratinized on cheeks, lips, and inferior surface of tongue
  • Thin, vascular mucosa inferior to tongue can rapidly absorb lipid-soluble drugs
  • Mucosae of cheeks
  • Supported by pads of fat and buccinator muscles
  • Continuous with those of lips
• The Oral Cavity

  • Oral vestibule
  • Space between cheeks (or lips) and teeth
  • Gingivae (gums)
  • Ridges of oral mucosa
  • Surround base of each tooth on alveolar processes of maxillae and mandible
• The Oral Cavity

  • Uvula
  • Dangling process at posterior margin of soft palate
  • Prevents food from entering pharynx too soon
  • Palatoglossal arch
  • Extends between soft palate and base of tongue
  • Fauces
  • Space between oral cavity and pharynx
  • Bounded by soft palate and base of tongue
  • Palatopharyngeal arch
  • Extends from soft palate to pharyngeal wall
• Tongue

  • Four primary functions
  • Mechanical digestion by compression, abrasion, and distortion
  • Manipulation to assist in chewing and to prepare food for swallowing
  • Sensory analysis by touch, temperature, and taste receptors
  • Secretion of mucins and lingual lipase
• Tongue

  • Anterior body
  • Posterior root
  • Frenulum of tongue
  • Along inferior midline
  • Extrinsic tongue muscles
  • Perform all gross movements
  • Intrinsic tongue muscles
  • Smaller
  • Perform precise movements
• The Oral Cavity

  • Teeth: assisted by tongue for chewing
  • Dentin: mineral matrix, similar to bone, no cells
  • Pulp cavity: interior chamber of teeth, blood vessels etc
  • Root of tooth: sits in bony socket, cement to anchor with periodontal ligament
  • Crown: protects soft tissue
  • Gingival sulcus surrounds the neck: keep debri out of from edge of roots
  • Enamel: provide biting surface
  • Cusps: sharp bits for mastification
• The Oral Cavity

  • Alveolar processes of maxillae and alveolar part of mandible
  • Form maxillary dental arcade and mandibular dental arcade
  • Types of teeth
  • Incisor teeth
  • Canine teeth
  • Premolar teeth
  • Molar teeth
• Teeth in children and adults

  • deciduous teeth: baby/primary teeth are temporary 5 on each side of our jaw in adulthood we lose them and get adult teeth
  • 8 on each side by adulthood
• Salivary glands

  • Three major pairs secrete into oral cavity
  • Parotid glands
  • Sublingual glands
  • Submandibular glands
  • Each pair has distinctive cellular organization
  • And produces saliva with slightly different properties
• Saliva

  • Salivary glands produce 1.0–1.5 liters each day 
  • 70% from submandibular glands
  • 25% from parotids
  • 5% from sublingual glands
  • 99.4% water
  • Remaining 0.6%
  • Electrolytes, buffers, glycoproteins, antibodies, enzymes, and wastes
• Regulation of salivary secretions

  • Salivary glands have parasympathetic and sympathetic innervation
  • Parasympathetic efferents
  • Originate in superior salivatory nucleus and inferior salivatory nucleus of medulla oblongata
  • Stimulated by any object in mouth, other brainstem nuclei, and activities of higher centers
  • Parasympathetic stimulation accelerates secretion by all salivary glands