Digestive system

• The digestive process


  1. Ingestion

  2. Propulsion

  3. Mechanical digestion

  4. Chemical digestion

  5. Absorption

  6. Defecation

  
• Ingestion


  Taking food into digestive tract via the mouth

  
• Propulsion


  Moving of food through the alimentary canal, some food mixing occurs as well. Swallowing is initiated voluntarily, peristalsis is the involuntary process of food propulsion by alternating contraction and relaxation of muscles in organ walls

  
• Mechanical digestion


  Chewing, mixing of food with saliva by tongue, churning of food in the stomach, segmentation (rhythmic mixing of food with digestive juices in small intestine) increases efficiency of absorption by repeatedly moving food over intestinal wall

  
• Chemical digestion


  Series of catabolic steps during which enzymes break complex food molecules into chemical building blocks, begins in mouth and ends in small intestine
• Absorption


  Passing of digestive food end products, vitamins, minerals, and water from the GI lumen into blood or lymph, small intestine is the major absorptive site

  
• Defecation


  Eliminates indigestible substances from large intestine via the anus in the form of feces

  
• Digestive regulatory mechanisms


  • Digestive activity is provoked by a range of mechanical and chemical stimuli
  • Walls of GI tract organs contain mechanoreceptors (stimulated by stretching of organ by food in the lumen) and chemoreceptors (stimulated by osmolarity changes, pH, and presence of substrates and digestion end products)

  
• Digestive regulatory mechanisms


  Sensors when stimulated initiate reflexes that activate or inhibit glands secreting digestive juices or hormones, and stimulate smooth muscles to mix lumen contents and move them along the tract

  
• Intrinsic controls of digestive activity


  Enteric nerve plexuses (gut brain) between muscle layers in the alimentary canal walls, short reflexes (stimuli arising in the GI tract), long reflexes (stimuli arising inside/outside GI tract involving CNS and extrinsic autonomic nerves), exciting mediators (ACh and substance P), inhibiting mediators (VIP and NO), hormone-producing cells releasing their products to interstitial fluid and distributed via blood

  
• Extrinsic controls of digestive activity


  Involve CNS and extrinsic autonomic nerves, parasympathetic input enhances secretory activity and motility, sympathetic input inhibits digestive activities

  
• Peritoneum


  Slippery serous membranes found in all ventral body cavities, visceral peritoneum covers external surface of most digestive organs, continuous with parietal peritoneum lining the body wall, separated by peritoneal cavity containing slippery serous fluid that lubricates the mobile digestive organs
• Mesentery


  Double layer of peritoneum, sheet of two serous membranes fused back to back, extends to the digestive organs from the body wall, provides routes for blood vessels, lymphatic vessels, and nerves to reach the digestive viscera, holds organs in place, stores fat

  
• Retroperitoneal organs


  Organs that lose part of their mesentery and adhere to dorsal abdominal wall, lie posterior to the peritoneum (most of pancreas and parts of large intestine)
• Intraperitoneal (peritoneal) organs


  Organs that keep their mesentery and are located in the peritoneal cavity

  
• Peritonitis is inflammation of the peritoneum, resulting from a wound, perforating ulcer, or burst appendix, peritoneal coverings tend to stick together around the infection site, can become widespread and lethal, treated by removing infectious debris and antibiotics
• Splanchnic circulation


  Arteries branching off the abdominal aorta to serve the digestive organs, receives one-quarter of cardiac output, increases after a meal. Hepatic portal circulation collects nutrient-rich venous blood draining from digestive viscera and delivers it to the liver
• Layers of the alimentary canal


  Mucosa (innermost, contains epithelium, lamina propria, and muscularis mucosae), submucosa (CT with blood/lymphatic vessels and nerve fibers), muscularis externa (inner circular and outer longitudinal layer of SMCs responsible for segmentation, peristalsis, and sphincters), serosa (outermost protective layer, replaced by adventitia in esophagus)
• Enteric nervous system


  In-house nerve supply staffed by enteric neurons, communicate to regulate digestive system activity, semiautonomous, form two major intrinsic nerve plexuses: submucosal (occupies submucosa, regulates glands and smooth muscle in mucosa) and myenteric (between muscle layers, provide major nerve supply, control GI tract motility)

  
• Enteric nervous system links to CNS


  Via afferent visceral fibers and autonomic motor fibers (both parasympathetic and sympathetic), motor fibers synapse with neurons in intrinsic plexuses, parasympathetic input enhances secretory activity and motility, sympathetic input inhibits digestive activities

  
• The mouth


  Only part of alimentary canal involved in ingestion, digestive functions reflect activity of teeth, salivary glands, and tongue, oral cavity is mucosa-lined, releases antimicrobial peptides (defensins)

  
• Lips and cheeks


  Keep food between teeth when chewing, core is made of skeletal muscles, red margin is reddened area lacking sweat/sebaceous glands
• The palate


  Roof of the mouth, anterior hard palate aids in chewing, posterior soft palate rises reflexively to close off the nasopharynx when swallowing, uvula is fingerlike downward projection
• The tongue


  Composed of interlinsic and extrinsic skeletal muscle fibers, functions to reposition food between teeth, mix food with saliva, initiate swallowing, and aid in speaking, intrinsic muscles change tongue shape, extrinsic muscles alter tongue position, lingual frenulum secures tongue to floor of mouth
• Tongue surface


  Bears papillae (projections of underlying mucosa), filiform papillae aid in licking semisolid foods and provide friction, fungiform/circumvallate/foliate papillae house taste buds, terminal sulcus distinguishes root from body, nodular lingual tonsil gives root a bumpy appearance
• Salivary glands


  Extrinsic glands lie outside oral cavity and produce majority of saliva, intrinsic (buccal) glands scattered in oral cavity mucosa supplement extrinsic glands, saliva functions to cleanse mouth, dissolve food chemicals, moisten food, and begin starch digestion
• Composition of saliva


  • 97-99.5% water, slightly acidic pH, electrolytes (Na+, K+, Cl-, PO4

  1. , HCO3

  • ), digestive enzymes (salivary amylase, lingual lipase), proteins (mucin, lysozyme, defensins, IgA), metabolic waste (urea, uric acid)
• Control of salivation


  Intrinsic glands secrete saliva continuously, food ingestion activates extrinsic glands, salivation mainly controlled by parasympathetic division of ANS, sympathetic division causes release of thick mucin-rich saliva, food ingestion stimulates chemoreceptors and mechanoreceptors in mouth to signal salivatory nuclei in brain stem to increase parasympathetic output, GI irritations and dehydration inhibit salivation
• Inhibition of saliva secretion results in increased dental cavities, difficulty in talking and swallowing, and halitosis (bad breath from anaerobic bacterial growth and metabolic activity)
• Teeth


  Tear and grind food during mastication, lie in sockets called alveoli, classified as incisors, canines, premolars, and molars
• Primary (deciduous) dentition


  20 teeth that appear first and are replaced by permanent teeth, first to emerge are lower incisors at 6 months, all emerge by 24 months, roots are resorbed as permanent teeth develop and they loosen/fall out between ages 6-12
• Permanent dentition

  32 teeth that develop underneath the primary dentitions, all usually erupt by end of adolescence except for wisdom teeth which sometimes never erupt or are completely absent
• Tooth structure


  Served by branches of trigeminal nerves, blood supplied by branches of maxillary artery, odontoblasts secrete and maintain dentin, enamel, dentin, and cementum resemble bone but are avascular, cementum and dentin contain collagen but enamel does not
• Dental caries (cavities) result from gradual demineralization of enamel and dentin by bacterial activity, dental plaque forms and bacteria convert sugar to acid that dissolves calcium salts, accumulated plaque can calcify into tartar that disrupts seals between gums and teeth increasing risk of gum inflammation
• Teeth


  • Served by branches of trigeminal nerves
  • Blood supplied by branches of maxillary artery
  • Odontoblast secrets and maintains dentin
  • Enamel, dentin, and cementum are all classified and resemble bone, but unlike bones, they are AVASCULAR
  • Cementum and dentin contain collagen, enamel does NOT
• Dental caries (cavities)


  Gradual demineralization of enamel and underlying dentin by bacterial activity
• Dental plaque formation


  1. Bacteria converts sugar to acid

  2. Dissolve calcium salts of the teeth

  3. Salts leach out

  4. Other organic material is digested by degrading enzymes released by bacteria
• Dental plaque accumulates


  • Calcifies and forms calculus (tartar)
  • Disrupts seals between gingivae and teeth
  • Deepens the sulcus
  • Increasing risk of gum infections (gingivitis)
• Calculus is not removed and gingivitis is not treated


  • Immune cells cause tissue damage
  • Destroys periodontal ligaments and carves pockets around the teeth
  • Activates osteoclasts
  • Dissolves bone material
  • Periodontal disease (periodonitis)
• Periodontal disease (periodonitis)


  • Accounts for 80-90% of tooth loss in adults
  • Risk factors include smoking, DM, and oral piercing
  • May increase the risk or cardiovascular disease, including atherosclerosis and strokes