Digestive System

Digestive System

1. function

   • to break down food and absorb nutrients into the body

2. alimentary canal

   • tube begins at the mouth and terminates at the anus
     • accessory organs
       • teeth, tongue, salivary glands, gallbladder, liver, and pancreas

3. digestive processes

   • Ingestion
     • mastication (chewing)
       • deglutition (swallowing)
         • propulsion (peristalsis)
           • mechanical digestion
             • chemical digestion
               • absorption
                 • defecation
               • starts in the mouth
                 • digestive secretions break down complex food molecules into their chemical building blocks (proteins separate into amino acids)
                   • these secretions vary in composition, but typically contain water, various enzymes, acids, and salts
                     • the process is completed in the small intestine
             • physical process of breaking down food into smaller fragments
               • mastication (chewing): breaks food into smaller fragments and mixes it with saliva
                 • mechanical churning of food in the stomach: breaks and mixes food with digestive juices, creating an acidic "soup" called chyme
                   • segmentation: occurs mainly in the small intestine. contractions that moves bolus back and forth, breaking up, and mixing the contents.
                     • segmentation mixes food with digestive juices and facilitates absorption
           • alternating contraction and relaxation of muscle layers creates a wave-like movement to propel food down the digestive tract
         • buccal phase (voluntary): tongue moves bolus into pharynx
           • pharyngeal-esophageal phase (involuntary): food enter the laryngopharynx (epiglottis closes trachea), then esophagus.
           • esophagus
             • the lower esophageal sphincter (cardiac) controls the movement of food from the esophagus to the stomach
             • gastro-esophageal junction
               • notice change of lining epithelium that correlates with change in function

4. layers of the alimentary canal

   • mucosa, submucosa, muscularis, and serosa
   • blood supply of the digestive system
     • celiac trunk: supplies the liver, stomach, and duodenum
       • superior and inferior mesenteric arteries: supply blood to the small and large intestines
   • neural control of the digestive system
     • the myenteric plexus receives stimulation from the vagus nerve.
       • the plexus is located in the muscularis layer of the alimentary canal and is responsible for motility (peristaltic waves) and also promotes secretion of digestive juices
   • peritoneum
     • parietal peritoneum: lines the abdominal cavity
       • visceral peritoneum: covers the external surface of the abdominal organs
         • intraperitoneal: within the cavity of the peritoneum
           • retroperitoneal: located outside of the peritoneum: ascending colon, descending colon, and pancreas
             • mesentery
               • membrane that attaches the intestines to the wall of the abdomen and maintains their position in the abdominal cavity to supply them with blood vessels, nerves, and lymphatics

5. mouth

   • lips, tongue, palate, gums, and teeth
   • ingests food, mechanical chewing of food, salivary amylase begins, chemical breakdown of starch, swallowing food and propelling it into pharynx
     • saliva cleans and lubricates the teeth and oral cavity. saliva moistens food. salivary mucus helps dissolve food. release of flavors stimulates taste buds allowing us to appreciate its taste. tongue helps create a bolus that can be swallowed
       • tongue: filiform, fungiform, and circumvallate.
         • taste buds: fungiform and circumvallate papillae contain taste buds
           • tooth: enamel, dentin, pulp, gingiva
             • salivary glands: parotid, submandibular, and sublingual. parasympathetic stimulation from vagus nerve increases salivation

6. pharynx: nasopharynx, oropharynx, and laryngopharynx

7. regions of stomach

   • cardia, fundus, body, pylorus
   • stomach anatomy
     • muscularis: additional innermost oblique smoot muscle layer gives the muscularis the ability to vigorously churn and mix food
       • rugae
         • pyloric sphincter
     • gastric pits and gastric glands
       • gastric pits lead to gastric glands that secrete gastric juice
         • glands: chief cells, parietal cells, enteroendocrine (G cells) cells, and mucous cells
       • gastric juice secretions and functions
         • mucus neck cell: secretes mucus, protects stomach from pepsin and HCI
           • parietal cell: secretes hydrochloric acid, intrinsic factor. converts pepsinogen to pepsin and kills microbes. absorption of vitamin B12
             • chief cell: pepsinogen, lipase. converted to pepsin for protein digestion. converts lipids into glycerol and fatty acids.
               • gastro-endocrine (G cell): gastrin. on arrival of food to stomach, it stimulates gastric glands to secrete more HCL and pepsinogen increases intestinal motility

8. small intestine

   • duodenum, jejunum, ileum
     • histology of duodenum brunner's glands
       • epithelium: simple columnar with goblet cells.
         • brunner's glands: located in submucosa. they secrete alkaline mucus to protect the epithelium from the acidic chyme of the stomach
           • intestinal glands
             • goblet, brush border, entero-endocrine, and paneth cells
               • goblet cells: secrete mucus. protection from acids.
                 • brush border cells: secrete maltase, surcrase, lactase, peptidase. digestion of sugars and proteins.
                   • entero-endocrine cells: secretes hormone stimulated by arrival of acidic chyme of duodenum, cholecytokinin (CCK) hormone, when fatty food reaches duodenum. stimulates pancreas to secrete bicarbonate. causes contraction of gall bladder to release bile, and pancreas juice into duodenum.
                     • paneth cells: lysozyme and bactericidal enzymes. phagocytosis/immune.
   • plicae circulares, villi, and microvilli
     • increase the surface area for digestion and absorption