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The Digestive System Digestion • Digestion • the mechanical and chemical breakdown of foods into nutrients that cell membranes can absorb • 2 Components of the digestive system: • Alimentary canal – mouth, pharynx, esophagus, stomach, small intestine, large intestine, anal canal • Accessory organs – secrete products into the canal; salivary glands, liver, pancreas, gallbladder Alimentary Canal • Around 9 meters (30 feet) long • Muscular tube that passes through the ventral cavity Alimentary Wall Structure • Mucosa-innermost layer • Submucosa- 2nd layer • Muscular layer-3rd • Circular fibers • Longitudinal fibers • Oblique fibers • Serosa-Outside layer Mouth Structure • Surrounded by lips, cheeks, tongue, and palate • Oral cavity • Vestibule • Cheeks • Lips Tongue • Frenulum • Secures tongue to floor of mouth • Papillae • Hyoid bone • Lingual tonsils Palate • • • • • Hard palate Soft palate Uvula Palatine tonsils Pharyngeal tonsils (adenoids) • Tonsils are lymphatic tissue used to protect you from infection Primary and Secondary Teeth • Primary teeth • Deciduous teeth • Erupt between 6 months and 2-4 years • 20 teeth • Secondary teeth • Appear around 6 years • 32 teeth All teeth have a coating of enamel (hardest substance in human body) Salivary Glands • Secrete saliva: • Moistens food particles • Helps bind food particles • Amylase (enzyme) begins chemical digestion of carbohydrates (used first for energy) • Dissolves food for tasting • Helps cleanse mouth and teeth • Mucus softens food & lubricates G.I tract Regions of the Pharynx • Nasopharynx • Open to nasal cavity • Passage for air during breathing • Oropharynx • Behind soft palate • Passage for air and food • Laryngopharynx • Passage for food to the esophagus Swallowing Reflex= deglutition 1. 2. 3. Food is chewed (mastication)and mixed with saliva to form a mass called a bolus. Bolus is forced into the pharynx. Swallowing reflex is stimulated by sensory receptors around the pharyngeal opening. 4. 5. 6. Soft palate rises and seals the nasal cavity off with the uvula to stop food from entering the nasal cavity. Hyoid bone and larynx are elevated, and the epiglottis of the larynx closes off the top of the trachea. Breathing is briefly inhibited. Swallowing Reflex 7. Tongue presses against the soft palate, sealing the oral cavity off from the pharynx. 8. Longitudinal muscles in the pharyngeal wall contract, moving the pharynx up toward the bolus. 9. Muscles in the lower pharynx relax, and the esophagus opens. 10. Peristalsis moves the bolus through the esophagus. Swallowing Reflex Esophagus Disorders: Acid Reflux Ulcers Heart Burn • Straight, collapsible tube • Approximately 25 cm long • Passageway from pharynx to stomach • Cardiac or esophageal sphincter (entry to stomach) • Mucous glands for lubrication Stomach Disorders: Ulcers Tortion • Receives food from the esophagus • Mixes food with gastric juices • Initiates protein digestion • Performs limited absorption of water, salts, alcohol, and lipid-soluble drugs • Moves food into the small intestine • Bolus now turns into chyme (creamy paste) • Food enters through esophageal sphincter and exits via pyloric sphincter Exploratory Surgery • Gastric pits • Gastric glands Gastric Secretions • Goblet cells • Chief cells – pepsinogen • Parietal cells – HCl • Gastric juice • HCl • Pepsin-digest protein • Regulated by ACh, gastrin, and cholecystokinin Mucosal layer of stomach prevents stomach acid from eating through itself and releasing acid into the abdomen Lower pH (Acids) Pancreas • Secretes pancreatic juice from acinar cells • Mixed gland • Pancreatic duct • Hepatopancreatic sphincter • Helps digest carbohydrates, proteins, fats & nucleic acids Pancreatic Secretions • Pancreatic juice contains several digestive enzymes: • • • • Pancreatic amylase Pancreatic lipase Nuclease Trypsin, chymotrypsin, and carboxypeptidase • Bicarbonate ions What does the Pancreas do? Liver • Carbohydrate, lipid, and protein metabolism • Storage of glycogen, iron, vitamins A, D, and B12 • Blood filtering • Detoxification • Secretion of bile (emulsifies/breaks down fats) How does the Liver work? Gallbladder • Pear-shaped sac on the inferior liver surface • Connects to the cystic duct which feeds into the common hepatic duct • Stores bile between meals • Reabsorbs water to concentrate bile • Releases bile into the small intestine • Common bile duct Small Intestine • Extends from pyloric sphincter to the large intestine • Receives secretions from the pancreas and liver • Completes digestion of nutrients in chyme and absorbs nutrients of digestion • Mixing movements and peristalsis – chyme moves through in 3-10 hours • Transports digestive residue to the large intestine Regions of the Small Intestine • Duodenum • Portion closest to stomach • 25 cm long • Most fixed portion of the small intestine • Jejunum-middle portion • Ileum • Final portion of small intestines prior to large intestines Intestinal Villi • Tiny projections on the inner wall off the small intestine • Densest in the duodenum • Increase surface area for absorption • Microvilli • Secrete peptidases, sucrase, maltase, lactase, intestinal lipase • Capillaries absorb simple sugars, amino acids, electrolytes (coenzymes that help control metabolism), and water Large Intestine • Ileocecal valve • 1.5 meters long • Proximal end functions primarily in water and electrolyte absorption • Distal end functions primarily to store feces • Little to no digestive function Regions of Large Intestine • Cecum • Vermiform appendix • Colon • • • • Ascending colon Transverse colon Descending colon Sigmoid colon • Rectum • Anal canal To Gas or not to Gas? Anal Canal Structure • Anal columns • Anus • Internal anal sphincter • External anal sphincter • Hemorrhoids Defecation Reflex • Can be initiated by person (deep breath and abdominal contraction) • Forces feces into rectum • Reflex involves relaxation of the internal anal sphincter and peristaltic waves through the descending colon • Can be prevented by contraction of the external anal sphincter Feces • Made of materials not digested or absorbed • • • • Water Electrolytes Mucus Bacteria • 75% water • Color from bile pigments altered by bacterial action • Odor from compounds produced by bacteria