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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 Components of the Digestive System Alimentary Canal • Around 9 meters long • Muscular tube that passes through the ventral cavity • Lumen Alimentary Wall Structure • Mucosa • Submucosa • Muscular layer – Circular fibers – Longitudinal fibers – Oblique fibers • Serosa Movements of the Alimentary Canal • Mixing movements • Propelling movements Mouth Structure • Surrounded by lips, cheeks, tongue, and palate • Oral cavity • Vestibule • Cheeks • Lips Tongue • • • • Frenulum Papillae Hyoid bone Lingual tonsils Palate • • • • • Hard palate Soft palate Uvula Palatine tonsils Pharyngeal tonsils (adenoids) 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 Tooth Types • Incisors – Chisel-shaped – Bite off large pieces of food • Cuspids – Cone-shaped – Grasp and tear food • Bicuspids and Molars – Flattened surfaces – Grind food General Tooth Structure • • • • • • • • • Crown Root Neck Enamel Dentin Pulp Root canals Cementum Periodontal ligament Salivary Glands • Secrete saliva: – Moistens food particles – Helps bind food particles – Begins chemical digestion of carbohydrates – Dissolves food for tasting – Helps cleanse mouth and teeth Salivary Cells • Serous cells – Produce watery fluid that contains amylase – Amylase splits starch and glycogen into disaccharides • Mucous cells – Secrete mucus to bind food particles and lubricate during swallowing Major Salivary Glands • Parotid glands – Largest – Mostly serous secretions • Submandibular glands – Mostly serous secretions • Sublingual glands – Smallest – Mostly mucous secretions 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 • Food is chewed 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. Swallowing Reflex • Soft palate rises to prevent 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 • Tongue presses against the soft palate, sealing the oral cavity off from the pharynx. • Longitudinal muscles in the pharyngeal wall contract, moving the pharynx up toward the bolus. • Muscles in the lower pharynx relax, and the esophagus opens. • Peristalsis moves the bolus through the esophagus. Esophagus • Straight, collapsible tube • Approximately 25 cm long • Passageway from pharynx to stomach • Cardiac sphincter • Mucous glands for lubrication Movement through Esophagus Peristalsis Stomach • J-shaped, pouchlike organ • Hangs under the diaphragm • 1 liter capacity • Rugae Stomach Functions • • • • 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 Stomach Regions • • • • Cardiac Fundic Body Pyloric Gastric Secretions • Gastric pits • Gastric glands – Goblet cells – Chief cells – pepsinogen – Parietal cells – HCl and intrinsic factor • Gastric juice • Regulated by ACh, gastrin, and cholecystokinin Mixing and Emptying Actions of the Stomach Pancreas • Secretes pancreatic juice from acinar cells • Mixed gland • Pancreatic duct • Hepatopancreatic sphincter Pancreatic Secretions • Pancreatic juice contains several enzymes: – – – – Pancreatic amylase Pancreatic lipase Nucleases Trypsin, chymotrypsin, and carboxypeptidase – Bicarbonate ions • Release regulated by secretin Liver • Located in upper right quadrant below the diaphragm • Color from rich supply of blood vessels • Divided into left and right lobes by fibrous capsule • Each lobe separated into hepatic lobules functional units of liver Hepatic Lobule Structure • Consists of many hepatic cells radiating out from a central vein • Hepatic sinusoids • Portal vein • Central veins • Kupffer cells • Bile canals • Common hepatic duct Liver Functions • Cells respond to insulin and glucagon to maintain normal glucose levels • Carbohydrate, lipid, and protein metabolism – – – – – • • • • Glucose Glycogen Noncarbs Glucose Makes cholesterol and fats Amino acids Urea Makes plasma proteins Storage of glycogen, iron, vitamins A, D, and B12 Blood filtering Detoxification Secretion of bile Bile • Yellowish-green liquid that contains: – Bile salts – for emulsification and absorption of fatty acids, cholesterol, vitamins A, D, E, and K – Bile pigments – bilirubin and biliverdin – Cholesterol – Electrolytes 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 • Stimulated by cholecystokinin Gallbladder and Liver Problems • Jaundice • Hepatitis • Gallstones 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 products 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 – 25 cm long – Most fixed portion of the small intestine • Jejunum • Ileum – Jejunum and ileum are not distinctly separate – Both are mobile Mesentery • Double-layered fold of peritoneal membrane • Suspends the jejunum and ileum from the posterior abdominal wall • Supports the blood vessels, nerves, and lymphatic vessels that supply the intestinal wall Greater Omentum • Filmy, double-layered fold of the peritoneal membrane • Drapes like an apron from the stomach over the transverse colon and the folds of the small intestine • May adhere to infected areas of the alimentary canal to wall it off Intestinal Villi • Tiny projections on the inner wall off the small intestine • Densest in the duodenum • Increase surface area for absorption • Lacteals – absorb fatty acids and glycerol • Goblet cells • Intestinal glands • Microvilli – Secrete peptidases, sucrase, maltase, lactase, intestinal lipase • Capillaries absorb simple sugars, amino acids, electrolytes, and water Large Intestine • Ileocecal valve • 1.5 meters long • Extends up right side, crosses obliquely to the left side, and descends into the pelvis • Opens to the outside of the body as the anus Regions of Large Intestine • Cecum – Vermiform appendix • Colon – – – – Ascending colon Transverse colon Descending colon Sigmoid colon • Rectum • Anal canal Anal Canal Structure • Anal columns • Anus – Internal anal sphincter – External anal sphincter • Hemorrhoids Large Intestine Anatomy • Lack villi • Teniae coli • Many goblet cells – Protect intestinal wall – Bind particles of fecal matter – Help control pH Large Intestine Functions • Proximal end functions primarily in water and electrolyte absorption • Distal end functions primarily to store feces • Little to no digestive function • More sluggish movements – peristaltic waves 2-3 times per day (mass movements) 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