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The Digestive System

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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

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Alimentary Canal Around 9 meters long Muscular tube that passes through the ventral cavity

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Alimentary Wall Structure Mucosa Submucosa Muscular layer Circular fibers Longitudinal fibers Oblique fibers Serosa

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Mouth Structure Surrounded by lips, cheeks, tongue, and palate Oral cavity Vestibule Cheeks Lips

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Tongue Frenulum Secures tongue to floor of math Papillae Hyoid bone Lingual tonsils

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Palate Hard palate Soft palate Uvula Palatine tonsils Pharyngeal tonsils (adenoids)

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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

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General Tooth Structure Crown Root Neck Enamel Dentin Pulp Root canals Cementum Periodontal ligament

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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

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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

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Swallowing Reflex 1.Food is chewed and mixed with saliva to form a mass called a bolus. 2.Bolus is forced into the pharynx. 3.Swallowing reflex is stimulated by sensory receptors around the pharyngeal opening. 4.Soft palate rises to prevent food from entering the nasal cavity. 5.Hyoid bone and larynx are elevated, and the epiglottis of the larynx closes off the top of the trachea. 6.Breathing is briefly inhibited.

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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.

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Swallowing Reflex

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Esophagus Straight, collapsible tube Approximately 25 cm long Passageway from pharynx to stomach Cardiac sphincter Mucous glands for lubrication

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Stomach 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

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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

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Pancreas Secretes pancreatic juice from acinar cells Mixed gland Pancreatic duct Hepatopancreatic sphincter

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Pancreatic Secretions Pancreatic juice contains several enzymes: Pancreatic amylase Pancreatic lipase Nucleases Trypsin, chymotrypsin, and carboxypeptidase Bicarbonate ions Release regulated by secretin

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Liver Carbohydrate, lipid, and protein metabolism Storage of glycogen, iron, vitamins A, D, and B12 Blood filtering Detoxification Secretion of bile

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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

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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

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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

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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, and water

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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

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Regions of Large Intestine Cecum Vermiform appendix Colon Ascending colon Transverse colon Descending colon Sigmoid colon Rectum Anal canal

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Anal Canal Structure Anal columns Anus Internal anal sphincter External anal sphincter Hemorrhoids

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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

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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