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The Digestive System Chapter 15

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The Digestive System Chapter 15 Food is vital for life because it is the source of energy that drives the chemical reactions occurring in every cell. Energy is needed for muscle contraction, conduction of nerve impulses, and secretory and absorptive activities of many cells. Food, however, is not in a state suitable for immediate use as an energy source. The organs that mechanically and chemically break down food into usable molecules are collectively called the digestive system. Contents: 1. Oral cavity 2. Pharynx 3. Esophagus 4. Stomach 5. Small intestine 6. Large intestine Accessory glands/organs = contribute to digestion but food does not pass through. 1. Salivary glands 2. Pancreas 3. Liver 4. Gall bladder Functions: 1. Ingestion – taking food/drink into the body 2. Digestion – mechanical and chemical breakdown of food 3. Peristalsis – movement of food through digestive system 4. Absorption – taking nutrients into bloodstream 5. Egestion – elimination of unwanted/ undigested foods from body 6. Sensory Reception – sense of taste (chemoreceptors) 1 1. Oral Cavity = mouth. INGESTION. Lined with stratified squamous. Function: protection from abrasion vestibule = space between lips/cheeks and teeth lips = skeletal muscle = orbiculans oris muscle cheeks = skeletal muscle = buccinators muscle fauces = boundary between oral cavity and oropharynx. Location: between posterior tongue and tip of uvula. A. tongue A large, flexible muscular organ. Covered with stratified squamous. Protection from abrasion. –superior surface contains chemoreceptors (taste buds) B. Teeth Normally, 32 teeth in adults. Tooth socket = alveolus Each tooth held in place by periodontal ligaments tooth formula = 2 1 2 3 2 1 2 3 2 2. Accessory Glands of the Oral Cavity - 3 pairs of salivary glands located external to oral cavity - each has a duct that carries saliva into mouth (exocrine gland) -ducted! - functions: 1) keep mouth moist 2) begin chemical digestion of food A. Parotid gland - largest salivary gland. Secretes watery saliva. When infected with virus à mumps http://www.youtube.com/watch?v=5mZBz746o2E B. Submandibular gland middle in size. Secretes both a watery and mucousy saliva http://www.youtube.com/watch?v=uY_W1oToNJA C. Sublingual gland – smallest in size. Secretes mucousy saliva. 3 3. Pharynx - common openings for the respiratory and digestive systems A. Nasopharynx - only associated with the respiratory system B. Oropharynx – Middle division of pharynx Boundaries: fauces (opening of mouth into pharynx)- tip of epiglottis Lined with stratified squamous Common area for air, food and drink C. Laryngopharynx Inferior division of pharynx Boundaries: tip of epiglottis àtop of esophagus Lined with stratified squamous When swallowing, smooth muscle pulls epiglottis down to cover/protect airway. Food/drink channeled into esophagus. 4. Esophagus A long muscular tube extending from the pharynx to stomach. Consists of both skeletal and smooth muscle. Located posterior to trachea and anterior to vertebral column. Lined with stratified squamous Sphincter muscle at both ends Upper esophageal sphincter * skeletal muscle Lower esophageal sphincter *smooth muscle 5. Stomach http://www.youtube.com/watch?v=ZvudWuvMjtA An expandable muscular sack. Has 3 layers of smooth muscle Lined with simple columnar 4 Wall of stomach contains several gastric glands that secrete: a) HCL b) Digestive enzymes c) Mucus 1. Cardiac region 2. fundus (dome) 3. Body a) lesser curvature b) greater curvature 4. pyloric region 5. pyloric sphincter (boundary between stomach and small intestine) 5 6. Small intestine - has 3 subdivisions a relatively long muscular tube about 10 feet diameter 1-2 inches all 3 subdivisions lined with simple columnar Lacteals found in small intestine wall. A. duodenum About 1 foot long Shaped like a capital “C” Inner surface highly folded with villi and microvilli to increase surface area Receives digestive enzymes from liver and pancreas 6 B. Jejunum About 3 feet long with many tightly folded curves = flexure Inner surface highly folded with villi and microvilli to increase surface area. C. Ileum About 6 feet long Has many flexures No obvious boundary between jejenum and ileum Inner surface highly folded with villi and microvilli to increase surface area. ileocecal valve = a sphincter muscle *boundary between small & large intestines. 7 7. Large Intestine - has 4 subdivisions A relatively short muscular tube About 5 feet long Diameter = 6 cm (3 inches) First 3 subdivisions lined with simple columnar. Exception: anal canal. A. Cecum A blind pouch or sack Function: storehouse of intestinal bacteria Appendix attached B. Colon * has 4 subdivisions* (see diagram) 1. Ascending colon 8 2. Transverse colon 3. Descending colon 4. Sigmoid colon 3 long thin bands of smooth muscle along entire length of colon When contract, causes the colon to pleat. Each pleat = haustrum. Haustra (pl) C. Rectum A short muscular tube about 4-5 inches long Located with in pelvic cavity Function: temporary storage of feces prior to egestion. D. Anal canal A short muscular tube about 1-2inches long. Lined with stratified squamous. Protection from abrasion. Two sphincter muscles 1) internal anal sphincter *smooth muscle ( involuntary) 2) external anal sphincter *skeletal muscle (potty training) 9 3) Anal canal ends at opening to external environment – anus. 8. Accessory organs of the digestive system A. Liver The largest internal organ of body Dozens of important physiologic functions Two major lobes (left and right) Two minor lobes (caudate and quadrate Major digestive function: produces and secretes bile Bile travels through common hepatic duct àcommon bile ductà duodenum 10 B. Gall bladder A small sac on inferior side of liver Functions: 1. temporary storage of bile overproduced by liver 2. concentration of bile while being stored. When needed, bile is released through cystic duct (unites with common hepatic duct) into duodenum C. Pancreas A long slender organ with a bumpy surface Located within C-shape of duodenum Two major junctions: 1. Endocrine – production of insulin and glucagon 2. Exocrine – produces several digestive enzymes. Enzymes travel through pancreatic duct into duodenum. Quiz #1 11 9. Digestion of Ingested Food - 2 methods of digestion: mechanical digestion = Physically breaking large pieces of food into many smaller pieces Increases surface area chemical digestion = Breaking chemical bonds within food particles Proteins à amino acids Carbohydrates à glucose 10. DIGESTION: Oral cavity, pharynx, and esophagus Mechanical digestion begins when teeth grind food into smaller pieces (mastication) Chemical digestion begins when salivary glands secrete saliva, which contains salivary amylase Begins chemical breakdown of carbohydrates Glands in tongue secrete tiny amount of enzyme lipase to begin chemical digestion of lipids Food is pressed into a loose ball by tongue (bolus) 12 11. Swallowing A. voluntary stage Bolus is pushed by tongue posteriorly through fauces into oropharynx. Bolus hits uvula, which is a “trip-wire” to initiate involuntary stage B. involuntary stage - *reflex Once in oropharynx, bolus is moved inferiorly through the rest of pharynx by a series of smooth muscles that encircle pharynx (pharyngeal constrictor muscles) Epiglottis is pulled down to protect airway Bolus pass through upper esophageal sphincter Peristalsis moves bolus down esophagus Bolus passes through lower esophageal sphincter into stomach 13 14 12. DIGESTION: Stomach A. mechanical digestion Smooth muscle in stomach wall contracts to push bolus back and forth in churning motion (mixing waves) Similar to swishing mouthwash! chyme = Liquified food in stomach. Has thin watery consistency B. chemical digestion Salivay amylase and lingual lipase still working but soon inactivated by stomach acid 1) HCl – denatures proteins (breaks 3-D structure 2) pepsin – digests proteins – small polypeptides 3) gastric lipase – digests fats/lipids àtriglycerides 4) mucus – protects stomach itself from being digested! 13. ABSORPTION: Stomach Small amount of water absorbed Most all alcohol absorbed 14. Liver Produces about 1 liter of bile /day Function: breaks large globs of fat into many smaller globs of fat (emulsification) A form of mechanical digestion which increases surface area so lipase can work more efficiently. Bile sent through a common hepatic duct àcommon bile ductà duodenum 15 Excess bile rerouted into gall bladder for storage. 15. Pancreas Secretes a combination of digestive enzymes called: pancreatic juice Pancreatic juice sent through pancreatic duct into duodenum It is basic so it neutralizes stomach acid. A. pancreatic amylase – continues carbohydrate digestion B. trypsin – continues protein digestion C. pancreatic lipase – continues lipid digestion 16. DIGESTION: Small intestine chyme passes through pyloric sphincter into small intestine A. mechanical digestion - 2 processes: Peristalisis – pushes chyme through entire small intestine Segmentation – swishing chyme back and forth in short segments of intestine Both forms of mechanical digestion increase chances of substrate mixing with enzymes. B. chemical digestion Digestive enzymes from liver, gall bladder and pancreas are delivered into duodenum Most all-remaining chemical digestion occurs in duodenum 1. amylase (carbohydrates) à glucose 16 2. pepsin and trypsin (proteins) à amino acids 3. lipase and bile (lipids) à glycerol and fatty acids 17. ABSORPTION: small intestine Most all-remaining absorption of nutrients occurs in jejunum and ileum
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