digestion revised - For a good overview of the digestive system go to Crash Course Digestion on Google and view all three parts of the A&P Digestive

digestion revised - For a good overview of the digestive...

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Unformatted text preview: For a good overview of the digestive system, go to Crash Course Digestion on Google and view all three parts of the A&P Digestive System offering. Organs of the Digestive System 2 groups of organs: Alimentary canal organs Accessory digestive organs. Alimentary canal (a.k.a. GI tract, digestive tract) organs are the mouth, pharynx, esophagus, stomach, small intestine, and large intestine. Accessory organs the teeth, tongue, gallbladder, salivary glands, liver & pancreas Digestive Processes 1. 2. 3. 4. 5. 6. Ingestion Propulsion Mechanical Digestion Chemical Digestion Absorption Defecation Digestive pathway The G.I. tract is a tube that is open at both ends so it is outside the body. Entry point --the mouth –chewing – swallowing – propulsion through the esophagus –stomach – small intestine— large intestine – rectum – anus– exit point. Mechanical digestion – chemical digestion —absorption – elimination. Sphincters Prevent Back-flow/Leakage Lower esophagus sphincter –esophagus /stomach AKA- cardiac sphincter – gastroesophageal sphincter- GIRD Pyloric sphincter – stomach /small intestine Ulcers Ileocecal sphincter – small intestine /large intestine Prevents bacteria growth Anal sphincters Mouth (a.k.a. Oral or Buccal Cavity) Mucosa lined Anterior opening is the oral orifice. Teeth and tongue aid in mastication. Continuous with oropharynx. Lips and Cheeks Both have a core of skeletal muscle Orbicularis oris in lips and buccinators in cheeks. Vestibule is the area btwn the gums and teeth and the lips and cheeks. W/i the teeth and gums is the oral cavity proper. Labial frenulum joins internal lip to the gum. Occupies most of the closed oral cavity. Consists primarily of skeletal muscle. Its movements mix food w/ saliva and form a compact mass called a bolus. Lingual frenulum connects the tongue to the floor of the mouth Tongue Superior surface has many projections of the underlying mucosa – papillae. 3 types of papillae: filiform, fungiform, and circumvallate Circumvallate & fungiform contain taste buds. Salivary Glands Produce saliva which cleans, dissolves, and moistens food. It also contains enzymes that start chemical digestion and antibodies that fight infection. Parotid gland is large and lies anterior to the ear btwn masseter and skin. Mumps? Teeth Lie in sockets in the gum-covered margins of the mandible and the maxilla. Function in mastication Primary dentition (baby teeth) forms 1st. They first appear at 6mo and all 20 have usually emerged by 24mo. Permanent teeth eventually develop and force the baby teeth to fall out. Usually 32 permanent teeth to a set. Teeth are classified by their shape and function as incisors, canines, premolars, and molars Incisors are adapted for cutting, canines for piercing, premolars and molars for grinding and crushing. Each tooth has 2 major regions – crown and root. Crown is the exposed part of the tooth above the gingiva. Crown is covered in enamel – the hardest substance in the body. Portion of the tooth embedded in the jawbone is the root. The region connecting the crown and root is the neck. The root is covered by cementum. The periodontal ligament attaches the cementum of the tooth to its bony socket. Dentin (2nd hardest material in body) underlies the enamel and forms the bulk of a tooth. Dentin surrounds a pulp cavity which contains blood vessels and nerves. When the pulp cavity extends into the root it becomes the root canal. Swallowing Deglutition 2 phases: buccal and pharyngoesophageal Buccal phase occurs in the mouth and is voluntary Pharyngoesophageal is involuntary and is mediated by the swallowing center in the medulla and lower pons. Bolus moves through the pharynx-esophagus b/c of the coordinated muscle activity of peristalsis. 4-8 sec. for solid food. 1-2 for liquid. Pharynx Typical oral epithelium 2 skeletal muscle layers. Inner longitudinal Outer circular. Outer layer is the pharyngeal constrictor muscles which contract to propel food into the esophagus. Esophagus tube” connects pharynx to stomach Goes through mediastinum and pierces diaphragm at the esophageal hiatus Joins the stomach at the lower esophageal “Food sphincter. Weak sphincter can result in “GERD” Collapsed when empty. Stomach Bag for food storage. Food is churned and mixed and turned into a paste called chyme. When empty, the stomach is collapsed and has large impermanent folds called rugae. Stomach Produces hydrochloric acid and the enzyme pepsin which starts the digestion of proteins Lined with an alkaline mucosa which protects it from HCl. Stomach Gastric peristalsis begins near the lower esopphageal sphincter and moves the stomach contents toward the pylorus. Each peristaltic wave spits 3 mL or less of acidic chyme into the duodenum. Gastric Glands 1. 2. Mucous Neck Cells: Produce a slightly acid mucus Found in the gland’s upper region. (Theory: they may be stem cells for parietal cells.) Parietal Cells: Secrete HCl and intrinsic factor. HCl is necessary to activate pepsin – a protein-digesting enzyme – and it helps reduce the bacterial population. Intrinsic factor is necessary for absorption of vitamin B12 3. 4. 5. Chief Cells: Produce pepsinogen which is the inactive form of pepsin. Also produce rennin, an enzyme involved in the curdling of milk. Enteroendocrine Cells: Release hormones. Products include gastrin, histamine, endorphins, serotonin, cholecystokinin, and somatostatin Stem Cells: Necessary for the production of all the other cell types What prevents the proteolytic enzymes and low pH from damaging the stomach? A thick coat of bicarbonate containing mucus coats the stomach wall. Mucosal epithelial cells are joined by tight junctions The deep glandular cells are impermeable to HCl. Damaged epithelial cells are shed & quickly replaced. Small Intestine Convoluted tube extending from the pyloric sphincter to the ileocecal valve Longest portion of GI tract – 20 feet. 3 subdivisions: Duodenum Jejunum Ileum Duodenum Duodenum – 1st loop only 10 inches long. Largest digestive organ of the body. Built for absorption. Pancreas produces pancreatic juices containing digestive enzymes and bicarbonate to buffer acid from stomach Lipase acts on fats - amylase acts on sugar -, trypsin, chymotrypsin and carboxypeptidase act on proteins . Bile A yellow-green alkaline solution containing bile salts, cholesterol derivatives whose chief function is to emulsify fats. Keeps fats suspended in the watery fluids so that enzymes from the pancreas can act to break them down. Gallbladder Thin-walled, green, muscular sac the size of a kiwi fruit found on the ventral surface of the liver. Stores bile produced in the liver and concentrates it by absorbing water and ions. Jejunum and Ileum Jejunum is 8 ft long and extends from duodenum to ileum. Ileum is about 12 ft long and extends from jejunum to the ileocecal valve. Jejunum and ileum are encircled and framed by the large intestine. jejunum - largely responsible for digesting and absorbing proteins, carbohydrates, and fats from the partially digested food material that leaves the stomach ileum - responsible for absorbing vitamin B-12 and bile salts presence of Peyer’s patches Large Intestine Frames the small intestine on 3 sides and extends from the ileocecal valve to the anal canal. Compared to the small intestine, its diameter is bigger but it’s shorter (about 5 feet). Major function is to absorb water from indigestible food residues and then eliminate them as semisolid feces. It also absorbs Vitamins K and B complex along with ions. The ascending colon travels up the right side of the abdominal cavity and makes a right turn (the right colic flexure) and travels across the abdominal cavity as the transverse colon. At the spleen it bends as the left colic flexure and descends on the left side as the descending colon. Inferiorly it enters the pelvis where it becomes the sigmoid colon. In the pelvis , the sigmoid colon joins the rectum which runs just in front of the sacrum. Anal canal is the last segment of the LI and begins where the rectum penetrates the levator ani muscle of the pelvic floor. The anal canal has an involuntary internal sphincter (smooth muscle) and a voluntary external sphincter (skeletal muscle). Bacterial Flora Enter via both the oral cavity tract and the anus Colonize the colon and ferment the indigestible carbohydrates releasing irritating gases and acids. Important because they synthesize B vitamins and most of the vitamin K required by the liver. Diverticulitis Inflammation of the diverticula Diverticula are pouchlike herniations commonly found in the colon. Fecal matter aggregates in the pouch and penetrates their thin wall causing inflammation and abscess formation in the surrounding tissue. Carbohydrate Digestion What is a carbohydrate? Cn(H2O)n Most ingested carbs are either: Monosaccharides Disaccharides Simple sugars such as glucose, fructose, galactose 2 monosaccarides chemically bonded together • E.g., Sucrose or lactose Polysaccharides Starch and glycogen – large carbohydrate polymers Carbohydrate digestion Begins in the mouth with Salivary Amylase Stops in the stomach because of high acid content. Continues in the Small Intestine. With Pancreatic Amylase brush border enzymes such as lactase and sucrase produce monosaccharides Monosaccharides are readily absorbed and need no further digestion. Carbohydrate Absorption Glucose and galactose are absorbed in intestinal epithelial cells via cotransport with sodium ions. Fructose is absorbed via facilitated diffusion. All monosaccharides enter villi capillaries and are taken to the liver by the hepatic portal vein. Lipid Digestion Lipid digestion begins and ends in the small intestine. Lipids are hydrophobic. Once emulsified with bile we can digest them with pancreatic lipase. Most ingested fats are broken down into glycerol, monoglycerides and fatty acids. Lipid Digestion Lipid Absorption Fatty acids and monoglycerides passively diffuse into intestinal epithelial cells where they form structures called chylomicrons. The chylomicrons are then extruded from the basal side of the cell where they enter a lacteal Glycerol and short chain fatty acids are absorbed into capillary blood and transported to the liver via the hepatic portal vein. Protein Digestion Digestion begins in the stomach Chief cells secrete pepsinogen (inactive) which as turned into pepsin (active) by the low pH environment. In the small intestine there are multiple proteolytic enzymes including trypsin, chymotrypsin, carboxypeptidase, and brush border enzymes aminopeptidase dipeptidase. The final end products of protein digestion are amino acids. Protein Digestion Protein Absorption Amino acids are taken by intestinal epithelial cells primarily via cotransport with sodium. They then enter villus capillaries and are routed to the liver via the hepatic portal vein. Hormones of the Digestive System The endocrine system regulates digestive function by secreting hormones. Recall that hormones are chemical messengers secreted into blood that modify the physiology of target cells. Target cells have receptors for that hormone and can thus respond to it. The most profound control is exerted by hormones produced within the gastrointestinal tract. Single secreting cells are scattered among other types of epithelial cells in the mucosa of the stomach and small intestine. Hormones primarily effect two digestive functions, secretion and motility. Secretory effects include: •Release of enzymes from the stomach •Release of pancreatic juices •Inhibition of release Motility effects include: Changes in peristaltic activity Contraction of the gallbladder for bile release Delay of gastric emptying to regulate the rate at which food enters the SI to maximize digestion and absorption The Hormone Families Gastrin family- gastrin and cholecystokinin (CCK) Secretin family- Secretin, Gastric Inhibitory Peptide(GIP), and Vasoactive Intestinal Peptide(VIP) Others - Ghrelin and Motilin Stimulation Changes in the size of the lumen of the digestive tube Ability of apical epithelium to continually “taste”or sample the environment and respond Most epithelial cells in the stomach secrete mucus, HCl or pepsinogen G cells synthesize and secrete the hormone gastrin A. Imholtz, 6-4-1999 Arrows indicate G-cells in the human stomach Gastrin Stimulates gastric acid secretion and gastric emptying. In the intestine it stimulates peristalsis, relaxes the ileocecal valve, and stimulates mass movements. Production Stimulated by the presence of peptides and amino acids in gastric lumen. Caution acid A weird, but interesting, case in frogs… • Frogs (Rheobatrachus silks): mother broods young in stomach • Young secrete substance that inhibits gastric acid production (PG­E2) • Prostaglandin E2 lowers gastric acid production and promotes mucus barrier • Clinical relevance: non­steroidal anti­inflammatory drugs lower PG levels and patients get peptic ulcers because they lose protective barriers • Tyler et al., Science 220:609­610 (1983) Eggs or young tadpoles swallowed 8 weeks Young frogs Cholecystokinin Stimulates secretion of pancreatic enzymes, and contraction and emptying of the gall bladder Inhibits gastric emptying Production Stimulated by the presence of fatty acids and amino acids in the small intestine. Secretin Stimulates secretion of water and bicarbonate from the pancreas, inhibits gastric gland secretion and gastric motility, and increases bile output. ProductionStimulated by acidic pH in the lumen of the small intestine Gastric Inhibitory Peptide First isolated from pig intestine Inhibits gastric secretion and motility when fats and large amounts of sugar are present in SI Enhances insulin release and increases storage activity in fat cells Vasoactive Intestinal Peptide(VIP) Relaxes smooth muscle, inhibits gastric acid secretion, increases pancreatic and bile flows Dilates intestinal capillaries increasing absorption Ghrelin Appears to be a strong stimulant for appetite and feeding; also a potent stimulator of growth hormone secretion. Stimulus not clear, but secretion peaks prior to feeding and diminishes with gastric filling. Motilin Apparently involved in stimulating housekeeping patterns of motility in the stomach and small intestine Stimulation, not clear, but secretion is associated with fasting and response to cephalic stimulation. Some Clinical Concerns Gastritis Peptic ulcer Inflammation of the stomach lining A sharply circumscribed loss of the mucous membrane of the stomach, duodenum, or any part of the GI tract exposed to gastric juices containing acid and pepsin. Also known as gastric ulcer Gastrectomy Surgical excision of all or part of the stomach Often performed to remove a chronic peptic ulcer, to stop hemorrhage in a perforating ulcer (an ulcer that penetrates the thickness of the organ wall) or to remove a malignancy. How would this affect protein digestion? How would this affect RBC synthesis? Some More Clinical Concerns Pyloric stenosis Narrowing of the pyloric sphincter at the outlet of the stomach that blocks the flow of food into the small intestine. Occurs as a congenital defect in 1 of 200 newborns and occasionally in older adults secondary to an ulcer or fibrosis at the outlet. Hiatal hernia A protrusion of a portion of the stomach upward thru the diaphragm. Occurs in 40% of the population and most people display few, if any, symptoms. Major problem is gastroesophageal reflux. • Heartburn Emesis Extreme stretching of the stomach or intestine, presence of irritants (bacterial toxins, excessive alcohol) cause sensory impulses to go to the emetic center of the medulla and initiate contraction of the diaphragm and abdominal wall muscles, relaxation of the cardiac sphincter, and rising of the soft palate. Why can excessive vomiting cause plasma alkalosis? ...
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  • Spring '11
  • Sledge
  • Physiology, Anatomy, stomach /small intestine, intestine /large intestine

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