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digestion

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digestion

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  1. Digestion

  2. Digestive Functions & Processes • Functions • ingestion = intake of food • digestion = breakdown of molecules • absorption = uptake of nutrients into blood or lymph • defecation = elimination of undigested material • Processes • motility = muscular contractions that break up food, mix it with enzymes & move it along • secretion = digestive enzymes & hormones • membrane transport = absorption of nutrients

  3. Stages of Digestion • Mechanical digestion is physical breakdown of food into smaller particles • teeth & churning action of stomach & intestines • Chemical digestion is series of hydrolysis reactions that break macromolecules into their monomers • enzymes from saliva, stomach, pancreas & intestines • results • polysaccharides into monosaccharides • proteins into amino acids • fats into glycerol and fatty acids

  4. Saliva • Functions of saliva • moisten, begin starch & fat digestion, cleanse teeth, inhibit bacteria, bind food together into bolus • Hypotonic solutions of 99.5% water and solutes: • amylase = begins starch digestion • lingual lipase = digests fat after reaches the stomach • mucus = aids in swallowing • lysozyme = enzyme that kills bacteria • immunoglobulin A = inhibits bacterial growth • electrolytes = Na+, K+, Cl-, phosphate & bicarbonate • pH of 6.8 to 7.0

  5. Swallowing

  6. The esophagusSalvia contains salivary amylase. It hydrolyzes the glucose polymers starch and glycogen. The epiglottis blocks the top of the windpipe when we swallow. Peristalsis moves the bolus along the epiglottis.

  7. The stomachThe stomach can hold 2L of food and water. The lining of the stomach secretes gastic acid. Gastric acid contains HCl and pepsin. Pepsin is an enzyme that hydrolyzes proteins. But it breaks proteins into smaller polypeptides.

  8. Functions of Hydrochloric Acid • Activates enzymes pepsin & lingual lipase • Breaks up connective tissues & plant cell walls • liquifying food to form chyme • Converts ingested ferric ions (Fe+3) to ferrous ions (Fe+2) that can be absorbed & utilized for hemoglobin synthesis • Destroys ingested bacteria & pathogens

  9. Small Intestine • Nearly all chemical digestion and nutrient absorption occurs in the small intestine

  10. Carbohydrates Proteins Fats

  11. The digestive enzymes in the table below are summarized according to type of food that they digest.

  12. The table below shows digestive enzymes grouped by source of the enzyme.

  13. Digestive function is affected by hormones produced in many endocrine glands, but the most profound control is exerted by hormones produced within the gastrointestinal tract. The gastrointestinal tract is the largest endocrine organ in the body and the endocrine cells within it are referred to collectively as the enteric endocrine system. Three of the best-studied enteric hormones are: • Gastrin: Secreted from the stomach and plays an important role in control of gastric acid secretion. • Cholecystokinin: A small intestinal hormone that stimulates secretion of pancreatic enzymes and bile. • Secretin: Another hormone secreted from small intestinal epithelial cells; stimulates secretion of a bicarbonate-rich fluids from the pancreas and liver.

  14. Hormones and Digestion:Feedback loops.The hormone Gastrin stimulates the production of gastric acid.The pyloric sphincter regulates the release of chyme (nutrient broth) into the small intestine.Bile contains bile salts that aid digestion and absorption of fats.

  15. Secretin hormone production is stimulated by acid chyme entering the duodenum. This hormone stimulates the pancreas to release bicarbonate to neutralize the acid. • CCK stimulates the gall bladder to release bile and the panceas to release pancreatic juices. • If the chyme is rich in fats causes the release of enterogastrone. This hormone inhibits peristalsis and the release of secretion of acid.

  16. The small intestine

  17. Essential Amino Acids • The human liver can synthesize 9 of the 20 amino acids used in proteins. Those that cannot be synthesized are called the essential amino acids. They must be supplied by diet.

  18. Bile Ducts and Gallbladder    Definition:Bile is produced by the liver cells and collects in the small bile ducts between the microscopic liver lobules which lead to the larger ducts. Outside the liver, two outflowing hepatic ducts unite to form the common hepatic duct. From here the bile enters the cystic duct and the common bile duct . The common bile duct joins the pancreatic duct to enter the duodenum. The opening is controlled by a valve which regulates the flow of gastric juices (bile, pancreatic enzymes) into the duodenum.

  19. Function: Bile is produced in the liver and aids the digestion of fatty food substances. Between 250 and 1000 millilitres of bile are produced each day. The most important constituents of bile are: water, bile salts, cholesterol , phospholipids . Between meals, the bile is stored in the gallbladder, which contracts once or twice as food is taken. This causes the bile to enter the duodenum via the common bile duct. 80 to 90% of the bile acids are later reabsorbed by the small intestine and re-enter the liver via the bloodstream (portal system).

  20. Fat Digestion & Absorption

  21. Absorption and Motility • Transit time is 12 to 24 hours • reabsorbs water and electrolytes • Feces consist of water & solids (bacteria, mucus, undigested fiber, fat & sloughed epithelial cells • Haustral contractions occur every 30 minutes • distension of a haustrum stimulates it to contract • Mass movements occur 1 to 3 times a day • triggered by gastrocolic and duodenocolic reflexes • filling of the stomach & duodenum stimulates motility • moves residue for several centimeters with each contraction

  22. Defecation • Stretching of the rectum stimulates defecation • intrinsic defecation reflex via the myenteric plexus • causes muscularis to contract & internal sphincter to relax • relatively weak contractions • defecation occurs only if external anal sphincter is voluntarily relaxed • parasympathetic defecation reflex involves spinal cord • stretching of rectum sends sensory signals to spinal cord • splanchnic nerves return signals intensifying peristalsis • Abdominal contractions increase abdominal pressure as levator ani lifts anal canal upwards • feces will fall away

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