chapter 3 part 2. digestion is the process of breaking down foods into nutrients to prepare for...
TRANSCRIPT
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Chapter 3Part 2
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Digestion is the process of breaking down foods into nutrients to prepare for absorption
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Visual, olfactory, and auditory senses Taste (smell)
◦ Sweet sour, salty, bitter, umami (savoriness) Presence of food in the mouth
◦ Activate secretory responses from Salivary glands Stomach Pancreas
◦ Contraction of gall bladder Bile
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Anatomy – Gastrointestinal (GI) tract
Flexible muscular tube from mouth to anus
Lumen - inner space of the tract.
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Anatomy –GI tract◦ Mouth - beginning of digestive system and
digestion Mastication (chewing) -stimulates taste buds, Swallowing - epiglottis closes to prevent food from
entering the pharynx.
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Anatomy - GI tract◦ Esophagus - tube carrying bolus to stomach. ◦ Sphincters – control release, keeps food moving
forward Peristalsis Upper and lower esophageal sphincters (cardiac
sphincter - LES)
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Anatomy - GI tract◦ Stomach – HCl and enzymes turn food into a
fluidy paste Holds 2 to 4 cups of food (6 L) Grinds the bolus to a semiliquid mass (chyme) Holding “tank”
Emptying closely regulated 5 ml/min (teaspoon) 1-4 h to empty Pyloric sphincter
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Anatomy - GI tract◦ Stomach
Prevention of autodigestion Mucus lining HCO3
-
Prevents HCl in the stomach from burning through the stomach lining
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Stomach Acid (HCl)◦ Destroys biological activity of protein◦ Activates digestive enzymes◦ Partially digests dietary protein◦ Assists in vitamin B12 absorption
B12 requires gastric acid to be released from food◦ Improves absorption of minerals
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Gastrin stimulates secretion of gastric hydrochloric acid, performing many functions.◦ Activates pepsin◦ Stimulates the release of hydrochloric acid◦ Kills pathogenic organisms◦ Improves absorption of iron and calcium◦ Inactivates hormones of plant and animal origin◦ Denatures food proteins, making them more vulnerable
to enzyme action
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Anatomy - GI tract◦ Small intestine - digestive juices
Gallbladder (Bile) and pancreas (enzymes that break down CHO, fats and proteins)
Three parts of small intestine Duodenum Ileum Cecum
◦ Large intestine (colon) Ileocecal valve to rectum and anus.
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Anatomy – GI tract◦ Approximately 10 feet long
Narrow◦ Duodenum (10 in)
Where enzymes and bile are added Regulatory center that senses:
Nutrient content (particularly fat) pH (HCO3
- from pancreas raises pH of chyme from stomach)
◦ Jejunum (4 ft)◦ Ileum (5 ft)
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Anatomy – GI tract◦ Physical structure – intestinal wall
Folded walls Villi – made up of absorptive cells Increase surface area greatly Rapid turnover absorptive cells (daily)
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The Muscular Action of Digestion – under autonomic control◦ Peristalsis - pushes the digestive contents along.
Muscles are circular, longitudinal, and diagonal muscles.
◦ Sphincter contractions open and close passageways Prevents reflux, controls the passage of contents.
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Fig. 3-2, p. 75
Longitudinal
Circular
Diagonal
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Salivary glands Pancreas Liver (gall bladder) Stomach Small intestine
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Begins in the mouth Saliva
◦ Electrolytes (Na, Cl)◦ Solvent◦ Enzymes (salivary amylase)◦ Mucus
Saliva moistens food◦ Begins breakdown of CHO
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Small Intestine ◦ Pancreatic amylase (duodenum)
Smaller CHO molecules◦ Brush border enzymes
Maltase, sucrase, lactase Disaccharides to monosaccharides
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Maximum rate of glucose absorption◦ 50 – 80 g/h (70 kg)◦ 4 g CHO/min or 240 g/h
Absorbed into circulation◦ Hepatic portal vein
Liver
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Large intestine◦ Water reabsorption◦ Some digestion
Mostly bacterial Vitamin absorption
Vitamins K, B12, thiamin and riboflavin
Compacts and prepares feces for defecation
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Mouth ◦ Lingual lipase
Stomach◦ Gastric lipase
Pancreas ◦ Pancreatic lipase◦ Bile
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Cholecystokinin (CCK)◦ Released from wall of duodenum◦ In response to fat and protein in the chyme◦ Causes the release of bile and pancreatic
enzymes Secretin
◦ Stimulates bile secretion by liver and release of bicarbonate into duodenum
Micelles ◦ Monoglyceride, FFA + bile salts
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Micelles are formed after bile acts on large fat droplets
Pancreatic and brush border enzymes breakdown micelles to fatty acids
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These fatty acids are then repackaged as chylomicrons, which are largely absorbed into the lymphatic system
They then travel to the heart and enter the circulation
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Pepsinogen - precursor Pepsin
◦Released from stomach wall Gastrin
◦Release of HCL ◦Controls pepsin release
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Trypsinogen Secreted by pancreas
Trypsin Tripeptides, dipeptides,
and single amino acids. Brush border enzymes Small intestine into
portal vein Active absorption
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Vitamin absorption Passive diffusion In the jejunum and ileum.
Fat-soluble vitamins With dietary lipids Transport
Chylomicrons, lipoproteins
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Water-soluble vitamins Diffuse into the blood Excess (exceeds renal capacity)
Pass into the urine Vitamin B12.
Intrinsic factor Intestine absorbs by endocytosis.
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Dietary factors Type of food source impacts absorption
Cellular factors The body does not absorb minerals very well. Availability depends on its chemical form
Heme vs nonheme
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Passive process of osmosis in small intestine
9 L of water each day.◦ 72% absorbed in the proximal small intestine ◦ 20% absorbed from the distal segment of the
small intestine ◦ 8% absorbed from the large intestine
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