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Physiology of absorption. Digestive System . Reduces particle size Helps to absorb micronutrients and trace elements Sets a physical and immunologic barrier. Digestion. Breaks down Carbohydrates (starch and sugar) → single sugar molecules Proteins → amino acids
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Digestive System • Reduces particle size • Helps to absorb micronutrients and trace elements • Sets a physical and immunologic barrier
Digestion Breaks down • Carbohydrates (starch and sugar) → single sugar molecules • Proteins → amino acids • Fats → fatty acids, glycerol
Enzymes • Proteins that break bonds
The Digestive System • Is a long tube from the mouth to the anus
Digestive System • Mouth to anus • Epithelium lines the lumen • Barrier to invaders • Submucosal layer • Muscularis • Taste and smell
Peristalsis • Muscular contractions that move food along throughout GI tract
The Mouth • Mastication: chewing • Saliva • Salivary glands produce about 1.5 liters of saliva daily • Salivary amylase (ptyalin) begins to break down starch (inactivated in the stomach) • Salivary lipase: begins fat digestion, but has minimal effect • Mucus to lubricate the food for easier swallowing • Lysozyme to kill bacteria
Tongue • Taste receptors: sweet, sour, salty, bitter, savory • Enzymes to help break down fatty acids
Swallowing • Bolus is the food swallowed at one time • Epiglottis blocks windpipe, prevents choking
The Esophagus • Long tube • Connects pharynx to the stomach • Peristalsis, muscle contraction • Lower esophageal sphincter • Heartburn http://www.gerd.com/media/grossovw.gif
The Stomach • Lower esophageal sphincter and pyloric sphincter control entry and exit from stomach • Storage capacity of ~4 cups • Secretion of hydrochloric acid, enzymes, including a protease, gastric lipase; mucus, the hormone gastrin, and intrinsic factor (B12) • Stomach secretes about 2000-2500 ml of gastric juice daily
Stomach • pH of stomach is low, 1 to 4; kills many bacteria and micro-organisms • Achlorhydria, gastrectomy, GI dysfunction or disease ↑ risk of bacterial overgrowth in the intestines • Protein digestion begins with pepsinogen which is converted to active pepsin in acidic stomach environment • Chief cells secrete acid-stable gastric lipase, which has minor effect on shorter-chain triglycerides
Stomach • Most of a liquid meal exits in 1 to 2 hours and most of a solid meal in 2 to 3 hours • Formation of chyme • Mucus layer protects stomach from digestion • Very little absorption of nutrients
Stomach Acid • Activates digestive enzymes • Partially digests dietary protein • Assists in calcium absorption • Makes dietary minerals soluble for absorption
The Small Intestine • Most digestion and absorption happens here • About 5-7 meters • Duodenum (.5 meters) • Jejunum (2-3 meters) • Ileum (3-4 meters)
The Small Intestine • Folded walls with villi projections • Absorptive cells are located on the villi • Increases intestinal surface area 600x • Rapid cell turnover
Nutrient Absorption • Passive diffusion: driven by concentration; fats, water, some minerals • Active absorption: uses energy; glucose and amino acids • Phagocytosis and pinocytosis: absorptive cells engulf compounds, generally larger molecules, as in immune substances in breast milk
Determine of notion “absorption” • Absorption is a complex of processes, which are provide transport of substances from digestive tract into internal surroundings of organism (blood, lymph, intercellular substances)
Absorption Digestion → small particles End products of digestion: • Carbohydrates → monosaccharides • Proteins → amino acids • Fats → glycerol, fatty acids
Absorption Through small intestine walls Absorbed into • Blood – water soluble nutrients • Lymph – fat soluble nutrients • Blood → liver → general circulation • Liver detoxifies and repackages
The Large Intestine • ~3 1/2 feet in length • No villi or enzymes present • Little digestion occurs • Absorption of water, some minerals, vitamins • Bacteria break down fiber; produce Vitamin K • Formation of feces for elimination
Rectum • Stool remains • Stimulates elimination • Muscle contraction • Anal sphincters • Voluntary control • Opens for elimination
Accessory Organs • Salivary glands • Pancreas • Gallbladder • Liver
Salivary Glands Saliva • Works in mouth • Moistens • Salivary amylase • Digests starch
The Pancreas • Produces glucagon and insulin (endocrine) • Manufactures digestive enzymes→ small intestine • Secretes pancreatic juices • Bicarbonate needed to neutralize chyme when it enters small intestine
The Liver • Produces bile (fat digestion) • Enterohepatic circulation
The Gallbladder Stores bile • Concentrates it • Releases to small intestine when needed
Digestion in the Stomach • Different nutrients leave the stomach at different rates: —Carbohydrates first, followed by protein, fat, and fibrous foods
Features of Digestion and Absorption • 92% to 97% of the diet is absorbed • Small intestine—key role in digestion and absorption • Intestines: 7 meters long —Villi and microvilli/brush border
Refeeding after Starvation • GI tract compromised after starvation • Feeding results in cellular proliferation, which restores normal gastrointestinal architecture and function after a few days
Lipids • The major portion of fat digestion takes place in the small intestine as a result of pancreatic lipase
Large Intestine • 5 feet long • Bacterial action to produce gases and organic acids • Nutrients formed here—vitamin K, thiamin, vitamin B12, riboflavin
Sites of Secretion and Absorption into Gastrointestinal Tract