Chapter
3 Digestion and Absorption
Taste and Smell: The Beginnings of Our Food Experience • The Chemosenses • • • •
Olfactory cells Gustatory cells Common chemical sense Cephalic phase responses
The Gastrointestinal Tract • Organization • Mouth to anus • Accessory organs • Salivary glands, liver, pancreas, gallbladder
• Functions • • • • • •
Ingestion Transport Secretion Digestion Absorption Elimination
Overview of Digestion • Physical movement • Peristalsis • Segmentation
• Chemical breakdown • Enzymes • Other secretions
Overview of Absorption • Absorptive mechanisms • Passive diffusion • Facilitated diffusion • Active transport
Assisting Organs • Salivary glands • Moisten food • Supply enzymes
• Liver • Produces bile
• Gallbladder • Stores and secretes bile
• Pancreas • Secretes bicarbonate • Secretes enzymes
Putting It All Together: Digestion and Absorption • Mouth • Enzymes • Salivary amylase acts on starch • Lingual lipase acts on fat • Saliva • Moistens food for swallowing
Putting It All Together: Digestion and Absorption • Stomach • Hydrochloric acid • Prepares protein for digestion • Activates enzymes
• Pepsin • Begins protein digestion
• Gastric lipase • Some fat digestion
• Gastrin (hormone) • Stimulates gastric secretion and movement
• Intrinsic factor • Needed for absorption of vitamin B12
Putting It All Together: Digestion and Absorption • Small intestine • Sections of small intestine • Duodenum, jejunum, ileum
• Digestion • Bicarbonate neutralizes stomach acid • Pancreatic & intestinal enzymes • Carbohydrates • Fat • Protein
Putting It All Together: Digestion and Absorption • Small intestine • Absorption • Folds, villi, microvilli expand absorptive surface • Most nutrients absorbed here • Fat-soluble nutrients go into lymph • Other nutrients into blood
Putting It All Together: Digestion and Absorption • Large intestine • Digestion • Nutrient digestion already complete • Some digestion of fiber by bacteria
• Absorption • Water • Sodium, potassium, chloride • Vitamin K (produced by bacteria)
• Elimination
Regulation of GI Activity • Nervous system • Regulates GI activity • Local system of nerves • Central nervous system
• Hormonal system • Increases or decreases GI activity
Circulation of Nutrients
• Vascular system • Lymphatic system
Influences on Digestion and Absorption • Psychological Influences • Chemical Influences • Bacterial Influences
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Nutrition and GI Disorders • Constipation • Hard, dry, infrequent stools • Reduced by high fiber, fluid intake, exercise
• Diarrhea • Loose, watery, frequent stools • Symptom of diseases/infections • Can cause dehydration
• Diverticulosis • Pouches along colon • High-fiber diet reduces formation
Nutrition and GI Disorders • Heartburn and Gastroesophageal Reflux • Reduced by smaller meals, less fat
• • • •
Irritable Bowel Syndrome (IBS) Colorectal cancer Gas Ulcers • Bacterial cause
• Dyspepsia
Cephalic Phase Responses
Sphincters in Action • Movement from one section of the GI tract to the next is controlled by muscular valves called sphincters. • When closed, the sphincter prevents the backflow of material from the duodenum to the stomach.
Peristalsis and Segmentation • Peristalsis and segmentation help to break up, mix, and move food through the GI tract. • Periodic muscle contractions at intervals along the GI tract that alternate forward and backward movement of contents, thereby breaking apart chunks of the food mass and mixing in digestive juices.
Enterohepatic Circulation • During this recycling process, bile travels from the liver to the gallbladder and then to the small intestine, where it assists digestion. • In the small intestine, most of the bile is reabsorbed and sent back to the liver for reuse.
Down the Wrong Pipe • When you swallow, the epiglottis closes off the air passage and prevents choking. • If you have ever had a drink go “down the wrong pipe” and you choked, likely, your epiglottis didn’t completely do it’s job.
The Stomach • The stomach churns and mixes food with stomach secretions. • Hydrochloric acid unfolds protein and stops salivary amylase action. • Pepsin begins protein digestion. • The pyloric sphincter controls movement of chyme from stomach to the small intestine.
Lactose Intolerance • Of all the world’s populations, Native Americans have the highest level of lactose intolerance, with 100 percent intolerance. • Ninety-eight percent of Southeast Asians are intolerant, while only 2 percent of the Swedish population are intolerant.
Short Bowel Syndrome • Those who suffer from short bowel syndrome have difficulty absorbing fat-soluble vitamins. • To enhance absorption, treatment includes taking a fat-soluble vitamin supplement that easily mingles with water.
• These patients may also need to take intramuscular shots of B12 because they are unable to absorb this water-soluble vitamin.
Chewing Gum and Colon Surgery • In a new study, it has been observed that chewing gum immediately following abdominal surgery, including colon surgery, can cause a marked decrease or stoppage of intestinal function (ileus). • Ileus can lead to a longer hospital stay, increased risk of infection, and breathing difficulties.
Gastrointestinal Flora Abound • Your entire body has about 100 trillion cells, but this is only one-tenth the number of protective microorganisms normally living in your body. • More than 500 bacterial species alone live in your GI tract.