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Digestion Physiology Chapter 26 Test (Chapters 25 & 26) – Tuesday, 3/30 (Tentative Date ) Primary Mechanisms of Digestive System (Table 26-1) • Ingestion – take food in • Digestion – break down complex nutrients into simpler nutrients • Motility – movement of GI tract; aids in digestion • Secretion – enzymes are required for digestion to take place • Absorption – movement across GI mucosa into the internal environment • Elimination – process of eliminating unabsorbed material Mechanical Digestion • Includes all motility of the digestive tract that cause the following changes: – Change in physical state of the food from large particles to smaller particles – Churning of the GI lumen to mix particles with digestive juices – Propelling food forward ending with elimination Mastication • • • • Mastication = chewing Requires tongue, cheek and lips Reduces particle size Mix food with saliva Deglutition • Deglutition = swallowing 1. Oral stage • • • Bolus is formed voluntarily Tongue pushes bolus against the palate and into the oropharynx Soft palate acts as a valve to prevent food from entering the nasopharynx 2. Pharyngeal stage • • Involuntary reflexes push bolus toward esophagus Epiglottis prevents food from entering trachea • Involuntary reflexes move bolus towards stomach 3. Esophageal stage Motility • Smooth muscle contractions take over in the lower portion of the esophagus • Peristalsis – progressive wavelike ripple of the muscle layer of a hollow organ – Bolus stretches the GI tract wall triggers contraction of circular smooth muscle bolus moves forward Motility • Segmentation – mixing movement; back and forward movement within a single region – Mechanically breakdown food particles – Mix food and digestive juices together – Facilitate absorption • Peristalsis and segmentation can occur in alternating sequence to churn/mix and progress food Peristalsis vs. Segmentation Regulation of Motility • Gastric Motility – Emptying the stomach takes approx 2-6 hours after a meal – Food is churned with digestive juices to form chyme • Ejection every 20 seconds into the duodenum • Controlled by hormonal and nervous mechanisms Regulation of Motility • Hormonal Control – Fats and nutrients in duodenum cause secretion of gastric inhibitory peptide (GIP) from the intestinal mucosa into the bloodstream • Slows peristalsis in stomach; decreasing passage of food into the duodenum Regulation of Motility • Nervous Control – Nerve receptors in duodenum are sensitive to acid and distention • Vagus nerve (CN X) causes inhibition (stops) gastric peristalsis Regulation of Motility • Intestinal Motility • Takes approx 5 hours for food to pass through the small intestine – Segmentation • Mixes chyme and digestive juices from liver, pancreas and intestinal mucosa • causes contact with intestinal mucosa to increase absorption – Peristalsis • Continues in the jejunum to move food into the large intestine • Stimulated by the hormone cholecystokinin-pancreozymin (CCK) – Secreted by intestinal endocrine cells in the presence of chyme Mechanical Digestion • Summarized in Table 26-2; page 775 Chemical Digestion • Consists of all the changes in the chemical composition of food • Result of hydrolysis – Compounds combine with water then split into simpler compounds – Enzymes catalyze the hydrolysis of foods Chemical Digestion • Six main types of chemical substances: – Carbohydrates, proteins, fats, vitamins, mineral salts, water – Only carbohydrates, proteins and fats must undergo chemical digestion to be absorbed Properties of Digestive Enzymes • Extracellular enzymes • Classified as hydrolases • Function optimally at a specific pH – Ex: amylase vs pepsin • Continually destroyed or eliminated Carbohydrate Digestion • Carbohydrates are saccharide compounds – Contain one or more saccharide groups • Polysaccharides – starches & glycogen • Disaccharides – sucrose, lactose and maltose • Monosaccharides – glucose, fructose & galactose Carbohydrate Digestion • Polysaccharides are hydrolyzed by amylases – Present in saliva and pancreatic juice • Sucrose, lactose and maltose are hydrolyzed by sucrase, lactase, and maltase – Located on epithelial cells lining villi in small intestine – End product (usually glucose) is absorbed in the small intestine Protein Digestion • Proteins are large molecules composed of twisted chains of amino acids • Proteases catalyze the hydrolysis of proteins into smaller compounds – Proteins proteoses or peptides amino acids • The peptides bonds holding amino acids together can vary = increased need for varying proteases Protein Digestion - Proteases • Pepsin – gastric juice (stomach) • Trypsin and chymotrypsin – pancreatic juice (pancreas) • Peptidases – intestinal brush border (small intestine) Proteoses Fat Digestion • Fats are insoluble in water so they must be emulsified prior to digestion – Emulsify = dispersed as small droplets – Lecithin and bile salts emulsify oils and fats in the small intestine by forming micelles (fig 26-8, page 778) • Micelles are further broken down by lipase Residuals of Digestion • Certain compounds cannot be digested in humans b/c we lack the enzyme required for hydrolysis • These compounds are excreted in the feces • Cellulose (dietary fiber), connective tissue from meat (collagen), undigested fats combined with calcium and magnesium, bacteria, pigments, water, mucous Chemical Digestion • Summarized in Table 26-3, page 779 Secretion • Release of substances from exocrine glands in the GI tract – Examples: Saliva, gastric juice, bile, pancreatic juice, intestinal juice Saliva • Secreted from salivary glands • Water component helps liquefies food chyme – Allows enzymes to mix with food particles • Mucus lubricates food to protect mucosa lining • Amylase – chemically digest (breakdown) complex carbohydrates • Lipase (small amounts) – digest lipids – Decreased function when fat are not emulsified • Sodium bicarbonate (NaHCO3) – Dissociated in water – Bicarbonate ions bind with H+ to increase pH Control of Salivary Secretion • Controlled by reflex mechanisms: – Olfactory & visual stimuli send afferent impulses to centers in the brainstem efferent impulses to salivary glands – Chemical and mechanical stimuli come from the presence of food in the mouth Gastric Juice • Gastric juice = water, mucous, enzymes & acid • Secreted by gastric glands in the stomach • Chief cells – secrete enzymes of digestive juices – Pepsin • Parietal cells – Secrete HCl • Decreases stomach pH Control of Gastric Secretion • Gastric secretion is controlled by 3 phases: 1. Cephalic phase (“psychic phase”) – – – Sight, smell, taste, thought of food activate control centers in medulla oblongata Parasympathetic fibers of the vagus nerve conduct impulses to gastric glands Vagal impulses stimulate production of gastrin • Gastrin stimulates gastric secretion Control of Gastric Secretion 2. Gastric phase: – Gastrin secretion is further stimulated by the presence of products of protein digestion & distention – Gastrin continues to stimulate the secretion of gastric juices (pepsin and HCl) Control of Gastric Secretion 3. Intestinal phase: – Gastric inhibitory peptide (GIP) in secreted in the small intestine in the presence of fats and carbohydrates • Decrease gastric motility and secretion – Secretin secreted in the small intestine in the presence of acid, digested proteins and fats • Inhibit gastric secretion • Simulate secretion of pancreatic enzymes • Stimulate ejection of bile into small intestine – CCK • • • • Secreted in the small intestine in the presence of chyme Stimulates ejection of bile from gallbladder Stimulates secretion of pancreatic juices Opposes action of gastrin; raises pH of gastric juice Pancreatic Juice • Secreted by exocrine portion of the pancreas • Mostly water • Enzymes: – Trypsin and chymotrypsin (proteases) – Lipases – Amylase (starch digesting enzyme) • Secrete bicarbonate into the GI lumen and H+ into the blood to buffer the effects parietal cell secretion (fig 26-10 and fig 26-11) Control of Pancreatic Secretion • Secretin & CCK – Stimulates the secretion of pancreatic fluid high in bicarbonate to neutralize acidity of chyme in the small intestine – See notes under “Control of Gastric Section – Intestinal phase” Bile • Secreted by liver and stored in gallbladder • Bile contains: – Lecithin and bile salts • Emulsify fats by creating a hydrophilic “shell” around tiny fats droplets – Sodium Bicarbonate – increase pH of chyme in small intestine – Excretions: • Cholesterol, products of detoxification, bilirubin (product of hemolysis) Control of Bile Secretion • Controlled by CCK and secretin • See Table 26-5, page 782 Intestinal Juice • Mucus – provides lubrication • Sodium bicarbonate – increases pH to allow intestinal enzymes to function at optimal level • Water – carries mucus and NaHCO3 **Study These Tables** Table 26-4: Digestive Secretions Table 26-5: Actions of Digestive Hormones Both on page 782 Absorption • Passage of substances (digested foods, vitamins, salts, water) across the mucosa into the blood • Majority of absorption takes place in small intestine where surface area is increased Elimination • Expulsion of digestive residuals from the digestive tract in the form of feces – Defecation – Normally rectum is empty – Massive peristalsis of feces into the rectum stimulate receptors and relax the external anal sphincter – Voluntary control Elimination • Constipation – Contents move through large intestine at a slower rate – Increased water absorption occurs resulting in hardened feces • Diarrhea – Result of increased motility of the small intestine – Water absorption does not occur GI Tract Disorders • Common S/S – Gastroenteritis • Gastritis – stomach inflammation • Enteritis – intestinal inflammation – Anorexia: chronic loss of appetite – Nausea: feeling of needing to vomit; may progress to vomiting – Emesis: vomiting – Diarrhea: elimination of liquid feces; abdominal cramps may also be present – Constipation: decreased motility of colon; difficulty in defecating GI Tract Disorders • Ulcers – In stomach or duodenum – Cause pain and may lead to perforation of the wall of the GI tract – Bleeding anemia – Causes: • Hyperacidity • H. phylori bacterium GI Tract Disorders • Stomach cancer – Linked to excessive alcohol use, chewing tobacco, eating heavily preserved foods – Early signs: • Heartburn, belching, nausea – Later signs: • Chronic indigestion, vomiting, anorexia, stomach pain, blood in feces GI Tract Disorders • Diverticulosis – presence of abnormal sac-like projections on the large intestine (diverticula) – When inflamed causes diverticulitis – S/S: pain, tenderness, fever GI Tract Disorders • Colitis – inflammation of the large intestine – s/s: diarrhea, abdominal cramps, constipation, bleeding, intestinal ulcers – Crohn’s Disease: autoimmune colitis – Treatment: surgical removal of affected portions of the intestine GI Tract Disorders • Irritable bowel syndrome (IBS) – “spastic colon” – Noninflammatory condition usually caused by stress – Diarrhea or constipation GI Tract Disorders • Colorectal cancer – Occurs after 50 – Associated with low-fiber, high-fat diet – Early signs: change in bowel habits, fecal blood, rectal bleeding, abdominal pain, unexplained anemia, weight loss, fatigue Liver Disorders • Hepatitis – inflammation of the liver – S/S: jaundice, liver enlargement, anorexia, abdominal discomfort, gray-white feces, dark urine – Causes: alcohol or drug abuse; bacterial or viral infection • Cirrhosis – degenerative liver condition – Tissue can no longer regenerate