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Unit I: Metabolism Digestive Processes Chapter 21 and 22 Mastication or Chewing surface area exposed to digestive enzymes • Contact of food with sensory receptors triggers chewing reflex – tongue, buccinator and orbicularis oris manipulate food – masseter and temporalis elevate the teeth to crush food – medial and lateral pterygoids swing teeth in side-to-side grinding action of molars Salivation • Total of 1 to 1.5 L of saliva per day • Cells filter water from blood and add other substances • Food stimulates receptors that signal salivatory nuclei in medulla and pons – parasympathetic stimulation salivary glands produce thin saliva, rich in enzymes – sympathetic stimulation produce less abundant, thicker saliva, with more mucus Swallowing / Deglutition 1. Buccal Phase • Compression of the bolus Soft palate • Tongue then forces the bolus Bolus Esophagus into the oropharynx • Voluntary 2. Pharyngeal Phase Tongue Bolus • Tactile receptors on the palatal arches and uvula are stimulated. • Swallowing center in the medulla oblongata then coordinates: • Elevation of the larynx • Folding of the epiglottis • Elevate the uvula and soft palate • Involuntary Swallowing / Deglutition 3. Esophageal Phase • Myenteric nerve plexis signals peristaltic waves • LES opens Peristalsis • Typical bolus = 9 seconds • Liquids = 2 seconds • A dry or poorly lubricated bolus may need secondary peristaltic waves • Bolus continues to the stomach Thoracic cavity Lower esophageal sphincter (LES) Stomach Peristalsis allows us to move a bolus regardless of our body position. Gastric Motility • Swallowing center signals stomach to relax • Receptive-relaxation response • Peristalsis – contraction every 20 seconds – stronger contractions after 30 minutes; ejects 3 ml – typical meal emptied from stomach in 4 hours Vomiting • Induced by – excessive stretching of stomach, psychological stimuli or chemical irritants (bacterial toxins) • Emetic center in medulla causes – retching • Lower esophageal sphincter to relax • stomach and duodenum to contract spasmodically – vomiting • when abdominal contraction forces upper esophageal sphincter to open Regulation of Gastric Secretion 1. Cephalic Phase 2. Gastric Phase • See, smell, taste, or think of food. • Directed by the CNS • Prepares the stomach to receive food • Production of gastric juice accelerates • reaching rates of about 500 mL/h • lasts only minutes. Medulla Oblongata Vagus nerve (N X) • Arrival of food in the stomach (1) distension of the stomach (2) an increase in the pH (3) the presence of undigested proteins • Lasts 3-4 hours • Gastrin stimulates contractions in the muscularis externa of the stomach. Submucosal and myenteric plexuses Distension Elevated pH via bloodstream Submucosal plexuses Mucous cells Chief cells Gastrin Mucous cells Chief cells Mucus Gastrin Pepsinogen Parietal cells HCl G cells G cells KEY Stimulation Chemoreceptors Mucus Pepsinogen Parietal cells HCl Stretch receptors Partly digested peptides Mixing waves Regulation of Gastric Secretion 3. Intestinal Phase • Chyme first enters the small intestine • Function: control the rate of gastric emptying • Enterogastric reflex: Stimuli:↓ stimulation of stretch receptors in stomach and • ↑stimulation of stretch receptors in intestine Enterogastric Myenteric plexus Purpose: Inhibit gastric activity reflex Chief • Stimulates contraction of via bloodstream cells pyloric sphincter Parietal Duodenal • Mucus production cells stretch and chemoreceptors CCK GIP Presence of lipids and carbohydrates Secretin Decreased pH Peristalsis KEY Inhibition Intestinal Hormones • Cholecystokinin (CCK) 1. Contraction of gallbladder 2. Secretions of pancreatic enzymes 3. Relaxation of hepatopancreatic sphincter • Secretin • Glucose dependent insulin peptide/ Gastric inhibitory peptide Overall effect: reduce gastrin secretion and passage of chyme Small Intestine Motility • Segmentation − stationary ring-like constrictions − not as in peristalsis • Purpose: – mix – churn • Pacemaker cells in Muscularis externa • Provides the most contact digestion http://youtu.be/PfnKvErPwY4 Carbohydrate Digestion • Salivary amylase oligosaccharide – 50% of dietary starch digested before it reaches small intestine • Pancreatic amylase oligosaccharide and maltose • Brush border enzymes monosaccharides Carbohydrate Absorption • 80% of glucose taken up by Naglucose transport proteins → Disaccharides Enzymes • Facilitated diffusion → blood capillaries of villus → Simple sugars (such as glucose or fructose) • Delivered to liver Epithelial cell Simple sugars Protein Digestion Proteins ingested ½ of amino acids digested come from dietary proteins Protease pepsin breaks down complex proteins into smaller peptide and polypeptide chains Pancreatic enzymes: Trypsin, chymotrypsin, and carboxypeptidase break down proteins into a mixture of dipeptides, tripeptides, and amino acids. amino acids The epithelial surfaces of the small intestine contain several brush border enzymes that release individual amino acids. Dipeptidase Facilitated diffusion and cotransport Amino acids Epithelial cell Facilitated diffusion and cotransport Amino acids Protein Absorption Behaves like monosaccharides: •Taken up by Na-dependent transport proteins → epithelial cells → •Facilitated diffusion → blood capillaries of villus → bloodstream Lipid Digestion 1. Lipase – Lingual lipase, gastric lipase Bile 2. 3. Lipid Absorption 1. 2. Nucleic Acids, Vitamins, and Minerals • Nucleases – brush border split them completely apart • Vitamins are absorbed unchanged – A, D3, E and K with other lipids – B complex and C by simple diffusion – B12 if bound to intrinsic factor • Minerals are absorbed all along small intestine – Na+ cotransported with sugars and amino acids – Cl- exchanged for bicarbonate reversing stomach – Iron and calcium absorbed as needed Water Balance • Digestive tract receives about 9 L of water/day −0.7 L in food, 1.6 L in drink, 6.7 L in secretions −8 L is absorbed by small intestine and 0.8 L by large intestine • Diarrhea –feces pass through too quickly if irritated –feces contains high concentrations of a solute (lactose) Absorption and Motility of Large Intestine Vitamin Absorption Vitamin K 1500mL/day enters Vitamin B5 Biotin Over 1 L of water is reabsorbed through osmosis. Organic waste products, such as urobilinogen, and various toxins. Only 200 mL of feces is ejected. 75% water, 25% solids • Transit time is 12 to 24 hours – reabsorbs water and electrolytes • Haustral contractions occur every 30 minutes – distension of a haustrum stimulates it to contract • Mass movements occur 1 to 3 times a day – filling of the stomach and duodenum Stimulation of somatic motor neurons Defecation Stimulation of motor neurons in sacral spinal cord Stimulation of myenteric plexus Parasympathetic Defecation Reflex Intrinsic Defecation Reflex Increased peristalsis throughout large intestine Stimulation of stretch receptors Start Increased local peristalsis DISTENSION OF RECTUM Relaxation of internal anal sphincter External sphincter can be voluntarily relaxed Involuntary contraction of external anal sphincter When external sphincter is relaxed: DEFECATION OCCURS stimulates inhibits