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The Digestive System

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The Digestive System The Digestive System Overview 1. Food breakdown • CHO, Protein, Fat 2. Anatomy 3. Digestive process ¾ Primary function: breakdown & transport nutrients, H2O, & electrolytes 1. Motility – propulsive or mixing movements 2. Secretion – energy requiring secretion of H2O, electrolytes, & enzymes, bile salts, or mucus Lumen Duct cells Exocrine gland cells Secretory product Figure 16.1 Page 592 Capillary 1 ¾ Primary function: breakdown & transport nutrients, H2O, & electrolytes 3. Digestion –CHO Carbohydrates Monosaccharides Polysaccharides (starch and glycogens) Maltose (glucose, galactose, fructose) Amylase Maltase Disaccharides Sucrose Sucrase Lactose Lactase Table 16.1 (1) Page 593 ¾ Primary function: breakdown & transport nutrients, H2O, & electrolytes 3. Digestion – proteins & fats Fats Monoglyceride Triglyceride Free fatty acids Lipase ¾ Primary function: breakdown & transport nutrients, H2O, & electrolytes 4. Absorption – primarily in small intestines 2 General Digestive Anatomy 1. Digestive Tract 2. Accessory Digestive Organs Nasal passages Mouth Salivary glands Pharynx Trachea Esophagus Liver Stomach Gallbladder Pancreas Duodenum Descending colon Transverse colon Ascending colon Jejunum Cecum lleum The Digestive Tract (inner layer to outer) 1. Mucosa Mucous membrane • Mucous membrane 9 Exocrine cells (digestive juices) 9 Endocrine cells (hormones) 9 Epithelial cells (absorption) • Degree of folding depends on location 9 Highest in small intestine 3 The Digestive Tract (inner layer to outer) 2. Submucosa Submucosa • Thick connective tissue Muscularis externa 9 Blood & nerve innervation 3. Muscularis externa • Inner circular layer • Outer longitudinal layer 4. Serosa • Outer connective tissue 9 Prevents friction Digestive regulation 1. Smooth-muscle function • Slow-wave action potentials (basal electrical rhythm) 9 Interstitial cells of Cajal • Regulate peristalsis and segmentation 2. Intrinsic nerve plexuses (enteric nervous system) – digestive nervous system • Respond to local stimuli for secretion (digestive juices & GI hormones) • Both excitatory & inhibitory Digestive regulation 3. Extrinsic nerves • Modify intrinsic activity and other various digestive organs (generally sympathetic &/or parasympathetic) 4. Gastrointestinal hormones • Carried throughout blood influencing other areas of digestive tract 4 Sensory Receptors 1. Chemoreceptors • Sense changes in chemical components within lumen 2. Mechanoreceptors • Respond to stretch and tension 3. Osmoreceptors • Changes in osmolarity External Local changes in influence digestive tract Receptors in digestive tract Extrinsic Intrinsic Gastrointestinal automatic nerve plexuses hormones nerves Smooth muscle (contraction for motility) Exocrine gland cells (secretion of digestive juices) Figure 16.4 Endocrine gland cells (secretion of gastrointestinal Page 595 and pancreatic hormones) The Digestive Process 5 The Digestion Process 1. Mouth • Chewing • Saliva secretion ¾ 3 major salivary glands • Salivary proteins 1. Amylase (CHO breakdown) 9 Polysaccharides to disaccharides 2. Mucus (lubrication) 3. Lysozyme (antibacterial) Cerebral cortex Other inputs Salivary Regulation Salivary center Conditioned in medulla reflex Pressure receptors Autonomic nerves Unconditioned and chemoreceptors reflex in mouth Salivary glands Salivary secretions Figure 16.5 Page 601 The Digestion Process (cont.) 2. Pharynx & Esophagus • Swallowing reflex 6 The Digestion Process (cont.) 2. Pharynx & Esophagus • Peristalsis Bolus Figure 16.7 Page 603 The Digestion Process (cont.) Stomach Esophagus Fundus Gastroesophageal sphincter Body Pyloric sphincter Figure Antrum 16.8 Duodenum Page 604 Stomach Functions 1. Storage 2. Gastric mixing & mucous secretion 3. Production of chyme 4. Secretes hydrochloric acid (HCl) • Reduces large food particles • Kills microorganisms ingested in food 5. Initial stages of protein breakdown • Pepsinogen forming pepsin 7 Figure 16.9 (1) Page 605 Basal electrical rhythm ~ 3 per minute Pyloric sphincter Duodenum Direction of movement of peristaltic Peristaltic contraction contraction Gastric emptying Figure 16.9 (2) Page 605 Peristaltic contraction Gastric mixing Regulation of Gastric Emptying ¾ Amount of chyme ¾ Neural response • Intrinsic nerve plexus (short reflex) Enterogastric • Autonomic nerves (long reflex) Reflex ¾ Hormonal • Enterogastrones (secretin & cholecystoinin – CCK) released from duodenal mucosa 9 Inhibit antral contractions 8 Regulation of Gastric Emptying ¾ Duodenum 1. Fat ~ can only be processed in small intestine 2. Acid (unneutralized) 9 Excess HCl not buffered by sodium bicarbonate 3. Hypertonicity ~ increased osmolarity due to abundance of amino acids and glucose 4. Distension ¾ Emotions Gastric Digestive Juices ~ 2 liters/day ¾ Responsibility of cells lining gastric mucosa 1. Oxyntic mucosa • Body • Fundus 2. Pyloric gland area (PGA) • Antrum Table 16.4 (1) Page 609 Oxyntic Gastric mucosa pit Mucosa Stomach lumen Pyloric gland area Submucosa 9 In oxyntic mucosa In pyloric gland area Surface epithelial cells Gastric pit Mucosa cells Alkaline mucus Gastric G cells gland Chief cells Gastrin pepsiongen D cells Somatostatin Parietal cells HCl Enterochomaffin- like (ECL) cells Histamine Chief & Parietal Cell Regulation ¾ Acetylcholine (Ach) • Stimulates both • Also stimulates G and ECL cells ¾ Histamine (paracrine) • Released from ECL cells & increases HCl secretion ¾ Somatostatin • Released from D cells • Provides negative feedback HCl & Pepsinogen PlasmaHCl secretion into lumen Gastric lumen Parietal cell Cellular metabolism Chief cell 10 ¾ HCl functions to: • Activate pepsinogen to form pepsin Autocatalysis Digestion Pepsinogen Pepsin Protein Gastric HCI lumen Peptide fragments • Breakdown of connective tissue • Denatures proteins • Kills microorganisms Gastric Mucosal Barrier Mucus coating Impermeable to HCI Tight junction Cells lining gastric mucosa (including those lining Submucosa gastric pits and glands) Page 16.12 Page 614 Food leaving the Stomach… Mixed with secretions from pancreas and liver 11 Page 16.13 Bile duct Page 616 Duodenum from liver Stomach Hormones (insulin, glucagon) Blood Endocrine portion of pancreas (Islets of Langerhans) Duct cells Acinar cells secrete aqueous secrete digestive NaHCO3 solution enzymes The glandular portions of Exocrine portion of panaceas the pancreas are grossly (Acinar and duct cells) exaggerated Pancreas ¾ Exocrine & endocrine tissue 1. Exocrine: secretes enzymes capable of breaking down CHO, fat, & protein 9 Proteolytic enzymes: protein Regulated by secretin 9 Pancreatic amylase: CHO & cholecystokinin 9 Pancreatic lipase: fat (CCK) 2. Endocrine (hormones): Insulin & glucagon Hormonal Regulation Acid in Fat and protein duodenal products in lumen duodenal lumen Secretion release CCK release from duodenal from duodenal mucosa mucosa (Secretin carried Neutralizes(CCK carried Digests by blood) by blood) Pancreatic duct Pancreatic acinar cells cells Secretion of Secretion of aqueous pancreatic digestive NaHCO solution into 3 enzymes into duodenal lumen duodenal lumen 12 Liver ¾ Digestive role: secretion of bile salts • Aid fat digestion & absorption Hepatic Hepatic vein artery Blood flow – Hepatic Portal System Hepatic portal vein Digestive capillaries Digestive tract Page 16.15 Page 618 Liver ¾ Bile secretion Bile salts Cholesterol Common bile duct Gallbladder: Bile storage Portal circulation Colon Duodenum Page 16.17 Terminal ileum Page 620 Small Intestine ¾ Primary site of digestion & absorption Duodenum primary Jejunum Ileum 13 Segmentation of chyme ¾ Initiated by pacesetter cells Small intestine – digestion & absorption ¾ Potential for increased surface area 1. Extensive folding 2. Villi & microvilli 9 Increased digestive enzyme release ¾ Pancreatic enzymes • Fat reduced to FFA (with help of bile salts) 9 Lipase • Proteins to AA 9 Aminopeptidases • CHO to di- and monosaccharides 9 Maltase, sucrase, lactase Lumen Na+-and energy- dependent Facilitated secondary diffusion active transport Energy required Epithelial cell of villus Capillary 14 Lumen Na+-and energy- dependent absorption Energy required Epithelial cell of villus Capillary Lipid emulsion Micelles diffusion Lumen Lumen Micelle Micelles Microvillus Fatty acids, monoglycerides Aggregate and Short or Passive absorption coated with medium lipoprotien chain Basement Epithelial membrane cell of villus (Exocytosis) Central lacteal Capillary Small intestine – digestion & absorption 1. Vitamin absorption (passive) • Water-soluble • Fat-soluble 2. Iron absorption – regulated • Absorbed into epithelial cells 9 Either used immediately for production of RBC or 9 Stored as ferritin 3. Calcium absorption – regulated • Active transport stimulated by Vitamin D 15 Biochemical Balance Digestive tract Stomach Blood lumen parietal cell Pancreatic duct cell Intestinal epithelial cell Large Intestine Colon Cecum Drying & storage Rectum 16.
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