Digestive System Outline PDF

THE DIGESTIVE SYSTEM I. The major functions are: -Digestion: breakdown of food into smaller molecules, via physical and chemical means -Absorption: bringing those molecules into the body to be used by cells A. Divided into two major parts: 1. Alimentary canal (gastrointestinal (...

THE DIGESTIVE SYSTEM I. The major functions are: -Digestion: breakdown of food into smaller molecules, via physical and chemical means -Absorption: bringing those molecules into the body to be used by cells A. Divided into two major parts: 1. Alimentary canal (gastrointestinal (GI) tract, gut, or digestive tract): continuous tube food passes through, from mouth to anus 2. Accessory organs: teeth, tongue, salivary glands, liver, gallbladder, and pancreas B. Abdominal cavity aka peritoneal cavity 1. Intraperitoneal: within peritoneal cavity 2. Retroperitoneal: behind peritoneal cavity 3. Mesentery: peritoneal layers containing blood/lymph vessels and nerves; helps anchor organs to abdominal wall a. Lesser omentum: connects stomach to liver b. Greater omentum: “beer belly” c. Mesocolons: connect large intestine to abdominal wall C. Digestive Processes 1. Ingestion: intake of food 2. Propulsion: ability to move substances 3. Mechanical digestion: physically breaking food down 4. Chemical digestion: chemically break food into smaller molecules 5. Absorption: bring those small molecules into blood/lymph vessels 6. Defecation (excretion): eliminating indigestible material D. Histology 1. Blood for digestive tissues from splanchnic circulation 2. All digestive organs have same 4 basic layers: a. Mucosa: deepest layer, MALT located here; functions to secrete (mucus, enzymes, hormones), absorb nutrients, and protect against infectious disease b. Submucosa: contains blood/lymph vessels and nerve fibers c. Muscularis externa: layers of muscle for movement. Sphincters form at junctions of organs d. Serosa (adventitia in esophagus): most superficial layer 3. Enteric Nervous System: subdivision of ANS, contains large amount of neurons; functions to regulate gland activity and GI tract motility II. ANATOMY A. The Mouth and Associated Organs 1. Mouth: mucosa-lined cavity bordered by lips, cheeks, palate, and tongue; responsible for ingestion -soft palate: closes off nasopharynx when swallowing -tongue: forms bolus; important for swallowing, speech, and taste 2. Salivary glands: secrete saliva to breakdown food; saliva contains water (>97%), digestive enzymes, mucus, and antimicrobials -Salivary amylase important digestive enzyme for carbohydrates -Major glands are parotid, submandibular, and sublingual 4. Teeth: 20 deciduous teeth, replaced by 32 permanent teeth a. Types: incisors for biting, canines for tearing, premolars and molars for grinding and crushing 0  b. Structural features: i. Crown: portion visible outside the gum ii. Root: portion embedded in the bone iii. Pulp cavity: inside the tooth, contains blood vessels and nerves iv. Enamel: covers the crown; composed mostly of hydroxyapatite crystals, which make it the hardest substance in body v. Dentin: calcified, protein-rich tissue that composes the bulk of the tooth B. Pharynx: throat; food passes through oropharynx, then laryngopharynx, to esophagus C. Esophagus: muscular tube connecting pharynx and stomach; at stomach junction forms gastroesophageal sphincter D. Digestive Processes: Mouth to stomach 1. Mastication: chewing 2. Deglutition: swallowing E. Stomach: stores food, and mixes it with water/gastric juice to form chyme 1. Gross anatomy: a. Major regions: cardiac, fundus, body, and pyloric b. Can expand up to 4L due to folds called rugae c. Pyloric sphincter: regulates chyme movement into small intestine d. Contains additional muscularis layer (oblique) to increase churning (physical breakdown) 2. Unique microscopic anatomy: mucosa contains gastric pits that secrete gastric juice, composed of: a. Mucous neck cells: secrete an acidic mucus b. Parietal cells: secrete hydrochloric acid (HCl) and intrinsic factor (needed for absorption of vitamin B12 in the small intestine) c. Enteroendocrine cells: secrete hormones d. Chief cells: secrete pepsinogen (inactive form of pepsin), an enzyme to breakdown proteins e. Regenerative cells: constantly undergoing mitosis 3. Regulation of gastric secretion: regulated hormonally and neutrally; broken into phases: a. Cephalic phase: preparing for digestion b. Gastric phase: swallowing food initiates gastric secretion of acetylcholine, histamine, and gastrin to stimulate digestion. This also stimulates HCl and intrinsic factor to be released c. Intestinal phase: chyme empties into duodenum which stimulates release of secretin and cholecystokinin. This also causes gastrin secretions to decline F. Small Intestine 1. Function: absorption 2. Gross Anatomy: extends from the pyloric sphincter to the ileocecal valve (large intestine junction) a. Duodenum: first section; receives chyme, pancreatic juice, bile via hepatopancreatic ampulla b. Jejunum: middle section c. Ileum: last section; ends at the ileocecal valve 3. Unique microscopic anatomy a. Plicae circulares: folds that slow chyme movement, and increase surface area b. Villi: fingerlike projections containing capillaries and lacteals, and increase surface area c. Microvilli: small fingerlike projections that make the brush border, and increase surface area d. Intestinal crypts: contains cells that secrete intestinal juice, composed of: i. Absorptive cells: secrete brush border enzymes for carbohydrate and protein digestion ii. Goblet cells: secrete mucous iii. Enteroendocrine cells: secrete hormones iv. Paneth cells: secrete antimicrobials 1  e. Peyer’s patches: MALT tissue, within ileum f. Brunner’s glands: secretes alkaline mucous to neutralize gastric acid, within duodenum G. Liver 1. Functions: a. Produces bile, to break down fats b. Regulates blood glucose; i. Glycogenesis: when high levels, glucose is converted to glycogen for storage ii. Glycogenolysis: when low levels, glycogen is broken down and released into blood c. Stores fat-soluble vitamins (A, D, B12) and minerals d. Synthesizes proteins e. Breaks down ammonia and other toxic substances f. Breaks down old RBC’s to be recycled 2. Gross anatomy: a. Four major lobes: right, left, caudate, quadrate b. Porta hepatis: entrance/exit for blood and lymph vessels, and common hepatic duct 3. Microscopic anatomy: a. Functional units are lobules, which contain hepatocytes to filter blood and produce bile b. Kupffer cells are main immune cell in the liver c. Bile breaks down fats in the intestines H. Gallbladder: stores and concentrates excess bile from liver I. Pancreas: pancreatic acini cells produce pancreatic juice, that contains: a. Pancreatic amylase for starch digestion b. Proteases for protein/peptide digestion c. Pancreatic lipase for lipid digestion d. Pancreatic nucleases for nucleic acid digestion e. Sodium bicarbonate to neutralize stomach acid J. Digestive processes in small intestine a. Up to 6 hours for chyme to get through small intestines, and most nutrients are absorbed here b. Segmentation is common motion; facilitates absorption c. Peristalsis is motion for propulsion K. Large Intestine 1. Function: absorbs water from indigestible food, then stores until defecation 2. Gross Anatomy: a. Cecum: first pouch of large intestine; appendix attaches here b. Colons: ascending, transverse, descending, sigmoid c. Haustra: pouches of large intestine d. Taenia coli: smooth muscle that forms haustra e. Anal canal: last 1 inch of rectum, contains internal anal sphincter (involuntary) and external anal sphincter (voluntary) 3. Microscopic Anatomy a. Absorptive cells: absorb water b. Goblet cells: secrete mucus c. Bacterial Flora: metabolize indigestible carbohydrates (e.g. cellulose, pectin, inulin) via fermentation, producing gases as by-products. Also produce vitamin K and some B vitamins 4. Digestive Processes: digestion is minimal and absorption is limited to water, vitamins B and K, and some electrolytes a. Haustral contractions: form of segmentation for motility b. Defecation: release of feces; aided by Valsalva’s maneuver (increase intra-abdominal pressure) 2 III. CHEMICAL DIGESTION: breaks down polymers into monomers via hydrolysis; this allows them to be absorbed by the body A. Carbohydrates: monomers are monosaccharides, polymers are disaccharides and polysaccharides 1. Amylases: digestive enzymes B. Proteins: amino acids are monomers, proteins are polymers 2. Proteases: digestive enzymes i. Pepsin: protein digestion in stomach; secreted by chief cells ii. Rennin: breaks down milk proteins in infants iii. Trypsin/chymotrypsin: protein digestion in small intestine C. Lipids: monomers are fatty acids, triglycerides/phospholipids/cholesterol are polymers 1. Only broken down in the small intestine 2. Lipases: digestive enzymes D. Nucleic Acids: nucleotides are monomers and DNA/RNA are polymers 1. Pancreatic nucleases: digestive enzymes IV. ABSORPTION: moving monomers from GI tract into the body A. Tight junctions between epithelial cells ensure molecules pass through cells, instead of around B. Carbohydrate absorption: glucose and galactose via secondary active transport (Na+ symport carrier proteins), then facilitated diffusion brings into blood; fructose moves only via facilitated diffusion C. Protein absorption: active transport into cells, then facilitated diffusion into blood D. Nucleic acid absorption: active transport via special transport carriers E. Lipid absorption: insoluble in water, therefore must be transported in micelles via chylomicrons 3