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-Biology 55 Digestive System DIVISIONS OF THE DIGESTIVE SYSTEM Gastrointestinal (GI) Tract – path that food follows mouth, pharynx, esophagus, stomach, small intestine, large intestine, rectum, anus Accessory Digestive Organs – salivary glands, liver, gallbladder, pancreas FUNCTIONS OF THE DIGESTIVE SYSTEM 1) ingestion – eating and drinking 2) digestion – breaking down food into molecules that can be absorbed 3) absorption – passage of nutrient molecules from GI tract into blood or lymph 4) defecation – elimination of feces (undigested or unabsorbed materials) ANATOMY OF THE DIGESTIVE SYSTEM Abdominal (peritoneal) Cavity – contains most of GI tract peritoneum – serous membrane lining cavity, organs, and forming double-layered mesentery supporting organs and containing vessels and nerves to organs greater omentum – fold of mesentery overlying intestines peritonitis – inflammation of peritoneum Layers of the GI Tract mucosa – mucous membrane lining inner lumen (central cavity) stratified squamous epithelium – protects mouth, pharynx, esophagus, and anal canal from abrasion simple columnar epithelium – secretes and absorbs materials in the stomach and small and large intestines submucosa – connective tissue containing blood and lymphatic vessels and nerves muscularis – layers of smooth muscle that move contents serosa – visceral peritoneum covering organs in abdominal cavity MOUTH (ORAL CAVITY) – ingests food, chews it, and mixes it with secretions to form a bolus (moist ball of food) Hard and Soft Palates – separate mouth from nasal cavity hard palate– maxillae and palatine bones soft palate – muscle that closes nasal cavity when swallowing Lips, Cheeks, and Tongue – skeletal muscle structures that manipulate food to hold it in the mouth and push it between the teeth to be chewed tongue also has taste buds to determine palatability of food 1 Salivary Glands – secrete saliva that lubricates and chemically digests food mucus – lubricates mouth and food salivary amylase – digests starch lysozyme – kills bacteria Teeth – cut and grind food into smaller particles (mastication) crown – visible portion above gumline enamel – extremely hard, calcium material protecting crown root – portion embedded in alveolus of maxilla or mandible periodontal ligament – fibrous CT that anchors tooth gingiva (gums) – mucosa attached to and protecting root of tooth pulp – connective tissue with blood vessels and nerves in center of tooth Deciduous Teeth – 20 baby teeth that fall out and are replaced Permanent Teeth – 32 adult teeth Types of Teeth: 1) incisors – chisel-shaped front teeth that cut food 2) cuspids (canines) – pointed teeth that tear food 3) bicuspids (premolars) – flat teeth that grind food 4) molars – flat, back teeth that crush and grind food caries (cavities) – holes in teeth caused by acids from foods and bacteria PHARYNX (throat) – muscular tube that carries food from mouth to esophagus (also carries air from nasal cavity or mouth to larynx) Swallowing – movement of food from mouth to stomach voluntary phase – tongue pushes bolus back into pharynx tongue moves up and back pushing bolus into pharynx swallowing reflex – initiated by touch sensations in pharynx soft palate closes nasal cavity (keeps food out of nose) epiglottis closes larynx (keeps food out of lungs) respiratory center inhibited (breathing stops) esophagus opens to allow entry of bolus ESOPHAGUS – muscular tube that carries food from pharynx to stomach peristalsis – waves of contraction and relaxation of circular and longitudinal muscles move bolus down esophagus esophageal sphincter – circular muscle that opens so food can enter stomach, then closes again to prevent gastroesophageal reflux heartburn – acid from stomach damages esophageal mucosa STOMACH (gastric/gastro-) – muscular and glandular sac that stores food and mixes it with gastric secretions to form a soupy mixture called chyme hydrochloric acid (HCl) – kills bacteria, denatures proteins, activates enzymes pepsinogen (active form – pepsin) – digests proteins intrinsic factor – needed to absorb vitamin B12 2 mucus – protects mucosa of stomach from HCl ulcers – erosions in stomach lining due to inadequate protective mucus Anatomy of Stomach 1) cardia – superior portion near esophageal sphincter 2) fundus – rounded portion that stretches to store large meals 3) body – main central portion that churns food and gastric juice 4) pylorus – narrowed part attached to intestine pyloric sphincter – circular muscle that opens to allow chyme to pass into intestine rugae – folds in mucosa of empty stomach; disappear as stomach fills SMALL INTESTINE – muscular and glandular tube that is the main site of chemical digestion and absorption of nutrients Anatomy of Small Intestine 1) duodenum – first portion that is attached to stomach 2) jejunum – middle portion 3) ileum – last portion that is attached to large intestine ileocecal valve – opening into cecum; regulated by a sphincter structures that increase surface area for digestion and absorption circular folds in mucosa and submucosa villi – finger-like projections of mucosa contain many capillaries and lacteals (lymphatic capillaries) microvilli – finger-like projections of epithelial cell membrane brush border (fuzzy appearance) with attached enzymes Peyer’s patches – large lymphatic nodules in ileum Functions of Small Intestine alkaline mucus – neutralizes HCl from stomach brush border enzymes – digest carbohydrates, proteins, nucleic acids duodenum receives pancreatic enzymes (from pancreas) digest carbohydrates, proteins, lipids, nucleic acids duodenum receives bile (from liver) – emulsifies fats segmentation – contractions that mix intestinal contents absorption into blood capillaries monosaccharides (carbohydrates) amino acids (proteins) water and ions into lymphatic lacteals fatty acids, glycerol (lipids) peristalsis – slow waves move nondigested contents to large intestine 3 LARGE INTESTINE – large tube composed of sacs called haustra that functions mainly in absorbing water and forming indigestible materials into feces bacterial flora – normal, symbiotic bacteria living in GI tract have enzymes that digest materials our enzymes cannot produces gases and odor produce vitamins – K, B vitamins absorption water, vitamins (K,B), bile salts mass movement – rapid contraction drives undigested materials into rectum feces – bacteria, unabsorbed or indigestible materials (fiber) Anatomy of the Large Intestine 1) cecum – blind pouch attached to ileum appendix – finger-like extension containing lymphoid nodules 2) ascending colon – right abdomen 3) transverse colon – crosses from right to left abdomen 4) descending colon – left abdomen 5) sigmoid colon – S-shaped entrance to rectum 6) rectum – expands to store feces 7) anus – exit for feces internal sphincter – involuntary (smooth muscle) external sphincter – voluntary (skeletal muscle) PANCREAS endocrine function – pancreatic islets regulate blood glucose level exocrine function – secretes pancreatic juice into small intestine pancreatic duct – carries pancreatic juice to duodenum sodium bicarbonate – helps neutralize HCl from stomach pancreatic enzymes (many) pancreatic amylase – digests carbohydrates trypsin, chymotrypsin – digest proteins lipase – digests lipids nuclease – digests nucleic acids LIVER hepatic portal system carries blood and absorbed materials from GI tract to liver for processing before they enter general circulation liver regulates composition of the blood Anatomy of Liver – composed of lobes, divided into lobules (groups of liver cells) Portal Triad – 3 structures next to lobules hepatic artery – supplies oxygenated blood to liver cells bile duct – collects bile secreted by liver cells hepatic portal vein – carries venous blood from GI tract to liver cells 4 Functions of Liver 1) maintains blood glucose level glucose stored as glycogen after meals glycogen stores broken down to glucose between meals 2) breaks down amino acids ammonia (toxic by-product) converted to urea 3) synthesizes plasma proteins 4) removes or breaks down many toxins in blood 5) produces and secretes bile bile pigments (yellow-green) derived from bilirubin bile salts – formed from cholesterol emulsify fats in small intestine – breaks hydrophobic clumps into smaller droplets that can be digested by lipase and absorbed 6) stores iron and vitamins (A,D,E,K) 7) phagocytosis and antigen presentation Kupffer cells – macrophages that remove old blood cells, debris, microbes GALLBLADDER – pear-shaped sac that stores bile produced by liver common bile duct – empties bile into duodenum REGULATION OF DIGESTIVE ACTIVITY cephalic phase (head) – taste, smell, sight, thought of food causes parasympathetic stimulation which increases digestive secretions and smooth muscle contractions(motility) gastric phase – food entering stomach stretch receptors cause nervous reflexes that increase activity of stomach and small intestine nutrients in stomach cause secretion of gastrin (hormone from stomach) which increases activity of the stomach intestinal phase – chyme entering duodenum chyme in duodenum cause secretion of hormones secretin – stimulates liver and pancreas functions cholecystokinin (CCK) – stimulates pancreas and gallbladder gastrocolic reflex – stretch of stomach and duodenum causes reflex mass movement in large intestine defecation reflex – triggered by stretch receptors in wall of rectum peristalsis in rectum and relaxation of internal sphincter conscious control over external sphincter (skeletal muscle) 5