Chapter 24: The Digestive System

(The Digestive System) Chapter 24 oral cavity (mouth) → pharynx → esophagus → stomach → small intestine → large intestine → anus Digestive/Alimentary System · primary function is to break down food into organic molecules (carbs, fats, proteins), absorb nutrients, and eliminate wastes 1. Ingestion: food enters oral cavity 2. Mechanical digestion: crushing, squashing, shearing of food 3. Chemical digestion: via enzymes and digestive juices 4. Secretion: releasing water, acids, enzymes, buffers etc. to aid in digestion 5. Absorption: movement of ions, organic molecules, water, vitamins across the digestive epithelium 6. Defecation: the elimination of wastes through feces (7) The lining of the digestive tract acts as a protective function against corrosive acids/enzymes, mechanical stresses, and bacteria Layers of the Digestive Tract ① Mucosa ↳ innermost layer (mucous membrane) ↳ moist epithelium ↳ lamina propria: layer of arcolar tissue with blood vessels, nerves, lymphatics ↳ muscularis mucosae: concentric layers of smooth muscle and elastic fibers ② Submucosa ↳ a layer of dense irregular connective tissue that binds the mucosa to the muscular layer ↳ submucosal nerve plexus Layers of the Digestive Tract ③ Muscularis Externa (muscular layer) ↳ smooth muscles of mechanical digestion ↳ myenteric nerve plexus ④ Serosa ↳ serous membrane lining most parts of digestive tract ↳ adventitia covers areas where serosa is not present (sheath of connective tissue) The Peritoneum · the peritoneal cavity is lined by a serous membrane called the peritoneum · the peritoneum has two parts: ↳ visceral peritoneum: covers organs ↳ parietal peritoneum: lines inner walls of abdominal and pelvic cavity · peritoneal cavity: the space between the visceral and parietal peritoneum that contains serous fluid for lubrication and to prevent friction · peritonitis: inflammation of the peritoneum · ascites: a build up of fluid in the peritoneal cavity Mesenteries · double layers of serous membrane that stabilize the positions of organs in the abdominopelvic cavity and associated vessels · lesser omentum: mesentery between stomach and liver · greater omentum: mesentery that extends inferiorly to stomach and anterior surface of intestines (hangs like an apron) · mesentery proper: thick mesentery stabilizes small intestine · mesocolon: tightly anchors ascending colon, descending colon, large intestine and rectum to body wall in fetal development Gastric Motility · movement during digestion - contraction and relaxation of muscle · pacesetter cells: in muscular layer, undergo spontaneous depolarization, triggering waves of contraction that spreads through the GI muscle ↳ autoregulation · peristalsis: rhythmic contractions that move material along GI tract · Segmentation: activity that churns and fragments the bolus and mixes it with intestinal secretions Digestive Regulation · Local factors ↳ mechanoreceptors and chemoreceptors monitor changes in PH, volume, stretch · Neural mechanisms ↳ controls digestive movement and peristalsis ↳ nerve plexuses (Enteric NS): through sensory, motor, and interneurons in myenteric nerve plexus ↳ acetylcholine (PNS): stimulates gastric secretions and movements ↳ gastric reflexes ↳ special centers of medulla (swallowing and vomiting centers) · Hormonal mechanisms ↳ digestive hormones can stimulate or inhibit sensitivity of smooth muscles of digestive tract ↳ produced by enteroendocrine cells eg. Ghrelin (appetite and peristalsis) Oral Cavity The Tongue · lined with oral mucosa (stratified squamous), the gums, lingual frenulum of tongue · mastication = chewing with Keratinized cells for extra protection · nutrients are NOT absorbed in the oral cavity, but chemical digestion does begin with saliva (salivary amylase) → PH 7 · functions: sensation, mechanical digestion, lubrication, chemical digestion · frenulum: folds that attach the lower and upper lips to · lingual papillae: projections on the surface of the tongue (vallate papillae are tastebuds) · functions: mechanical digestion, manipulation of food, sensory analysis, secretions of mucins and lingual lipase · cranial nerve XII controls the extrinsic and intrinsic muscles of the tongue Teeth · bulk of each tooth is made of dentin ↳ similar to bone, but no cells · cement: stronger than dentin, covers the dentin of the root · enamel: stronger than cement, covers the dentin of the crown ↳ occlusive (biting) surface · pulp cavity: interior chamber that receives blood vessels and nerves through root canal · teeth are collectively referred to as dentition · two sets of dentition: ↳ 20 deciduous teeth (baby teeth) ↳ 32 permanent teeth - 3 pairs of molars, 3rd set is wisdom teeth Saliva and Salivary Glands · function: lubricate mouth, moisten food, dissolve chemicals to stimulate taste buds, starts digestion with salivary amylase (saliva keeps neutral PH, antibodies + lysozyme for bacteria) · 3 pairs of salivary glands: ↳ Parotid Glands - largest gland - parotid duct: secretes salivary amylase ↳ Sublingual Glands - located on floor of mouth - sublingual duct: secrete mucous that acts as a buffer and lubricant ↳ Submandibular Glands - located in submandibular groove - submandibular duct: secretes 70% of Saliva (mixture of mucins, buffers, and salivary amylase) From Oral Cavity to Stomach · ingested material forms into a bolus in the oral cavity · the pharynx propels the food to the esophagus Esophagus · enters the diaphragm through esophageal hiatus · lined with stratified squamous epithelium · mucosa and submucosa contain folds that expand when swallowing · innervated by esophageal plexus (SNS + PNS fibers) · esophageal sphincter: between esophagus and stomach to prevent reflux Swallowing · deglutition = swallowing · 3 phases: ① Buccal phase ② Pharyngeal phase ③ Esophageal phase · initiation of swallowing can be voluntary or involuntary · tactile receptors of palatal arch + uvula become stimulated by passing food, which initiates the swallowing reflex ↳ relayed to swallowing center in medulla · primary peristaltic waves: contraction of pharyngeal muscles to move bolus down · secondary peristaltic waves: needed for a dry/poorly lubricated bolus b/c it travels slowly down to the stomach Stomach · expandable tubelike organ in upper left quadrant of peritoneal cavity · functions: temporarily stores food, mechanical digestion through muscular contractions, chemical digestion w/acids and enzymes · ingested substances mix with gastric secretions to produce chyme · rugae: folds in the mucosa that expand when full · parts of stomach: ↳ Cardia: smallest region, lots of mucous to protect esophagus ↳ fundus: contact with the diaphragm ↳ body: largest region, acts as a mixing tank ↳ pylorus: exit portion between body and duodenum Stomach Muscle Layers · all internal portions of the stomach are lined with simple columnar epithelium ↳ secretory sheet · churning motion due to the different orientations of the muscles · alkaline mucous layer protects the epithelia against the acids produced ↳ because the mucous is so thick, absorption does not take place in the stomach Gastric Secretions · gastric pits: shallow depressions that actively divide to replace cells killed by acid of stomach, and secrete products of the gastric glands · Chief cells: secrete inactive pepsinogen (converted to active pepsin by gastric acid) ↳ pepsin breaks down proteins, works best in acidic environment · parietal cells: secrete intrinsic factor (for vitamin B12 absorption), and indirectly secrete HCI (secreted as H+ and Cl-) → keeps gastric PH at 1.5-2.0 Gastric Secretions Cont'd · enteroendocrine cells: secrete gastrin and histamine ↳ Gastrin: stimulates chief cells and parietal cells and stimulates contractions of gastric wall ↳ Histamine: stimulates HCI secretion · d-cells: secrete somatostatin (inhibits gastrin and secretion of HCl) · P/D1 cells: Secretes ghrelin (hungry) and leptin (full) · goblet cells: secretes watery mucous and bicarbonate (buffer) for protection of epithelia Chemical Digestion Notes · When food reaches the stomach, salivary amylase and lingual lipase will keep digesting carbs and fats until PH falls below 4.5 · digestion of proteins begins when PH reaches ~2.0 · nutrients are not absorbed in the stomach Regulation of Digestion ① Cephalic Phase ② Gastric Phase ③ Intestinal Phase Central Reflexes · gastroenteric and gastroileal reflexes accelerate movement along small intestine · enterogastric reflex controls the rate at which chyme enters the duodenum Vomiting · aka emesis · vomiting center in medulla · pyloric sphincters will relax → duodenum → sphincter → stomach → sphincter → back up esophagus Small Intestine · extends from pyloric sphincter to ileocecal valve · stabilized by mesentery proper · blood supply: superior mesenteric arteries and superior mesenteric vein · receives chyme from the stomach · lining has circular folds with intestinal villi and microvilli (brush border) ↳ lamina propria has extensive capillary supply, nerve endings, central lymph vessel called a lacteal · secretions: ↳ goblet cells secrete mucins, submucosal glands secrete lots of mucous ↳ paneth cells: release defensing and lysosomes ↳ endocrine cells: secrete gastrin, CCK, and secretin Divisions of the Small Intestine ① Duodenum ↳ first portion after stomach ↳ mixing portion (recieves chyme and digestive secretions from pancreas and liver) ↳ primary function is to receive and neutralize chyme ② Jejunum ↳ bulk of chemical digestion and absorption occur ↳ circular fold and villi decrease as you move toward ileum ③ Ileum ↳ longest segment ending in ileocecal valve ↳ distal ileum lacks circular folds and contains peyers patches ↳ peyers patches: abundant lymphoid tissue to prevent bacteria entering from rectum Chemical Digestion of Chyme · intestinal juices moisten chyme, buffer acidity, and keep it very moist · brush border enzymes: enzymes at tip of microvilli that digest foods exposed to them ↳ enteropeptidase (precursor to tripsinogen) · lipids/fatty acids: packaged into chylomicrons and absorbed into lacteals · activity is triggered by: ↳ presence of food ↳ ANS (PNS) → PNS stimulates activity, SNS inhibits activity ↳ stretch receptors ↳ hormones (gastrin, secretin, CIP, CCK) Intestinal Hormones · Gastrin: promotes gastric motility and production of gastric acids and enzymes ↳ made in stomach in response to undigested protein · Secretin: Secreted when chyme enters duodenum, increases release of buffers from pancreas, increases bile production, slows motility · Gastric Inhibitory Peptide (GIP): Secreted when fats and carbs enter small intestine, inhibits gastric activity and stimulates the release of insulin · Cholecystokinin (CCK): secreted when chyme enters duodenum, increases production of digestive enzymes, relaxes hepatopancreatic sphincter, stimulates contraction of gallbladder for more bile · Vasoactive Intestinal Peptide (VIP): dilates area capillaries to assist in removal of absorbed nutrients · Enterocrinin: secreted when chyme enters duodenum, stimulates protective alkaline mucous production by submucosal cells The Pancreas · lies retroperitoneal · pancreatic arteries → pancreas → drains into splenic vein · endocrine pancreas: insulin and glucagon from pancreatic islet cells · exocrine pancreas: involves acinar cells (secretes pancreatic enzymes), and epithelial cells of pancreatic ducts (secretes water and ions) that together make pancreatic juices · pancreatic juices: alkaline mix of digestive enzymes, water, and ions ↳ enzymes break down ingested material, water & ions dilute and neutralize chyme · large pancreatic duct delivers pancreatic secretions to the duodenum at the duodenal ampulla · secretion controlled by ANS activation (vagus nerve), arrival of food in the duodenum, and hormones pancreatic duct → bile duct → empties into duodenal ampulla The Pancreas Contd · secretions controlled by hormones of duodenum: ↳ secretin: triggers release of pancreatic watery buffering solution ↳ cholecystokinin (CCK): stimulates increased production of pancreatic enzymes · pancreatic enzymes: ↳ pancreatic alpha-amylase (carbohydrates) ↳ pancreatic lipase (lipids/fatty acids) ↳ nucleases (RNA/DNA) ↳ proteolytic enzymes (proteins) The Liver · largest visceral organ, covered by visceral peritoneum · two main lobes separated by falciform ligament ↳ falciform and round ligament comes from fetal development (umbilical vein) · liver cells = hepatocytes · functional unit = lobules ↳ in each lobule there are sinusoid capillaries, Kupfer cells, and canaliculi that contains bile · blood supply: ↳ hepatic artery and hepatic vein ↳ portal vein (from GI tract) · lobules have a hexagonal structure w/portal triad · within a lobule, sinusoids between adjacent plates empty into a central vein ↳ interlobar circulation → sinusoids → central vein → hepatic vein · sinusoids lack basement membrane, so small + large particles can enter (even large plasma proteins) ↳ contains kupffer macrophages Hepatic Portal System · blood enters the liver sinusoids from small branches of the hepatic portal vein and hepatic artery proper · portal triad: contains interlobar vein, interlobar artery, and interlobar bile duct · liver hepatocytes absorb solutes from plasma and secrete plasma proteins Portal Hypertension · the liver is a low pressure system, and pressures can increase dramatically by restriction of blood flow · increased pressures lead to portal hypertension ↳ increased risk of ascites (fluid in peritoneal cavity) Enterohepatic circulation: the cycling of bile salts from the liver and small intestine repeatedly Liver Functions: ① Metabolic Regulation ↳ extracts toxins and nutrients before blood reaches systemic circulation ↳ Carbohydrate metabolism (glucogenesis), lipid metabolism, amino acid metabolism, waste removal, vitamin storage ② Hematological Regulation ↳ plasma protein synthesis, removal of circulating hormones, antibodies, and toxins ③ Bile Production ↳ synthesis of bile (made of water, ions, bilirubin, cholesterol, bile salts ↳ bile salts: needed for digestion of lipids through emulsification Bile Duct System · liver secretes bile into passageways called bile canaliculi · bile canaliculi connect into the left and right bile ducts, which then unite to form the common hepatic duct · common hepatic duct sends bile to: ↳ the bile duct which deposits into duodenal ampulla ↳ the cystic duct to store bile in the gallbladder The Gallbladder · functions: stores and concentrates bile (by removing H2O) before secretion · bile released into duodenum when stimulated by CCK ↳ CCK relaxes hepato pancreatic sphincter and stimulates contraction of gallbladder to secrete bile ↳ fatty foods make you release more CCK and more bile · three regions of gallbladder: ↳ fundus, body, neck gallstones: super concentrated bile (by removing water) can lead to solid bits that can increase in size too big to be excreted Large Intestine · blood supply: mesenteric arteries and veins · lining of intestine lacks villi but has lots of goblet cells and mucous for lubrication ↳ thinner wall for reabsorption of water · three regions: ① Cecum ↳ pouch-like portion that stores materials from ileum through ileocecal valve ↳ appendix attached at bottom ② Colon ↳ smooth muscles provide support and pouches of haustra allow colon to elongate and expand ↳ 4 segments: ascending colon, transverse colon, descending colon, sigmoid colon ③ Rectum ↳ expandable organ for temporary storage of feces ↳ anal canal contains anal columns ↳ anal sphincters: - internal: involuntary sphincter - external: voluntary sphincter Function of Large Intestine · primary function is reabsorption of water · minimal digestion · Vitamin synthesis (vitamin K, biotin, vitamin B5) · control of defecation through gastroileal + gastroenteric reflexes for powerful peristaltic mass movements · distention of rectum → defecation reflex → relaxing of internal + external sphincters constipation: increased time in large intestine leads to too much fluid reabsorption which makes bowel movements difficult diarrhea: decreased time in large intestine leads to too little fluid loss and watery feces Protective Functions of GI Tract: · Keratinized layers of oral mucosa and rectum for barrier · saliva neutralizes bacterial acids, contains antibodies and lysosomes · GI lymphatic tissues (tonsils, peyers patches, paneth cells, appendix) · stomach acid · kupffer macrophages Age-Related Changes · epithelial cell division declines → increased susceptibility to damage · decreased smooth muscle tone → weaker peristalsis, decreased motility · cumulative damage · increased risk of cancers · increased risk of dehydration