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• 6 Processes of Digestion – Ingestion – Mechanical Digestion Ingestion Mechanical digestion • Chewing (mouth) • Churning (stomach) • Segmentation (small intestine) Chemical digestion Food Pharynx Esophagus Propulsion • Swallowing (oropharynx) • Peristalsis Stomach (esophagus, stomach, small intestine, large intestine) Absorption Lymph vessel Small intestine Large intestine Defecation Blood vessel Mainly H2O Feces Anus • Chewing, swallowing • Peristalsis • Segmentation – Chemical Digestion • Enzymes breaking chemical bonds – Secretion • Goblet cells secreting mucus • Organs secreting enzymes – Absorption • Nutrients pass into blood stream – Defecation • Removal of solid waste Salivary glands Accessory Organs to S. Intestines stomach Small Intestine Blood stream Large Intestine Histology of Digestive Tract • Stratified squamous – Mouth, pharynx & esophagus • Simple Columnar – Stomach thru L.I. – Goblet cells • Villi Microvilli Mouth • Main Functions – Ingestion – Physical digestion • Tongue, teeth – Chemical digestion • Saliva and salivary glands Accessory Organs of Mouth • Tongue – Taste buds – Responsible for gripping food – Mixes food with saliva – Initiates swallowing From mouth (a) Peristalsis: Adjacent segments of alimentary tract organs alternately contract and relax, which moves food along the tract distally. (b) Segmentation: Nonadjacent segments of alimentary tract organs alternately contract and relax, moving the food forward then backward. Food mixing and slow food propulsion occurs. Salivary Glands Parotid, submandibular and sublingual • Functions – Produce saliva • • • • 99% water Contains enzyme amylase Mucin – forms mucus Growth Factor • Functions of Saliva – Cleanses mouth • Growth factor and lysosomes – Dissolves food for taste – Moistens food; creates bolus – Enzyme amylase begins the chemical digestion of starch Amylase • Starch (polysaccharide) disaccharides Pharynx and Esophagus • Pharynx – Back of throat – Passage way for food • Esophagus – Food chute – Stratified squamous tissue Stomach • Four Primary Functions – Temporary storage tank for food – Mechanically break down food • Food is converted into creamy paste called chyme – Chemical breakdown of protein begins – Production of intrinsic factor; necessary for absorbing Vitamin B12. Regions of the Stomach • Cardiac region – Upper area around cardiac sphincter • Fundus – Dome-shaped part of stomach • Body • Pylorus- inferior portion • Pyloric sphincter: controls the emptying of the stomach into the S.I. Stomach Physiology • Rugae • Peristalsis – Controlled by pacemaker cells – Becomes more forceful closer to the pyloric region • Gastric Pit – Gastric gland • Secretes mucus and enzyme pepsinogen • Parietal cells – Secrete HCl and instrinsic factor – HCl is needed to convert pepsinogen into pepsin – Kills bacteria that is ingested with food – Rennin • Chief cells – produce pepsinogen; inactive form of pepsin Pepsinogen Pepsin Converting Pepsinogen to Pepsin HCl Mitochondria Parietal cell Chief cell Enteroendocrine cell (c) Location of the HCl-producing parietal cells and pepsin-secreting chief cells in a gastric gland Regulation of Gastric Activity 1. Cephalic phase – prepares stomach to receive food – Sight, smell, taste of food – Controlled by vagus nerve and parasympathetic system – Only lasts a short time 2. Gastric Phase – Stimulation of stretch receptors and chemoreceptors stimulate the release of hormone gastrin – Gastrin causes release of HCl and pepsinogen – Increase in stomach contraction intensity – Small amounts of caffeine and alcohol stimulate chemoreceptors causing an increase in gastric secretions 3. Intestinal phase – Begins when food starts to enter S.I. – Controls the rate of gastric emptying – Slows down gastric activities by releasing three hormones • • • Cholecystokinin (CCK) Secretin Gastric inhibitory peptide (GIP) Small Intestine • Digesting & Absorbing Organ • Anatomy – Consists of 3 sections • Duodenum • Jejunum • Ileum – ileocecal spincter: controls flow into colon – Mesentary Modifications for Absorption • Folds – Mixes chyme and slows down movement • Villi – Fingerlike projections increases surface area • Microvilli Lining of Small Intestine Regulation with Intestinal Hormones • Secretin: released when pH falls in duodenum (Why would this happen?) – Result: causes water and buffers to be secreted into S.I. by pancreas • Cholecystokinin (CCK): secreted when protein and lipid dense chyme arrives in S.I. – Result: increases the secretion of digestive enzymes from the pancreas and bile from the liver • Gastric Inhibitory Peptide (GIP): released when food begins to enter S.I. – Result: inhibit digestive activity in stomach; causes release of insulin from pancreas 1 Chyme entering duodenum causes release of cholecystokinin (CCK) and secretin from duodenal cells. 4 Bile salts and, to a lesser extent, secretin transported via bloodstream stimulate liver to produce bile more rapidly. 5 2 CCK (red dots) and secretin (yellow dots) enter the bloodstream. 3 CCK induces secretion of enzyme-rich pancreatic juice. Secretin causes secretion of HCO3–-rich pancreatic juice. CCK (via bloodstream) causes gallbladder to contract ; bile enters duodenum. 6 During cephalic and gastric phases, vagal nerve stimulation causes weak contractions of gallbladder. Chemical Digestion • Brush Border Enzymes – Intestinal juice is water, mucus and enzymes – Sucrase – Lactase – Maltase – Peptidases/proteases Lactose Intolerance • Segmentation: movement of food through S.I.; moves very slowly so chemical digestion can occur • Chemical Digestion: – Very few Enzymes come from S.I. – Need assistance from accessory organs • Absorption: the building blocks of food are passed into blood stream by Active Transport – Most absorption is completed b4 it reaches the ileum • Ilieum’s main function is to return bile salts to liver – Mesentary Accessory Organs: Pancreas • Endocrine Gland – Produces hormones • Exocrine Gland – Produces digestive enzymes • Empties into duodenum • Pancreatic duct fuses with Common bile duct and enters the duodenum Pancreatic Secretions • Secretes (Exocrine Gland) – Bicarbonate ions (NaHCO3-); turns acidic chyme from stomach into basic solution (ph 78) – Enzymes • • • • Trypsinogen (inactive form) Amylase Lipase Nucleases Conversion of Trypsinogen into Trypsin • Increase in pH from bicarbonate ions and another enzymes converts trypsinogen into its active form trypsin • Trypsin is a protease and continues protein digestion Digestion of Lipids • Lipase breaks down lipids into fatty acids and glycerol. Fatty acids and glycerol can then be absorbed into the blood stream Bile assists in this process by physically breaking down fats into smaller droplets. • Endocrine Gland – Islets of Langerhans are specialized cells • Responsible for secreting 2 hormones – Glucagon – Insulin Anatomy of Liver and gall bladder • Liver – Hepatic duct allows bile to leave the liver • Gall Bladder – Lies under liver; appears like a slightly inflated green balloon – Stores bile • Bile leaves the liver via the cystic duct • Cystic duct and hepatic duct join to make Common Bile Duct. • Bile Production – Only digestive process – Physically separates fats into smaller droplets; allows lipase to digest faster – (Remember fats don’t dissolve in water) • I.e. scatters fat droplets throughout solution • Metabolic Regulation – Processes all nutrients absorbed into blood – Helps regulate cholesterol levels – Forms nonessential amino acids; byproduct is ammonia (toxic) – Converts ammonia into urea (less toxic) – Stores glucose as glycogen (polysaccharide) – Stores fat soluble vitamins (A,D,E,K) – Detoxifies alcohol and drugs (both legal and illegal) • Blood leaving liver has fewer waste products than the blood that enters it. • Assists in Regulating Blood – Produces plasma proteins – Conserves iron from old RBC’s • Cirrhosis – Chronic inflammation of liver – Results from severe alcoholism or chronic sever hepatitis – Liver cells die are replaced scar tissue and fat deposits. • Hepatitis – Inflammation of liver – Caused by a virus, sewage contaminated water or feces-mouth • Cholesterol in Bile salts crystallize; • Gets stuck in duct; causes extreme pain Protein digestion Foodstuff Protein Large polypeptides Small polypeptides, small peptides Amino acids (some dipeptides and tripeptides) Enzyme(s) and source Pepsin (stomach glands) in presence of HCl Pancreatic enzymes (trypsin, chymotrypsin, carboxypeptidase) Brush border enzymes (peptidases, proteases) Site of action Path of absorption • Amino acids are absorbed by cotransport with Stomach sodium ions. • Some dipeptides and tripeptides are absorbed via cotransport with H++ Small and hydrolyzed to amino intestine acids within the cells. • Amino acids leave the epithelial cells by Small facilitated diffusion, enter intestine the capillary blood in the villi, and are transported to the liver via the hepatic portal vein. Carbohydrate digestion Foodstuff Enzyme(s) and source Site of action Starch and disaccharides Oligosaccharides and disaccharides Lactose Maltose Sucrose Galactose Glucose Fructose Salivary amylase Pancreatic amylase Brush border enzymes in small intestine (lactase,maltase and sucrase) Mouth Small intestine Small intestine Path of absorption • Glucose and galactose are absorbed via cotransport with sodium ions. • Fructose passes via facilitated diffusion. • All monosaccharides leave the epithelial cells via facilitated diffusion, enter the capillary blood in the villi, and are transported to the liver via the hepatic portal vein. Fat digestion Foodstuff Enzyme(s) and source Site of action Unemulsified fats Emulsification by the detergent action of bile salts ducted in from the liver Pancreatic lipases Monoglycerides Glycerol and fatty acids and fatty acids Small intestine Path of absorption • Fatty acids and monoglycerides enter the intestinal cells via diffusion. • Fatty acids and monoglycerides are recombined to form triglycerides and then combined with other lipids and Small intestine proteins within the cells. • Some short-chain fatty acids are absorbed, move into the capillary blood in the villi by diffusion, and are transported to the liver via the hepatic portal vein. Nucleic acid digestion Foodstuff Enzyme(s) and source Nucleic acids Pentose sugars, N-containing bases, phosphate ions Pancreatic ribonuclease and deoxyribonuclease Brush border enzymes (nucleosidases and phosphatases) Site of action Path of absorption • Units enter intestinal cells by active transport via Small intestine membrane carriers. • Units are absorbed into capillary blood in the villi Small and transported to the intestine liver via the hepatic portal vein. What remains in Small Intestine? • • • • Indigestible food materials (cellulose) Lots of bacteria Water Passes through ileocecal valve into Large Intestine Large Intestine/Colon Anatomy • Cecum • Appendix • Colon – – – – Ascending Transverse Descending Sigmoid • Rectum • Anal Canal Microscopic Anatomy • No villi or microvilli • No cells which secrete enzymes • Many goblet cells lining the simple columnar tissue for lubrication • Rectum tissue becomes stratified squamous Main functions • Absorbs water and electrolytes (Na, Cl, K) • Absorbs water soluble vitamins • Eliminates undigested food; defecation Haustra = wall pockets (seen in picture) Other Inhabitants • Bacteria – Responsible for fermenting undigested food – Product of these reactions is gas (H2, N2, CO2, H2S, CH4) – Assist in synthesizing vitamin B and vitamin K.