The Digestive System PDF

The digestive system I. Functions II. Structure III. Steps in digestion IV. Accessory organs V. Digestion mechanisms for carbohydrates, lipids and proteins VI. Regulation of function  Functions of the digestive system motility: move food through GI tract secretion: water, digesti...

The digestive system I. Functions II. Structure III. Steps in digestion IV. Accessory organs V. Digestion mechanisms for carbohydrates, lipids and proteins VI. Regulation of function  Functions of the digestive system motility: move food through GI tract secretion: water, digestive enzymes, hormones digestion: chemical breakdown of food absorption: (GI-blood/lymph-cells) excretion:  Mammals (human) Fig. 15.15  4 layers of the GI tract mucosa: absorption and secretion submucosa: vasculature muscularis: circ. and long. muscle serosa: outer protective covering mucus By Goran tek-en, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=31413106  Steps in digestion mouth: mastication esophagus: 10” muscular tube, peristalsis – peristalsis: waves of contraction from top to bottom – bolus moves through subesophageal sphincter into stomach Figure 15.25  stomach muscular activity mixes bolus with digestive enzymes to produce chyme initial digestion of protein (pepsin) kills bacteria (pH ~2) transports chyme into small intestine no absorbtion (except alcohol and some drugs) no starch digestion (salivary amylase is inactivated) Fig. 15.17 (infoldings)  mucus Fig. 5.17 HCl, intrinsic factor pepsinogen G cells : gastrin secretions + water = gastric juice (2/3 L/day)  gastric secretions mucus (goblet cells) lubrication, protection gastrin (G cells) hormone, more later HCl (parietal cells) acidification of gastric juice – denaturation of proteins – helps convert pepsinogen to pepsin – pH 2 optimal for pepsin activity – kills bacteria  gastric secretions, cont. intrinsic factor (parietal cells) – required for vitamin B12 absorption in small intestine – the most vital function of the stomach pepsinogen (chief cells)  pepsinogen-pepsin pepsinogen is a zymogen (inactive form of an enzyme) low pH converts pepsinogen into pepsin pepsin cleaves ingested proteins into short peptides  small intestine 12 ft. long (partially contracted) 3 regions – duodenum, ~10”, digestion and secretion – jejunum, ~5 ft., absorption – ileum ~6 ft., absorption functions – digestion, secretion, absorption mucosal layer – villi and microvilli (brush border) – large surface area fig. 15.19 Villi and microvilli Absorptive (enterocyte) cell fig. 15.20  Large Intestine Mechanical movements: haustra Digestion: intestinal flora (CHO, AA). Absorption: vitamins (vita B12 via intrinsic factor, salts, water Secretion: mucus Defecation Fig. 15.25  Fig. 15.26 Smooth muscle pacemaker cells: interstitial cells of Cajal  intestinal motility is further regulated by the ANS/hormones symp: decreases gastro-intestinal motility para: increases gastro-intestinal motility Enteric nervous system: an additional level of neural control “2nd brain” Release of hormones: from intestinal epithelium into chyme, paracrine effects. Fig. 15.27 Enteric Nervous System Niesler et al, 2021  accessory organs liver gall bladder pancreas Accessory organs  the liver consists of hepatocytes, form sinusoids each hepatocyte contacts blood capillaries very permeable Kupffer cells (phagocytic) hepatic portal system hepatic portal system: cap-vein-cap substances absorbed in S.I. caps hepatic liver caps hepatic portal vein (sinusoids) vein inferior vena cava  functions of liver 1. bile production (250 ml-1.5 L/day) – bile salts: organic ions – bilirubin and biliverdin: from old RBCs, breakdown product of heme – phospholipids – cholesterol (liver plays major role in regulating levels – inorganic ions – Important in fat digestion  functions of liver, cont. 2. energy storage (glycogen) 3. gluconeogenesis 4. vitamin and iron storage 5. protein synthesis 6. detoxification:  detox mechanisms 1. excretion into bile (ex: bilirubin) 2. phagocytosis by Kupffer cells 3. chemical modifications Accessory organs Gall bladder and ducts  gall bladder storage organ for bile can be stimulated to secrete bile Hepatic duct to cystic duct to gall bladder Hepatic duct to common bile duct to duodenum Gall bladder via cystic duct to common bile duct to duodenum Accessory organs  Pancreas sits underneath stomach endocrine and exocrine function endocrine: insulin/glucagon exocrine function: digestive enzymes via pancreatic duct (most of the pancreas)  pancreas, exocrine function digestive enzymes secreted from acini into pancreatic juice most are secreted as zymogens: activated in the small intestine  trypsin active other inactive enzymes (zymogens) enzymes pancreatic juice trypsinogen trypsin enterokinase microvilli epithelial cell of duodenum  pancreatic enzymes trypsin enterokinase secreted chymotrypsin trypsin as elastase trypsin zymogens carboxypeptidase trypsin phospholipase trypsin lipase none not amylase none secreted choesterolesterase none as zymogens ribonuclease none deoxyribonuclease none  digestion of carbohydrates primarily ingest: starch, sucrose, lactose – starch: glucose polymer – sucrose: glucose + fructose – lactose: glucose + galactose mouth: salivary amylase, inactivated by stomach pH small intestine lumen: pancreatic amylase  digestion of CHO, continued small intestine brush border: everything broken down to monosacharides (glc, gal, fruc) transported across intestinal epithelium enters blood stream Fig. 15.37  digestion of protein stomach: pepsin short AA chains lumen of SI endopeptidases and exopeptidases endopeptidases: cleave AA’s in middle exopeptidases: cleave AA’s off ends  digestion of protein, cont. di- and tripeptides are transported across apical surface cellular dipeptidases and tripeptidases further digest to single AAs AA transporters move AAs across basal surface enter bloodstream  exopeptidase endopeptidase brush border enzymes dipeptidase tripeptidase  Transport of AAs across AA-X SI epithelium X= Na+ or H+ Na+ AA-X H+ H+ dipep / tripep  Fat digestion occurs in small intestine emulsification by bile salts enzymatic breakdown formation of micelles entry into epithelial cell Reformation of triglyceride in smooth ER  fat digestions (cont.) Formation of the chylomicron exocytosis absorption into the lacteal transport to bloodstream breakdown of triglycerides cells How bile salts interact with lipid globule triglycerides = Bile Salts  from liver and gall bladder micelles of bile salts, bile duct cholesterol and lecithin colipase + free fatty pancreatic acids mixed lipase micelles monoglycerides fat droplets (triglycerides) Fig. 15.38  Apolippoproteins chylomicron Triglycerides and cholesterol By Xvazquez - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=4024624 Regulation of the digestive system three phases of activities 1. cephalic phase: a conditioned reflex to sight and smell of food $ parasympathetic (vagus) – $ parietal cells (HCl) – $ chief cells (pepsinogen) – $ G cells (gastrin) Regulation of the digestive system three phases of activities 2. Gastric phase: further stimulation of gastric secretions stimulate by: – distention of stomach by food – composition of chyme (polypeptides, AAs)  stomach blood lumen polypeptides + G cell gastrin and AAs distention - HCl + parietal cell + chief cell pepsinogen + Gastric phase Regulation of the digestive system three phases of activities 3. Intestinal phase: inhibition of stomach motility and secretion. $ by food moving into the SI – distension of the SI activates a neural reflex that stimulates the SI and pancreatic activity – fat in food stimulates hormone secretion  The intestinal phase of gastric regulation fat in food food + + stomach duodenum stretch-activated _ neural reflex gastric motility (myenteric plexus) and secretion Also stimulates secretion of Pancreatic somatostatin secretion + cholecystokinin (CCK) from small Intestine Regulation of pancreatic secretion 1) pH effects: chyme duodeneum secretion of secretin pH< 4.5 + duodenum by SI pH* pancreas *critical for optimal pancreatic transport of HCO-3 enzyme function into pancreatic juice  Regulation of pancreatic secretion 2) Effects of fat chyme duodenum + +CCK secretion fat content duodenum pancreas digestion production and of fat duodenum secretion of protein & CHO pancreatic enz.  Regulation of bile secretion 1) Bile production: bile secreted continuously by liver but level increased after a meal. secretin (released from duodenum in increase rate response to low pH) Liver of bile CCK (released from duodenum production in response to fat)  Regulation of bile secretion 2) contraction of gall bladder CCK (released from duodenum in response gall bladder contraction to fat) small bile into intestine bile duct Fig. 15.36