Physiology Ch. 6 Gastrointestinal System 2023/2024 PDF

Physiology (0603302) Ch.6. Gastrointestinal system Summer Semester-2023/2024 Dr. Mohammad A. Abedal-Majed School of Agriculture The Unive...

Physiology (0603302) Ch.6. Gastrointestinal system Summer Semester-2023/2024 Dr. Mohammad A. Abedal-Majed School of Agriculture The University of Jordan 1 I want to finish before I lose Dr. Mohammad A. Abedal-Majed 1 my appetite  The digestive system Gastrointestinal system (GI system): the system that is responsible for providing us with nutrients and electrolytes form outside environment to maintain homeostasis. I want to finish before I lose Dr. Mohammad A. Abedal-Majed 2 my appetite  Structure of the Gastrointestinal system **The digestive system is composed of two main parts: A)GI tract mouth, pharynx, esophagus, stomach, small intestine, large intestine, rectum ending with anus B)Accessory Glandular Organs salivary glands, liver, gallbladder, exocrine and endocrine glands I want to finish before I lose Dr. Mohammad A. Abedal-Majed 3 my appetite  Mouth: **contain muscles that help in the mechanical digestion.  The salivary glands in mouth secret saliva that is essential for lubrication, degradation of food, and carbohydrate digestion. So carbohydrates digestion starts at the mouth by salivary amylase. The food will pass through pharynx, which is: 1. passage way for food 2. Common way that connect GI with respiratory system Esophagus: rapid passage for food and nutrients occurs here. Generally, no absorption occurs, except for some drugs. I want to finish before I lose Dr. Mohammad A. Abedal-Majed 4 my appetite  Stomach: Contain a special lining that helps in secrete the digestive juices (pepsinogen, HCl acid), that digest proteins. Digestive juices/enzymes here originate from structures called gastric pits Gastric pits contain cells responsible from secreting HCL& pepsinogen Due to strong muscular wall, the stomach produce mechanical movements that help in mix the food with the digestive juice. Food then passes through the pylorus sphincter of the stomach to reach the small intestine I want to finish before I lose Dr. Mohammad A. Abedal-Majed 5 my appetite  Small intestine: Contains enzymes that come from the pancreas and from the small intestine itself. These enzymes help in more digestion of food. Contain villa cells that increase the surface area for absorption and digestion Villa cells covered the epithelial cells that renews itself every 2-3 days from the stem cells found in the crypts Contain microvilli (brush boarder) which have enzymes that help in digestion of carbohydrate and protein. The cells that line the villi and microvilli are called enterocytes. I want to finish before I lose Dr. Mohammad A. Abedal-Majed 6 my appetite Crypt  Large intestine: A very little amount of food will be absorbed (eg: water and some electrolytes) There is a beneficial bacterium that helps in produce (vit K, some lipid molecules) patients who have a problem in this bacteria (dysbiosis) may develop some diseases like diabetes, obesity. Formation the feces to be eliminated outside the body. Diet rich in fibers will ease the movements through the large intestines and thus decreases the risk of constipation I want to finish before I lose Dr. Mohammad A. Abedal-Majed 7 my appetite Crypt  Structure of the Gastrointestinal system - consists of 4 primary layers Circumferential Circumferential/ Longitudinal 1 -Mucosa -inner most layer of the GI tract. -layer of epithelial cells lining the inside of the GIT (?) - It has Protective role, secretion and absorption Arteriole Intestine 2-Submucosa -layer of loose connective tissue beneath the mucosa Contain some important network of neurons that called submucousal plexus which gives GI tract distensibility and elasticity I want to finish before I lose Dr. Mohammad A. Abedal-Majed 8 my appetite  Structure of the Gastrointestinal system 3-Muscle -there are 2 perpendicular layers of smooth muscle A-thick inner circumferential layer (contraction of these muscles causes constricts or decreases the diameter of the lumen/narrowing of the tube) B-thin outer longitudinal layer (contraction of these muscles causes shortens the tube) **All GI composed of smooth muscles which is under involuntary control (we can’t control it). Exceptions: mouth and end of the rectum which are skeletal muscles which are voluntary muscles (we can control it) In the muscular wall between the inner and outer muscular layers, there is another neuronal network called myenteric plexus: Note: both Submucosal and myenteric plexuses are called the enteric nervous system, which is responsible for regulation of GI tract functions 4-Serosa -Anothet thin layer of connective tissue covering the outside of the GIT. Secretes a watery, slippery fluid; lubricates and prevents friction with surrounding organs I want to finish before I lose Dr. Mohammad A. Abedal-Majed 9 my appetite  Processing of food by the DS 1. Ingestion : process of eating 2. Digestion: Chemical and mechanical breakdown of food complexes into absorbable units (Motility) (the digestive tract’s muscular contractions); involves alternating contractions of muscles in body walls of GI organs I want to finish before I lose Dr. Mohammad A. Abedal-Majed 10 my appetite  Processing of food by the DS Two types of movements phasic contraction: A-Phasic contraction 1. Propulsive movements: propel or push the contents forward through the digestive tract (oral to anal) called (Peristaltic contraction/ peristalsis) that moves into one direction 2. Mixing movements: A. facilitate digestion by mixing food with the digestive juices B. facilitate absorption by exposing all parts of the intestinal contents to the absorbing surfaces of the digestive tract and enzymes Also called segmental contractions. The ends of the tract: motility involves skeletal muscle B-Tonic contraction I want to finish before I lose Dr. Mohammad A. Abedal-Majed 11 my appetite  GI motility Some muscles contract and relax in seconds – Phasic Contractions (Peristalsis and Segmentation): ↓ transit time with ↑ Peristalsis motility Some maintain contractions over minutes or hours – Tonic Contractions (Sphincter) ↑ transit time with ↑ retentive motility *GIT smooth muscle - depolarization -spontaneous waves of partial depolarization originate from interstitial cells of Cajal (ICC) -specialized smooth muscle cells that are the “pacemakers” of the GIT -connected to one another & other smooth muscle cells via tight junctions -allows for the movement of ions & the propagation of membrane depolarizations between cells --Slow waves are changes in resting membrane potential -propagate aborally (away from the mouth) -occur ~ 20 times/min in the small intestine -occur ~ 3 times/min in the stomach & colon -amplitude & frequency modulated by the ENS (enteric nervous system) I want to finish before I lose Dr. Mohammad A. Abedal-Majed 12 my appetite Motility: Stomach GI motility I want to finish before I lose Dr. Mohammad A. Abedal-Majed 13 my appetite  GI motility Peristaltic waves: Body Antrum Body Thin muscle weak contraction No mixing Antrum Thick muscle powerful contraction A Mixing B Contraction of pyloric sphincter 1 Only small quantity of gastric content (chyme) entering duodenum 2 Further mixing as antral contents forced back towards body I want to finish before I lose Dr. Mohammad A. Abedal-Majed 14 my appetite  Processing of food by the DS 3. Secretion: Process by which the glands associated with the gastrointestinal tract release water and substance into the tract Digestive juices consist of water, electrolytes, enzymes, bile salts, and mucus. ---Energy-dependent process, Requires neural or hormonal stimulation I want to finish before I lose Dr. Mohammad A. Abedal-Majed 15 my appetite  GIT Secretion - The digestive secretions are required for digestion & absorption -The total volume of digestive secretions is large (greater than the volume of ingested fluid) -synthesis & secretion of digestive fluids is controlled by endocrine, paracrine, & neural regulation I want to finish before I lose Dr. Mohammad A. Abedal-Majed 16 my appetite  Saliva secretion -most mammals have 3 pairs of major salivary glands -all major glands drain into a main duct that has a single opening in the mouth -parotid glands -secrete serous saliva (enriched with proteins and water) -mandibular glands -secrete a mixed saliva with both mucous & enzyme amylase -lingual glands -secrete a mixed saliva with both mucous & enzyme amylase I want to finish before I lose Dr. Mohammad A. Abedal-Majed 17 my appetite  Saliva secretion *Functions 1) Moistening –facilitates swallowing 2) Neutralization –of acids in food and bacteria -saliva contains water, ions, mucus, antibodies & antimicrobial enzymes -a healthy mouth contains a thriving population of bacteria -saliva doesn’t eliminate all bacteria but regulates & limits the population 3)Saliva participates in digestion -salivary amylase (starch digesting enzyme) (minimal) -lingual lipase (fat digesting enzyme) (minimal) 4)Taste –solvent for molecules that stimulate taste buds I want to finish before I lose Dr. Mohammad A. Abedal-Majed 18 my appetite  Gastric gland cells 1. Exocrine cells a) Parietal cell b) Chief cell c) Mucous neck cell 2. Pyloric gland: contain G cells that secrete gastrin, mucus 3. Cardiac gland : Alkaline Mucus cell 4. Endocrine cells: G Cell secrete gastrin which stimulate parietal cell D cell secrete somatostatin which inhibit parietal cell I want to finish before I lose Dr. Mohammad A. Abedal-Majed 19 my appetite  Composition of gastric secretions 1. HCl 2. Pepsinogen 3. Mucus ( forms a protective barrier) 4. Intrinsic factor  combines with vitamin B12 to make it absorbable I want to finish before I lose Dr. Mohammad A. Abedal-Majed 20 my appetite  HCL secretion H+ and Cl- transported independently H+: primary active transport by H+-K+- ATPase (3 X 106 more H+ in lumen compared to plasma) H+ derived from carbonic anhydrase which catalyzes the reaction of CO2 + H2O to H+ + HCO3- Cl-: secondary active transport (energy source is HCO3-) and facilitated diffusion Functions of HCl: activates digestive enzymes, breaks down connective tissue, denatures proteins, kills ingested microorganisms I want to finish before I lose Dr. Mohammad A. Abedal-Majed 21 my appetite I want to finish before I lose Dr. Mohammad A. Abedal-Majed 22 my appetite  Pepsinogen secretion Contained in zymogen granules in chief cells and released by regulated exocytosis Pepsinogen is inactive form of the enzyme – why is this important? HCl cleaves pepsinogen to produce the active enzyme pepsin Pepsin also cleaves pepsinogen (autocatalysis) I want to finish before I lose Dr. Mohammad A. Abedal-Majed 23 my appetite  Gastric Mucosal Barrier Protects against mechanical injury Protects epithelial cells from self-digestion by pepsin and acid injury Loss of barrier results in development of peptic ulcer (H. pylori induced) (Helicobacter pylori) I want to finish before I lose Dr. Mohammad A. Abedal-Majed 24 my appetite  Intrinsic factor  Secreted from parietal cells in humans  Forms a complex with vitamin B12 in the gut  The complex is resistant to digestion and therefore enables absorption of vitamin B12  Lack of intrinsic factor causes Vit B12 deficiency (anaemia) – as all the Vit B12 is digested and therefore can not be absorbed I want to finish before I lose Dr. Mohammad A. Abedal-Majed 25 my appetite  Secretion of the pancreas Endocrine - insulin & glucagon Exocrine - enzymes and bicarbonate (essential for digestion) Alpha cells; Glucagon Beta cells; Insulin Delta cells; Somatostatin F cells; pancreatic polypeptide I want to finish before I lose Dr. Mohammad A. Abedal-Majed 26 my appetite  Secretion of the pancreas Exocrine Secretion in the Pancreas Acinar cells secrete pancreatic digestive enzymes Enzymes: stored in zymogen granules and secreted in inactive form Duct cell secrete aqueous alkaline solution Alkaline solution neutralizes acidic chyme entering duodenum (Protects duodenal mucosa) pancreatic enzymes work best in pH neutral environment I want to finish before I lose Dr. Mohammad A. Abedal-Majed 27 my appetite  Secretion of the pancreas summary I want to finish before I lose 28 my appetite Dr. Mohammad A. Abedal-Majed  Secretion of the Liver Secretory Function of Liver = Bile Salts Bile salts secreted by liver transported to duodenum via bile ducts Entry of bile (bile salts, cholesterol, bilirubin) into duodenum regulated by Sphincter of Oddi Gallbladder bile is concentrated and recycled After entry into small intestines, 95% of bile salts are reabsorbed and returned to liver via enterohepatic circulation Functions of bile 1. Emulsification of fats 2. Increased absorption of lipids into enterocytes (include vitamin A, D, E, K) 3. Cholesterol excretion (only route) 4. Excretion of breakdown products of haemoglobin (bilirubin) I want to finish before I lose Dr. Mohammad A. Abedal-Majed 29 my appetite  Secretion of the Liver Bile salts and phospholipids convert large fat globules into smaller pieces with polar surfaces emulsification Emulsified fat globules are small enough that lipase enzymes gain access to degrade triglycerides to monoglycerides and fatty acids, Monoglycerides and fatty acids 1- enter the absorptive cells by simple diffusion 2-or aggregate to form loosely held micelles, which readily break down. I want to finish before I lose Dr. Mohammad A. Abedal-Majed 30 my appetite  Secretion of the Liver *Bile secretion - regulation -the gallbladder stores & concentrates bile -the sphincter of Oddi is located @ the junction of the common bile duct & duodenum -closed when little or no food is present in the intestinal lumen -bile cannot enter the intestinal lumen & is diverted into the gallbladder -stimulation of bile secretion → fat in the duodenal lumen → stimulates CCK secretion from I cells → relaxation of the sphincter of Oddi & contraction of the gallbladder → bile enters duodenum I want to finish before I lose Dr. Mohammad A. Abedal-Majed 31 my appetite  Small Intestine secretion water, salt and mucus Aids in lubrication and process of enzymatic digestion (no digestive enzymes secreted) Digestive Enzymes in enterocytes ( intestinal absorptive cells) Attached to “brush border” on apical side of epithelial cells I want to finish before I lose Dr. Mohammad A. Abedal-Majed 32 my appetite  Processing of food by the DS 4. DIGESTION: Biochemical breakdown of the structurally complex foodstuffs of the diet into smaller, absorbable units A. Mechanical digestion prepares food for chemical digestion includes chewing, mixing with saliva by tongue action, churning in stomach... B. Chemical digestion accomplished by enzymes in digestive juices I want to finish before I lose Dr. Mohammad A. Abedal-Majed 33 my appetite  Digestion: Overview I want to finish before I lose Dr. Mohammad A. Abedal-Majed 34 my appetite  Carbohydrate Digestion I want to finish before I lose Dr. Mohammad A. Abedal-Majed 35 my appetite  Protein Digestion Trypsin Chymotrypsin Carboxypeptidase I want to finish before I lose Dr. Mohammad A. Abedal-Majed 36 my appetite  Protein Digestion I want to finish before I lose Dr. Mohammad A. Abedal-Majed 37 my appetite  Fat Digestion Bile salts act like detergents (emulsification) Converts large fat droplet into smaller micelles Provides access of triglycerides to lipase (enzyme involved in triglyceride digestion I want to finish before I lose Dr. Mohammad A. Abedal-Majed 38 my appetite  Fat Digestion Monoacylglycerol lipase Hormone sensitive lipase Adipose triglyceride lipase I want to finish before I lose Dr. Mohammad A. Abedal-Majed 39 my appetite  Processing of food by the DS 5. Absorption : passage of food particles into the blood-lymph. The small absorbable units that result from digestion, along with water, vitamins, and electrolytes, are transferred from the digestive tract lumen into the blood or lymph Note: Most absorption occurs in small intestine 6. Defecation: elimination of indigestible food substances I want to finish before I lose Dr. Mohammad A. Abedal-Majed 40 my appetite  REGULATION OF GASTROINTESTINAL TRACT FUNCTIONS relating to, or occurring in the intestines I want to finish before I lose Dr. Mohammad A. Abedal-Majed 41 my appetite  Regulation of the GIT A) Intrinsic System Endocrine I want to finish before I lose Dr. Mohammad A. Abedal-Majed 42 my appetite  Regulation of the GIT A) Intrinsic System Endocrine Hormone Production Site Action Release Stimulus Stimulates bicarbonate Duodenum and upper Secretin secretion and inhibits acid Acid, fat, and protein jejunum secretion Stimulates acid secretion and growth of stomach Protein, increased high Gastrin Stomach and duodenum epithelium (marker for gastric acidity cancer) Stimulates pancreatic enzyme secretion and Duodenum, jejunum, and Cholecystokinin gallbladder contractions; Fats and proteins ileum inhibits food intake and gastric emptying Inhibits gastric secretions Gastric inhibitory Duodenum and jejunum and stimulates insulin Fat and glucose polypeptide secretion Motilin Duodenum and jejunum Stimulate motility Acetylcholine I want to finish before I lose Dr. Mohammad A. Abedal-Majed 43 my appetite  Regulation of the GIT B) Extrinsic System Endocrine (aldosterone) -mineralocorticoid produced within the zona glomerulosa (outer zone of the adrenal cortex) -steroid hormone synthesized by multiple enzymatic conversions of cholesterol -synthesis stimulated by angiotensin II, ↑ plasma [K+], & ↓ plasma [Na+] -mediates actions by binding the mineralocorticoid receptor (MR) in the cytoplasm of target cells -the MR is a nuclear hormone receptor that regulates gene transcription -↑ renal reabsorption of Na+/H2O & secretion of K+ -↓ Na+/H2O excretion -↑ K+ excretion -↑ GIT absorption of Na+/H2O & secretion of K+ I want to finish before I lose Dr. Mohammad A. Abedal-Majed 44 my appetite