Secretory Functions of the Gastrointestinal System PDF

Secretory Functions of the Alimentary Tract Dr. Arzu Temizyürek Secretory glands in the GI tract subserve two primary functions: Digestive enzymes are secreted from the mouth to the distal end of the ileum. Mucous glands, from the mouth to the anus, provide mucus for lubrication and protection o...

Secretory Functions of the Alimentary Tract Dr. Arzu Temizyürek Secretory glands in the GI tract subserve two primary functions: Digestive enzymes are secreted from the mouth to the distal end of the ileum. Mucous glands, from the mouth to the anus, provide mucus for lubrication and protection of all parts of the alimentary tract General Principles of GI Tract Secretion Anatomical Types of Glands 1. Mucous glands (Goblet cells) mucous protects the surfaces from excoriation and digestion. 2. Crypts of Lieberkuhn contain specialized secretory cells 3. Tubular glands an acid- and pepsinogen-secreting gland of the stomach (oxyntic gland). 4. Salivary glands, pancreas, and liver glanda secretions for digestion or emulsification of food. General Principles of GI Tract Secretion • Basic Mechanism of Stimulation of the Alimentary Tract Glands 1. Contact of food with the epithelium stimulates secretion—function of enteric nervous stimuli 2. Local epithelial stimulation also activates the Enteric Nervous System (ENS) of the gut wall and includes; - tactile stimulation, - chemical irritation, - distension of the gut wall General Principles of GI Tract Secretion Autonomic Stimulation of Secretion 1. Parasympathetic stimulation increases alimentary tract glandular secretion. n.Glossopharyngeus & n.vagus • Escp. upper portion of the tract salivary glands, esophageal glands, gastric glands, pancreas, and Brunner’s glands in the duodenum • Some parts of the large intestine General Principles of GI Tract Secretion Autonomic Stimulation of Secretion 2. Sympathetic Stimulation Dual Effect on the Alimentary Tract Glandular Secretion Rate. (because vasoconstructive effect on vessels) 1. Sympathetic nerves has a slight to moderate effect on increasing the secretion of some of the local glands 2. If parasympathetic or hormonal stimulation is already causing copious secretion, sympathetic system reduces the secretion General Principles of GI Tract Secretion Autonomic Stimulation of Secretion Regulation of Glandular Secretion by Hormones In the stomach and intestine, several GI hormones regulate the volume and character of the secretions. The hormones are produced in response to food in the lumen of the gut stimulate secretion the gastrointestinal hormones are polypeptides or polypeptide derivatives Basic Mechanism of Secretion by Glandular Cells 1. Secretion of organic substances • Diffusion or actively transported nutrients by the blood in into the base of the glandular cell. • Energy from the ATP provided by the nutrients + mitocondria is used to synthesize the organic secretory substance - synthesis occurs in the; o endoplasmic reticulum o Golgi complex o Ribosomes (especially for protein) General Principles of GI Tract Secretion Basic Mechanism of Secretion by Glandular Cells Secretion of organic substances • The secretory materials are transported through the tubules of the endoplasmic reticulum • In the Golgi complex, secretory vesicles are formed • Vesicles remain stored until nervous or hormonal control signals cause the cells to extrude the vesicular contents through the cells’ surface Exocytosis • • • • Hormon binds to its receptor Initiating cell signaling mechanisms Allowing Ca ions to enter the cell Fusion General Principles of GI Tract Secretion Basic Mechanism of Secretion by Glandular Cells 2. Water and electrolyte secretion • Nerve stimulation causes active transport of chloride ions to the cell interior. • The resulting increase in electronegativity by excess negatively charged chloride ions causes positive ions such as sodium ions also to move to the interior of the cell. General Principles of GI Tract Secretion Basic Mechanism of Secretion by Glandular Cells 2. Water and electrolyte secretion • The new excess of both negative and positive ions inside the cell creates an osmotic force causing the cell itself to swell. • The pressure in the cell then initiates minute openings of the secretory border of the cell, causing flushing of water, electrolytes, and organic materials out of the secretory end of the glandular cell. General Principles of GI Tract Secretion Basic Mechanism of Secretion by Glandular Cells 3. Lubricating and protecting properties of mucous ➢ Mucus composed mainly of water, electrolytes, and a mixture of several glycoproteins. ➢ Mucus is different in different parts of the gastrointestinal tract. Serves as an excellent lubricant and a protectant for the wall of the gut. General Principles of GI Tract Secretion Basic Mechanism of Secretion by Glandular Cells 3. Lubricating and protecting properties of mucous Mucus; • • • • • • Has adherent qualities It coats the wall of the gut Has low resistance for slippage Support the feces formation Resistant to digestion by the gastrointestinal enzymes Has amphoteric properties Secretion of Saliva Saliva Contains a Serous Secretion and a Mucus Secretion 1. Principle glands are the parotid, submandibular, and sublingual (also the tiny buccal glands) 2. Two types of protein secretion; - Serous secretion contains ptyalin (amylase) for digesting starches - Mucus secretion contains mucin for lubrication and surface protective properties Serous secretion Buccal glands Mucus secretion Serous + Mucus secretion Secretion of Saliva Secretion of Ions in Saliva 1. Saliva contains large quantities of K+ and HCO3 − ions , the concentrations of Na+ and Cl−ions are less. 2. Acini secrete a primary secretion containing ptyalin or mucin in a solution of ions 3. Ductal epithelium secretes the bicarbonate ion into the lumen Secretion of Saliva Consists of two Stages. 1. Involves acini Ptyalin an/or mucin in a solution of ions 2. Salivary ducts Secretion of Saliva Secretion of Ions in Saliva As the primary secretion flows through the ducts, two major active transport processes take place; First • Na+ ions are actively reabsorbed from all the salivary ducts and K+ ions are actively secreted in exchange for the sodium. Na ion concentration in the saliva becomes K ion concentration in the saliva becomes Secretion of Saliva Excess sodium reabsorption causes electronegativity. Therefore, Cl is reabsorbed passively (out of the duct) Cl ion concentration in the salivary fluid falls Secretion of Ions in Saliva Second • HCO3 − ions are secreted by the ductal epithelium into the lumen of the duct. Function of Saliva for Oral Hygiene Saliva helps to prevent the deteriorative processes in several ways: ❑ The flow of saliva helps wash away pathogenic bacteria & food particles ❑ Saliva contains several factors that destroy bacteria • Thiocyanate • Several proteolytic enzymes (lysozome) ❑ Contains antibodies that can destroy oral bacteria Nervous Regulation of Salivary Secretion 1.Controlled by parasympathetic pathways Brainstem Nervous Regulation of Salivary Secretion 2.Salivation can be stimulated or inhibited by signals coming from higher brain centers Anterior Hypothalamus appetite area In response to signals from the taste and smell areas of the Cerebral cortex & Amigdala Nervous Regulation of Salivary Secretion 3.Salivation can occur in response to reflexes in the stomach and upper small intestines Irritating food in the stomach or Nauseated because of GI anbnormality Nervous Regulation of Salivary Secretion 4.Sympathetic stimulation can increase salivation a slight amount 5. Blood supply to the glands Secretion of Saliva Esophageal Secretion mucous 1. Entirely mucus and provide lubrication for swallowing 2. Contains both simple and compound mucus glands The compound glands located near The compound gland in the the esophagogastric junction upper esophagus prevents protect the esophageal wall from mucosal excoriation by newly digestion by acidic gastric juices that often reflux from the stomach entering food Gastric Secretion Characteristics of the Gastric Secretions In addition to mucus-secreting cells the stomach mucosa has two types of tubular glands: 1. Gastric glands the gastric glands (oxyntic-acid forming) a. pepsinogen b. Hydrochloric acid (HCl) c. intrinsic factor d. mucus 2. Pyloric glands a. Mucus b. Gastrin The oxyntic glands are located on the inside surfaces of the body and fundus of the stomach The pyloric glands are located in the antral portion of the stomach Secretions from the Oxyntic (Gastric) Glands It is composed of three types of cells mucus Hydrochloric acid Pepsinogen Gastric Secretion Basic Mechanism of HCl Secretion • the parietal cells secrete an acid solution in response to stimulation • pH ≈ 0.8 (H+ ion concentration is high) • parietal cell contains large branching intracellular canaliculi where HCl acid is formed anatomy of the canaliculi in a parietal (oxyntic) cell Gastric Secretion Surface Mucous Cells Continous layer of a special type of mucous cells called “surface mucous cells.” They secrete a very viscid mucus that coats the stomach mucosa with a gel layer of mucus providing a major shell of protection for the stomach & contributing to lubrication of food transport. • This mucus is alkaline. STIMULATION OF GASTRIC ACID SECRETION Parietal Cells of the Oxyntic Glands Are the Only Cells That Secrete Hydrochloric Acid ❑ Secretion of acid is under continuous control by both endocrine and nervous signals. Gastrin Enterochromaffin-like cells Histamin In response to proteins in the foods being digested Stimulation of Acid Secretion by Gastrin. ❑ Gastrin is a hormone secreted by gastrin cells (pyloric glands), also called G cells. ❑ Stimulary effect of food protein reached the antral end of stomach GASTRIN cells Enterochromaffin-like cells (ELC) HISTAMIN Stimulates gastric hydrochloric acid secretion. REGULATION OF PEPSINOGEN SECRETION peptic cells in the oxyntic glands In response to two main types of signals: (1)Acetylcholine released from the vagus nerves or from the gastric enteric nervous plexus (2) Acid in the stomach REGULATION OF PEPSINOGEN SECRETION The rate of secretion of pepsinogen, the precursor of the enzyme pepsin that causes protein digestion, is strongly influenced by the amount of acid in the stomach. Gastric Secretion Phases of Gastric Secretion - Neural and hormonal mechanisms regulate the release of gastric juice - Stimulatory and inhibitory events occur in three phases; 1. Cephalic (reflex) phase: prior to food entry (while it is being eaten) 2. Gastric phase: once food enters the stomach 3. Intestinal phase: as partially digested food enters the duodenum Cephalic Phase Neurogenic signals originate from cerebral cortex & appetite centers of amygdala and hypothalamus Excitatory events include: Sight or thought of food Stimulation of taste or smell receptors -Parasympathetic impulses increase gastric secretion Inhibitory events include: Loss of appetite or depression Decrease in stimulation of the parasympathetic division Gastric Phase (1) Long vagovagal reflexes from the stomach to the brain and back to the stomach (2) Local enteric reflexes (3) The gastrin mechanism Secretion of gastric juice Phases of Gastric Secretion 2. Gastric phase • Excitatory events include: • Stomach distension → Activation of stretch receptors (neural activation) → Acetylcholine Ach)( release) • Activation of chemoreceptors by peptides, caffeine, and rising pH • Release of gastrin to the blood  Inhibitory events include:  A pH lower than 2  Emotional upset that overrides the parasympathetic division Phases of Gastric Secretion 3. Intestinal Phase • Excitatory phase • Very short phase • Initiated by chyme entry into duodenum • Stretch receptors stimulate release of intestinal (enteric) gastrin • Chemoreceptors detect fatty acids & glucose in the duodenum • Stimulate enteric gastrin release • low pH; partially digested food enters the duodenum and encourages gastric gland activity Intestinal Phase  Inhibitory phase distension of duodenum,  presence of fatty, acidic, or hypertonic chyme, and/or irritants in the duodenum Initiate e reverse ENTEROGASTRIC REFLEX Enterogastric reflex triggers 3 reflexes 1. Inhibits local reflexes and vagal reflex 2. Activates sympathetic n. System to close the pyloric sphincter 3. Releases enterogastrones that inhibit gastric secretion Pancreatic Secretions Pancreas - Large compound gland - Enzymes secreted by the pancreatic acini cells - Sodium bicarbonate secreted by small ductules Sodium bicarbonate + enzyme --------> trough pancreatic duct duedonum Pancreatic Secretions Pancreatic enzyme + Sodium Bicarbonate Small ductules Trough the pancreatic duct Hepatic duct Trough the papilla of Vater surrounded by the sphincter of Oddi. Duodenum Pancreatic Secretions • Pancreatic juice produced in response to the presence of chyme (upper portions of the small intestine) • exocrine function • • Secretes pancreatic juice which breaks down all categories of foodstuff • Acini (clusters of secretory cells) contain zymogen granules with digestive enzymes endocrine function • release of insulin & glucagon • insulin is secreted directly • into the blood- by the islets • of Langerhans Pancreatic Secretions • Pancreatic juice • 1.2-1.5 L/day • Mostly water, some salts, bicarbonate, enzymes • alkaline, pH 7.1-8.2 • buffers acidic gastric juice stops pepsin activity creates proper alkaline pH for enzymes acting in the intestine - Pancreatic juice - Bile - intestinal juices neutralize the gastric acid Pancreatic Secretions Pancreatic Digestive Enzymes a. For proteins: trypsin Proteins to chymotrypsin carboxypeptidase Peptides to b. For carbohydrates: pancreatic amylase Peptides Amino acids c. For fats: pancreatic lipase cholesterol esterase phospholipase d. For nucleic acid : ribonuclease and deoxyribonuclease Pancreatic Secretions Pancreatic Digestive Enzymes ─ Proteolytic enzymes are released in inactive form and activated in the duodenum; *trypsinogen, * chymotrypsinogen, * procarboxypolypeptidase. They become activated only after they are secreted into the intestinal tract. ─ Secretion of trypsin inhibitor prevents the digestion of the pancreas itself Pancreatic Secretions Secretion of Bicarbonate Ions ➢ the other components of pancreatic juice, - bicarbonate ions - water are secreted by the epithelial cells of the ductules Bicarbonate provides alkali in the pancreatic juice to neutralize the HCl coming into the duodenum from the stomach Regulation of Pancreatic Secretion  Neural control from parasympathetic division of ANS via vagus nerve  Autoregulation by sensing the presence of fatty acids and amino acids in the acidic chyme  Hormonal control by the secretion of enteroendocrines from duodenum  Secretin – stimulates secretion of water, HCO3 Cholecystokinin (CCK) – stimulates secretion of enzymes Pancreatic Secretions Regulation of Pancreatic Secretion a. Acetylcholine-released from the parasympathetic vagus nerve endings and other cholinergic nerves associated with the enteric nervous system b. Cholecystokinin-secreted by the duodenal and upper jejunal mucosa when food enters the small intestine c. Secretin- secreted by the duodenal and jejunal mucosa when highly acid food enters the small intestine c. Multiplicative effects of “a, b, and c” Pancreatic Secretions Acetylcholine and Cholecystokinin stimulate the acinar cells of the pancreas production of large quantities of pancreatic digestive enzymes but relatively small quantities of water and electrolytes to go with the enzymes BUT Secretin Stimulates secretion of large quantities of water solution of sodium bicarbonate by the pancreatic ductal epithelium Multiplicative Effects of Different Stimuli Thus, pancreatic secretion normally results from the combined effects of the multiple stimuli, not from one alone Phases of Pancreatic Secretion Pancreatic Secretions Phases of Pancreatic Secretion a. Cephalic Phases Acetylcholine secretion stim.enzyme secr. Little secr. Flows through panc.ducts because small amounts of water & electrolytes b. Gastric Phases the nervous stimulation of enzyme secretion continues a. Intestinal Phase - Secretin stimulates secretion of bicarbonate ions + water - Cholecystokinin production which stimulates more digestive enzymes by the acini (similar to vagal stimulation) Control Of Peristalsis By Nervous and Hormonal Signals • Hormonal factors include ─ ─ ─ ─ ─ gastrin, CCK, insulin, motilin, Serotonin ─ secretin ─ glucagon enhance intestinal motility inhibit small intestinal motility Peristaltic Rush Although peristalsis in the small intestine is normally weak, intense irritation of the intestinal mucosa, as occurs in some severe cases of infectious diarrhea, can cause both powerful and rapid peristalsis. SECRETION OF BILE Secretion of Bile by the Liver Bile • secretion rate : 600-1000 ml/day • yellow, brownish, or olive-green liquid • pH 7.6-8.6 • • • • is produced in liver is stored in gallbladder is ejected into small intestine when gallbladder contracts aids in the digestion and absorbtion of lipids Secretion of Bile by the Liver Functions of Bile  Bile salts emulsify lipids to prepare them for digestion (detergent effect)  Bile salts also form very small complexes with lipids (micelles) to help their absorbtion  Bile is responsible for excretion of several important waste products from the blood (especially bilirubin and excesses of cholesterol) Secretion of Bile by the Liver Physiologic Anatomy of Biliary Secretion Bile is secreted in two steps by the liver 1. Initial secretion by the hepatocytes; contains bile salts, cholesterol, and other organic compounds; into the bile canaliculi 2. Bile empties into terminal bile ducts, then the hepatic duct, and finally the common bile duct into the duodenum or diverted through the cystic duct into the gallbladder 3. Bile is stored and concentrated in the gallbladder The Gallbladder  Thin-walled, green muscular sac on the ventral surface of the liver  Stores and concentrates bile by absorbing its water and ions ─ The maximum volume ; 30 to 60 milliliters bile ─ 12 hours of bile secretion ( ≈ 450 milliliters) can be stored in the gallbladder ─ because water, sodium, chloride, and most other small electrolytes are absorbed through the gallbladder mucosa, concentrating the remaining bile constituents that contain the bile salts, cholesterol, lecithin, and bilirubin  Releases bile via the cystic duct, which flows into the bile duct Secretion of Bile by the Liver Composition of Bile Liver Bile Gallbladder Bile Water 97.5 g/dl 92 g/dl Bile salts 1.1 g/dl 6 g/dl Bilirubin 0.04 g/dl 0.3 g/dl Cholesterol 0.1 g/dl 0.3-0.9 g/dl Fatty Acids 0.12 g/dl 0.3-1.2 g/dl Lecithin 0.04 g/dl 0.3 g/dl 145 mEq/L 130 mEq/L Potassium Ions 5 mEq/L 12 mEq/L Calcium Ions 5 mEq/L 23 mEq/L Chloride Ions 100 mEq/L 25 mEq/L Bicarbonate Ions 28 mEq/L 10 mEq/L Sodium Ions Secretion of Bile by the Liver Liver secretion and gallbladder emptying Secretion of Bile by the Liver Emptying of the Gallbladder-Role of CCK a. Most potent stimulus for the gallbladder to undergo rhythmic contractions when food enter the small intestine Vagal stimulation and the intestinal enteric nervous system causes weak contractions of the gallbladder b. Acidic, fatty chyme causes the duodenum to release: - Cholecystokinin (CCK) and secretin into the bloodstream Bile salts and secretin transported in blood stimulate the liver to produce bile Secretion of Bile by the Liver Cholecystokinin causes: ▪ The gallbladder to contract ▪ The hepatopancreatic sphincter to relax As a result, bile enters the duodenum Bile Salts Function in Digestion and Absorption a. Have a detergent action on fat particles which causes the emulsification of the fat • the surface tension of the particles decreases and allows agitation in the intestinal tract to break the fat globules into minute sizes. b. Help in the absorption of lipids by forming micelles Secretion of Bile by the Liver Bile salts; - are formed in the hepatic cells - formed from cholesterol cholesterol is insoluble in pure water - bile salts and lecithin in bile combine with the cholesterol to form micelles. - if the bile becomes concentrated the bile salts and lecithin become concentrated with the cholesterol. Formation of gallstones - Under abnormal conditions, the cholesterol may precipitate in the gallbladder, resulting in the formation of cholesterol gallstones. Regulation of Bile Release Digestion in the Small Intestine • As chyme enters the duodenum: • Carbohydrates and proteins are only partially digested • No fat digestion has taken place  Chyme is released slowly into the duodenum  Because it is hypertonic and has low pH, mixing is required for proper digestion  Required substances needed are supplied by the liver  Virtually all nutrient absorption takes place in the small intestine Intestinal Motility and Secretions The movements of the small intestine; - mixing contractions (Segmentation Contractions) • distention of the small intestine • stretching of the intestinal wall - propulsive contractions elicit contractions Intestinal Motility and Secretions Small Intestine: Segmentation • primary action of small intestine when food is present • a form of mechanical digestion • a mixing activity • alternate contraction, relaxation of antagonistic smooth (circular and longitudinal) muscle segments in the intestine • controlled by the autonomic nervous system Intestinal Motility and Secretions Small Intestine: Peristalsis • as absorption continues, distension decreases and true peristalsis starts • a movement which propels chyme onward ileocecal • these weak movements which occur only after most nutrients have been absorbed Control of Peristalsis by Nervous and Hormonal Signals • Peristalsis are governed mainly by the myenteric plexus. ─ ─ ─ ─ ─ ─ stimulated by distension and acidic chime gastroenteric reflex is initiated local reflexes increase Ach release vasoactive intestinal polypeptide (VIP) stimulates production of intestinal secretions basal motility is controlled by autorhythmic pacemakers local hormones and parasympathetic ANS reflexes increase motility Control of Peristalsis by Nervous and Hormonal Signals • hormonal factors include ─ gastrin, ─ CCK, ─ insulin, enhance intestinal motility ─ motilin, ─ Serotonin ─ ─ secretin glucagon inhibit small intestinal motility Secretion of the Small Intestine Secretion of Mucus by Brunner’s Glands in the Duodenum a. Secrete large amounts of mucus in response to:  tactile or irritating stimuli on the mucosa,  vagal stimulation,  secretin b. Mucus protects the duodenal wall from the acidic chyme coming from the stomach c. Can be inhibited by sympathetic stimulation Secretion of the Small Intestine Secretion of Intestinal Digestive Juices by the Crypts of Lieberkuhn a. Surface covered by goblet cells (mucus) and enterocytes (secrete water and electrolytes) b.Mechanism of secretion of the watery fluid is unclear but involves the active secretion of chloride ions and bicarbonate ions - Secretion rate 1800 ml/day - pH 7.5 -8 Secretion of the Small Intestine Secretion of Intestinal Digestive Juices by the Crypts of Lieberkuhn c. Digestive enzymes in the small intestine secretion: 1. several peptidases; small peptides amino acids, 2. sucrase, maltase, isomaltase, and lactase ; disaccharides monosaccharides; 3. small amounts of intestinal lipase; neutral fats glycerol and fatty acids. d. Regulation is entirely local by the enteric nervous system - reflexes initiated by tactile or irritative stimuli from the chyme Functions of the Large Intestine 1. absorption of water and electrolytes from the chyme to form solid feces 2. storage of fecal matter until it can be expelled. • Other than digestion of enteric bacteria, no further digestion takes place • Production of vitamins • Its major function is propulsion of fecal material toward the anus • Though essential for comfort, the colon is not essential for life Motility of the Large Intestine • Mixing movements : Haustral contractions • Slow segmenting movements that move the contents of the colon • Haustra sequentially contract as they are stimulated by distension • Propulsive Movements: Mass Movements • mass movements after meals is facilitated by gastrocolic and duodenocolic reflexes → result from distention of the stomach and duodenum. • during or immediately following a meal, 3-4 times day • strong peristaltic waves from middle of transverse colon • push contents into the rectum * Presence of food in the stomach: • Activates the gastrocolic reflex • Initiates peristalsis that forces contents toward the rectum Secretion of the Large Intestine Secretion of Mucus by the Large Intestine a. Many crypts but no villi in the large intestine b. Epithelial cells secrete almost no enzymes but consist of many mucus cells - contain bicarbonate ions c. Rate of mucus formation is regulated by the direct tactile stimulation of the epithelial cells. ❖ Stimulation of pelvic nerves can cause mucus secretion