Physiology of the Digestive System Part 1 PDF

PHYSIOLOGY OF THE DIGESTIVE SYSTEM INTRODUCTION  Digestive system Alimentary canal Associated glands Mouth Salivary glands Anus Liver, gallbladder Pancreas  Functions of the GIT Digestion Secreti...

PHYSIOLOGY OF THE DIGESTIVE SYSTEM INTRODUCTION  Digestive system Alimentary canal Associated glands Mouth Salivary glands Anus Liver, gallbladder Pancreas  Functions of the GIT Digestion Secretion Absorption Endocrine function Immune function 1- Digestion Breakdown of food materials Small particles Absorption and utilization by the body cells Digestion mostly occurs in the proximal part of the small intestine. Duodenum and first part in jujenium Digestion is either mechanical or chemical (by enzymes) 2- Absorption The transfer of the contents of the gut from lumen into the mucosa, and then to the blood. Absorption mostly occurs in the small intestine especially “jejunum”.  3- Secretion: Secretion of digestive juices 7000-7500 ml are secreted daily into the gut  Water balance in GIT (ml/day) In Out Ingested 2000 mL Reabsorbed GIT secretions - By jejunum 5500 mL - Salivary secretion 1500 mL - By ileum 2000 mL - Gastric secretion 2500 mL - By colon 1300 mL - Bile 500 mL - Pancreatic secretion 1500 mL - Intestinal secretion 1000 mL Total input 9000 mL Total reabsorbed 8800 mL Balance in stool = 9000-8800 = 200 mL  4- Endocrine function Release of hormones: e.g.: 1-Gastrin (from the stomach) 2-Secretin (From the upper small intestine) 3-Cholecystokinin (CCK) (From the upper small intestine)  5- Immune function lymphocytes secrete immunoglobulins (or antibodies) to protect the body against microorganisms in the lumen of the gut. Peyer's patches are collection of lymphocytes in the intestine  Innervation of the digestive tract Extrinsic nervous Intrinsic nervous system system Enteric nerve plexus 1- Parasympathetic nervous system 2- Sympathetic A- Auerbach nervous system plexus B- Meissner plexus  Enteric nerve plexus Myenteric plexus Submucosal plexus (Auerbach's plexus) (Meissner's plexus) Stimulates motility Stimulates secretion  Extrinsic innervation of the gut Parasympathetic Sympathetic nervous system nervous system (The vagus nerve and pelvic nerve) 1- It inhibits the motility & secretion. 1- It stimulates the motility 2- It contains excitatory and secretion. fibers to GIT sphincters 2- It contains inhibitory fibers to GIT sphincters. Salivary glands  Salivary glands Salivary glands are exocrine glands, with ducts opening into the oral cavity. Salivary glands The largest one Parotid 25 % Submandibular The main salivary gland 70 % Sublingual 5%  Composition of saliva Volume: 1-1.5 liters/day pH: 6.8 – 7.2 (nearly neutral) Constituents: - Water (99.5%) - Inorganic constituents e.g. electrolytes (0.2%) - Organic constituents e.g. mucin and enzymes (0.3%)  Functions of saliva 1. Moistens and lubricates food to facilitate its swallowing & facilitates speech. 2. Protective functions: a) Mechanical cleaning of food residues & shed epithelial cells which prevents putrefaction b) Bactericidal action: potassium thiocyanate ions and the lysozymes. c) Antibodies: can destroy the bacteria in the oral cavity. d) Protects against irritant substances and neutralizes excessively cold or hot food.  Functions of saliva 3. Has a buffering action via its bicarbonate system, phosphate system, & proteins (mucin), to protect the buccal cavity from excessive acids or alkalis. Excessive alkalis precipitation of calcium salts around the teeth calcium salts + organic matter hard concretions (tartar), bacteria flourish underneath the tartar chronic inflammation of the gums. Excessive acids dissolves the enamel & dentine of teeth after prolonged exposure (dental caries).  Functions of saliva 4.Has a role in the digestion of starch by salivary amylase (ptyalin) , 5. Dissolves some food materials to facilitate the stimulation of taste buds (helps taste sensation). 6. Has a role in water balance because dryness of the buccal and pharyngeal mucosa initiates the sensation of thirst.  Multifunctionality Cleaning Phosphate & Bicarbonate moisturizing mucin Buffering Amylases The pH Digestion Salivary secretion Water balance Protection Lysozyme, K thiocyanate Lubrication Ig A For immunoglobulin swallowing & speech Mucin Control of salivary secretion By Nervous regulation  1- Unconditioned reflexes moving the tongue Presence of food in inside the mouth the mouth, During speech Chemical stimulation of the taste receptors Mechanical stimulation of touch or temperature receptors Afferent Center Salivatory nuclei (in the medulla oblongata) Efferent : Autonomic nerves Response Salivary secretion  Innervation of salivary secretion Parasympathetic Sympathetic (Main effect) (Minor effect) -Profuse secretion of watery Small amounts of viscid saliva saliva with a relatively low rich in organic constituents content of organic material (enzymes, mucin) (enzymes, mucin) - rich in electrolytes - Marked vasoconstriction: - Marked vasodilatation: decrease in blood flow. -Blood flow increases during activity  2- Conditioned reflexes These are acquired reflexes; They depend on training. Stimulation of any sensory nerve endings in the body with absence of food from the buccal cavity (optic, auditory, & olfactory nerves) can be converted by training into conditioned stimuli for salivation. Condition stimuli: Seeing, hearing, thinking, officiating Center: cerebral cortex then the impulse travels to the salivary centers in the medulla oblongata.  Clinical relevance q Xerostomia: dryness of the mouth q Ptyalism: excessive salivation PHYSIOLOGY OF THE ESOPHAGUS & SWALLOWING  Mixed The esophagus is a muscular tube. Functions of the esophagus 1- Transfer 2- Anti-reflux function function 3- Secretion of mucin  Deglutition (Swallowing) Deglutition is the transfer of food from the mouth to the stomach. The start of deglutition is voluntary, but it is completed involuntarily (swallowing reflex). Centre: is the swallowing center in the medulla oblongata.  Phases of swallowing The first: oral (voluntary) phase Bolus: in mouth Chyme: after mouth The tongue elevates and rolls posteriorly propelling the bolus into the pharynx. The bolus is “voluntarily” squeezed or pushed to the oropharynx by pressure of the tongue upward and backward against the palate.  Phases of swallowing The second (pharyngeal) (involuntary) phase: 1. The muscles of the pharynx contract (swallowing reflex is initiated). 2. Receptive relaxation of the upper esophageal sphincter. Protective reflexes to prevent the passage of food into the trachea: 1. Soft palate elevates to close the nasopharynx and prevent food from entering the nasal passages. 2. The larynx is elevated, and the epiglottis covers the glottis to prevent food from entering the trachea 3. Breathing is interrupted (reflex apnea) Bolus transfer from the mouth through the pharynx requires multiple events Bolus moves through Bolus enters Bolus in mouth esophagus pharynx and UES Pharynx contracts UES opens Glottis closed by epiglottis/ Apnea Larynx elevated Nasopharynx closed Esophageal peristaltic contractions Tongue thrusts up & back 0 0.2 0.4 0.6 0.8 1 1.2 Time (S)  Phases of swallowing Oral phase Pharyngeal phase Pharyngeal phase Esophageal phase  The third (esophageal; involuntary) phase In this involuntary phase, food passes along the esophagus to the stomach by esophageal peristaltic contractions.  Peristalsis = propulsive movement What is peristalsis? Where does peristalsis occur? How does peristalsis occur? What is the function of peristaltic movement?  Peristalsis = propulsive movement From oral to conal Involuntary contraction and relaxation of the muscles of the alimentary canal, create wave-like movements that push the contents of the canal forward. Mechanism Contraction of the muscles behind the bolus Receptive relaxation if the muscles in front of the bolus >>>>> Forward movement of food  Esophageal peristaltic contractions The peristalsis in the upper third of the esophagus are rapid because the muscles of the upper third are striated and supplied by the vagus nerve directly, while the movement in the lower third is slow, because the muscles are smooth, which are excited by the vagus nerve indirectly through the local plexus of Auerbach. The middle third contains both types of muscles.  Phases of swallowing a) & b) oral phase c) & d) pharyngeal phase e) & f) esophageal phase. N.B. Difficulty in swallowing is called Dysphagia e.g. damage of the swallowing center or bilateral vagal nerve injury Upper and lower esophageal sphincters Functions of sphincters: vUES prevents Influx of air vLES prevents Reflux of gastric contents to the esophagus Stomach  Anti-reflux function of Lower esophageal sphincter (LES; cardiac sphincter) It is the lowest 3-5 cm of the esophagus. VIP/ NO This sphincter is tonically Relaxes the LES contracted (myogenic) between meals to prevent reflux of gastric contents into the esophagus. It relaxes during swallowing by inhibitory signals from the vagus nerve → Auerbach plexus → Nitric oxide (NO) and vasoactive intestinal peptide (VIP) release (receptive relaxation)  Clinical relevance If the LES does not relax satisfactorily, this results in a condition called achalasia. If the LES does not close satisfactorily, this results in a condition called Gastroesophageal reflux disease (GERD) The Stomach  Functions of the stomach A. Secretory functions The amount of gastric juice is about 2.5- 3 liters /day It is the most acidic body secretion (pH=1-2) Contains: mucin, HCl, pepsinogen, and intrinsic factor B. Motor functions C. Endocrine function: gastrin hormone  A. Secretory functions (Gastric secretion) 1. Secretes mucin, which protects the mucosa against ulceration. 2. Secretes digestive enzymes e.g. pepsinogen (by the chief cells), a proteolytic enzyme that starts protein digestion to peptides. 3. Secretes HCl (by the parietal cells), which acidifies the gastric juice 4. Releases the intrinsic factor (by the parietal cells) for vitamin B12 absorption from the intestine. Mechanism of HCl formation CA H+/K+ ATPase Bicarbonate chloride exchanger  Mechanism of HCl formation HCl is secreted by the parietal cells of the gastric glands. 1. CO2 (from the blood or cell metabolism) in the parietal cells reacts with water to form carbonic acid (H2CO3), catalyzed by carbonic anhydrase enzyme. 2. Dissociation of H2CO3 into H+ and HCO3- occurs. CO2 + H2O → H2CO3→ H+ + HCO3 -  Mechanism of HCl formation 3. Active transport of H+ across the membrane of the parietal cells to the gastric lumen occurs (proton pump). The energy for this active transport is derived from aerobic glycolysis, since acid secretion ceases under anaerobic conditions. N.B. Diamox, a carbonic anhydrase inhibitor depresses gastric secretion of HCI. 4. Bicarbonate (HCO3-) ions pass from parietal cells to the blood in exchange for chloride ions (Cl-) that enter the cell by bicarbonate-chloride exchanger. 5. Cl- ions are secreted by the parietal cells at the luminal side of the gastric mucosa through Cl- channels and are coupled with the H+ ions, giving a strong solution of hydrochloric acid (HCl) in the canaliculus. The HCl is then secreted outward through the open end of the canaliculus into the lumen of the gland.  Stimulation of parietal cells to secrete HCl 1. H2 receptors: stimulated by histamine (secreted by Enterochromaffin Like (ECL) cells in the gastric mucosa) & blocked by H2 blocker cimetidine. 2. Muscarinic (M3) receptors : +++ by Ach. & blocked by the muscarinic blocker atropine. 3. Gastrin receptors: Stimulated by gastrin hormone released from antral mucosa. 4. Prostaglandin and Somatostatin inhibit HCl secretion N.B. Anti-prostaglandin drugs increase gastric HCL secretion  Functions of gastric HCl 1. Activates the inactive pepsinogen to pepsin & gives its optimum pH for its action. 2. Has an antibacterial action by killing microorganisms 3. Facilitates the absorption of calcium and iron from the upper small intestine.  B. Motor functions 1. Stores large quantities of food (by receptive relaxation in the fundus and body) 2. Mixes and grinds food with gastric secretions (by peristalsis in the antrum). 3. Regulates the rate of evacuation of chyme into the duodenum (by peristalsis in the antrum: pyloric pump; gastric emptying) , to prevent rapid passage of food into the small intestine  Vomiting It is outward expulsion of the gastric contents (also intestinal contents), through the esophagus, pharynx and mouth.  Vomiting occurs via the following steps: 1. Relaxation of the body and fundus of the stomach, opening of the esophageal sphincters. 2. Contraction of the diaphragm, the anterior abdominal wall muscles and the pelvic floor ⇒ ⇒ ⇒ rise of the intra-abdominal pressure. 3. Contraction of the pyloric portion of the stomach. The elevated intra-abdominal pressure squeezes the relaxed stomach ⇒ ⇒ forces the contents through the relaxed esophagus. N. B. What are the protective mechanism against entry of the vomitus to the respiratory passages?  Causes of vomiting 1) Central causes a. Direct stimulation of the vomiting center in the medulla oblongata by increased intracranial pressure due to brain tumors or other lesions. b. Indirect stimulation of the vomiting center through impulses from the chemoreceptor trigger zone (CTZ), which is unprotected by the blood brain barrier. The CTZ is stimulated by certain circulating chemical agents in the blood (emetics). i) Exogenous emetics: e.g. emetine and morphine. ii) Endogenous emetics: e.g. high blood urea  Causes of vomiting 2) Peripheral causes (Reflex vomiting) Reflex stimulation of the vomiting center may occur by: a) Irritation of the mucosa of the upper GIT by mechanical stimulation of the posterior part of the tongue. b) Irritation of any part of GIT, kidney, heart or inner ear by inflammation, tumor or ulceration. e.g. appendicitis, gastritis, colitis, renal stones and motion sickness  Effects of severe vomiting? 1- Dehydration 2- Alkalosis 3- chronic vomiting causes malnutrition and weight loss Doaa Samy