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Chapter 16 The Digestive System Dr R.E Gordon HSB N310, X4499 [email protected] Chapter 16 The Digestive System Human PhysiologybybyLauralee HumanPhysiology LauraleeSherwood...

Chapter 16 The Digestive System Dr R.E Gordon HSB N310, X4499 [email protected] Chapter 16 The Digestive System Human PhysiologybybyLauralee HumanPhysiology LauraleeSherwood Sherwood©2010 ©2010Brooks/Cole, Brooks/Cole,Cengage CengageLearning Learning Digestive System Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning  THE DIGESTIVE SYSTEM 1. General aspects 1.1 Basic digestive processes 1.2 Components of the digestive system 1.3 General regulating mechanisms 2. Mouth 2.1 Components of the mouth 2.2 Chewing 2.2 Salivary secretion 2.3 Digestion and Absorption in the mouth 3. Pharynx and oesophagus 3.1 Swallowing 4. Stomach 4.1 Gastric motility 4.2 Gastric secretion 4.3 Digestion in the stomach 4.4 Absorption by the stomach Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning 5. Pancreatic and biliary secretions 5.1 Pancreas 5.2 Biliary system 6. Small intestine 6.1 Small-intestine motility 6.2 Small-intestine secretion 6.3 Digestion in the small intestine 6.4 Absorption by the small intestine 7. Large intestine 7.1 Large-intestine motility 7.2 Large-intestine secretion 7.3 Absorption by the large intestine 8. Overview of the gastrointestinal hormones Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning  THE DIGESTIVE SYSTEM 1. General aspects 1.1 Basic digestive processes 1.2 Components of the digestive system 1.3 General regulating mechanisms 2. Mouth 2.1 Components of the mouth 2.2 Chewing 2.2 Salivary secretion 2.3 Digestion and Absorption in the mouth 3. Pharynx and oesophagus 3.1 Swallowing 4. Stomach 4.1 Gastric motility 4.2 Gastric secretion 4.3 Digestion in the stomach 4.4 Absorption by the stomach Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning  Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning  Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning 1. General aspects of digestion 1.1 Basic digestive processes Primary function – Transfer nutrients, water, and electrolytes from ingested food into body’s internal environment Four digestive processes – Motility – Secretion – Digestion – Absorption Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning Motility Definitions: Motility: Muscular contractions that mix and move forward the contents of the digestive tract Tone: a constant low level of contraction Two types of digestive motility – Propulsive movements Push contents forward through the digestive tract – Mixing movements Serve two functions – Mixing food with digestive juices promotes digestion of foods – Facilitates absorption by exposing all parts of intestinal contents to absorbing surfaces of digestive tract Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning NOTE: In most GIT parts: Motility involves contraction of smooth muscle in walls of GIT Therefore: Motility controlled by involuntary mechanisms Exceptions In mouth, top part of oesophagus and beginning and external anal sphincter: Motility involves skeletal muscle contraction Therefore: Chewing, swallowing, defaecation have voluntary components Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning Secretion – Secreted by exocrine glands into the lumen of the digestive tract – Consist of water, electrolytes, and specific organic constituents (extracted from plasma) – Secretion of digestive juices requires energy – Neural or hormonal stimulation → exocrine glands → secretions into digestive tract lumen – Normally reabsorbed in one form or another back into blood after their participation in digestion – GIT hormones help control digestive motility and exocrine gland secretion Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning Digestion Definition: Digestion: Biochemical breakdown of structurally complex foodstuffs into smaller, absorbable units – Accomplished by enzymatic hydrolysis – Complex foodstuffs and their absorbable units Carbohydrates (di- and polysaccarides, glycogen, cellulose) → monosaccharides (glucose, fructose, galactose) Proteins → amino acids (also some small polypeptides) Fats → monoglycerides and free fatty acids Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning Absorption - Digestion is completed in the small intestine - Most absorption occurs in the small intestine - Small units resulting from digestion, along with water, vitamins, electrolytes transferred from digestive tract lumen into blood or lymph Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning 1.2 Components of the digestive system Digestive tract (4,5 m Accessory digestive organs contractile state) – Salivary glands – Continuous from mouth to – Exocrine pancreas anus – Biliary system – Consists of Liver Mouth Gallbladder Pharynx Oesophagus Stomach Small intestine – Duodenum – Jejunum – Ileum Large intestine – Cecum – Appendix – Colon – Rectum Anus Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning Digestive System Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning NOTE: The lumen of the GIT is continuous with external environment Therefore: contents in lumen of GIT not part of the body NB because conditions in GIT lumen cannot be tolerated in the body (conditions such as e.g. very low pH in stomach; digestive enzymes would digest body’s own tissues; microorganisms in colon; immune system that would attack foodstuffs which are foreign to the body) Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning The wall of the digestive tract: Same general structure throughout length from oesophagus to anus Four major tissue layers – Mucosa Innermost layer – Submucosa – Muscularis externa – Serosa Outer layer Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning Layers of Digestive Tract Wall Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning Structure of the Gastrointestinal Tract Wall Mucosa Lines luminal surface of digestive tract Highly folded surface greatly increases absorptive area Three layers – Mucous membrane Serves as protective surface Modified for secretion and absorption Contains – Exocrine gland cells – secrete digestive juices – Endocrine gland cells – secrete blood-borne gastrointestinal hormones – Epithelial cells – specialized for absorbing digestive nutrients – Lamina propria Houses gut-associated lymphoid tissue (GALT) – Important in defense against disease-causing intestinal bacteria – Muscularis mucosa Sparse layer of smooth muscle Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning Submucosa Thick layer of connective tissue Provides digestive tract with distensibility and elasticity Contains larger blood and lymph vessels Contains nerve network known as submucosal plexus Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning Muscularis Externa Major smooth muscle coat of digestive tube In most areas consists of two layers – Circular layer Inner layer Contraction decreases diameter of lumen – Longitudinal layer Outer layer Contraction shortens the tube Together contractile activity produces propulsive and mixing movements Myenteric plexus – Lies between the two muscle layers Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning  Enteric Nervous System A specialized division of the nervous system associated only with the GIT Connected to the CNS via the Parasympathetic NS (stimulates digestion) and Sympathetic NS (inhibits digestion) Composed of two major nerve plexuses (groups) which send both sensory and motor information throughout the GI tract to control digestion Submucosal nerve plexus (submucosa layer) associated with mechano-and chemoreceptors in the mucosa controls the endo-and exocrine secretion of the mucosa Myenteric nerve plexus (muscularis layer) controls the contraction of smooth muscle Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning Serosa Secretes serous fluid – Lubricates and prevents friction between digestive organs and surrounding viscera Continuous with mesentery throughout much of the tract – Attachment provides relative fixation – Supports digestive organs in proper place while allowing them freedom for mixing and propulsive movements Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning  Control of the digestive system Neural and hormonal mechanisms coordinate glands Hormonal mechanisms enhance or inhibit smooth muscle contraction Local mechanisms coordinate response to changes in pH or chemical stimuli Hormonal regulation NOTE: Short reflexes: LOCAL stimuli → intrinsic nerve plexuses → LOCAL motility or secretion changed All elements located in wall of digestive tract Long reflexes: Superimposed on local controls EXTERNAL influences → extrinsic nerves → correlate activity between different regions of GIT or modify GIT activity Involve CNS GIT hormones: Local changes in digestive tract, short or long reflexes → secretion of gastrointestinal hormones 2 kinds of receptors in plasma membranes of digestive tract: Sensory receptors – monitor luminal content and wall tension Effector cells – have receptor proteins that bind with and respond to GIT hormones, neurotransmitters, local chemical mediators Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning  Receptor activation 3 types of sensory receptors 1. Chemoreceptors - chemical 2. Mechanoreceptors – stretch / tension 3. Osmoreceptors - osmolarity Receptor activation neural reflexes hormone secretion (short and long) EFFECTOR CELLS (have receptor proteins that bind with and respond to GIT hormones, neurotransmitters, local chemical mediators) changes in motility, secretion (digestive juices and GIT hormones) Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning  THE DIGESTIVE SYSTEM 1. General aspects 1.1 Basic digestive processes 1.2 Components of the digestive system 1.3 General regulating mechanisms 2. Mouth 2.1 Components of the mouth 2.2 Chewing 2.2 Salivary secretion 2.3 Digestion and Absorption in the mouth 3. Pharynx and oesophagus 3.1 Swallowing 4. Stomach 4.1 Gastric motility 4.2 Gastric secretion 4.3 Digestion in the stomach 4.4 Absorption by the stomach Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning 2. Mouth (Oral Cavity) 2.1 Components of the mouth Lips – Form opening – Help procure, guide, and contain food in the mouth – Important in speech – Well-developed tactile sensation Palate – Forms roof of oral cavity (separates mouth from nasal passages) – Allows breathing and chewing or suckling to take place at same time Uvula (seals off nasal passages during swallowing) Tongue (voluntary controlled skeletal muscle) – Forms floor of oral cavity – Composed of skeletal muscle – Movements aid in chewing and swallowing – Plays important role in speech – Taste buds Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning  Pharynx – Cavity at rear of throat – Common passageway for digestive and respiratory systems – Tonsils Within side walls of pharynx Lymphoid tissue Teeth – Responsible for chewing (mastication) 2.2 Chewing – First step in digestive process – Degree of occlusion affect efficiency of chewing – Can be voluntary, but mostly rhythmic reflex – Functions of chewing To grind and break food up into smaller pieces (↑ surface area for salivary enzyme action; facilitate swallowing) To mix food with saliva To stimulate taste buds (sensation of taste; reflexly increases salivary, gastric, pancreatic, and bile secretion in feedforward way) Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning 2.3 Salivary Secretion – Produced (1-2 liters/day) by three major pairs of salivary glands – Not essential for digesting and absorbing foods – Composition 99.5% H2O 0.5% electrolytes and protein (amylase, mucus, lysozyme) Amylase – polysaccharides to maltose & amylopectin to α-limit dextrins Lingual lipase Rich in bicarbonate – buffers acids Mucus - lubrication Lysozyme - destroys bacteria Immunoglobulin A – neutralization of pathogens Nitric acid – defense against pathogens Salivary lactoferrin – binds iron (bacteria need iron to multiply) – Functions of water Facilitates swallowing by moistening food Washes the mouth Solvent for molecules that stimulate taste buds Lubricates the mouth for speech Helps keep mouth and teeth clean Definition: Xerostomia = diminished salivary secretion Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning 34  Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning Control of salivary secretion Secretion is continuous Basal rate of 0.5ml/min due to stimulation by parasympathetic nerve Secretion increased by 2 types of salivary reflexes: Simple salivary reflex: Conditioned salivary reflex: Food in oral cavity External stimuli Chemo- and pressure receptors Cerebral cortex Salivary centre in medulla ANS Salivary glands ↑ salivary secretion Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning  Control of Salivary Secretion Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning NOTE: NO hormonal control of salivary secretion Salivary secretion only digestive secretion entirely under neural control Conditioned salivary reflex is a learned response based on previous experience Parasympathetic stimulation → ↑↑ watery saliva rich in enzymes Sympathetic stimulation → ↑ saliva, thick rich in mucus Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning 2.4 Digestion and Absorption in the mouth Digestion: amylase Polysaccharides → disaccharides Digestion by salivary amylase continues in the body of stomach Absorption: NO absorption of foodstuffs Some drugs can be absorbed eg nitroglycerin Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning  THE DIGESTIVE SYSTEM 1. General aspects 1.1 Basic digestive processes 1.2 Components of the digestive system 1.3 General regulating mechanisms 2. Mouth 2.1 Components of the mouth 2.2 Chewing 2.2 Salivary secretion 2.3 Digestion and Absorption in the mouth 3. Pharynx and oesophagus 3.1 Swallowing 4. Stomach 4.1 Gastric motility 4.2 Gastric secretion 4.3 Digestion in the stomach 4.4 Absorption by the stomach Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning 3. Pharynx and Oesophagus Oesophagus – Fairly straight muscular tube – Extends between pharynx and stomach – Sphincters at each end Pharyngoesophageal sphincter (skeletal muscle) – Keeps entrance closed to prevent large volumes of air from entering oesophagus and stomach during breathing Gastroesophageal sphincter – Prevents reflux of gastric contents – Peristaltic waves push food through oesophagus – Secretions (mucus) are entirely protective Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning 3.1 Swallowing Motility associated with pharynx and oesophagus Sequentially programmed all-or-none reflex Bolus voluntarily forced by tongue into pharynx → swallowing reflex Most complex reflex in body Can be initiated voluntarily but cannot be stopped once it has begun Process divided into two stages: Oropharyngeal stage (bolus from pharynx into oesophagus) Oesophageal stage (bolus through oesophagus into stomach) Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning Oropharyngeal stage: Bolus → pressure receptors in pharynx → impulses to swallowing centre in medulla → activates muscle contraction in the proper sequence → larynx and trachea sealed off → contraction of pharyngeal muscles → pharyngoesophageal sphincter opens → bolus forced into oesophagus NOTE: Initiated voluntarily, but once begun it cannot be stopped Lasts about 1 second Bolus prevented from: re-entering mouth by tongue against palate entering nasal passage by elevated uvula entering trachea by elevation of the larynx, closure of the vocal folds, epiglottis tilting backwards over glottis As bolus enters oesophagus → pharyngoesophageal sphincter closes, respiratory airways open → breathing resumes Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning Oesophageal stage: Swallowing centre triggers a peristaltic wave → forces bolus ahead of it through the oesophagus into the stomach If bolus becomes stuck → distension of the oesophagus intrinsic nerve plexuses reflexly ↑ salivary secretion at the site of the distension secondary peristaltic waves + ↑ saliva swallowed bolus forced into stomach NOTE: Takes about 5-9 seconds Secondary peristaltic waves do not involve the swallowing centre but is mediated by intrinsic nerve plexuses at the site of the distension Secondary peristaltic waves are involuntary Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning Peristalsis in the Oesophagus Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning NOTE: Gastroesophageal sphincter  is smooth muscle  stays tonically contracted except during swallowing → prevents reflux of acidic gastric contents into oesophagus Secretion of oesophagus is mucus to protect oesophageal wall from mechanical damage as well as to protect against acid and enzymes in gastric juice if reflux occurs Digestion and absorption in the pharynx and oesophagus: Time (6 – 10 seconds) too short for any digestion or absorption Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning  Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning  GERD = Gastro Oesophageal Reflux Disorder Causes:  increased pressure on the stomach may cause GERD, e.g. obesity  Improper closing of LES, increased pressure on LES, substances that may relax the LES.  THE DIGESTIVE SYSTEM 1. General aspects 1.1 Basic digestive processes 1.2 Components of the digestive system 1.3 General regulating mechanisms 2. Mouth 2.1 Components of the mouth 2.2 Chewing 2.2 Salivary secretion 2.3 Digestion and Absorption in the mouth 3. Pharynx and oesophagus 3.1 Swallowing 4. Stomach 4.1 Gastric motility 4.2 Gastric secretion 4.3 Digestion in the stomach 4.4 Absorption by the stomach Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning 4. Stomach J-shaped sac-like chamber lying between oesophagus and small intestine Divided into three sections – Fundus (thin muscle layer) – Body (thin muscle layer) – Antrum (thicker muscle layer) Three main functions – Store ingested food until it can be emptied into small intestine – Secretes hydrochloric acid (HCl) and enzymes that begin protein digestion – Mixing movements convert pulverized food to chyme Pyloric sphincter – Serves as barrier between stomach and upper part of small intestine Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning  Stomach Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning 4.1 Gastric Motility Four aspects – Filling Involves receptive relaxation – Enhances stomach’s ability to accommodate the extra volume of food with little rise in stomach pressure – Triggered by act of eating – Mediated by vagus nerve – Storage Takes place in body of stomach Peristaltic contractions from fundus to body to antrum – Mixing Takes place in antrum of stomach (retropulsion) – Emptying Largely controlled by factors in duodenum Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning  Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning Gastric Emptying Factors in stomach 1. Volume of chyme: stomach empties chyme at rate proportional to volume of chyme it has. 2. Degree of fluidity of chyme Factors in duodenum – Fat (most potent factor that inhibits gastric emptying) Fat already in duodenum → ↓ further gastric emptying of additional fatty stomach contents – Acid Unneutralized acid → inhibits further emptying of acidic gastric contents until neutralization can be accomplished – Hypertonicity ↑ Osmolarity of duodenal contents → gastric emptying reflexly inhibited – Distension Too much chyme in duodenum inhibits emptying of even more gastric contents Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning Importance of these factors in delaying gastric emptying: Fat: Digestion and absorption only in small intestine – entry of extra fat prevented until small intestine has had time to process fat already there Acid: Unneutralized acid irritates the duodenal mucosa and inactivates pancreatic digestive enzymes secreted into the duodenal lumen Hypertonicity: Digestion of protein and starch → ↑ aa’s and glucose. If digestion > absorption → ↑↑ aa’s and glucose molecules → ↑ osmolarity → ↑ osmosis of water into duodenal lumen → ↑ intestinal distension → could lead to circulatory disturbances Distension: ↑ chyme in duodenum → distension → inhibits more chyme entering duodenum → enough time to process chyme already there Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning  Factors trigger either – Neural response Mediated through both intrinsic nerve plexuses (short reflex) and autonomic nerves (long reflex) - enterogastric reflex – Hormonal response Involves release of hormones from duodenal mucosa collectively known as enterogastrones which inhibit antral contractions – Secretin – Cholecystokinin (CCK) Additional factors that that influence gastric motility (act through ANS) – Emotions Sadness and fear – tend to decrease motility Anger and aggression – tend to increase motility – Intense pain – tends to inhibit motility Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning  Vomiting Vomiting is the forceful expulsion of contents of the stomach and often, the proximal small intestine. It is a manifestation of many conditions, many of which are not primary disorders of the gastrointestinal tract. Regardless of cause, vomiting can have serious consequences, including acid- base derangements, volume and electrolyte depletion, malnutrition and aspiration pneumonia. Chapter 16 The Digestive SystemHuman Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning  Vomiting Definition: Vomiting (emesis) = forceful expulsion of gastric contents out through the mouth NOTE: It is NOT reverse peristalsis The stomach does not actively participate The stomach, oesophagus, gastroesophageal sphincter and pharyngoesophageal sphincter are relaxed Major force for expulsion is contraction of the diaphragm and abdominal muscles Coordinated by a vomiting centre in the medulla Preceded by profuse salivation, sweating, rapid heart rate and the sensation of nausea (characteristic of generalized discharge of the ANS) Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning Causes of vomiting: Tactile stimulation of the back of the throat Irritation or distension of the stomach and duodenum Elevated intracranial pressure Motion sickness Chemical agents (emetics) Psychogenic vomiting induced by emotional factors Effects of vomiting: Dehydration Circulatory problems Metabolic alkalosis Getting rid of noxious substances if ingested Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning  CAUSES OF NAUSEA/VOMITING Early pregnancy Myocardial infarction Psychogenic vomiting Peritonitis Bulemia Acute obstruction Acute gastritis Neurologic Gastric retention emergency Viral gastroenteritis Drug toxicity Acute gastroenteritis Cancer therapy Drug withdrawal Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning  How to minimize nausea and vomiting (if patient is eating orally): Not lying down directly after a meal, shortening food preparation time (to avoid odours), eating foods that are appealing, eating in a comfortable place, avoiding warm odorous places, wearing comfortable clothing, drinking herbal tea with honey and/or ginger, or peppermint tea, sucking on a peppermint candy, and brushing teeth after a meal. Sour liquids like lemonade are often better tolerated than water. Adequate amounts of sleep and rest are also needed, as fatigue may worsen N&V. Vitamin B6 (pyridoxine) suppl of 200mg / day may also be beneficial. Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning  THE DIGESTIVE SYSTEM 1. General aspects 1.1 Basic digestive processes 1.2 Components of the digestive system 1.3 General regulating mechanisms 2. Mouth 2.1 Components of the mouth 2.2 Chewing 2.2 Salivary secretion 2.3 Digestion and Absorption in the mouth 3. Pharynx and oesophagus 3.1 Swallowing 4. Stomach 4.1 Gastric motility 4.2 Gastric secretion 4.3 Digestion in the stomach 4.4 Absorption by the stomach Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning 4.2 Gastric Secretion (2 l/day) Two distinct areas of gastric mucosa that secrete gastric juice – Oxyntic mucosa Lines body and fundus – Pyloric gland area (PGA) Lines the antrum Infoldings of gastric mucosa forming gastric pits with gastric glands at the base of the pits Between gastric pits are surface epithelial cells – secrete a layer of thick, viscous, alkaline mucus which covers surface of mucosa Three types of gastric exocrine secretory cells line infoldings – Mucous cells Line gastric pits and entrance of glands Secrete thin, watery mucus – Chief cells Secrete enzyme precursor, pepsinogen – Parietal cells Secrete HCl and intrinsic factor (pg 584 read up on stem cells in stomach) Human Physiology by Lauralee Chapter 16 The Digestive System Sherwood ©2010 Brooks/Cole, Cengage Learning  Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning  Enterochromaffin-like cells Secretes histamine in response to parasympathetic activity and gastrin and increases parietal cell activity D cells: Secretes somatostatin when pH drops to inhibit further parietal cell secretions G cells: Secrete gastrin to stimulate parietal cells, also relaxes ileocecal sphincter, increases pyloric sphincter activity and lower stomach motility Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning  Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning  Stomach Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning  Gastrointestinal Secretions Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning HCl Secretion Secreted by the parietal cells into lumen of gastric pits → empty into lumen of stomach No HCl is secreted in the pyloric gland area Functions of HCl – Activates pepsinogen to active enzyme pepsin and provides acid (pH 2) medium for optimal pepsin activity – Aids in breakdown of connective tissue and muscle fibers → large food particles into smaller particles – Denatures protein (uncoils proteins → more peptide bonds exposed for enzymatic attack) – Kills most of the microorganisms ingested with food (salivary lysozyme also kills microorganisms) Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning  Mechanism of acid secretion Chloride is secreted against both a concentration and electric gradient. Ability of the parietal cell to secrete acid is dependent on active transport. The key player in acid secretion is an H+/K+ ATPase or "proton pump" Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning  Mechanism of acid secretion Hydrogen ions are generated within the parietal cell from dissociation of water. The hydroxyl ions formed in this process rapidly combine with carbon dioxide to form bicarbonate ion, a reaction catalysed by carbonic anhydrase. Bicarbonate is transported out of the basolateral membrane in exchange for chloride. Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning  Mechanism of acid secretion The outflow of bicarbonate into blood results in a slight elevation of blood pH known as the "alkaline tide". This process serves to maintain intracellular pH in the parietal cell. Chloride and potassium ions are transported into the lumen by conductance channels. Hydrogen ion is pumped out of the cell, into the lumen, in exchange for potassium through the action of the proton pump; potassium is thus effectively recycled. Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning Regulation of HCl Production Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning  Pepsinogen Produced and stored (in inactive form) in chief cells in zymogen granules (to prevent digestion of proteins of chief cells) Released by exocytosis into gastric lumen Pepsinogen pepsin HCl Definition: Autocatalytic process = the active form of an enzyme activates other molecules of the same enzyme Pepsin acts on pepsinogen molecules → pepsin (autocatalytic process) Protein peptide fragments pepsin Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning Mucus Secreted by surface epithelial cells and mucous cells of gastric pits Mucus layer covers gastric mucosa Is alkaline (pH 7) Does not interfere with HCl and pepsin action in the gastric lumen Is a protective barrier because it protects against:  mechanical injury (lubricating properties)  self-digestion (inactivates pepsin)  acid injury (neutralizes HCl) Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning Intrinsic Factor Secreted by the parietal cells Essential for the absorption of vit B12  Absorption of vit B12 takes place in terminal ileum  Vit B12 essential for rbc formation  Vit B12 binds to intrinsic factor → vit B12-intrinsic factor combination binds to receptor → endocytosis of vit B12-intrinsic factor-receptor complex NOTE: Absence of intrinsic factor → no vit B12 absorbed → ↓ rbc produced → pernicious anaemia Treatment of pernicious anaemia is injections of vit B12 Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning Regulatory pathways (see table 16.3) Gastrin (hormone) Secreted by G-cells in pyloric gland area in response to protein products and ACh Stimulates motility and secretion of HCl, pepsinogen and histamine Histamine (paracrine) Released by ECL cells in response to gastrin and Ach Stimulates HCl secretion Somatostatin (acts as paracrine) Released by D cells in response to acid Inhibits HCl, gastrin and histamine secretion Acetylcholine (Ach) (neurotransmitter) Released in response to short and long reflexes and vagus stimulation Stimulates HCl and pepsinogen, gastrin and histamine secretion Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning  Phases of Gastric Secretion Cephalic phase Increased secretion of HCl and pepsinogen in response to stimuli acting in the head (thinking about, smelling, tasting, chewing, swallowing) before food reaches stomach. Vagal stimulation. See figure 16.4 pg 586 Gastric phase Begins when food actually reaches the stomach Presence of protein (most potent), distension, caffeine & alcohol increases gastric secretions. People with ulcers avoid caffeine and alcohol. See figure 16.4 pg 586 Intestinal phase Inhibitory phase Helps shut off flow of gastric juices as chyme begins to empty into small intestine. See figure 16.5 pg 587 Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning Gastric secretion decreases as food empties from stomach to intestine 1. As meal empties stomach, protein is withdrawn. 2. After food leaves stomach, gastric juice accumulates & pH drops. Somatostatin released to inhibit gastric secretion. 3. Fat, acid, distension & hypertonicity inhibit gastric secretion. Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning Gastric mucosal barrier mucus protective barrier Mucus-secreting cells HCO3- neutralizes acid Luminal membranes impermeable to H+ Mucosal lining Tight junctions no acid to submucosa Peptic ulcer Barrier broken →acid and enzymes damage gastric wall → peptic ulcer Gastric reflux into oesophagus or ↑↑ acid in duodenum → ulcers Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning Gastric Mucosal Barrier Enables stomach to contain acid without injuring itself Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning  Peptic ulcers (gastric or duodenal) Normal conditions: Gastric and duodenal mucosa is protected from acid and pepsin by: - Mucus secretion -Bicarbonate production -Removal of excess acid by normal blood flow -Rapid renewal and repair of epithelial cell injury. If there is a breakdown in any of these protective and repairing mechanisms (usually if >1 mechanism), then it can lead to the development of a peptic ulcer.  Symptoms of peptic ulcers Can be asymptomatic / abd pain / discomfort Gastric ulcers also cause N&V, anorexia, weight loss. COMPLICATIONS: Hemorrhage and perforation  morbidity & mortality Can perforate even into the peritonela cavity and penetrate the pancreas or other organs or erode an artery and cause massive bleeding. Melena = black, tarry stools, common in people with ulcers. Indicates acute or chronic upper GIT bleeding.  Primary causes of Peptic Ulcers Helicobacter pylori infection (treatment: Antibiotics) Gastritis  check pt’s Vit B12 status due to lowered intrinsic factor production Aspirin / other NSAIDs Corticosteroids Severe illness can cause stress ulcers (burns, trauma, shock, surgery, renal failure, radiation therapy) ‘Life stress’ can lead to increased alcohol and smoking  these factors can cause damage. Excessive alcohol increases gastric secretions Smoking lowers mucosal blood flow, lowers bicarbonate secretion, increases inflammation  H pylori is an extremely common pathogen, found in over half of the world’s population; is a gram-negative bacillus often found in the stomach. H. pylori produces urease enzymes that metabolize urea to ammonia and CO2 – The ammonia neutralizes the gastric acid and protects the bacillus. The ammonia also damages the gastric epithelial cells, contributing to the formation of an ulcer (Mostly in the duodenum). – Many individuals with gastric H. pylori never develop any symptoms of peptic ulcer disease. However, in individuals who do develop peptic ulcer disease, H. pylori is almost always present treatment of peptic ulcer that does not eradicate H pylori is associated with rapid recurrence of acid-peptic disease in most patients. 4.3 Digestion in the stomach In body of stomach: Salivary amylase continues carbohydrate digestion. Peristaltic contractions weak → no mixing with gastric secretions → little protein digestion In antrum of stomach: Strong peristaltic contractions → food mixed with gastric secretions → much protein digestion Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning 4.4 Absorption by the stomach Ethyl alcohol Aspirin NOTE: 1. Alcohol absorption more rapidly in small-intestine (↑↑ surface area) Therefore: Eating fat-rich foods when drinking alcohol → gastric emptying delayed (fat inhibits gastric emptying) → alcohol longer in stomach where absorption is slower than in small intestine → rise in blood alcohol slow 2. Most drugs absorbed in small intestine Therefore: Aspirin acts quicker because it is absorbed by the stomach Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning 5. Pancreatic and biliary secretions 5.1 Pancreas 5.2 Biliary system 6. Small intestine 6.1 Small-intestine motility 6.2 Small-intestine secretion 6.3 Digestion in the small intestine 6.4 Absorption by the small intestine 7. Large intestine 7.1 Large-intestine motility 7.2 Large-intestine secretion 7.3 Absorption by the large intestine 8. Overview of the gastrointestinal hormones Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning 5. Pancreatic and biliary secretions 5.1 Pancreas Located behind and below the stomach Contains exocrine and endocrine tissue Endocrine function – Islets of Langerhans Found throughout pancreas Secrete insulin and glucagon Exocrine function – Secretes pancreatic juice consisting of Pancreatic enzymes actively secreted by acinar cells that form the acini Aqueous alkaline solution actively secreted by duct cells that line pancreatic ducts Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning Fig. 16-12, p. 613  enzymes 5.1.1 Pancreatic Secretion (1 – 2 l/day) aqueous alkaline solution Enzymes Proteolytic enzymes – Digest protein Trypsinogen - converted to active form trypsin Chymotrypsinogen – converted to active form chymotrysin Procarboxypeptidase – converted to active form carboxypeptidase Pancreatic amylase – Converts polysaccharides into the disaccharide maltose Pancreatic lipase – Only enzyme secreted throughout entire digestive system that can digest fat Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning Proteolytic enzymes: Produced and stored (in inactive form) to prevent digestion of proteins of acinar cells Released into duodenal lumen (in inactive form) In duodenum: Trypsinogen trypsin Enteropeptidase / Enterokinase (in duodenal mucosa) Trypsin acts on: trypysinogen molecules trypsin (autocatalytic process) chymotrypsinogen chymotrypsin procarboxypeptidase carboxypeptidase NOTE: If trypsinogen is activated in pancreas: trypsin actions blocked by trypsin inhibitor Pancreatic insufficiency: digestion of proteins is impaired but still occurs due to gastric and small intestine enzymes Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning Pancreatic amylase: Polysaccharides maltose & α-limit dextrins Pancreatic amylase NOTE: Pancreatic amylase stored and secreted in an active form because secretory cells do not contain polysaccharides Pancreatic insufficiency: digestion of carbohydrates is impaired but still occurs due to salivary and small intestine enzymes Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning Pancreatic lipase Triglycerides monoglycerides and free fatty acids Pancreatic lipase NOTE: Only enzyme in GIT for fat digestion Pancreatic lipase secreted and stored in an active form because secretory cells do not contain triglycerides Pancreatic insufficiency → steatorrhea (excessive undigested fat in faeces) Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning  Steatorrhoea Steatorrhoea is accompanied with weight loss Steatorrhoea is the presence of excess fat in faeces. Stools may be bulky and difficult to flush, have a pale and oily appearance and can be especially foul- smelling Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning  Steatorrhoea An oily anal leakage or some level of faecal incontinence may occur. Weight loss is due to the lack of fat digestion (no lipase) and subsequently absorption Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning  Steatorrhoea Occurs when there is not enough healthy absorptive area, or rapid transit, or interrupted flow of pancreatic enzymes and bile, or after extensive bowel resection. Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning  Pancreatic aqueous alkaline secretion Actively secreted by duct cells that line pancreatic ducts Largest component of pancreatic secretion Functions:  Neutralizes acidic chyme (for optimal function of pancreatic enzymes)  Prevents acid damage to duodenal mucosa Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning 5. Pancreatic and biliary secretions 5.1 Pancreas 5.2 Biliary system 6. Small intestine 6.1 Small-intestine motility 6.2 Small-intestine secretion 6.3 Digestion in the small intestine 6.4 Absorption by the small intestine 7. Large intestine 7.1 Large-intestine motility 7.2 Large-intestine secretion 7.3 Absorption by the large intestine 8. Overview of the gastrointestinal hormones Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning  5.1.2 Regulation of pancreatic exocrine secretion Phases of digestion Regulation Cephalic phase A little parasympathetic stimulation → small ↑ pancreatic secretion Gastric phase Gastrin → a little parasympathetic stimulation → small ↑ pancreatic secretion Intestinal phase Enterogastrones (Secretin and cholecystokinin) → ↑↑↑ pancreatic secretion NOTE: Most secretion of pancreatic secretion during intestinal phase of digestion Primarily hormonal mechanisms Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning Secretin Acid in duodenal lumen ↑ Secretin secretion pancreatic duct cells ↑ aqueous NaHCO3- solution into duodenal lumen Acid in duodenal lumen neutralized Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning Cholecystokinin (CCK) Fat and protein products in duodenal lumen ↑ CCK released from duodenal mucosa Pancreatic acinar cells ↑ digestive enzymes into duodenal lumen ↑ fat and protein digestion Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning  Summary of Pathways Controlling Digestive System Activities Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning NOTE: All three types (proteolytic enzymes, amylase, lipase) are packaged together in zymogen granules and release together (exocytosis) Secretin and CCK exert trophic effects on the exocrine pancreas Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning 5.2 The biliary system (= liver, gallbladder, associated ducts) 5.2.1 Liver Liver is largest and most important metabolic organ in the body Body’s major biochemical factory Importance to digestive system – secretion of bile salts Functions not related to digestion – Metabolic processing of the major categories of nutrients – Detoxifying or degrading body wastes and hormones, drugs, and other foreign compounds – Synthesizes plasma proteins – Stores glycogen, fats, iron, copper, and many vitamins – Activates vitamin D – Removes bacteria and worn-out red blood cells – Excretes cholesterol and bilirubin Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning Structure of the liver Functional unit = lobule Lobule:  hexagonal around a central vein  At each corner: branch of hepatic artery, a hepatic portal vein, a bile duct  Sinusoids (= capillary spaces) run between rows of hepatocytes  Hepatocytes in plates of 2 cell layers thick  A bile canaliculus runs between the cells within each hepatic plate, carry bile to bile duct at corner of the lobule  Bile ducts converge to form common bile duct which takes bile to duodenum  Kupffer cells in sinusoids remove and destroy old rbc’s and bacteria Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning Fig. 16-15, p. 617 Liver blood flow: The liver receives blood from hepatic artery (from heart) and blood from hepatic portal system (= venous blood coming from the digestive tract) Therefore:  Gets its oxygen and nutrients from arterial blood  Products absorbed from digestive tract first go to liver to be processed, stored, or detoxified before entering general circulation Venous blood from digestive tract carried by portal vein → liver sinusoids (= capillaries) → venules → hepatic vein → inferior vena cava → right atrium of heart Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning Bile – Actively secreted (250 ml – 1 litre / day) by liver and actively diverted to gallbladder between meals – Sphincter of Oddi (at opening of bile duct into duodenum) prevents bile from entering duodenum between meals – Stored and concentrated in gallbladder – Consists of Bile salts Cholesterol Lecithin Bilirubin Aqueous alkaline fluid NO digestive enzymes – After meal, bile enters duodenum (↑ secretion by liver + gallbladder emptying) Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning Bile salts Derivatives of cholesterol After participation in fat digestion and absorption, most are reabsorbed in terminal ileum into the blood (enterohepatic circulation) Important for digestion and absorption of fats convert large fat globules form micelles together with into a liquid emulsion cholesterol and lecithin ↑ surface area for lipase action. digested lipids carried through watery luminal contents to their sites of absorption Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning NOTE: Lipase cannot penetrate the layer of bile salts on fat droplet. Colipase (secreted with lipase by pancreas) displaces bile salts & lodges on surface of fat droplet, where it binds to lipase anchoring it. Bile duct obstructed by gallstone results in greyish white faeces. Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning Fig. 16-17, p. 619  A MICELLE Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning  Bilirubin Is a waste product excreted in bile Primary bile pigment (yellow) No role in digestion Small amount of bilirubin reabsorbed in intestine & excreted in urine – yellow colour of urine Modified by bacterial enzymes – brown colour of faeces Jaundice: Bilirubin formation > excretion → jaundice 3 causes: a) Prehepatic jaundice: liver normal, but excessive breakdown of rbc’s b) Hepatic jaundice: normal breakdown of rbc’s but liver is diseased c) Posthepatic jaundice: bile duct obstructed, bilirubin cannot reach GIT Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning Regulation of bile secretion Chemical mechanism: (bile salts) Bile salts reabsorbed after digestion and absorption → enterohepatic circulation → to liver → ↑ bile secretion Hormonal mechanism: (secretin) Secretin → ↑ aqueous bile secretion (no increase in bile salts) Neural mechanism: (vagus nerve) During cephalic phase of digestion: vagus → ↑ bile secretion Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning  Hepatitis Inflammatory disease of the liver caused by variety of factors Viral infection Obesity (Fatty liver disease) Exposure to toxic agents Severity: Mild, reversible symptoms, acute massive liver damage Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning  Cirrhosis A condition in which the liver does not function properly due to long-term damage. This damage is characterized by the replacement of normal liver tissue by scar tissue Commonly caused by alcohol, hepatitis and non- alcoholic fatty liver disease Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning 5.2.2 Gallbladder Between meals bile stored and concentrated in gallbladder Not essential role in digestion Therefore: if removed → bile secreted between meals stored in the common bile duct Contraction of the gallbladder: Fat in duodenum CCK contraction of gallbladder relaxation of sphincter of Oddi Bile discharged into duodenum Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning 5. Pancreatic and biliary secretions 5.1 Pancreas 5.2 Biliary system 6. Small intestine 6.1 Small-intestine motility 6.2 Small-intestine secretion 6.3 Digestion in the small intestine 6.4 Absorption by the small intestine 7. Large intestine 7.1 Large-intestine motility 7.2 Large-intestine secretion 7.3 Absorption by the large intestine 8. Overview of the gastrointestinal hormones Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning  6. Small intestine Site where most digestion and absorption take place Three segments – Duodenum – Jejunum – Ileum Ileocecal valve and ileocecal sphincter prevent contamination of the small intestine by bacteria from colon Motility includes – Segmentation – Migrating motility complex Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning Fig. 16-20, p. 622  Segmentation 6.1 Small intestine motility Migrating motility complex Segmentation – Primary method of motility in small intestine – Consists of ringlike contractions (every few cm) along length of small intestine – Within seconds, contracted segments relax and previously relaxed areas contract – Action mixes and slowly propels chyme throughout small intestine lumen Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning – initiated by pacemaker cells in small intestine which produce basic electrical rhythm (BER) – Circular smooth muscle responsiveness is influenced by distension of intestine, gastrin, and extrinsic nerve activity – Doudenum segments in response to local distension – Gastroileal reflex: presence of chyme in the stomach → gastrin → segmentation of ileum starts – Functions Mixing chyme with digestive juices secreted into small intestine lumen Exposing all chyme to absorptive surfaces of small intestine mucosa Slowly moves chyme through small intestine (enough time for digestion and absorption) Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning  Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning  Segmentation Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning  Migrating motility complex (= weak peristaltic waves) – Sweeps intestines clean between meals – Between meals: motilin → migrating motility complex – Feeding → inhibition of motilin → migrating motility complex ceases Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning Migrating motility complex (MMC) MMC cycles through following phases in repetitive pattern every 1.5 hours during fasting: 1. 40 – 60 minutes of quiet, very few contractions. 2. 20 - 30 minutes, some peristaltic contractions, time varying between contractions. 3. shortest phase, intense peristaltic contractions repeat for 5 – 10 minutes. Pyloric sphincter relaxes & opens. While fasting person notices gurgling noises often thought of as stomach growling. Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning 5. Pancreatic and biliary secretions 5.1 Pancreas 5.2 Biliary system 6. Small intestine 6.1 Small-intestine motility 6.2 Small-intestine secretion 6.3 Digestion in the small intestine 6.4 Absorption by the small intestine 7. Large intestine 7.1 Large-intestine motility 7.2 Large-intestine secretion 7.3 Absorption by the large intestine 8. Overview of the gastrointestinal hormones Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning 6.2 Small intestine secretion (Succus entericus) Small intestine secretion (about 1.5 l/day) water and electrolytes mucus (secreted by crypts of Lieberkühn) (secreted by cells on villus surface) NOTE: NO enzymes secreted However: enzymes form integral part of brush-border membrane Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning  6.2 Small intestine secretion (Succus entericus) Paneth cells produce:  Lysozyme, bacteria-lysing enzyme.  Defensins, small proteins with antimicrobial properties. Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning  Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning 6.3 Digestion in the small intestine Carbohydrate digestion In lumen: carbohydrates Pancreatic amylase disaccharides and some monosaccharides In brush border plasma membrane: disaccharides Maltase, sucrase- isomaltase (α-limit dextrins), lactase monosaccharides NOTE: monosaccharides = absorbable units Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning Lactose Intolerance Children < 4 years adequate lactase. Adults lactase activity lost or diminished. Undigested lactose draws water into intestinal lumen. Bacteria in large intestine have lactose-splitting ability, attack lactose as energy source, producing much carbon dioxide and methane gas. Distension of intestine by fluid & gas produces cramping (pain) & diarrhea. Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning  Dietary Changes for Lactose Intolerance Take milk products with other foods in meals Lactose intolerant people can tolerate fermented milk products such as yoghurt. (the bacteria in the products digest lactose for their own use thus reducing the lactose content). Hard cheeses such as cheddar and cottage cheese often well tolerated. Protein digestion In lumen: Protein Pancreatic proteolytic enzymes small peptide fragments and some amino acids In brush border plasma membrane: In epithelial cells: peptide fragments peptide fragments Intracellular aminopeptidases peptidases amino acids amino acids NOTE: amino acids = absorbable units Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning Fat digestion In lumen: triglycerides pancreatic lipase monoglycerides and free fatty acids NOTE: Monoglycerides and free fatty acids = absorbable units Fat digestion completed in small intestinal lumen Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning Structure of a villus A villus is composed of: 1. Epithelial cells covering surface of the villus Within luminal brush borders carriers for absorption of nutrients and electrolytes membrane bound digestive enzymes (to complete carbohydrate and protein digestion) 2. A connective tissue core 3. A capillary network 4. A terminal lymphatic vessel (= central lacteal) Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning  Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning 6.4 Absorption by the small intestine – Absorbs almost everything presented to it – Only calcium and iron absorption regulated according to body’s needs – Most occurs in duodenum and jejunum – Terminal ileum – absorption of bile salts and vit B12 – Adaptations that increase small intestine’s surface area Inner surface has permanent circular folds Microscopic finger-like projections called villi Brush border (microvilli) arise from luminal surface of epithelial cells – Lining is replaced about every three days Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning Absorption: Substance from lumen → pass through epithelial cell → diffuse through the interstitial fluid within the connective tissue core of the villus → cross wall of capillary or lymph vessel Absorption of carbohydrates (figure 16.22) and protein (figure 16.23): Secondary active transport into blood, absorption of glucose & galactose into duodenum epithelial cells linked to active Na+ transport Absorption of fat (figure 16.24): Passive diffusion into lymph Inside epithelial cells: monoglycerides and free fatty acids resynthesized into triglycerides → form droplets → covered with layer of lipoprotein (=chylomicrons) → exocytosis into interstitial fluid of villus → enter central lacteals by passive diffusion NOTE: Capillaries have basement membrane that prevents chylomicrons from entering, but lymph vessels do not have this membrane Therefore: chylomicrons enterbylymph Human Physiology and©2010not blood Chapter 16 The Digestive System Lauralee Sherwood Brooks/Cole, Cengage Learning Carbohydrate Absorption Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning Protein Absorption Absorption of vitamins: Water soluble vitamins: passive absorption Fat-soluble vitamins: absorbed passively with end products of fat digestion Some vitamins absorbed by carriers Vit B12 unique - absorbed by carrier in combination with intrinsic factor (in terminal ileum) Absorption of calcium: Regulated according to body’s needs Parathyroid hormone (PTH) increases to a fall in [Ca2+] in the blood for vit D activation → Vit D enhances Ca2+ absorption About ⅔ absorbed of the 1000mg Ca2+ ingested, rest passes out in faeces Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning  Absorption of iron (figure 16.25): Regulated according to body’s needs Dietary iron haeme iron (in meat) inorganic iron (in plants) (bound) (Fe3+ and Fe2+) After absorption iron either used for rbc production or irreversibly stored as ferritin in small intestine epithelial cells (ferritin lost in faeces with epithelial cells when these cells are replaced every 3 days) NOTE: Fe2+ absorbed more easily - Vit C enhances iron absorption (reduces Fe3+ to Fe2+) Hepcidin (hormone) regulates iron homeostasis ↑↑↑ iron gives faeces dark almost black colour Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning  Iron absorption Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning After absorption Absorbed carbohydrates, amino acids → venules → hepatic portal vein → to liver → metabolic processing → hepatic vein → vena cava → heart → systemic circulation → body Absorbed fat → central lacteal → thoracic duct → into venous blood → heart → systemic circulation → body Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning Celiac Disease (gluten enteropathy) Small intestine abnormally sensitive to gluten. Exposure to gluten activates T-cell response that damages intestinal villi:  Villi is reduced.  Mucosa is flattened  Brush border becomes short & stubby. Resulting in less surface area for absorption. Absorption of all nutrients is impaired. Condition treated by avoiding gluten. Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning  Celiac Disease Characteristics:  Genetic susceptibility  Exposure to gluten  Environmental ‘trigger’  Autoimmune response What is GLUTEN?? Specific peptide fractions of proteins found in wheat (gliadin and glutenin) Barley (hordein) Rye (secalin)  Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning Diarrhea Leads to a loss of fluid and an acid-base imbalance Passage of highly fluid faecal matter, often with increased frequency Causes dehydration, electrolyte imbalances (resulting in hyponatraemia, hypokalaemia), metabolic acidosis (due to loss of alkaline fluids through diarrhea. occurs as body compensates for the loss by retaining acidic compounds), loss of nutrient material (over time body will show signs of deficiency of nutrients, and nutrient needs are increased in diarrhea). Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning  Diarrhea Causes of diarrhea: Local irritation of gut wall by bacterial or viral infection or emotional stress → excessive small intestine motility → time in small intestine too rapid for adequate absorption of fluid → diarrhea Excess osmotically active particles in digestive lumen → excessive fluid enters lumen → diarrhea Toxins of bacterium that causes cholera and certain other micro- organisms → excessive secretion of fluid by small intestine mucosa → diarrhea NOTE: Oral rehydration therapy could prevent water loss Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning  Oral Rehydration Solution A type of fluid replacement used to prevent and treat dehydration, especially due to diarrhoea and vomiting. It involves drinking water with modest amounts of SUGAR and SALT Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning  Diarrhea Diarrhoea can be acute (4 weeks) or chronic (>4 weeks). Acute diarrhoea is usually due to an infectious cause. The most common non-infectious causes are side effects of medications. Generally, diarrhoea is caused by an osmotic imbalance, hyper-motility, inflammation, or excessive secretion. diarrhoea is due to osmotic load, too much secretion or not enough absorption Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning  Diarrhea Osmotic (malabsorptive) diarrhoea: is due to malabsorbed nutrients or poorly absorbed electrolytes that retain water in the lumen. Secretory diarrhoea: results when secretagogues maintain elevated rates of fluid transport out of epithelial cells into the GI tract lumen. (Cholera toxin) Hypermotility diarrhoea: occurs when fluid passes through the intestines too quickly for normal absorption. (bacterial overgrowth) Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning  Diarrhea Inflammatory diarrhoea: such as that characterized by ulcerative colitis, is due to damage of the intestinal epithelial cells. Infectious diarrhoea: e.g. Enterotoxic E. coli infection is contracted by oral ingestion of contaminated food or water. Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning  Diarrhea Treatment of diarrhea: (if patient is eating orally) SOLUBLE FIBRE will lead to gel formation and slower GIT transit. Peeled and grated apples, bananas, carrots and oats (soluble fibre) ORS ZINC supplementation (NB for recovery from symptoms) Probiotics, prebiotics Avoid rich, creamy or fatty foods. Avoid milk, rather have lactose-free milk for a few days. 5. Pancreatic and biliary secretions 5.1 Pancreas 5.2 Biliary system 6. Small intestine 6.1 Small-intestine motility 6.2 Small-intestine secretion 6.3 Digestion in the small intestine 6.4 Absorption by the small intestine 7. Large intestine 7.1 Large-intestine motility 7.2 Large-intestine secretion 7.3 Absorption by the large intestine 8. Overview of the gastrointestinal hormones Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning 7. Large intestine Consists of – Cecum – Appendix – Colon (ascending, transverse, descending, sigmoid colon) – Rectum Primarily a drying and storage organ Colon received about 500 ml chyme from small intestine consisting of indigestible food residues, unabsorbed biliary components, and remaining fluid Colon – Extracts more water and salt from contents – What remains to be eliminated is called faeces Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning  Large Intestine Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning 7.1 Large intestine motility Taeniae coli – 3 separate longitudinal bands of muscle – Shorter than underlying circular smooth muscle → underlying layers form haustra Haustra – Pouches or sacs – Actively change location as result of contraction of circular smooth muscle layer Haustral contractions – Main motility – mixing movement – Initiated by autonomous rhythmicity (BER) of colonic smooth muscle cells – Regulated by reflexes of intrinsic plexuses – Non-propulsive, slowly shuffle contents back-and-forth. Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning  Mass movements – Massive contractions – Food enters stomach → gastrocolic reflex mediated by gastrin and extrinsic autonomic nerves → mass movements of large intestine – Moves colonic contents into distal part of large intestine Defecation reflex – Initiated when stretch receptors in rectal wall are stimulated by distension – Causes internal anal sphincter to relax and rectum and sigmoid colon to contract more vigorously – If external anal sphincter (skeletal muscle under voluntary control) is also relaxed, defaecation occurs Defecation assisted by contraction of abdominal muscles and a forcible expiration against a closed glottis (↑ intra-abdominal P) Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning Constipation Contents retained for longer periods of time than normal → ↑↑ water absorbed → faeces hard and dry (= constipation) Prolonged distension → abdominal discomfort, dull headache, loss of appetite, nausea and mental depression Possible causes:  Ignoring the urge to defaecate  Decreased colon motility (aging, emotion, low-bulk diet)  Obstruction by tumour or colonic spasm  Impairment of defaecation reflex (injury of nerve pathways) Appendicitis: faecal matter lodged in appendix → inflammation → swelling → could rupture → infectious contents into abdominal cavity Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning  Diverticular disease Diverticula: small, bulging pouches often in the lower part of colon. Diverticula are common, especially after age 40, seldom cause problems. Diverticulosis: presence of diverticula. Incidence increases with age. Prolonged constipation can cause herniations to form (increased pressure in colon causing diverticula). When these little pouches get filled with faecal matter, inflammation can start, leading to diverticulitis. = spectrum of inflammation, abscess formation, acute perforation, acute bleeding, obstruction, sepsis.  Diverticular disease  Clinical manifestations of acute episode: abdominal pain, fever, vomiting, change in bowel habits, vomiting, bright red blood from rectum.  Risk factors: age > 40, diet low in fibre & high in fat, red meat, obese, sedentary lifestyle among others  Rx  antibiotics, oral intake as tolerated and sometimes bowel rest (drinking only certain liquids or not eating or drinking anything) is advised.  General fibre recommendation for diverticular disease: 25 g/day for women and 38 g/day plus 2-3L water per day.  (https://www.youtube.com/watch?v=UYUiplMgcUM)  Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning 7.2 Large intestine secretion Consists of an alkaline mucus solution:  NaHCO3 neutralizes acids produced by local bacterial fermentation → protects against chemical injury  Mucus provides lubrication → protects against mechanical injury NOTE: NO digestive enzymes secreted NO antibacterial agents secreted Colonic microorganisms beneficial because: Crowd out pathogenic microbes → enhance intestinal immunity Promote colonic motility Help maintain colonic mucosal integrity Make nutritional contributions e.g.  Vit K  Acid → ↑ absorption of calcium, magnesium and zinc Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning 7.3 Absorption by the large intestine Salt and water Remaining in the faeces: undigested cellulose, bilirubin, unabsorbed food residues and bacteria Intestinal gases or flatus (from swallowed air and from bacterial fermentation in the colon) are absorbed or expelled Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning Absorption sites of the GIT  Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning 5. Pancreatic and biliary secretions 5.1 Pancreas 5.2 Biliary system 6. Small intestine 6.1 Small-intestine motility 6.2 Small-intestine secretion 6.3 Digestion in the small intestine 6.4 Absorption by the small intestine 7. Large intestine 7.1 Large-intestine motility 7.2 Large-intestine secretion 7.3 Absorption by the large intestine 8. Overview of the gastrointestinal hormones Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning 8. Overview of the gastrointestinal hormones Gastrin – Release is stimulated by presence of protein in stomach – Secretion inhibited by accumulation of acid in stomach – Functions Acts in several ways to increase secretion of HCl and pepsinogen Enhances gastric motility, stimulates ileal motility, relaxes ileocecal sphincter, - all this induces mass movements in colon Helps maintain well-developed, functionally viable digestive tract lining Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning Secretin – Presence of acid in duodenum stimulates release – Functions Inhibits gastric emptying to prevent further acid from entering duodenum until acid already present is neutralized Inhibits gastric secretion to reduce amount of acid being produced Stimulates pancreatic duct cells to produce large volume of aqueous NaHCO3 secretion Stimulates liver to secrete alkaline rich bile which assists in neutralization process Along with CCK, is trophic to exocrine pancreas Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning CCK – Functions Inhibits gastric motility and secretion Stimulates pancreatic acinar cells to increase secretion of pancreatic enzymes Causes contraction of gallbladder and relaxation of sphincter of Oddi Along with secretin, is trophic to exocrine pancreas Implicated in long-term adaptive changes in proportion of pancreatic enzymes in response to prolonged diet changes Important regulator of food intake Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning GIP – Glucose-dependent insulinotrophic peptide – Released in response to: glucose, aa, FA – Stimulates insulin release by pancreas – Inhibits gastric acid secretion Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning  THE END Good luck with the test and exams! Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning