HSC1008 Anatomy and Physiology 2 Digestive System 1 PDF

HSC1008 ANATOMY AND PHYSIOLOGY 2 DIGESTIVE SYSTEM 1 ANDY LEE (PHD) ASSISTANT PROFESSOR FACULTY HALL #04-17 OFFICE PHONE: 6592 2524 [email protected] LEARNING OUTCOMES At the end of the lesson, you should be able to: Describe the 4 basic digestive processes Discuss the main f...

HSC1008 ANATOMY AND PHYSIOLOGY 2 DIGESTIVE SYSTEM 1 ANDY LEE (PHD) ASSISTANT PROFESSOR FACULTY HALL #04-17 OFFICE PHONE: 6592 2524 [email protected] LEARNING OUTCOMES At the end of the lesson, you should be able to: Describe the 4 basic digestive processes Discuss the main functions of the oral cavity and the mechanisms involved Discuss the main functions of the stomach, liver and pancreas Explain the control of gastric secretion and motility Discuss the mechanism and control of bile and pancreatic secretions Describe the role of the main GI hormones in regulation of digestive function REVISION (ANATOMY) A JOURNEY THROUGH THE DIGESTIVE SYSTEM https://youtu.be/_QYwscALNng 4 BASIC DIGESTIVE PROCESSES Motility Secretion Digestion Absorption  Motility  Contraction of smooth muscle in the digestive tract  1) Mixing and 2) Moving the contents in the GIT  Secretion  Both exocrine and endocrine secretions  Mucus – along entire digestive tract  Saliva, acid, enzymes, bile, bicarbonate, hormones etc 4 BASIC DIGESTIVE PROCESSES  Digestion  Food that we consume are generally large molecules  Cannot cross the plasma membrane of the intestinal epithelial cells  Chemically break down complex macromolecules into smaller absorbable molecules  Absorption  Digested material transferred from GIT lumen into blood or lymph 4 BASIC DIGESTIVE PROCESSES Refer to Table 16-1 (Sherwood) for a summary REGULATION OF DIGESTIVE FUNCTION 1. Autonomous smooth muscle function 2. Intrinsic nerve plexuses 3. Extrinsic autonomic nerves 4. GI hormones  Receptors in digestive tract  Chemoreceptors, mechanoreceptors & osmoreceptors Fig 16-3; Sherwood ORAL CAVITY (MOUTH)  Entrance to the digestive tract  Muscular lips help procure, guide and contain food in the mouth  Functions  Mastication  Initiation of swallowing (deglutition)  Salivary secretion  Taste MASTICATION  Involves the mechanical action of slicing, tearing, grinding, and mixing of ingested food by the teeth  Functions  Breaks food into smaller pieces  Facilitate swallowing  Increases surface area of food particles  Mixes food with saliva  Expose food to taste buds  Increases secretions to prepare for the arrival of food SALIVA  Composition  99.5% water, 0.5% electrolytes, proteins and mucus  Functions 1. Initiates digestion of dietary starch by salivary amylase 2. Initiates digestion of some lipids by lingual lipase 3. Some antibacterial effect (lysozyme and salivary IgA) 4. Dissolving molecules (taste buds) 5. Helps in swallowing, speech and oral health PHARYNX AND ESOPHAGUS  Pharynx – cavity at rear of throat  Common passageway for digestive and respiratory tract  Swallowing: All-or-none reflex  Oropharyngeal stage  Initiated by tongue voluntarily pushing bolus of food to pharynx  Note functions of uvula, tongue, glottis and epiglottis  Esophageal stage  Primary peristaltic wave sweeps from the beginning to end of the esophagus, forcing the bolus ahead of it toward the stomach  SWALLOWING Video on Swallowing Nasal passages Uvula Pharynx Glottis at entrance Epiglottis of larynx Fig 16-5; Sherwood STOMACH Esophagus Fundus Gastroesophageal  J-shaped chamber sphincter 1. Stores food 2. Begins protein Pyloric Body digestion sphincter 3. Mixing movements produces chyme Pyloric Duodenum antrium GASTRIC STORAGE, MIXING AND EMPTYING 1. Storage  Body of stomach  Receptive relaxation – Expansion of stomach cavity with little change in intragastric pressure 2. Mixing  Mainly in the pyloric antrium where the peristaltic contractions are stronger and more vigorous  Retropulsion – Churning action breaks the food into smaller pieces producing chyme 3. Emptying  Controlled propulsion of chyme into the duodenum with each peristaltic wave GASTRIC EMPTYING AND MIXING Fig 16-7; Sherwood DIGESTION IN STOMACH GASTRIC MOTILITY AND EMPTYING Table 16-2; Sherwood VOMITING  Not a function of retroperistalsis in the stomach  Contraction of respiratory muscles and abdominal muscles  Stomach, esophagus and sphincters relaxed  Controlled by vomiting center (area postrema – medulla of brainstem)  Causes - throat stimulation, irritation of stomach, elevated intracranial pressure etc  Body and Fundus GASTRIC SECRETION Table 16-3; Sherwood Pyloric atrium GASTRIC ACID SECRETION  Hydrochloric acid is secreted by parietal cells and activates pepsinogen  Mechanism of H+ and Cl- Secretion 1. CO2 +H20 ↔ HCO3- + H+ 2. H+-K+ ATPase pump 3. Luminal K+ channel allow K+ to passively leak back 4. Cl--HCO3- antiporter 5. Cl- channel Fig 16-8; Sherwood FUNCTIONS OF HCL autocatalysis  Formation of pepsin Pepsinogen Pepsin  Breakdown of connective tissue and HCl muscle fibers Protein (polypeptide) Peptide fragments  Denaturation of protein  Killing of microorganisms CONTROL OF GASTRIC SECRETION 1. Cephalic Phase  Prepare stomach for arrival of food 2. Gastric Phase  Increased secretion started in previous phase  Initiates digestion of proteins 3. Intestinal Phase  Control rate of gastric emptying See Table 16-4 and 16-5 of Sherwood CEPHALIC PHASE  Prepare stomach for arrival of food  Short phase (minutes)  Directed by CNS via vagus nerve and submucosal plexus  Increased production of gastric juice and release of gastrin Fig 24-15; Martini GASTRIC PHASE  Increased secretion of gastric juice  Long phase (3 – 4hr)  Stimulation of stretch and chemoreceptors  Effects of gastrin  Release of histamine and its local effect  Sustained increased production of gastric juice and increased motility (mixing) Fig 24-15; Martini INTESTINAL PHASE  Control rate of gastric emptying  Long duration  Distension of duodenum trigger enterogastric reflex  Stimulation of CCK,GIP and secretin release  Feedback inhibition of pepsinogen and HCl production and gastric motility Fig 24-15; Martini GENERAL EFFECT OF SOME GI HORMONES ON GASTRIC FUNCTION  Gastrin  ↑ HCl, pepsinogen, gastric motility  Secretin, Gastric inhibitory peptide (GIP) and Cholecystokinin (CCK)  ↓ Gastric motility and secretion  These hormones have other effects on the digestive system which will be covered later INTESTINAL PHASE  Distension of duodenum trigger enterogastric reflex  ↓ gastric contraction  ↓ gastrin production  Arrival of chyme in duodenum (lipids, carbohydrates, low pH) trigger release of hormones  Secretin, CCK etc  ↓ gastric contraction  ↓ gastric secretions FUNCTIONS OF THE LIVER Bile salts 1. Secretion of bile salts RBCs Process nutrients 2. Metabolic processing of nutrients 3. Detoxifying or degrading 4. Synthesizing plasma proteins Cholesterol Detoxifying Bilirubin 5. Storing glycogen, fats, vitamins etc 6. Activating vitamin D 7. Hormone secretion Hormone Produce proteins 8. Excrete cholesterol and bilirubin 9. Remove bacteria and worn-out RBCs Vit D Store nutrients LIVER BLOOD FLOW The liver receives blood from two sources: 1a Arterial blood, which provides the liver’s O2 supply and carries blood-borne metabolites for hepatic processing, is delivered by the hepatic artery. 1b Venous blood draining the digestive tract is carried by the hepatic portal vein to the liver for processing and storage of newly absorbed nutrients. 2 Blood leaves the liver via the hepatic vein. BILE  Bile is continuously secreted by the liver (into the duodenum) and is diverted to the gallbladder between meals  Contains bile salts, cholesterol, lecithin, bilirubin etc  Does not contain digestive enzymes  Bile is concentrated in the gallbladder  Fig 16-15; Sherwood ENTEROHEPATIC CIRCULATION  Bile salts are recycled through the enterohepatic circulation  Bile salts: derivatives of cholesterol  Enterohepatic circulation: recycling of bile salts between the small intestine and the liver FUNCTIONS OF BILE  Aid fat digestion and absorption  Detergent action of bile salts: bile salts’ ability to convert large fat globules into a lipid emulsion consisting of many small fat droplets suspended in the aqueous chyme Fig 16-16; Sherwood FUNCTIONS OF BILE  Aid fat digestion and absorption  Formation of micelles: bile salts and lecithin aggregate in small clusters with their fat- soluble parts huddled together in the middle to form a hydrophobic (“water-fearing”) core Fig 16-17; Sherwood FUNCTIONS OF BILE  Excretion of water insoluble substances  Cholesterol and the waste product bilirubin for example  Bilirubin has no role in digestion  Bile salts stimulate bile secretion  CCK promotes gallbladder emptying (contraction of gallbladder) and relaxation of Sphincter of Oddi /Hepatopancreatic Sphincter (HPS) Gallbladder contraction ↑ CCK ↑ Plasma ↑ bile flow into secretion CCK duodenum HPS Relaxation  Hepatitis and cirrhosis are the most common liver disorders PANCREAS PANCREAS  The pancreas is a mixture of exocrine and endocrine tissue  Elongated gland that lies behind and below the stomach  Exocrine function  Pancreatic enzymes by the acinar cells  Aqueous alkaline solution Fig 16-10; Sherwood PANCREATIC ENZYMES  Pancreatic Amylase  Secreted in active form  Breaks down complex carbohydrates like starch and glycogen  Pancreatic Lipase  Main enzyme for fat digestion in GIT  Hydrolyses dietary triglycerides into monoglycerides and free fatty acids  Pancreatic Proteolytic Enzymes (Proteases)  Trypsin, chymotrypsin and carboxypeptidase  Secreted as inactive forms (trypsinogen, chymotrypsinogen and procarboxypeptidase)  Trypsinogen activated by enteropeptidase  Trypsin activates other enzymes and is also autocatalytic PANCREAS  Aqueous alkaline solution  Pancreatic enzymes optimal in neutral or slightly alkaline environment  Neutralize acidic chyme  High concentration of bicarbonate (HCO3-)  Pancreatic insufficiency  Deficient of pancreatic enzymes  Dietary fat digestion is seriously impaired HORMONAL CONTROL OF PANCREATIC SECRETION Fig 16-12; Sherwood OVERVIEW OF THE GI HORMONE  Gastrin  CCK  Stimulated by protein in stomach  Stimulated by chyme in duodenum  ↑ secretion of HCl and  Inhibits gastric motility and secretion pepsinogen  Stimulates pancreatic enzyme  Enhances gastric motility secretion  Secretin  Stimulates contraction of gallbladder and relaxation of hepatopancreatic  Stimulated by acid in duodenum sphincter  Inhibits gastric emptying and  Gastric inhibitory peptide (GIP) gastric secretion  Stimulates pancreas to produce  Promotes metabolic processing of HCO3- nutrients once they are absorbed Major hormones of the duodenum (Figure 24-22; Martini) FUNCTION OF MAJOR DIGESTIVE TRACT HORMONES (Figure 24-23; Martini) REFERENCES  Chapter 16. Sherwood, L. (2016) Human Physiology: From Cells to Systems. 9th edition. Cengage  Chapter 24. Martini, F. H., Nath, J.L., & Bartholomew, E. F. (2018). Fundamentals of Anatomy and Physiology. 11th Global Edition. Pearson. HSC1008 ANATOMY AND PHYSIOLOGY 2 DIGESTIVE SYSTEM 2 ANDY LEE (PHD) ASSISTANT PROFESSOR FACULTY HALL #04-17 OFFICE PHONE: 6592 2524 [email protected] LEARNING OUTCOMES  At the end of the lesson, you should be able to:  Discuss the main functions of the small and large intestines  Discuss the digestion and absorption of carbohydrates, proteins, fats, water, salts, vitamins and minerals  Discuss the control of defecation SMALL INTESTINE  3 parts  Duodenum, Jejunum and Ileum  Site where most digestion and absorption takes place  ~ 5 – 6 meters long  Motility  Peristalsis  Segmentation contractions PERISTALSIS  Waves of muscular contractions  Moves the bolus along the length of the digestive tract  Oral to anal direction  Myenteric plexus is essential for this Fig. 24-4; Martini PERISTALSIS https://youtu.be/o18UycWRsaA SEGMENTATION CONTRACTION  Mixes and slowly propels the chyme  Oscillating, ringlike contractions every few centimeters  Functions  Mixing chyme with digestive juices  Exposing chyme to all absorptive surfaces  Higher rate at the start of small intestines, duodenum (12/min); and lower rate at the end, terminal ileum (9/min) Fig 16-18; Sherwood SEGMENTATION CONTRACTION https://youtu.be/hKQ8eFpUKLs MYENTERIC PLEXUS  Between longitudinal and circular muscle layers  Controls mainly gastrointestinal movements  Stimulated by receptors in digestive tract (recall) Fig. 24-5; Martini SMALL INTESTINE – MOTILITY  The migrating motility complex sweeps the intestine clean between meals  The ileocecal juncture prevents contamination of the small intestine by colonic bacteria  Ileocecal valve and sphincter SMALL INTESTINE – SECRETIONS  Pancreatic enzymes perform most of the digestive activities  Enzymes secreted by the enterocytes in the small intestines are localized at the brush border  Enteropeptidase  Disaccharidases (Maltase, lactase, sucrose- isomaltase)  Aminopeptidases CARBOHYDRATES Fructose Glucose Galactose  Monosaccharides  Fructose, glucose and galactose  Disaccharides  Sucrose, maltose and lactose  Polysaccharides Sucrose Maltose Lactose  Starch, glycogen etc PROTEINS  Amino acids  Peptides  Short chain of amino acid monomers link by peptide bonds Peptide fragments  Polypeptides  Long chain of amino acids (>10)  Protein polypeptide  1 (>50 a.a) or more polypeptide chain SUMMARY FOR 3 MAJOR CLASSES OF NUTRIENTS ADAPTATION FOR ABSORPTION Fig 16-19; Sherwood MECHANISMS OF ABSORPTION  Diffusion  Simple diffusion  Monoglycerides and fatty acids etc  Facilitated diffusion  Fructose, amino acids etc  Active Transport  Primary active  Sodium  Secondary active  Glucose, amino acids and peptides etc MECHANISMS OF ABSORPTION  Diffusion  Simple diffusion  Monoglycerides and fatty acids etc  Facilitated diffusion  Fructose, amino acids etc  Active Transport  Primary active  Sodium  Secondary active  Glucose, amino acids and peptides etc Fig 3-14; Sherwood MECHANISMS OF ABSORPTION  Diffusion  Simple diffusion  Monoglycerides and fatty acids etc  Facilitated diffusion  Fructose, amino acids etc  Active Transport  Primary active  Sodium  Secondary active  Glucose, amino acids and peptides etc Fig 3-16; Sherwood MECHANISMS OF ABSORPTION  Diffusion  Simple diffusion  Monoglycerides and fatty acids etc  Facilitated diffusion  Fructose, amino acids etc  Active Transport  Primary active  Sodium  Secondary active  Glucose, amino acids and peptides etc Fig 3-17,18; Sherwood DIGESTION AND ABSORPTION IN SMALL INTESTINE CARBOHYDRATE DIGESTION AND ABSORPTION  Mainly starch and glycogen  Enzymes  Salivary amylase  Pancreatic amylase ---> disaccharides  Disaccharidases ---> monosaccharides  Cellulose (plant cell wall) is not digestible by humans  Major end-product is glucose CARBOHYDRATE DIGESTION Fig 16-22; Sherwood CARBOHYDRATE ABSORPTION Fig 16-22; Sherwood PROTEIN DIGESTION AND ABSORPTION  Start in the stomach  Pepsinogen --- gastric acid ---> pepsin  Proteins ---> polypeptides  Continue in the small intestines (main site of digestion is in the duodenum)  Trypsin, chymotrypsin ---> peptides  Aminopeptidases ---> amino acids PROTEIN DIGESTION Fig 16-23; Sherwood  PROTEIN ABSORPTION Primary Active Na+ / K+ Pump Secondary Active Na+ / Amino Acid Symport Na+ / H+ Antiport Tertiary Active H+ / Small peptides Symport Fig 16-23; Sherwood FAT DIGESTION Fig 16-24; Sherwood FAT ABSORPTION  Digested fat is absorbed passively and enters the lymph Fig 16-24; Sherwood ABSORPTION OF SODIUM CHLORIDE  Sodium Chloride  Na+ – K+ ATPase at the basolateral membrane creates the electrochemical gradient (low in the Na+ cell) 1. Through sodium channels 2. Co-transport carriers 1. Na+ – Cl- symporter 2. Na+ – H+ antiporter 3. Na+ – glucose symporter 4. Na+ – amino acid symporter ABSORPTION OF WATER  Passive absorption  Active reabsorption of electrolytes and nutrients create osmotic gradient  Na+ – K+ ATPase also creates a localised concentrated area of high osmotic pressure  In duodenum, water may flow into lumen due to hyperosmotic chyme  In jejunum and ileum, water is absorbed ABSORPTION OF VITAMINS  Water soluble vitamins  Vitamin B complex, Vitamin C etc  Mostly by carrier mediated mechanisms  Fat soluble vitamins  Vitamin A, D, E, K etc  Carried in the micelles and absorbed with other lipids  Vitamin B12  Absorbed by receptor-mediated endocytosis  Requires the formation of a complex by binding with gastric intrinsic factor ABSORPTION OF IRON AND CALCIUM  Iron and calcium absorption is regulated  Iron essential for hemoglobin production  Slightly < 10% of daily ingested iron (15-25mg) absorbed; women > man  Heme iron > Fe2+ (ferrous iron) > Fe3+ (ferric iron)  Vit C (Ascorbic acid) ↑ iron absorption by reducing Fe3+ to Fe2+.  Activation of Vit. D in the kidney or liver is required for the effective absorption of calcium  ~ 2/3 of daily intake of calcium (~1000mg) is absorbed SMALL INTESTINE  Most absorbed nutrients immediately pass through the liver for processing  Anything absorbed into the digestive capillaries first must pass through the hepatic biochemical factory  Lipid bypass this as they enter the lymphatic system  Extensive absorption by the small intestine keeps pace with secretion SMALL INTESTINE – MAINTAINING BIOCHEMICAL BALANCE  Biochemical balance among the stomach, pancreas, and small intestine is normally maintained  Body normally does not experience a net gain or loss of acid or base during digestion  Diarrhea results in loss of fluid and electrolytes  Passage of a highly fluid fecal matter, often with increased frequency of defecation  Dehydration, loss of nutrient and metabolic acidosis etc  1. Excessive small intestinal motility; 2. Excess osmotically active particle in lumen; 3. Toxins from Vibrio cholera etc LARGE INTESTINE Fig 16-26; Sherwood LARGE INTESTINE  The large intestine is primarily a drying and storage organ  Forms and store feces  Motility  Secretion  Mucus and some ions  Digestion  No but bacteria present help digest food and produce Vitamin K, Vitamin B12 and folate etc  Absorption  Water and ions MOTILITY  Contractions of the haustra slowly shuffle the colonic contents back and forth  Initiated by the rhythmic contractions of the colonic smooth muscles  Slow and nonpropulsive  Mass movements propel feces long distances into the distal part of the large intestines  Large segments of the ascending and transverse colon contract simultaneously DEFECATION REFLEX  Feces are eliminated by the defecation reflex  Initiated by distension of the rectum which stimulates stretch receptors in the rectal wall  Muscle contraction with relaxation of both the internal and external anal sphincter muscle  Voluntary straining movements increases intraabdominal pressure  Contraction of abdominal muscles  Forcible expiration against closed glottis DEFECATION REFLEX  2 positive feedback loops  Short intrinsic myenteric defecation reflex  Stretch receptors stimulate the plexus to initiate increased local peristaltic contractions in sigmoid colon and rectum  Parasympathetic defecation reflex (long reflex)  Stimulate mass movements in descending and sigmoid colon Fig. 24-26; Martini CONSTIPATION  When feces become too dry  Abdominal discomfort, dull headache, loss of appetite, nausea etc  Causes  Ignoring urge to defecate  Decreased colonic motility  Obstruction  Impairment of defecation reflex GASES AND SECRETIONS  Intestinal gases are absorbed or expelled  Flatus: gas passes out  Swallowed air  Bacterial fermentation  Large-intestine secretion is entirely protective  Colonic secretion consists of an alkaline (NaHCO3) mucus solution  The colon contains myriad beneficial bacteria  Microbiota and microbiome ABSORPTION OF WATER AND SALT  The large intestine absorbs salt and water, converting the luminal contents into feces  Some absorption takes place within the colon but not to the same extent as in the small intestine  Na+ actively absorbed  Cl- follow down electrical gradient  Water follows osmotically Fig. 24-28; Martini REFERENCES  Chapter 16. Sherwood, L. (2016) Human Physiology: From Cells to Systems. 9th edition. Cengage  Chapter 24. Martini, F. H., Nath, J.L., & Bartholomew, E. F. (2018). Fundamentals of Anatomy and Physiology. 11th Global Edition. Pearson.