Digestive System 2024 (4pm PDF)

Illustration from Fig. 24.04, Tortora & Grabowski “Principles of Anatomy and Physiology” 10th edition. John Wiley & Sons, New York. ISBN 978-0471415015 b...

Illustration from Fig. 24.04, Tortora & Grabowski “Principles of Anatomy and Physiology” 10th edition. John Wiley & Sons, New York. ISBN 978-0471415015 b 201 system Digestive 8 Topic Angela Tsai 4pm MEDSCI 142 COURSE GUIDE © 2024 4pm 202 MEDSCI 142 COURSE GUIDE © 2024  203 4pm Lecture 1 of 3 The Digestive System 1 Angela Tsai Materials from Lectures 1 and 2 are examinable in Theory Test 2. Introduction The gut is a soft muscular tube about 5 metres long in which food is stored, broken down into small molecules, and absorbed. Two large glands (pancreas and liver) are associated with the gut tube, and need to be considered with it. In this first lecture we discuss first the mouth, then outline some general features of the entire gut tube. This will give us the foundation for a detailed look at the tube region-by-region, beginning with the esophagus. Resources This topic is well covered in Tortora & Derrickson, Chapter 24. Intended learning outcomes (ILOs) After thoughtfully reviewing this session, you should be able to: 1. Label and describe the main parts of a tooth. 2. Briefly describe the papillae and intrinsic muscles of the tongue. 3. Name the three major salivary glands and describe the function of their secretions. 4. Describe methods by which surface area of the epithelium lining the gut tube may be increased. 5. List or label the four layers that generally make up the wall of the gut tube. 6. Describe the tissue components of each layer, and comment on their functions. MEDSCI 142 COURSE GUIDE © 2024 4pm 204 1 A typical tooth. T&D Fig 24.7 Enamel crystalline rods or prisms of calcium phosphate & carbonate no cells hardest tissue of the body no sensation Dentin similar to bone but cells (odontoblasts) occur nearby in the pulp instead of scattered through the dentin Pulp soft tissue blood vessels, nerves, lymphatics Cementum calcified connective tissue covering the root Periodontal ligament C. Quilter, University of Auckland a collagen fibres linking the bone of the socket (alveolar bone) to the cementum rapid turnover 2 Tongue C. Quilter, University of Auckland a 3 Salivary glands Major salivary glands: Mod. T&D Fig 24.6 three pairs of glands: parotid (serous only), sublingual (mixed, mostly mucous) and submandibular (mixed) Parotid secrete following parasympathetic Submandibular stimulation induced by seeing, smelling, Sublingual tasting or thinking about food saliva = water + mucus + enzymes Serous cells Mucous cells enzymes = amylase (breaks down starchy (watery) (viscous) debris around teeth) and lysozyme (antibacterial) MEDSCI 142 COURSE GUIDE © 2024  205 4pm 4 Organs of the digestive system. T&Db Fig 24.1 Digestion: chemical breakdown of ingested food into absorbable molecules. Requires secretion. Absorption: movement of nutrients, water and electrolytes through the epithelial lining of the gut into blood or lymph. Secretion: Transport: 5 Gaining surface area C. Quilter, University of Auckland a MEDSCI 142 COURSE GUIDE © 2024  4pm 206 6 Four tunics of the gut tube see also T&D Fig 24.2 Throughout the length of the gut tube we can recognise four tunics making up the wall. The example shown below is the small intestine. Other regions of the tube differ principally in the specialised features of the mucous membrane; the outer three tunics are less variable. Beginning at the lumen (inside of the tube) the four tunics are: Mucosa (mucous membrane) consisting of: epithelium, (specialised for protection or absorption or secretion or combinations of all three). lamina propria, (a soft fibrous bed of loose connective tissue on which the epithelium rests; carries nerves and blood capillaries, populated with defensive cells). muscularis mucosae, (two thin layers of smooth muscle, inner circular and outer longitudinal). This provides the mucosa with some movement independent of the external muscle coat (e.g. contractions squeeze secretions from glands or lymph along lacteals). Submucosa, a thick bed of loose connective tissue carrying larger blood vessels, lymphatic vessels and nerves (submucosal plexus). It connects the mucosa to the external muscle coat, but allows some movement between the two. Muscularis externa (external smooth muscle), in two layers to produce peristalsis. The inner layer is circular, the outer is longitudinal. The myenteric nerve plexus occurs between the layers. Serosa, a slippery outer covering for the gut tube (except oesophagus). It is two- layered, with outer mesothelium sitting on a bed of connective tissue. The serosa is also known as the visceral peritoneum (refer lab 1 notes). Where a structure is not in contact with the body cavity, the outermost connective tissue layer is referred to as the adventitia. C. Quilter, University of Auckland a MEDSCI 142 COURSE GUIDE © 2024  207 4pm Lecture 2 of 3 The Digestive System 2 Angela Tsai Introduction The stomach is a storage bag which during the last 400 million years has taken on a few digestive functions. Food enters it in large lumps, fragmented by chewing; but leaves it as a steady trickle released through the pyloric sphincter into the intestine. The liver is an enormous outgrowth of gut epithelium which acts partly as an excretory organ (of bile), partly as a nutrient storage-and-release organ, and partly as a blood-purifying organ. In order to carry out their many roles the liver cells (hepatocytes) need good access to two blood streams, one carrying oxygen and the other nutrients. Every cell also needs access to a bile drainage system. This makes for a complex microscopic architecture. Resources This topic is well covered in Tortora & Derrickson, Chapter 24. Intended learning outcomes (ILOs) After thoughtfully reviewing this session, you should be able to: 1. Describe the main functions of the digestive system and explain how these vary along the length of the gut tube. 2. Describe the ways in which the standard gut tube is modified in the esophagus, and relate this to its function. 3. Describe or label gross anatomical features of the stomach, including its four regions. 4. Describe or label gastric pits and glands of the mucosa, and comment on the cell types which occur there. Explain how these contribute to the functions of the stomach 5. Briefly list the major functions of the liver. 6. Explain using notes or a diagram how hepatocytes are placed in relation to the blood and bile spaces around them. 7. Explain or label the structure of one liver lobule and the vessels which convey blood or bile to or from it. MEDSCI 142 COURSE GUIDE © 2024 4pm 208 1 Oesophagus AP 3rd ed. pp. 1163-1164 AP 2nd ed. pp. 1329-1330 14th ed. p. 899 13th ed. p. 980 12th ed. p. 934 A muscular tube about 25cm long, extending from pharynx to stomach. Travels posterior to trachea. The tube is normally empty with its lumen collapsed, and expands to accommodate food / water. Functions of the esophagus transport (travel time 5 seconds for food, 1 second for fluid, due to peristalsis) protection no absorption, little secretion, no digestion Four layers of the gut tube all as usual, except: Epithelium of mucous membrane thick, many-layered (stratified squamous) with sacrificial outer layers for protection against abrasive fragments of food cells are replaced by division in basal layers, then slow migration outwards. Old cells are shed from the surface. Entire epithelium is renewed every 7 days. Stratified squamous epithelium C. Quilter, University of Auckland a External muscle contains, in addition to the usual smooth muscle, some skeletal muscle in the upper third of the esophagus to allow rapid contraction and voluntary control of swallowing Serosa over most of its length the esophagus does not lie in a body cavity, so lacks a serosa. Instead it is covered with a fibrous adventitia which attaches it to neighbouring organs eg. trachea MEDSCI 142 COURSE GUIDE © 2024  209 4pm 2 Stomach AP 3rd ed. pp. 1167-1168 AP 2nd ed. pp. 1332-1334 14th ed. pp. 901-902 13th ed. p. 982 12th ed. p. 937 A J-shaped bag on the left side, an enlargement of the gut tube. Capacity about 1.5 litres. Primary function is storage, since food can be eaten more quickly than it can be digested and absorbed. Four regions (cardia, fundus, body, pylorus) Well developed muscular sphincter at outlet (pyloric sphincter) When empty, lined with longitudinal folds (rugae) Four layers of the gut tube all as usual in the stomach, except: Epithelium of mucosa forms many pits lined with mucus-secreting cells, plus gastric glands which open into the pits External muscle three layers rather than two, (with addition of innermost oblique layer) Functions of the stomach Storage Secretion of acid, enzymes, mucus (total volume 2 - 3 litres per day). Food + gastric juice = chyme Digestion of proteins by pepsin Absorption of water, ions, some drugs (aspirin, alcohol) Protection (against its own secretions and microbes) Transport (mixing waves, every 20 sec) T&D Fig 24.11a b Fundus Cardia (mostly mucous glands) Secrete acid, enzymes, and mucus Body Pylorus (mostly mucous glands) MEDSCI 142 COURSE GUIDE © 2024  Surface mucous cells Secrete insoluble alkaline mucus which protects the mucosa from 4pm acid and pepsin. Undifferentiated cells Stem cells dividing to generate new epithelium. Parietal cells 3 Mucosa of the stomach Secrete HCl which kills microbes and living cells. Also secretes MEDSCI 142 COURSE GUIDE © 2024 intrinsic factor. Mucous neck cells Secrete soluble acid mucus at mealtimes. Cell lineage 210 Chief cells and migration, Secrete pepsinogen and gastric (just for interest) See also T&D Fig 24.12 lipase. Pepsinogen is converted into the protein-splitting enzyme pepsin by acid in the lumen of the MOVE gland. UP Surface mucous cell Enteroendocrine cells Gastrin enters blood stream as a hormone. (1) stimulates secretion of acid and pepsinogen; (2) increases muscular contractions of stomach; Note: in this diagram the size of the epithelial (3) relaxes the pyloric sphincter. Parietal Stem cell cell cells is exaggerated for clarity, and many small blood vessels and nerves are omitted. Mucous neck cell MOVE DOWN Chief cell C. Quilter, University of Auckland a  211 4pm 4 Liver AP 3rd ed. pp. 1177-1178 AP 2nd ed. pp. 1344-1345 14th ed. p. 909 13th ed. p. 990 12th ed. p. 945 The liver is an enormous gland, made of epithelial cells (hepatocytes) derived from embryonic endoderm. See T&D Fig 24.15 for its relations to the gall bladder, duodenum and pancreas. All liver functions are carried out by hepatocytes which are multi-talented cells, each performing more than 500 different metabolic functions, including: glycogen/glucose storage and release, recycling of red blood cells, bile synthesis and secretion, synthesis of plasma proteins and removal of toxins from blood. See T&D for a full list of liver functions. Every hepatocyte requires: access to nutrient-laden blood drained from the intestinal wall access to oxygentated blood from the systemic circuit access to ducts which drain bile to the gall bladder 5 Hepatocytes C. Quilter, University of Auckland a Imagine each hepatocyte as a brick......... Blood Lymph Bile Fenestrated endothelial cell of liver sinusoid. These cells act as a filter and allow passage of the watery Lymph space part of the blood (lymph) but exclude red blood cells. Bile of Disse canaliculus Tight junction Sinusoid Microvilli Hepatocyte Endothelial cell MEDSCI 142 COURSE GUIDE © 2024 4pm 212 6 Liver lobules see also T&D Fig 24.15 Plates of hepatocytes are stacked together forming liver lobules. Each lobule is hexagonal in cross- section, about 2mm long by 1mm wide. C. Quilter, University of Auckland a Liver lobules are hexagonal. The diagram at right shows cross- sections through two lobules Incoming oxygenated systemic blood (hepatic artery) Incoming nutrient-laden blood (hepatic portal vein) Outgoing blood (central vein) Outgoing bile (canaliculus leading into bile duct) Nuclei of hepatocytes Intercellular bile canaliculi Fig 25.21: Liver histology Human Anatomy, 7th ed., pp. 684 | Martini, Timmons & Tallitsch, 2009 Pearson Benjamin Cummings, San Francisco. ISBN 978-0321-50042-7 MEDSCI 142 COURSE GUIDE © 2024  213 4pm Lecture 3 of 3 The Digestive System 3 Angela Tsai Introduction In this session we begin by discussing the pancreas, another epithelial gland which releases its important secretions downstream of the stomach. Then we move on to the intestine where most of the body’s digestive and absorptive activities takes place. Resources This topic is well covered in Tortora & Derrickson, Chapter 24. Intended learning outcomes (ILOs) After thoughtfully reviewing this session, you should be able to: 1. Write brief notes about the exocrine function of the pancreas. 2. Explain or label the exocrine acini and their branching system of ducts 3. List the three regions of the small intestine, and describe their main functions. 4. Explain the ways in which the standard gut tube is modified in the intestine, and identify the adaptations which increase surface area. 5. Describe or label intestinal villi and glands, and comment on the cell types found there. 6. List the seven regions and describe the main functions of the large intestine. 7. Explain the ways in which the standard gut tube is modified in the colon, especially the external muscle coat. 8. Describe or label mucosal glands of the colon, and comment on the cell types found there. MEDSCI 142 COURSE GUIDE © 2024 4pm 214 1 Pancreas AP 3rd ed. pp. 1174-1175 AP 2nd ed. pp. 1341-1343 14th ed. pp. 906-907 13th ed. p. 988 12th ed. p. 942 The pancreas is a dual-function organ: an exocrine gland manufacturing the precursors of digestive enzymes and secreting them as alkaline “pancreatic juice” via a duct system leading to the duodenum; (it is the exocrine function which is important for this lecture). Enzyme precursors are converted to their active form once they arrive in the duodenum. Most food substances are digested by pancreatic enzymes (proteins, carbohydrates, lipids and nucleic acids). an endocrine gland. Islets of Langerhans make up 1% of the pancreas and secrete hormones into the bloodstream. The hormones (insulin & glucagon) regulate blood glucose levels. Intercalated duct (“twig”) If we compare the pancreas to a tree......... each leaf is a secretory unit (acinus) Acinus.... the duct system forms (one “leaf”) twigs and branches Secretory cell (serous type).... and the main pancreatic duct is the trunk Acini (“leaves”) Gallbladder Common bile duct Intercalated ducts Spleen (“twigs”) Pancreas Interlobular ducts (“small branches”) Main pancreatic duct Duodenum (“trunk”) Images C. Quilter, University of Auckland a MEDSCI 142 COURSE GUIDE © 2024  215 4pm 2 Small intestine T&D AP 3rd ed. pp. 1183-1184 AP 2nd ed. pp. 1350-1351 14th ed. pp. 913-914 13th ed. p. 995 12th ed. p. 949 The small intestine is a tube about 3 cm in diameter and a little over 3m long. Most digestion and absorption occurs here. At its upper end it receives exocrine secretions from the liver (i.e. bile, stored in the gallbladder) and pancreas (pancreatic juice, containing digestive enzyme precursors). Three regions: duodenum (L duodeni = 12) refering to its length in finger-breadths, ~25 cm long. C-shaped tube, not suspended by mesentery, receives biliary and pancreatic ducts. jejunum (L jejunus = empty), ~1 m long. ileum (L ileum = twisted), ~2 m long. Four layers of the gut tube are all as usual, except: the mucosa is specialised to greatly increase the surface area available for secretion (required for digestion) and absorption. the submucosa just downstream of the pyloric sphincter contains mucous glands (glands of Brunner). 3 Increasing surface area Orders of folding to increase surface area (note the differences in scale): gross convolutions plicae (“ply-see”) are circular folds ~1 cm high. Each plica is covered with mucosa and has a core of submucosa. villi (“vill-eye”) are mini finger-like projections ~1 mm high. Covering is epithelium, core is lamina propria. microvilli are microscopic projections ~1 micrometre high. They form a “brush border” on the surface of individual absorptive cells. Each microvillus is covered with cell membrane and filled with cytoplasm. MEDSCI 142 COURSE GUIDE © 2024 4pm 216 4 Mucosa of the small intestine See also T&D Fig. 24.19 Columnar absorptive cells, (enterocytes) Absorb the small molecules resulting from digestion Goblet cells Secrete mucus Enteroendocrine cells Secrete hormone secretin (and others) into capillaries of the lamina propria Undifferentiated cells Stem cells dividing to generate new epithelium Paneth cells Secrete bactericidal enzyme lysozyme and are phagocytic Transport of carbohydrates (absorbed as monosaccharides), proteins (absorbed as amino acids), water and electrolytes. Venules in the submucosa are tributaries of the hepatic portal vein. Transport of absorbed lipids. Lymphatic vessels eventually drain into the venous system. C. Quilter, University of Auckland a Note: The epithelium is short-lived. Cell division occurs deep in the glands, then the entire sheet (except Paneth cells) moves slowly up the gland walls and up the sides of the villi. Old cells are shed from the villous tip. The entire journey lasts ~5 days. The lymph vessel within each villus = lacteal. Lymph is “milked” along the lacteal by contraction of smooth muscle fibres in the lamina propria which shorten the villus. These fibres arise from the muscularis mucosae. Release of secretin from enteroendocrine cells is stimulated by arrival of acid chyme. Secretin in bloodstream stimulates release of pancreatic juice. Cholecystokinin similar. Gastrin is secreted by enteroendocrine cells of small intestine and stomach; it stimulates gastric glands to secrete acid and enzymes. MEDSCI 142 COURSE GUIDE © 2024  217 4pm 5 Large intestine T&D Fig. 24.23a A soft muscular tube about 1.5 m long with 7 parts. Its functions are: absorption of salts and water, (1 litre/day compared with 8 litre/ day absorbed in the small intestine) conversion of chyme into feces. Bacteria ferment remaining carbohydrates bacteria produce some vitamins (B & K) which are absorbed secretion of mucus to lubricate feces defecation Note: the ileocecal valve controls intermittent flow of chyme from ileum into cecum. the cecum is a dilated pouch, (much larger in other mammmals, especially herbivores). Here bacteria (not enzymes) are responsible for digestion. Humans produce no enzymes capable of digesting the cellulose of plants. the appendix is a vestigial worm-like appendage of the cecum. faeces contain bacteria (30%), undigested dietary fibre (30%), cells shed from the intestinal lining, and mucus. Their colour is from pigments derived from bacterial decomposition of bilirubin. Four layers of the gut tube all as usual in the large intestine, except: Mucosa has no villi but many intestinal glands (also called “crypts of Lieberkuhn”). surface epithelial cells are enterocytes (absorptive) similar to small intestine. intestinal glands contain mostly goblet cells (mucus), no Paneth cells. cell renewal as in the small intestine, entire epithelium replaced every 4-5 days. many clusters of lymphocytes (lymphoid nodules) in the lamina propria, reflecting large bacterial content of colon. External muscle outer longitudinal muscle is thickened in three strips. The strips (teniae coli) contract to pull the intestinal tube into sac-like pockets (haustra coli). Haustra change shape and position. MEDSCI 142 COURSE GUIDE © 2024 4pm 218 6 Mucosa of the colon See also T&D Fig. 24.24 C. Quilter, University of Auckland a Columnar absorptive cells, (enterocytes) Similar to those in the small intestine, absorbing mostly water Goblet cells Secrete mucus to lubricate the passage of faeces Undifferentiated cells Stem cells dividing to generate new epithelium White blood cells The colonic glands are deeper, and Mostly lymphocytes which provide defence packed together more closely than against bacteria invading from the lumen of this drawing indicates. the colon 7 Rectum and anus T&D Fig. 24.23b The rectum makes up the final ~20 cm of the gut tube. Its last 2 cm are called the anal canal. The canal is closed by two sphincters. The inner one is an involuntary smooth-muscle sphincter; the outer one is under voluntary control (skeletal muscle). The urge to defecate is felt when the rectum fills to about 25% of its capacity. Stretching of the rectal wall initiates a reflex contraction of teniae coli in the descending colon and rectum. Shortening of this part of the gut tube further increases pressure in the rectum. The internal anal sphincter relaxes (opens) as part of the reflex. If the time is convenient the external sphincter is then relaxed voluntarily; if not it is maintained in the contracted state and the defecation reflex subsides. Children take some time to learn how to control the external sphincter. MEDSCI 142 COURSE GUIDE © 2024  Summary: Regional variations Mucosa Epithelium Lamina Propria Muscularis mucosae Submucosa 219 Muscularis externa Serosa / adventitia Microvilli Modifications involving the mucosa Modifications involving the submucosa Gross convolutions 4pm MEDSCI 142 COURSE GUIDE © 2024 4pm 220 MEDSCI 142 COURSE GUIDE © 2024