Griffith College Lecture Notes - Digestive System PDF

LECTURE 6 The Digestive System 1: Structure & Function of the Digestive System 1808NRS: Human Anatomy & Physiology 2 Gold Coast Campus 1 Learning activities In addition to viewing this lecture, please: Complete...

LECTURE 6 The Digestive System 1: Structure & Function of the Digestive System 1808NRS: Human Anatomy & Physiology 2 Gold Coast Campus 1 Learning activities In addition to viewing this lecture, please: Complete Course Content: Topic 3.1 – Structure & function of the ✔ Digestive System ✔ Review with Tutorial Workbook 4: Digestive System ✔ Complete required readings ✔ Attend and participate in your tutorial & lab 2 1  Lecture outcomes By the end of this lecture, you should be able to: Identify ALL of the anatomical & histological structures of the GIT Describe the physiological processes involved in digestion, absorption and excretion Describe the factors involved in motility throughout the GIT Discuss the various secretions produced throughout the GIT and their functional role Describe the digestion, absorption and functional importance for the major nutrients and vitamins 3 Terminology Mucosa Serosa Mesentery Duodenum Jejunum Ileum Caecum Peritoneum Villi ‘ase Colon 4 2  General introduction Terms digestive system and gastrointestinal system use interchangeably Gastrointestinal (GI) tract, also called alimentary canal, includes: mouth, pharynx, oesophagus, stomach, small and large intestine Accessory digestive organs: teeth, tongue, salivary glands, liver, gallbladder and pancreas Functions ◦ ingestion (intake) ◦ secretion (digestive enzymes, mucus) ◦ mixing and propulsion ◦ digestion (breakdown) ◦ absorption of nutrients, minerals & water ◦ defecation (elimination) Pg 917 5 General introduction (2) Parotid gland Mouth (oral cavity) Sublingual gland Salivary Tongue Submandibular glands gland Oesophagus Pharynx Stomach Pancreas Pg 916 Liver (Spleen) Gallbladder Transverse colon Duodenum Descending colon Small Jejunum Ascending colon intestine Ileum Cecum Large Sigmoid colon intestine Rectum Appendix Anus Anal canal 6 3  General Histology of GI tract 1. Mucosa - lines the lumen; composed of Epithelium (different types in different locations but mostly simple columnar epithelium and mucus-secreting cells lamina propria (loose areolar connective tissue with capillaries for nourishment and absorption, lymphatics MALT (Mucosa- Associated Lymphatic Tissue) Muscularis mucosae (smooth muscle that produces local movements of mucosa) 2. Submucosa - dense connective tissue with glands, blood/lymph vessels (MALT); also submucosal plexus (neural network) – controls secretion 3. Smooth muscle - responsible for segmentation and peristalsis; also contains myenteric plexus (neural network) – controls motility 4. Serosa - visceral peritoneum 7 General Histology of GI tract (2) 1. Mucosa - lines the lumen; composed of Epithelium (different types in different locations but mostly simple columnar epithelium and mucus- secreting cells (goblet cells)) lamina propria (loose areolar connective tissue with capillaries for nourishment and absorption, lymphatics MALT (Mucosa- Associated Lymphatic Tissue) Muscularis mucosae (smooth muscle that produces local movements of mucosa) 2. Submucosa - dense connective tissue with glands, blood/lymph vessels (MALT); also submucosal plexus (neural network) – controls secretion 3. Smooth muscle - responsible for segmentation and peristalsis; also contains myenteric plexus (neural network) – controls motility 4. Serosa – Also called visceral peritoneum, outermost layer of GIT connecting to the peritoneal cavity 8 4  General Histology of GI tract (2) Pg 920 9 General Histology of GI tract (2) 10 5  General Histology………. Keeping in mind the 4 layers of tissue in the GIT.. For nutrient absorption to occur, substances would need to cross which layer/s? Mucosa Sub-mucosa Smooth muscle – muscularis externa Serosa 11 General Histology………. Keeping in mind the 4 layers of tissue in the GIT.. For nutrient absorption to occur, substances would need to cross which layer/s? Mucosa Sub-mucosa Smooth muscle – muscularis externa Serosa 12 6  Peritoneum Serous epithelial membrane that covers the wall of the abdominal cavity as well as abdominal organs on the outside Two layers: parietal peritoneum – covers abdominal wall visceral peritoneum – covers outer surface of abdominal organs Peritoneal cavity – space between parietal and visceral peritoneum Peritoneal folds: mesentery (small intestines suspended on it) mesocolon (colon suspended on it) greater omentum 13 Peritoneum/peritoneal cavity eg pancreas 14 7  Mouth: Ingestion & digestion Mouth = oral cavity Main parts: ◦ cheeks ◦ hard and soft palate (form roof) ◦ uvula ◦ tongue ◦ teeth Pg 922 Oral cavity is lined with stratified squamous epithelium which is kept moist by saliva Site for mechanical and chemical digestion 15 Salivary glands: Chemical digestion Saliva: 1-1.5L produced by salivary glands (parotid, submandibular and sublingual) contains H2O, enzyme = salivary amylase (starts starch breakdown), IgA & lysozymes etc… Salivation is triggered by taste, smell, sight or thought of food Salivation stimulated by parasympathetic nervous system Pg 924 16 8  Teeth: mechanical digestion Three parts: crown, neck and root(s) * Gingivitis & Candida Upper and lower jaws are covered by gingivae (gums) Teeth types ◦ incisors - chisel shaped for cutting ◦ canines - fanglike for tearing ◦ premolars & molars - for crushing & grinding Pg 926 17 Digestion in the mouth Mechanical digestion: mastication or chewing breaks food into smaller pieces mixes pieces of food with saliva Chemical digestion: breakdown of food through enzyme activity Enzyme catalyses the hydrolytic breakdown of complex polysaccharides into simpler ones + molecule of water the main enzyme in the mouth is amylase - begins starch (glu α1-6 homopolysaccharide) digestion into simpler sugars this digestion stops in the stomach since stomach acidity destroys amylase Why is amylase destroyed in the stomach? 18 9  Digestion in the mouth Mechanical digestion: mastication or chewing breaks food into smaller pieces mixes pieces of food with saliva Chemical digestion: breakdown of food through enzyme activity Enzyme catalyses the hydrolytic breakdown of complex polysaccharides into simpler ones + molecule of water the main enzyme in the mouth is amylase - begins starch (glu α1-6 homopolysaccharide) digestion into simpler sugars this digestion stops in the stomach since stomach acidity neutralises (destroys) amylase Why is amylase destroyed in the stomach? Amylase = enzyme = protein. Proteins are denatured by acid 19 Oesophagus Muscular tube (25cm) that Pg 930 connects pharynx and stomach Upper and lower oesophageal muscular sphincters control passage of food Passes through diaphragm at the opening called oesophagus hiatus Function: transports food into the stomach during swallowing = Peristalsis swallowing (deglutition) is aided by presence of moist surface due to production of some mucus and swallowing of saliva 20 10  Anatomy of stomach Regions of stomach: cardia, fundus, body, pyloric region (opening into duodenum controlled by circular pyloric sphincter muscle) Stomach is located under diaphragm in an area known as left Pg 931 hypochondrium Layers: mucosa - simple columnar epithelium; has large folds known as rugae that make the surface rough (to increase the mixing and mechanical breakdown effects) Submucosa – blood vessels smooth muscle arranged in several layers (3 layers) serosa – smooth visceral peritoneum 21 Histology of the stomach Gastric mucosa is composed of simple columnar epithelium that folds inside the wall forming numerous gastric glands opening on mucosal surface via gastric pits Most cells in the gastric mucosa are mucus cells that produce mucus (*Neurofen & Helicobactor) Chief cells within gastric glands produce enzymes pepsinogen (to be activated into pepsin) and gastric lipase Parietal cells within gastric glands produce hydrochloric acid (HCl) and intrinsic factor (protein required for absorption of vitamin B12…… *Pernicious anemia) G cells at the bottom of gastric glands are endocrine – produce hormone gastrin  stimulates release of more gastric juice and increases gastric motility 22 11  Intrinsic Factor  Aids in Vit B12  increase Fe2+ absorption Gastrin  stimulate the other three stomach cells 23 Digestion in the stomach Mechanical digestion achieved through peristaltic movements (mixing waves) food turned into thick soupy liquid called chime Chemical digestion hydrochloric acid from parietal cells: o kills microbes; denatures (unfolds) proteins from food making enzymatic proteolysis easier; transforms pepsinogen (inactive enzyme) into pepsin, inactivates salivary amylase pepsin (proteolytic enzyme): o breaks down protein chains into smaller peptide fragments o most effective in the very acidic environment (pH=2) 24 12  Gastric motility and emptying Pyloric Pyloric valve Pyloric valve closed valve slightly closed opened Pg 939 Grinding Retropulsion Propulsion The most vigorous The pyloric valve delivers small Peristaltic waves move from peristalsis and amounts of chyme into the the fundus toward the pylorus. mixing action occur duodenum, while forcing most of close to the pylorus its contained material backward into the stomach 4-6hrs empty time; fats slows emptying 25 Regulation of gastric secretion/motility *Relates to Nerves & Hormones Cephalic phase starts with sight, smell, taste, thought of food (hypothalamus) parasympathetic activation leads to stimulation of gastric secretion and motility (via cranial nerve X – vagus) – increased gastric secretions Gastric phase activated via stretch receptors and chemoreceptors (monitor pH) in the stomach after the food comes in outcomes: increased peristalsis (mixing), release of gastrin  increased gastric secretions Intestinal phase enterogastric reflex and hormones enterogastrones (secretin, cholecystokinin and vasoactive intestinal peptide) decrease stomach secretions, motility and emptying – inhibits gastric secretions 26 13  Anatomy of the small intestine Around 6-7 m long and 2-4 cm in diameter Very large surface area where most of nutrient Stomach absorption occurs Duodenum Duodenojejuna Mesentery flexure Small intestine has 3 sections: Ascending colon ◦ Duodenum Ileocecal junction Jejunum ◦ Jejunum Cecum Appendix ◦ Ileum Ileum ◦ Pyloric sphincter to ileocaecal valve - connects to caecum Contraction and relaxation of smooth muscle in intestinal wall generates peristaltic activity  important in mixing of content and its slow movement forward (mechanical digestion) Mechanoreceptors & chemoreceptors stimulate liver & pancreas to release juices into duodenum 27 Histology of Small Intestine Small intestine is lined with simple columnar epithelium specialised for absorption; contains numerous mucus-producing cells Structures that increase surface area: ◦ circular folds (~1cm tall) - also force the content to slowly spiral through the lumen ◦ villi - 1 mm tall finger-like mucosal processes covered by intestinal epithelium (contain blood capillaries and small lymphatic vessels called lacteals) ◦ microvilli - microscopic extensions of cell membrane on the apical (upper) surface ◦ microvilli form so-called brush border; it contains some digestive enzymes embedded in microvilli membrane – brush border enzymes 28 14  Histology of small intestine (2) Vein carrying blood to hepatic portal vessel (to the Liver) with absorbed nutrients Muscle layers Lumen Circular folds Microvilli Villi (brush border) Pg 942 Absorptive cells Lacteal Goblet cell Blood capillaries Vilus Mucosa Enteroendocrine cells associated (secrete hormones such as lymphoid tissue secretin, CCK) Intestinal crypt Venule Mucus secreting Lymphatic vessel gland Submucosa 29 Absorption in the small intestine Absorption can be achieved via simple diffusion, facilitated diffusion, active transport and osmosis (water). What is normally absorbed: ◦ monosaccharides ◦ amino acids ◦ monoglycerides, fatty acids (after absorption inside epithelial cells triglycerides are rebuilt and coated with protein(why????) to form chylomicrons which are passed into lymph) ◦ electrolytes ◦ vitamins (vitamin B12 requires intrinsic factor for its absorption) ◦ water 30 15  Fat globule Pg 961 Bile salts Large fat globules are emulsified (physically broken up into smaller fat droplets) by bile salts in the duodenum. Fat droplets Digestion of fat by the pancreatic enzyme lipase yields coated with free fatty acids and monoglycerides. These then bile salts associate with bile salts to form micelles which “ferry” them to the intestinal mucosa. Micelles made up of fatty acids, monoglycerides Fatty acids and monoglycerides leave micelles and and bile salts diffuse into epithelial cells. There they are recombined and packaged with other lipoid substances and proteins to form chylomicrons. Epithelial cells of Chylomicrons are extruded from the small epithelial cells by exocytosis. The intestine chylomicrons enter lymphatic lacteals. They are Lacteal carried away from the intestine by lymph. 31 Lipoproteins * Blood cholesterol Which one is good?? 32 16  Anatomy of large intestine Unique features: ◦ teniae coli - three longitudinal smooth muscle bands in the wall ◦ haustrae - pocketlike sacs caused by the tone of the teniae coli ◦ The teniae Coli constrict to produce the haustrum (this is why the large intestines looks segmented). Once one haustrum distends & fills with chyme, it will constrict and push the contents to the next haustrum ◦ epiploic appendages - fat-filled pouches of visceral peritoneum Parts: caecum, ascending colon, transverse colon, descending colon, sigmoid colon, rectum and anus Ascending colon and descending colon are retroperitoneal Transverse colon and sigmoid colon are anchored via mesocolons (mesenteries) Anal canal = last 2-3 cm of GI tract 33 Anatomy of large intestine (2) Right colic Left colic (hepatic) (splenic) flexure flexure Transverse Transverse mesocolon colon Epiploic Superior appendages mesenteric artery Descending Haustrum colon Ascending Cut edge of colon mesentery IIeum Teniae coli IIeocecal valve Sigmoid Cecum colon Vermiform appendix Rectum Anal canal External anal sphincter Pg 954 34 17  Processes in large intestine Mucosa of large intestine has no villi (only microvilli) and only glandular cells are goblet cells (produce mucus) Mechanical digestion – slow peristaltic waves for churning and mixing Chemical processes - bacterial fermentation of undigested carbohydrates (into CO2 and CH4 gas), undigested proteins (into simpler substances such as indoles = odor) and bilirubin from bile into other pigments (stercobilin). Stercobilin is a biproduct of heme breakdown and is the cause of the brown colour of human feces Absorption of water and some electrolytes forms solid faeces (largely contains undigested materials such as cellulose, but also numerous bacteria) 35 Absorption of water 9 litres of fluid dumped into GI tract each day Small intestine reabsorbs 8 litres Large intestine reabsorbs 90% of that last litre Absorption is by osmosis through cell walls into vascular capillaries inside villi 36 18  Defecation Pg 954 Colonic peristaltic activity slowly moves faeces into rectum When full, rectal stretch receptors signal PNS centres in sacral spinal cord PNS nerves activate muscles of rectum & relax internal anal sphincter External sphincter is Anal canal has two sphincters: voluntarily controlled – internal (smooth muscle) and external defecation can be (skeletal muscle…Stratified Squamous postponed… epithelium *Blood Supply? Think medication administered rectally…… 37 Control of defecation Pg 957 38 19  Review of Accessory Organs 39 Anatomy of liver & gallbladder Liver Pg 941 weighs 1.5 kg Located below diaphragm in right hypochondrium & has 4 primary lobes Gallbladder pear shaped sac, located underneath right lobe stores bile Cystic duct connects gall bladder to common hepatic duct (formed from left and right hepatic ducts) 40 20  Liver histology Liver cells (hepatocytes) are arranged in functional units (roughly hexagonal in shape) called lobules Central vein is located in the lobule centre Liver cells are well positioned to take substances such as nutrients and medications from blood for metabolism Scavenger Kupffer cells (hepatic macrophages) in sinusoids phagocytose microorganisms & foreign matter from blood Bile canaliculi are found between layers of hepatocytes – they drain bile produced by hepatocytes into bile ducts 41 Liver histology (2) Pg 946 Lobule Central vein Connective tissue septum 42 21  Blood supply to the liver Liver receives blood from: ◦ hepatic portal vein bringing nutrient rich blood from GIT * Except where? ◦ From the lower 2/3 of the rectum  is drained into the iliac vein (not the hepatic portal) that drains to the vena cava Blood from both the hepatic portal vein and hepatic artery mix in liver sinusoids and flows towards the central vein Central veins join to form single hepatic vein  goes into inferior vena cava Sidenote: The “hepatic portal vein” is technically not a true vein, as it moves blood to the liver not the heart. 43 Main functions of the liver Carbohydrate metabolism (maintenance of blood glucose level: glycogenesis, glycogenolysis, gluconeogenesis) Lipid metabolism (synthesis of lipoproteins and cholesterol) Protein metabolism (conversion of one amino acid into another, synthesis of plasma proteins such as albumin and blood clotting factors) Processing of drugs and hormones (detoxifying) Processing and excretion of bilirubin (biproduct created by the breakdown of hemoglobin) & excreted into bile ◦ Sidenote: It is the presence of bilirubin that is responsible for the yellow colour seen in bruises. Synthesis of bile acids (needed for digestion of lipids) Nutrient Storage (glycogen, iron, vitamins A, B12, D, E, K) Phagocytosis (via Kupffer cells) Activation of vitamin D taken in food (together with kidneys) 44 22  Bile & bile secretion Bile production is 800-1,000 mL/day; pH is 7.6-8.6 Composition: water, ions, bile acids, cholesterol, lecithin (type of phospholipid), bile pigments (conjugated bilirubin) Important function: emulsification of large lipid globules Bilirubin from bile after conversion into stercobilin in large intestine gives feces its brown colour Hydrophilic region Lecithin Hydrophobic region Bile acid Fat globule 45 Control of bile secretion Bile is temporarily stored and concentrated in the Pg 946 gallbladder Bile flow: gallbladder  cystic duct  common bile duct  duodenum 80% of bile acids is reabsorbed in the terminal ileum; returns to the liver via the portal vein and are re-excreted into the bile -20% of the bile acids is excreted in the feces and this is the body’s only way of eliminating excess cholesterol (liver synthesises new bile acids from cholesterol to replace those lost in feces) 46 23  Pancreas Mostly retroperitoneal, deep to the stomach Pancreas is both exocrine and endocrine gland: o Exocrine (ducts): glandular cells produce 1.2-1.5 L/day of digestive pancreatic juice o Endocrine (directly into blood): pancreatic islets (islets of Langerhans) contain cells that produce hormones glucagon and insulin that control blood glucose level Pancreatic juice is drained into the duodenum via the main pancreatic duct and to some extent accessory duct Pancreatic duct joins common bile duct that comes from the liver forming common duct that joins duodenum 47 Pancreas Pg 941 Pg 948 48 24  Pg 949 Pancreatic juice Composition: water, salts and bicarbonate (hydrocarbonate) ions pancreatic juice is alkaline  neutralises gastric chyme, stops the action of pepsin; creates the proper pH for pancreatic and intestinal digestive enzymes Digestive enzymes pancreatic amylase  carbohydrate starch breakdown trypsin, chymotrypsin and procarboxypeptidase  protein breakdown pancreatic lipase  most of triglyceride breakdown deoxyribonuclease and ribonuclease  nucleic acids breakdown 49 Control of pancreatic secretion Pg 949 Chyme entering duodenum causes release of cholecystokinin (CCK) and secretin from duodenal enteroendocrine cells Secretin – stimulates release of H2O & HCO3- from the pancreas CCK (red dots) and secretin (yellow dots) enter the bloodstream CCK induces secretion of enzyme-rich pancreatic juice Secretin causes secretion of HCO3–-rich pancreatic juice CCK is the principle stimulus for delivery of pancreatic enzymes and bile into the small intestines! 50 25  REVIEW DIGESTION 51 Digestion of nutrients Chemical digestion of carbohydrates: Mouth and duodenum: salivary and pancreatic amylase break down starch into maltose (glucose-glucose), maltotriose (glucose-glucose-glucose) and - dextrins (oligosaccharides with 5-10 glucose units) Small intestine: intestinal lining brush-border enzymes -dextrinase, sucrase, lactase and maltase digest the oligosaccharides (-dextrins) & disaccharides (sucrose, lactose, maltose) into monosaccharides (glucose, fructose and galactose) 52 26  Digestion of nutrients (2) Chemical digestion of proteins: Stomach: HCl denatures and unfolds proteins preparing them for easier chemical digestion, pepsin breaks proteins into small peptides Duodenum: pancreatic enzymes trypsin, chymotrypsin and carboxypeptidase continue breaking proteins into small peptides Small intestine: various peptidase enzymes in intestinal lining brush border finish digestion of peptides into amino acids 53 Digestion of nutrients (3) Chemical digestion of lipids: Duodenum o emulsification by bile (physical breakdown into smaller droplets) o pancreatic lipase splits lipids into fatty acids & monoglycerides (glycerol + 1 fatty acid) Chemical digestion of nucleic acids: Duodenum o pancreatic ribonuclease digests RNA, deoxyribonuclease digests DNA into nucleotides Small intestine o brush-border enzymes break down nucleotides into nitrogenous bases, pentose sugars and phosphates 54 27  Lecture outcomes You should now be able to: Identify ALL of the anatomical & histological structures of the GIT, including the accessory organs Describe the physiological processes involved in digestion, absorption and excretion Describe the factors involved in motility throughout the GIT Discuss the various secretions produced throughout the GIT and their functional role Describe the digestion, absorption and functional importance for the major nutrients and vitamins 55 Remember… In addition to viewing this lecture, please: Complete Course Content: Topic 3.1 – Structure & function of the ✔ Digestive System ✔ Review with Workbook 4 ✔ Complete required readings ✔ Attend and participate in your tutorial & lab 56 28  References McKinley, M.P., O’Loughlin, V.D., & Bidle, T.S. (2016). Anatomy & physiology: An integrative approach (2e. ed.). New York, NY: McGraw-Hill Education. 57 End 58 29