Human Anatomy and Physiology - The Digestive System PDF

Human Anatomy and Physiology Elaine N. Marieb and Suzanne M. Keller The Digestive System Learning Objectives 1. Identify the organs of the digestive system 2. Describe the basic processes performed by the digestive system 3. Describe the structure and function of layers of digest...

Human Anatomy and Physiology Elaine N. Marieb and Suzanne M. Keller The Digestive System Learning Objectives 1. Identify the organs of the digestive system 2. Describe the basic processes performed by the digestive system 3. Describe the structure and function of layers of digestive canal 4. Describe the nerve supply of the digestive canal 5. Describe the components of the digestive system Digestive System: The components The digestive system consists of a group of organs that breakdown the food we eat into smaller molecules that can be used by body cells Two groups of organs compose the digestive system The digestive canal or gastrointestinal (GI) tract, includes the mouth, pharynx, esophagus, stomach, small intestine, and large intestine. The accessory digestive organs that include the teeth, tongue, salivary glands, liver, gall bladder, and pancreas Digestive System: The components Digestive System: Functions The digestive system performs six basic processes: 1. Ingestion: taking food into mouth 2. Secretion: release of water, acid, buffers, and enzymes into lumen of digestive canal 3. Mixing and propulsion: churning and movement of food through digestive canal 4. Digestion: mechanical and chemical breakdown of food 5. Absorption: passage of digested products from digestive canal into blood plasma and lymph plasma 6. Defecation: elimination of feces from digestive canal Layers of the Digestive canal: From lower esophagus to anal canal, the same four layers arrangement from deep to superficial, the mucosa, submucosa, muscular layer, and serosa/adventitia Layers of the Digestive canal: 1. The Mucosa is the innermost layer is the mucosa, or mucous membrane, a moist epithelial membrane that lines the alimentary canal lumen from mouth to anus. Its major functions are to: Secrete mucus, digestive enzymes, and hormones Absorb the end products of digestion into the blood Protect against infectious disease 2. The submucosa contains blood and lymphatic vessels. It also contains a neural network known as the submucosal neural plexus (Meissner plexus) Layers of the Digestive canal: 3. The muscular layer (or muscularis externa) component depends on its location in the digestive canal: Voluntary control is provided by skeletal muscle in the mouth, pharynx, superior and middle esophagus, and the external anal sphincter Other areas of the digestive canal contain smooth muscle arranged in 2 layers, inner circular and outer longitudinal, with a myenteric neural plexus located in between 4. The Serosa, called peritoneum is the largest serous membrane of the body, contains layer of simple squamous epithelium (mesothelium) underlying areolar connective tissue  In esophagus, serosa is replaced with adventitia  Peritoneum Divided into the parietal (outer) layer which binds the alimentary canal to the abdominal wall and the visceral (inner) which covers organs in the cavity. Peritoneal fluid is found between the 2 layers Peritonitis: Inflammation of the peritoneum due to bacterial infection, injury, or surgery Blood Supply: The Splanchnic Circulation The splanchnic circulation: branches off from the aorta to supply the digestive organs and the hepatic portal circulation Arterial supply: the celiac trunk serve the liver, spleen, and stomach – the mesenteric arteries serve the small and large intestines The digestive organs receive 25% of the total cardiac output The hepatic portal circulation collects nutrient-rich venous blood draining the digestive viscera and delivers it to the liver Blood Supply: The Splanchnic Circulation Nerve supply of the digestive canal Enteric Nervous System: Intrinsic nervous system ~100 million neurons, together forming the enteric nervous system ENS (gut brain) Submucosal plexus - lies in the submucosa and controls digestive canal secretion & enfolding Myenteric plexus – lies between the 2 smooth muscle layers and controls digestive canal motility Autonomic Nervous System Parasympathetic – increases digestive canal secretion and motility Sympathetic – decrease secretion and motility The Mouth The mouth is formed by cheeks, lips, hard and soft palates, oral cavity, teeth, salivary glands and tongue Salivary Glands (an Accessory Digestive organ): The salivary glands lie outside the mouth but release saliva via duct into the oral cavity Salivary glands in the mouth include major glands (parotid, submandibular, sublingual). Minor glands are scattered throughout the mouth Parasympathetic stimuli increase saliva secretion Sympathetic stimuli decrease saliva secretion Mumps is inflammation and enlargement of parotid glands with fever, malaise (general discomfort), and extreme throat pain during swallowing Salivary Glands (an Accessory Digestive organ): Salivary glands and saliva: characteristics The Salivary gland is composed of two types of secretory cells: serous and mucous cells Saliva is mainly water 97 – 99.5% with a pH (6.5 – 7.0) the solutes include ions such as Na, K, Cl, bicarbonate and phosphate Organic substances such as urea, uric acid, mucus, IgA, lysozyme, and salivary amylase The concentration and ingredients in secretions from the major salivary glands are not identical but may differ from each other. Functions of saliva: Dissolves food, activates salivary amylase, buffers acidic foods, prevents microbial attachment (IgA), lubricates food, and kills microbes (lysozyme) Tongue (an Accessory Digestive organ): The tongue is an accessory organ composed of skeletal muscle covered with mucous membrane. Each half of tongue contains identical component of extrinsic and intrinsic muscles Functions: Used to reposition food when chewing, Mix food with saliva Initiate the voluntary part of swallowing, Help form consonants for speech. The Pharynx The pharynx is a funnel-shaped tube that extends from the internal nares to the esophagus posteriorly and to the larynx anteriorly It is composed of skeletal muscle and lined with mucous membrane, with three parts: the nasopharynx, the oropharynx, and the laryngopharynx The Esophagus (The GIT) The esophagus is a muscular tube that connects the pharynx to the stomach Bolus passes through the esophagus guarded by upper esophageal sphincter (UES) and lower esophageal sphincter (LES). Both sphincters open when relaxed. The submucosa secretes mucus that lubricates esophagus for smooth passage of bolus The outer layer of esophagus is fibrous adventitia Stomach (the GIT) Stomach is ‘J’-shaped enlargement that converts bolus into chyme, a creamy paste. It has 5 distinct regions: Cardia, Fundus, Corpus, Antrum and Pylorus (see next slide) When empty, the mucosa lies in large gastric folds. The pylorus separates the stomach and duodenum via the pyloric valve Functions of Stomach: Mixes saliva, food, and gastric juice to form chyme Serves as a reservoir for food before release into the small intestine Secretes gastric juice, which contains HCl, pepsin, intrinsic factor, and gastric lipase Secretes gastrin into the blood Stomach (the GIT) Histology of the Stomach The stomach mucosa is a layer of simple columnar cells called surface mucous cells. Epithelial cells extend down into lamina propria and form gastric glands Several glands open into narrow channels called gastric pits Secretions from glands flow to pits and then to stomach lumen  The Mucosal Barrier Three factors create this barrier: 1. A thick coat of bicarbonate-rich mucus 2. Epithelial cells are joined together by tight junction (prevent leaking) 3. Damaged epithelial mucosal cells are shed quickly and replaced by division undifferentiated stem cells Anything that breaches the gel-like mucosal barrier causes inflammation If it persists it will promote peptic ulcers Gastric glands and Cell Types in the Stomach Small Intestine (the GIT): Regions of Small Intestine: A. The Duodenum “Mixing bowl” that receives chyme from the stomach and digestive secretions from the pancreas and liver Functions of the duodenum To receive chyme from the stomach To neutralize acids before they can damage the absorptive surfaces of the small intestine Begins digestion and absorption B. The Jejunum Is the middle segment of the small intestine The site for maximal digestion and absorption C. The Ileum The final segment of the small intestine Ends at the ileocecal valve Copyright ©2021 John Wiley & Sons, Inc. 25 Small Intestine (the GIT): Structural modifications of the Small Intestine Mucosa and submucosa layers in small intestine have unique features: 1. Circular folds (in the mucosa and submucosa), 2. Intestinal Villi - finger-like projections of mucosa, contains the lacteals or lymphatic capillary 3. Microvilli - fuzzy appearance, called brush borders, and contains enzymes Cell types of the Small Intestine Copyright ©2021 John Wiley & Sons, Inc. 27 Functions of the Small Intestine 1. In addition to peristalsis, there are two motility patterns in the SI: a) After a meal: segmentation – mixing of chyme with bile, pancreatic and intestinal juices b) Between meals: a form of peristalsis called migrating motor complexes - MMC (housekeeping function) – role of the hormone motilin 2. Completes digestion of carbohydrates, proteins, and lipids; begins and completes digestion of nucleic acids 3. Absorbs about 90% of nutrients and water that pass through the digestive system Copyright ©2021 John Wiley & Sons, Inc. 28  Large Intestine (the GIT) Extends from the ileocecal valve to the anus contains cecum, appendix, colon, rectum, and anal canal Functions of the Large Intestine 1. Haustral churning, peristalsis, and mass movement drive contents of colon into rectum 2. Bacteria in colon convert proteins into amino acids, break down amino acids, and produce some B complex and vitamin K 3. Absorption of some water, ions, and vitamins 4. Formation of feces from undigested food material 5. Defecation Copyright ©2021 John Wiley & Sons, Inc. 30 Anatomy of the Large Intestine (Colon) Copyright ©2021 John Wiley & Sons, Inc. 31 Histology of the Large Intestine The large intestine wall has 4 layers like the rest of the digestive canal: mucosa, submucosa, muscular layer, and serosa. Muscular and serosa layers have unique features: Taeniae coli, Haustra, and omental appendages. Tonic contractions of the bands – Tenaie coli, gather the colon into a series of pouches called haustra and the serosa layer has omental appendages (fat-filled pouches) that may serve as a protective and defensive mechanism. Copyright ©2021 John Wiley & Sons, Inc. 32 Anal canal (the GIT) Large intestine ends at anal canal Anal canal is the last 2-3 cm of the digestive canal that opens outside the body The anus is guarded by internal (involuntary) and external anal sphincters (voluntary), which keep it closed except during elimination of feces Copyright ©2021 John Wiley & Sons, Inc. 33 Defecation Reflex Copyright ©2021 John Wiley & Sons, Inc. 34 Pancreas (an Accessory Digestive organ): The pancreas is C-shaped and retroperitoneal. Pancreatic juice neutralizes acidic chyme from the stomach and contains water, electrolytes, and enzymes that break down all food types. Enzymes in the juice are proteases, lipases, amylase, and nucleases, and secretion is regulated by local hormones and the parasympathetic nervous system It empties its contents into the duodenum Histology of the Pancreas Pancreas contains glandular epithelial cells 99% of the pancreas are pancreatic acini, which release pancreatic juice The remaining 1% are pancreatic islets, producing hormones such as glucagon, insulin, somatostatin and pancreatic polypeptide Liver and GB (an Accessory Digestive organ): Accessory organs associated with small intestines The liver produces bile, which is important in the emulsification of fats The gallbladder stores bile until it is needed, and is released by hormone cholecystokinin (CCK) Functions of Liver and Gall bladder: Carbohydrate, lipid, and protein metabolism Processing of drugs and hormones Bilirubin excretion Bile salt synthesis Storage Phagocytosis Vitamin D activation Copyright ©2021 John Wiley & Sons, Inc. 37  Histology of the Liver Liver cells (hepatocytes) form complex 3-D shapes (plates) that are branched and irregular called hepatic laminae. Plates are surrounded by endothelia-lined spaces called hepatic sinusoids which deliver blood into central vein and then on to hepatic vein. Hepatocytes secrete bile, which is collected by small ducts called bile canaliculi and merge into a bile duct. Bile duct + branch of hepatic artery + branch of hepatic portal vein form the Portal triad Blood supply of the Liver Copyright ©2021 John Wiley & Sons, Inc. 39 Accessory Digestive organ: Gall Bladder Bile duct and main pancreatic duct unite in the wall of the duodenum to form a bulblike point called hepatopancreatic ampulla, and the entry of the bile and the pancreatic juice is controlled by the hepatopancreatic sphincter (known as the sphincter of Oddi) Copyright ©2021 John Wiley & Sons, Inc. 40 Summary