Digestive System PDF

THE DIGESTIVE SYSTEM The Digestive System The Gastrointestinal System consists of the: ❖Digestive Tract ❖Accessory Organs Oral Cavity Teeth Pharynx Tongue Esophagus Salivary Glands Stomach Liver Small Intestines...

THE DIGESTIVE SYSTEM The Digestive System The Gastrointestinal System consists of the: ❖Digestive Tract ❖Accessory Organs Oral Cavity Teeth Pharynx Tongue Esophagus Salivary Glands Stomach Liver Small Intestines Gallbladder Large Intestines Pancreas  GENERAL FUNCTIONS 1. Ingestion 2. Mechanical Processing 3. Chemical Digestion 4. Secretion 5. Absorption 6. Egestion  LAYERS OF THE WALL OF THE DIGESTIVE TRACT From innermost to outermost layer 1. Mucosa 2. Submucosa 3. Muscularis Externa 4. Serosa Mucosa Epithelial Layer & Lamina Propria Epithelial Layer  The layer is folded to facilitate a large surface area for digestion.  The epithelium may be simple or stratified.  Stratified Squamous- Oral Cavity, Pharynx (orpharynx and laryngopharynx), Esophagus, Anal Canal  Simple Columnar- Stomach, Small Intestine and almost all the length of the large intestine. Stratified and Simple Epithelium Small intestine Lamina Propria Loose connective tissue Blood vessels Sensory nerve endings Lymphatic vessels Smooth muscle cells Scattered lymphoid tissues Secretory cells of mucus glands Muscularis Mucosa (circular and longitudinal muscle layer) Submucosa ❖ A layer of dense connective tissue that lies below the mucosa consisting of: Blood vessels Lymphatic Vessels Exocrine glands - buffers and enzymes Sub mucosal plexus/Plexus of Meissner Sensory neurons, parasympathetic ganglionic neurons and sympathetic postganglionic fibers innervating mucosa and submucosa The Submucosa Muscularis Externa Dominated by smooth muscle cells Forms an inner circular and outer longitudinal layer Contraction facilitates peristalsis and segmentation Contraction also facilitates mechanical processing Muscularis Externa Facilitates Peristalsis & Segmentation Muscularis Externa ◦Enteric Nervous System (ENS) coordinates movements of the muscles of the muscularis externa; innervated by the ANS ◦Sympathetic fibers also innervate mucosa and Myenteric Plexus ◦Myenteric Plexus lies between circular and longitudinal muscle ◦Parasympathetic stimulation increases muscle tone and activity Serosa  Serous membrane covering the Muscularis Externa  Found along most portions of tract except in the oral cavity, pharynx, esophagus and rectum.  These areas (without a serosa) consist of a dense network of collagen fibers which attaches the digestive tract to the adjacent structures. This layer is known as an ADVENTITIA Digestive Organs and Their Functions ❖Oral Cavity ❖Pharynx ❖Esophagus ❖Stomach ❖Small Intestine ❖Large Intestine  ORAL CAVITY Functions: Analysis of material before swallowing Mechanical processing by teeth, tongue and palate surfaces Lubrication with mucus and salivary secretions Limited digestion of carbohydrates and lipids (Salivary Amylase & Lingual Lipase) Salivary Glands Salivary Gland ❖ Submandibular Glands -produce buffers, mucins & salivary amylase ❖ Sublingual Glands -contain mucus cells ❖ Parotid Glands -contain only serous cells –salivary amylase Functions of Saliva Helps keep oral surfaces clean Moistening and lubricating of mouth and food. Aids in tasting. Aids in Swallowing. Helps in the metabolism of carbohydrates. Helps to maintain the calcium phosphate matrix of the teeth.  Four Major Components of Saliva: 1. Mucus 2. Amylase 3. Lingual Lipase 4. Alkaline Electrolyte Solution Note: Saliva contains thiocyanates and lysozymes that can attack and destroy oral bacteria. It also contains antibodies that destroy oral bacteria. Control of Salivary Secretions  ANS control PNS increases secretion SNS produces small amounts of thick saliva  Other brain stem nuclei & higher centers Chewing gum Smelling or thinking about food Irritating stimuli in esophagus, stomach & intestine Teeth  Pharynx ◦Review structure in respiratory notes ◦Note that the Muscularis Externa is composed of skeletal muscle  Esophagus ◦ A hollow muscular tube that transfers solid food and liquids to the stomach. ◦ Extends from cricoid cartilage, along posterior surface of trachea, through diaphragm to stomach ◦ Cardiac Sphincter Muscles regulate the passage of bolus into the stomach Histology of Esophagus  Mucosa – nonkeratinized stratified squamous epithelium  Mucosa and submucosa thrown into folds  Submucosa contains mucus secreting glands  Muscularis Externa – circular & longitudinal muscle; 1st third contain skeletal muscle, the middle third of skeletal & smooth muscle and inferior third of smooth muscle  No Serosa, but an adventitia of connective tissue Oesophagus Histology The Cardiac Sphincter/ Lower Esophageal Sphincter The Mechanism of Swallowing ◦Consists of three phases: ◦Oral Phase ◦Pharyngeal Phase ◦Esophageal Phase The Mechanism of Swallowing ◦ Oral Phase ◦ Voluntary Phase ◦ Hard palate compresses bolus ◦ Tongue forces bolus into oropharynx ◦ Soft palate elevated  The Mechanism of Swallowing ◦ Pharyngeal Phase ◦ Bolus enters pharynx ◦ Tactile receptors on palatal arches and uvula stimulated by bolus ◦ Swallowing center of medulla oblongata receives information ◦ Pharyngeal muscles stimulated to contract ◦ Larynx elevates ◦ Respiratory centers inhibited  The Mechanism of Swallowing ◦ Esophageal Phase ◦ Bolus enters esophagus ◦ Bolus pushed to stomach by peristalsis ◦ Cardiac sphincter muscles open ◦ Bolus enters stomach The Stomach Functions: Bulk storage of ingested food. Mechanical breakdown of ingested food. Disruption of chemical bonds in food. Production of intrinsic factor (for vitamin B12 absorption). The Stomach LES = lower oesophageal sphincter Rugal Folds of the Stomach The Stomach Histology ◦ Epithelium – simple columnar with goblet cells ◦ Gastric pits open onto gastric surface ◦ Gastric glands below gastric pits in fundus and body Stomach Mucosa Showing Gastric Pits Gastric Glands Consist of: Parietal Cells produce Intrinsic factor & HCl Chief Cells produce Pepsinogen Hydrochloric Acid ◦ Functions: ◦ Kills microbes. ◦ Denatures proteins and inactivates most enzymes in food. ◦ Breaks down plant cell walls and connective tissues in meat. ◦ Provides an acidic environment for activation and function of pepsin. Chief Cells ◦ Secrete pepsinogen ◦ Pepsinogen is converted to pepsin in the gastric lumen by HCl (pH 1.5 – 2.0) ◦ Renin and Gastric Lipase produced in infants Pyloric glands ◦ Produce: ◦ Mucus ◦ Hormones by Enteroendocrine cells ◦ Gastrin by G cells – stimulates parietal and chief cells and promotes gastric mixing ◦ Somatostatin by D cells – inhibits gastrin release; overridden by neural & hormonal stimuli Small Intestines and Accessory Organs ❖Extend from the pyloric sphincter to the cecum. (Approx. 2.5 cm in diameter & 7m long) Consists of three parts: ◦ DUODENUM ◦ JEJUNUM ◦ ILEUM ◦ NB -9O% of absorption occurs in the small intestine. Small Intestines and Accessory Organs ❖Enzymes are secreted into the small intestines by the PANCREAS These are: ◦ Pancreatic Amylase ◦ Pancreatic Lipase ◦ Nucleases ◦ Proteolytic Enzymes ◦ The liver and gall bladder secrete bile into the small intestine. Histology ❖Structural features of the Small intestines: ◦ Plicae circulares ◦ Villi ◦ Microvilli ❖Components of the villi : ◦ Lamina propria has capillaries that transport gases and absorbed nutrients to the hepatic portal vein. ◦ Lacteals transport substances too large to diffuse into the bloodstream eg.Chylomicrons (protein-lipid packages). ◦ Muscularis mucosae and smooth muscle of intestinal villi move villi for maximum absorption of nutrients and squeeze lacteals therefore moving lymph out of villi Structural Features of the Small Intestine Structural Features of the Small Intestine Histology ❖Structure of Duodenum, Jejunum and ileum. Duodenum- has few plicae, numerous villi and mucous glands Jejunum- has plicae and villi (prominent in its proximal portion). ileum- has scattered villi but lacks plicae altogether; lymphoid tissue (Peyer’s Patches) at terminal portion Intestinal Secretions ◦ Intestinal secretions contain ◦ Mucus ◦ Brush border enzymes ◦ Enterokinase ◦ Maltase ◦ Sucrase ◦ Lactase ◦ Dipeptidases ◦ Peptidases Digestion in the Small Intestine The Pancreas ❖When acidic chyme enters the duodenum, pancreatic juice via the pancreatic duct enters as well. ❖Functions of the pancreas ❖Endocrine cells of the pancreatic islets produce insulin and glucagon ❖Exocrine cells including acinar and epithelial duct cells produce pancreatic juice Digestion in the Small Intestine The Pancreas Pancreatic juice consists of: ◦Sodium bicarbonate (pH 7.5 -8.8) ◦ Buffer solution secreted when chyme enters duodenum and secretin produced ◦PANCREATIC ENZYMES: ◦ CHOLECYSTOKININ accelerates the production and secretion of pancreatic enzymes, the relaxation of the hepatopancreatic sphincter and the contraction of the gallbladder, resulting in the ejection of bile and pancreatic juice into the duodenum Digestion in the Small Intestine The Pancreas ◦ PANCREATIC ENZYMES ◦ Pancreatic amylase – carbohydrates to tri- and di- saccharides ◦ Pancreatic lipase – lipids to fatty acids and monoglycerides ◦ Nucleases – nucleic acids to simple sugars and nitrogen bases Digestion in the Small Intestine The Pancreas ◦ PANCREATIC ENZYMES (Proteolytic enzymes) ◦ Trypsinogen, chymotrypsinogen, procarboxypeptidase and proelastase (INACTIVE) ◦ In the duodenum enterokinase from the brush border converts trypsinogen to trypsin ◦ Trypsin activates the other enzymes to chymotrypsin, carboxypeptidase and elastase ◦ These enzymes produce dipeptides, tripeptides and amino acids from proteins Digestion in the Small Intestine The Liver ◦ The liver has numerous functions which may be categorised as: ◦ Metabolic regulation ◦ Carbohydrate, lipid and amino acid metabolism, waste product removal, vitamin & mineral storage, drug inactivation ◦ Hematological regulation ◦ Bile production Digestion in the Small Intestine The Liver ◦ Bile consists of: ◦ Water & various ions – dilute & buffer acids in chyme ◦ Bilirubin ◦ Cholesterol ◦ Bile salts – enhances lipid digestion Digestion in the Small Intestine The Liver ◦ Bile is secreted from the liver in the presence of the hormone Secretin when lipids enter the duodenum ◦ Functions of Bile ◦ Emulsification of fats – increase surface area for pancreatic lipase to work ◦ Facilitates absorption of lipids by intestinal epithelium Digestion in the Small Intestine The Gallbladder Digestion in the Small Intestine The Gallbladder ◦ Stores bile ◦ Concentrates bile ◦ Bile is released from the gallbladder in the presence of CCK ◦ CCK stimulates the hepatopancreatic sphincter to relax and the gallbladder to contract forcing bile into the duodenum End of Digestion in Small Intestines ◦ Brush border Enzymes complete digestion ◦ Maltase – maltose to glucose ◦ Sucrase – sucrose to glucose and fructose ◦ Lactase – lactose to glucose and galactose ◦ Dipeptidases and Peptidases – dipeptides and tripeptides to amino acids Absorption in the Small Intestine ❖Substances absorbed in the small intestines are: ◦ Monosaccharides-absorbed by the duodenum and upper jejunum; co-transport with sodium ◦ Amino Acids- absorbed by the end of the jejunum; co- transport with sodium and facilitated diffusion Absorption in the Small Intestine ◦Fatty acids and Monoglycerides ◦ absorbed in the duodenum, jejunum and ileum ◦ diffusion in water-soluble micelles which diffuse into epithelial cells of villi ◦ Fatty acids and monoglycerides combine to form triglycerides ◦ Triglycerides combine with cholesterol, lipoprotein and phospholipids to form chylomicrons ◦ Chylomicrons then diffuse into lacteals Absorption in the Small Intestine ◦Vitamins - C and B by passive diffusion - B12 with intrinsic factor active transport - A,D,E & K with micelles Large Intestine ❖Parts of the Large Intestine ◦ Cecum ◦ Colon - Ascending, Transverse, Descending and Sigmoid ◦ Rectum ◦ Anal canal Large Intestine ◦ Simple columnar epithelium with goblet cells ◦ Mucus ◦ Protects intestinal wall and holds fecal matter together ◦ Contains sodium bicarbonate that neutralizes acids produced by bacterial metabolism Large Intestine ◦ Produces large amounts of water and electrolytes when irritated ◦ Movement of fecal matter is due to peristalsis, segmentation and contraction of longitudinal muscle bands