Gastro-intestinal tract 1 Physiology 232 BMS Dr/Nahla Yacout 2016/2017

Objectives Enumerate the organs of the digestive system & explain the digestive processes Explain types of salivary glands, & their secretions Explain composition & functions of saliva Explain the control of salivary secretion Explain digestion in mouth, pharynx & esophagus Enumerate the glands present in stomach & their secretions Explain functions of HCl & mucus Explain the three phases of gastric juice secretion & digestion in stomach Know a brief account on gastric ulcers

Digestive system The digestive system takes in food, breaks it down into nutrient molecules, absorb these molecules into the blood stream, & then gets rid of the undesirable molecules The organs of the digestive system are divided into two groups: Organs of the alimentary canal (Gastrointestinal tract) Mouth, Pharynx, Esophagus, Stomach, Small intestine, & Large intestine The large intestine leads to a terminal opening called (Anus) Accessory organs Teeth, Tongue, Salivary glands, Gall bladder, Liver, Pancreas

Digestive processes Processing of food by the digestive system involves six activities: Ingestion, Propulsion, Mechanical digestion, Chemical digestion, Absorption & Defecation Ingestion: Is taking food into the digestive system through the mouth Propulsion: Moving food through the GIT Includes: Swallowing & Peristalsis (Is the contraction & relaxation of muscles, in order to squeeze food along the tract) Mechanical digestion: Includes: Chewing, Mixing of food with saliva by the tongue, Segmentation (Mixing the food with digestive juices & increasing the efficiency of absorption by moving the food over the intestinal wall)

Chemical digestion: Is breaking down the complex food molecules by enzymes Absorption: Is the passage of digested end products (Plus vitamins, water) from the GIT to the blood Small intestine is the major site for absorption Defecation: Eliminates the indigestible substances from the body through the anus in the form of feces

Mucosa Is the layer that lines the alimentary canal starting from the mouth to the anus Functions 1. Secretion of mucus, digestive enzymes & hormones 2. Absorption of digested food into the blood 3. Protection against diseases

Mouth Is the only part that is involved in ingestion, But is involved also in other digestive processes, because in mouth food is chewed & mixed with saliva containing enzymes that begins the process of chemical digestion …. Also, it begins propulsion by swallowing & carrying food though the pharynx, esophagus to the stomach Salivary glands Are a number of glands within the oral cavity that secrete saliva We have 3 types of salivary glands: Parotid glands – Sublingual glands – Submandibular glands Salivary glands are composed of two types of cells: Mucous cells – Serous cells Mucous cells: Secrete mucus Serous cells: Secrete watery secretion containing enzymes & ions

Parotid glands: Contain only Serous cells Sublingual glands: Contain mostly Mucous cells Submandibular glands: Contain equal numbers of Mucous cells & Serous cells Functions of saliva Cleans the mouth Moisten food & helps in chewing & swallowing Contains enzymes that begin the chemical breakdown of starch (Starts digestion of starch) Helps in tasting food Protects the mouth from bacteria, & prevents tooth decay

Composition of saliva: 1. Electrolytes (Na+ , K+ , Cl+ ) 2 Composition of saliva: 1. Electrolytes (Na+ , K+ , Cl+ ) 2. Salivary amylase enzyme 3. Lingual lipase enzyme …. Secreted from Sublingual salivary glands (Fat – digesting enzyme) 4. Proteins 5. Antibodies 6. Metabolic wastes

Control of salivary secretion: Salivary secretion is controlled by sympathetic & parasympathetic nervous system Sympathetic Parasympathetic Causes vasoconstriction & secretion of small amounts of thick saliva It may inhibits completely salivary secretion causing a dry mouth ….. (Xerostomia) Causes vasodilatation & secretion of large amount of watery saliva Reflex secretion occurs when there is food in mouth, or even the smell, thought of food stimulates the flow of saliva

Pharynx Is divided into three parts: Nasopharynx Oropharynx Laryngopharynx Nasopharynx …. Important in respiration Oropharynx & Laryngopharynx …. Are common passages for respiratory & digestive systems Food passes from the oral cavity into the pharynx & then esophagus

Esophagus Is a muscular tube about 25cm long After food moves through the laryngopharynx, it passes into the esophagus (There is a flap called Epiglottis that closes over the larynx to force food to enter esophagus not the larynx & trachea) Esophagus passes between the muscles of the diaphragm, just it has passed the diaphragm, it curves upwards before opening into the stomach The lower end of esophagus is closed by sphincter called (Esophageal sphincter …. Between esophagus & stomach) …. To prevent the reflux of gastric contents into the esophagus

Digestion in mouth, pharynx & esophagus The mouth is involved in most digestive processes, it: Ingests Begins mechanical digestion by chewing Begins propulsion by chewing Once the food enters the mouth, saliva will be produced containing Salivary amylase enzyme Salivary amylase will start digestion of carbohydrates (Starch) Then by swallowing, food will enter pharynx & then esophagus by peristalsis which act only as pathways for the food Once swallowing occurs, Epiglottis will cover the larynx to prevent the food entry in it Then food passes from esophagus to the stomach Once the food enters the stomach, esophageal sphincter closes to prevent the back flow of the food into the esophagus again

Stomach Below the esophagus, GIT expands to form the stomach Inside the stomach, digestion of proteins begins, & food is converted to a creamy paste called (Chyme) At the lower part of stomach (Between stomach & duodenum …. 1st part of small intestine), there is a sphincter called (Pyloric sphincter) Once the food passes from stomach to the duodenum, pyloric sphincter closes to prevent the back flow of food from duodenum to stomach again

Stomach There are glands present in the stomach that secrete the gastric juice These glands contain 4 types of cells: Mucous cells ….. Secrete mucus Parietal cells ….. Secrete hydrochloric acid (HCl) & intrinsic factor (Is required for vitamin B12 absorption in the small intestine) Chief cells ….. Secrete pepsinogen (Inactive form of the enzyme that is converted to pepsin to digest proteins) Enteroendocrine cells ….. Secrete some chemical messengers as histamine, serotonin & hormones as gastrin Gastric juice is highly acidic (Because of the presence of high concentration of HCl) …. This high acidity can digest the stomach itself For the stomach to protect itself from digestion: It secretes mucus that forms a thick coat on the wall & prevents HCl from reaching the stomach wall

Functions of HCl: Converts pepsinogen (Inactive form) into pepsin (Active form) to digest the proteins Kill many bacteria ingested with food Functions of mucus: Prevents injury of stomach wall by lubricating the stomach contents Acts as a barrier between the stomach wall & gastric juice to prevent formation of peptic ulcer

There are 3 phases for gastric juice secretion: Cephalic phase: It occurs before the food enters the stomach Taste, sight or thought of food will stimulate the (Vagus nerve) Vagus nerve will cause gastric secretion Gastric phase: It occurs when the food enters the stomach Presence of food in the stomach will stimulate the entero-endocrine cells in gastric glands to secrete Gastrin Gastrin: Is a hormone that stimulates gastric glands to secrete more gastric juice

Intestinal phase: When the partially digested contents of the stomach reaches the small intestine, a complex called (Enterogastrone …. Consists of two hormones Secretin & Cholecystokinin) is produced by the cells present in the intestine This complex will decrease the secretion of gastric juice & also decrease the gastric motility

Digestion in stomach Stomach is involved in most of the digestive processes Once food enters the stomach, HCl & pepsinogen are secreted HCl activates pepsinogen & converts it into pepsin to digest the proteins (Protein digestion starts in the stomach) Lingual lipase enzyme may digest some lipids in the stomach

Gastric ulcers Damage to the underlying tissue, which causes erosions of the stomach wall The most important symptom of gastric ulcer is: Epigastric pain that occurs 1 – 3 hours after eating & is often relieved by eating again Causes of gastric ulcers: High HCl production in stomach Low mucus secretion in stomach Infection with Helicobacter pylori bacteria, which destroys the mucosa of stomach Complications of gastric ulcers: Hemorrhage – Shock Perforation May lead to development of malignant tumor