Digestive System Lecture Notes PDF

Digestive System 1 st part Dr. Mai Adawi Lecturer Of Physiology FOM-SCU 2023-2024 FUNCTIONAL ANATOMY OF DIGESTIVE SYSTEM: Primary digestive organs Accessory digestive organs  Organization Primary...

Digestive System 1 st part Dr. Mai Adawi Lecturer Of Physiology FOM-SCU 2023-2024 FUNCTIONAL ANATOMY OF DIGESTIVE SYSTEM: Primary digestive organs Accessory digestive organs  Organization Primary Accessory 1. Mouth 1. Salivary glands 2. Pharynx 2. pancreas 3. Esophagus 3. Liver 4. Stomach 4. Gallbladder. 5. Small intestine 6. Large intestine. Primary Function of GIT  Motility: This refers to the movement of food through the digestive tract.  Secretion: This includes both exocrine and endocrine secretions  Digestion : the breakdown of food molecules into their smaller subunits, which can be absorbed.  Absorption: the passage of digested end products into the blood or lymph.  Nervous Supply Of The GI tract GI tract has two types of nerve supply: I. Enteric nerve supply Enteric nervous system is present within the wall of GI tract from esophagus to anus. Nerve fibers of this system are inter-connected and form two major networks called a. Submucousal plexus = Meissner plexus b. Myenteric plexus = Auerbach plexus  Nervous Supply Of The GI tract Functions of Meissner plexus Function of Meissner plexus is the regulation of secretory functions of GI tract.  Nervous Supply Of The GI tract Functions of Auerbach plexus - Major function of this plexus is to regulate the movements of GI tract. - Some nerve fibers of this plexus accelerate the movements by secreting the excitatory neurotransmitter substances like acetylcholine, serotonin and substance P. - Other fibers of this plexus inhibit the GI motility by secreting the inhibitory neurotransmitters such as vasoactive intestinal polypeptide (VIP) and enkephalin  Nervous Supply Of The GI tract II. Extrinsic nerve supply a) Sympathetic Nerve Fibers Preganglionic sympathetic nerve fibers arise from lateral horns of spinal cord between fifth thoracic and second lumbar segments (T5 to L2). From here, the fibers leave the spinal cord, pass through the ganglia of sympathetic chain without having any synapse and then terminate in the celiac and mesenteric ganglia.  Nervous Supply Of The GI tract Functions of sympathetic nerve fibers Sympathetic nerve fibers inhibit the movements and decrease the secretions of GI tract by secreting the neurotransmitter noradrenaline. It also causes constriction of sphincters  Nervous Supply Of The GI tract II. Extrinsic nerve supply a) Parasympathetic Nerve Fibers Parasympathetic nerve fibers to GI tract pass through some of the cranial nerves and sacral nerves. - The preganglionic and postganglionic parasympathetic nerve fibers to mouth and salivary glands pass through facial and glossopharyngeal nerves.  Nervous Supply Of The GI tract II. Extrinsic nerve supply a) Parasympathetic Nerve Fibers - Preganglionic parasympathetic nerve fibers to esophagus, stomach, small intestine and upper part of large intestine pass through vagus nerve. -- Preganglionic nerve fibers to lower part of large intestine arise from second, third and fourth sacral segments (S2, S3 and S4) of spinal cord and pass through pelvic nerve  Nervous Supply Of The GI tract Functions of parasympathetic nerve fibers parasympathetic nerve fibers accelerate the movements and increase the secretions of GI tract. The neurotransmitter secreted by the parasympathetic nerve fibers is acetylcholine (Ach).  Mouth and Salivary Glands  FUNCTIONS OF MOUTH 1. Ingestion of food materials 2. Chewing the food and mixing it with saliva 3. Appreciation of taste of the food 4. Transfer of food (bolus) to the esophagus by swallowing 5. Role in speech 6. Social functions such as smiling and other expressions. Salivary Glands  Major glands are: 1. Parotid glands 2. Submaxillary or submandibular glands 3. Sublingual glands. Salivary Glands „ inor SALIVARY GLANDS M 1. Lingual Mucus Glands 2. Lingual Serous Glands 3. Buccal Glands 4. Labial Glands 5. Palatal Glands  Mouth and Salivary Glands  CLASSIFICATION OF SALIVARY GLANDS  Salivary glands are classified into three types, based on the type of secretion: 1. Serous Glands: secrete thin and watery saliva. Parotid glands and lingual serous glands are the serous glands. 2. Mucus Glands: secrete thick, viscous saliva with high mucin content. Lingual mucus glands, buccal glands and palatal glands belong to this type. 3. Mixed Glands: made up of both serous and mucus cells. Submandibular, sublingual and labial glands are the mixed glands.  Saliva  PROPERTIES OF SALIVA 1. Volume: 1000 mL to 1500 mL of saliva is secreted per day and it is approximately about 1 mL/minute. 2. Reaction: Mixed saliva from all the glands is slightly acidic with pH of 6.35 to 6.85 3. Specific gravity: It ranges between 1.002 and 1.012 4. Tonicity: Saliva is hypotonic to plasma Saliva  COMPOSITION OF SALIVA  Mixed saliva contains 99.5% water and 0.5% solids.  Saliva  Functions of salive 1. PREPARATION OF FOOD FOR SWALLOWING „2. APPRECIATION OF TASTE 3. ROLE IN SPEECH 4. REGULATION OF BODY TEMPERATURE 5. REGULATION OF WATER BALANCE 6. EXCRETORY FUNCTION : It excretes substances like mercury, potassium iodide, lead, and thiocyanate. Saliva also excretes some viruses such as those causing rabies and mumps  Saliva  Functions of saliva 7. DIGESTIVE FUNCTION: Saliva has three digestive enzymes, namely salivary amylase, maltase and lingual lipase. A- Salivary Amylase: Salivary amylase is a carbohydrate-digesting (amylolytic) enzyme. It acts on cooked or boiled starch and converts it into dextrin and maltose. B- Maltase: Maltase is present only in traces in human saliva and it converts maltose into glucose. C- Lingual Lipase: Lingual lipase is a lipid-digesting (lipolytic) enzyme. It hydrolyzes triglycerides into fatty acids and diacylglycerol  Saliva  Functions of saliva 8. CLEANSING AND PROTECTIVE FUNCTIONS i. Due to the constant secretion of saliva, the mouth and teeth are rinsed and kept free off food debris, shed epithelial cells and foreign particles. In this way, saliva prevents bacterial growth by removing materials, which may serve as culture media for the bacterial growth. ii. Enzyme lysozyme of saliva kills some bacteria such as staphylococcus, streptococcus and brucella. iii. Proline-rich proteins present in saliva posses antimicrobial property and neutralize the toxic substances.  Saliva  Functions of saliva 8. CLEANSING AND PROTECTIVE FUNCTIONS v. Lactoferrin of saliva also has antimicrobial property. v. Proline-rich proteins and lactoferrin protect the teeth by stimulating enamel formation. vi. Immunoglobulin IgA in saliva also has antibacterial and antiviral actions. vii. Mucin present in the saliva protects the mouth by lubricating the mucus membrane of mouth.  Esophagus  The esophagus is the portion of the GI tract that connects the pharynx to the stomach.  It is a muscular tube approximately 25 cm long  The esophagus is sealed off by two sphincters, one at either end of the tube: the upper esophageal sphincter and the lower esophageal sphincter.  Each of these sphincters is normally closed except during the process of swallowing  Lower esophageal sphincter : prevents the reflux of the corrosive gastric contents into the esophagus  Esophagus  Swallowed food is pushed from pharynx to the anal end of the esophagus by a wavelike muscular contraction called peristalsis Stomach PARTS OF STOMACH  In humans, stomach has four parts: 1. Cardiac region 2. Fundus 3. Body or corpus 4. Pyloric region. STRUCTURE OF STOMACH WALL  Stomach wall is formed by four layers of structures: 1. Outer serous layer: Formed by peritoneum 2. Muscular layer: Made up of three layers of smooth muscle fibers, namely inner oblique, middle circular and outer longitudinal layers STRUCTURE OF STOMACH WALL  Stomach wall is formed by four layers of structures: 3. Submucus layer: Formed by areolar tissue, blood vessels, lymph vessels and Meissner nerve plexus. 4. Inner mucus layer: Lined by mucus-secreting columnar epithelial cells. The gastric glands are situated in this layer. GLANDS OF STOMACH – GASTRIC GLANDS  Gastric glands are classified into three types, on the basis of their location in the stomach: 1. Fundic glands or main gastric glands or oxyntic glands: Situated in body and fundus of stomach Main cells of fundic glands 1. 1. Chief cells or pepsinogen cells 2. 2. Parietal cells 3. 3. Mucus neck cells 4. 4. Enterochromaffin (EC) cells GLANDS OF STOMACH – GASTRIC GLANDS  Gastric glands are classified into three types, on the basis of their location in the stomach: 2. Pyloric glands: Present in the pyloric part of the stomach These glands are formed by G cells, mucus cells, EC cells and ECL cells. 3. Cardiac glands: Located in the cardiac region of the stomach with many mucus cells, EC cells, ECL cells and chief Cells. GLANDS OF STOMACH – GASTRIC GLANDS  Enteroendocrine Cells Enteroendocrine cells are the hormone-secreting cells present in the glands or mucosa of gastrointestinal tract, particularly stomach and intestine. The enteroendocrine cells present in gastric glands are G cells, EC cells and ECL cells.  GASTRIC JUICE  PROPERTIES OF GASTRIC JUICE  Volume : 1200 mL/day to 1500 mL/day.  Reaction : Gastric juice is highly acidic with a pH of 0.9 to 1.2. Acidity of gastric juice is due to the presence of hydrochloric acid (HCL).  Specific gravity : 1.002 to 1.004  GASTRIC JUICE  COMPOSITION OF GASTRIC JUICE  SECRETION OF GASTRIC JUICE  SECRETION OF HYDROCHLORIC ACID  It is an ACTIVE PROCESS.  Carbon dioxide is derived from metabolic activities of parietal cell or from the blood.  It combines with water to form carbonic acid in the presence of carbonic anhydrase (This enzyme is present in high concentration in parietal cells). Carbonic acid splits into hydrogen ion and bicarbonate ion.  The hydrogen ion is actively pumped into the canaliculus of parietal cell.  SECRETION OF GASTRIC JUICE  SECRETION OF HYDROCHLORIC ACID  Simultaneously, the chloride ion is also pumped into canaliculus actively. The chloride is derived from sodium chloride in the blood.  The hydrogen ion combines with chloride ion to form hydrochloric acid.  To compensate the loss of chloride ion, the bicarbonate ion from parietal cell enters the blood and combines with sodium to form sodium bicarbonate.  The entire process is summarized as:  CO2+ H2O + NaCl → HCl + NaHCO3  SECRETION OF GASTRIC JUICE  Factors Stimulating the Secretion of Hydrochloric Acid 1. Gastrin 2. Histamine 3. Vagal stimulation.  Factors Inhibiting the Secretion of Hydrochloric Acid 1. Secretin 2. Gastric inhibitory polypeptide 3. Peptide YY  FUNCTIONS OF GASTRIC JUICE 1. DIGESTIVE FUNCTION Pepsin  Pepsin is secreted as inactive pepsinogen. Pepsinogen is converted into pepsin by HCL. Optimum pH for activation of pepsinogen is below 6.  Pepsin converts proteins into proteoses, peptones and polypeptides. Gastric Lipase  Gastric lipase is a weak lipolytic enzyme when compared to pancreatic lipase. It is a tributyrase and it hydrolyzes tributyrin (butter fat) into fatty acids and glycerols  FUNCTIONS OF GASTRIC JUICE 1. DIGESTIVE FUNCTION Gelatinase Urase Gastric amylase Rennin  FUNCTIONS OF GASTRIC JUICE 2. FUNCTIONS OF HYDROCHLORIC ACID Hydrochloric acid is present in the gastric juice: i. Activates pepsinogen into pepsin ii. Kills some of the bacteria entering the stomach along with food substances. This action is called bacteriolytic action iii. Provides acid medium, which is necessary for the action of hormones.  FUNCTIONS OF GASTRIC JUICE 3. HEMOPOIETIC FUNCTION  Intrinsic factor of Castle, secreted by parietal cells of gastric glands plays an important role in erythropoiesis.  It is necessary for the absorption of vitamin B12 (which is called extrinsic factor) from GI tract into the blood.  Vitamin B12 is an important maturation factor during erythropoiesis. It`s absence in gastric juice causes deficiency of vitamin B12, leading to pernicious anemia.  FUNCTIONS OF GASTRIC JUICE 4- PROTECTIVE FUNCTION – FUNCTION OF MUCUS i. Protects the stomach wall from irritation or mechanical injury, due to its high viscosity. ii. Prevents the digestive action of pepsin on the wall of the stomach, particularly gastric mucosa. iii. Protects the gastric mucosa from hydrochloric acid of gastric juice because of its alkaline nature. hormone stimulus Source of secretion Action Gastrin Proteins in antrum of stomach Stimulates gastric secretion stomach and motility Secretin Acids in small intestine Stimulates secretion of watery duodenum and alkaline pancreatic secretion Cholecystokini Fat in small intestine Contracts gallbladder n duodenum Stimulates pancreatic secretion with enzymes Gastric Glucose in small intestine Stimulates insulin secretion inhibitory duodenum peptide (GIP) Thank you