Digestive System Lecture Notes PDF | Charmo University 2024

L6: Digestive System Part 1 Human Physiology Charmo University 2024  Digestive System (gastrointestinal tract GIT) The digestive system provides nutrients for cell maintenance and growth. The GIT can organised into: 1. Alimentary canal (Digestive tract) (mouth, pharynx, oesophagus...

L6: Digestive System Part 1 Human Physiology Charmo University 2024  Digestive System (gastrointestinal tract GIT) The digestive system provides nutrients for cell maintenance and growth. The GIT can organised into: 1. Alimentary canal (Digestive tract) (mouth, pharynx, oesophagus, stomach, small intestine and large intestine, rectum, and anal canal). 2. Accessory digestive organs (Teeth, tongue, salivary glands, the liver, the gallbladder and the pancreas).  The activities of the digestive system The activity of the digestive system can be categorised into 5 processes: 1. Ingestion 2. Propulsion 3. Digestion 4. Secretion 5. Absorption 6. Defecation  The activities of the digestive system 1.Ingestion: the oral cavity allows food to enter the digestive tract and have mastication (chewing) occurs , and the resulting food bolus is swallowed. 2. Propulsion: swallowing and peristalsis Peristalsis: involuntary, alternating waves of contraction and relaxation of muscles in the organ walls Segmentation: regions of the small intestine contracting and relaxing independently, allowing the small intestine to digestive and absorb more efficiently. Peristalsis and Segmentation Esophagus Intestin  The activities of the digestive system 3. Digestion a. Mechanical digestion : muscular movement of the digestive tract that physically break down food into smaller particles. b. Chemical digestion : hydrolysis reactions aided by enzymes (mainly in the stomach and small intestine) chemically break down food particles into nutrient molecules , small enough to be absorbed. Some nutrients are absorbed without being digested: vitamins, free amino acids, minerals, cholesterol, and water. Carbohydrates → monosaccharides Proteins → amino acids Fats → glycerol and fatty acids  The activities of the digestive system 4. Secretions: consist of water, electrolytes, and specific organic constituents Secretions are released into digestive tract lumen on appropriate neural or hormonal stimulation 5.Absorption: the uptake of nutrient molecules into the epithelial cells of the digestive tract and then into the blood or lymph 6.Defecation: the waste products of digestion are excreted from the body as faeces.  Mouth & Oral Cavity Food is broken down into smaller particles by mechanical digestion achieved by the cutting and grinding action of the teeth during the process of mastication. § One chemical digestive process occur where amylase enzyme in saliva breaks down polysaccharide into disaccharides. § The tongue , made of skeletal muscle, manipulates the food during mastication. it also contains taste buds to detect taste sensations. § Food particles are mixed with saliva during mastication, resulting in a moist lump called bolus for easier passage into or pharynx.  The Salivary glands 3 paired salivary glands made of ducts and acini that make between 1-1.5 liters/day with a pH of 6-7 1. Parotid glands 2. Submandibular or submaxillary glands 3. Sublingual glands 12/18/23 GIT physiology 17 2nd year Lab & Nutrition  Composition of Saliva 1. Mostly water 99.5% 2. Ionic content: low in Na+ and Cl-, high in K+ and HCO3- 3. Enzymes: lingual lipase and a-amylase (ptyalin). 4. Mucins 5. Lysozymes: proteolytic enzyme 6. Immunoglobulin A (IgA) 12/18/23 GIT physiology 2nd year Lab & Nutrition  Functions of Saliva Digestive functions: Amylase breaks down starch, and lingual lipase breaks down triglycerides (active in the stomach). It dissolves food materials so it can be sensed by taste buds. Lubrication: Keeps mouth moist and thus facilitates movements of lips and tongue during speech. Moistens food and thus facilitates swallowing. Protection: Keeps mouth and teeth clean by dissolving and washing food particles from between the teeth. Has an anti-bacterial action. 12/18/23 Buffers acidic gastric secretions. GIT physiology 19 2nd year Lab & Nutrition Salivary Glands  Pharynx and Esophagus Swallowing Motility associated with pharynx and esophagus Initiated when bolus is voluntarily forced by tongue to rear of mouth into pharynx Can be initiated voluntarily but cannot be stopped once it has begun Process divided into two stages 1. Oropharyngeal stage 2. Esophageal stage (moves bolus from mouth through pharynx and into esophagus) Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning  Esophagus Esophagus Fairly straight muscular tube Extends between pharynx and stomach Peristaltic waves push food through esophagus Secretions (mucus) are entirely protective Function: swallowing or deglutition, which is the process of moving food from the mouth through the esophagus into the stomach No digestion or absorption Secretions: mucus  Esophagus Has sphincters at each end 1. Pharyngoesophageal sphincter Keeps entrance closed to prevent large volumes of air from entering esophagus and stomach during breathing 2. Gastroesophageal sphincter Prevents reflux of gastric contents 12/18/23  Swallowing Swallowing can be initiated voluntarily but then it is under reflex control. Swallowing reflex: Receptors: touch receptors in pharynx. Afferent: sensory impulses from receptors. Centre: swallowing centre in brain stem. Efferent: parasympathetic nerves to muscles of pharynx and esophagus. 12/18/23 GIT physiology 26 2nd year Lab & Nutrition  Mechanism of Swallowing 3 stages: 1. Oral or Voluntary: bolus of food is passed into the pharynx by upward and backward movement of tongue against palate. This stimulates the touch receptors that initiate the swallowing reflex. 2. Pharyngeal: involuntary passage of bolus through the pharynx into esophagus. Respiratory passageways are closed & respiration is inhibited (protective reflexes). 3. Esophageal: involuntary passage of bolus from esophagus 12/18/23 to stomachGITby peristaltic movements27 of physiology esophagus. 2nd year Lab & Nutrition  Stomach § A pouch-like organ primarily designed for food storage (for 2-4 hours) , some mechanical and chemical digestion also occur. § Contains two sphincters at both ends to regulate food movement – cardiac sphincter near the esophagus ,and pyloric sphincter near the small intestine. § Divided into 4 regions : 1. cardiac stomach (or cardiac), 2. fundic stomach (or funded) , 3. body of stomach 4. pyloric stomach (or Pylorus). § Contain thick folds called rugae at its layer , for providing larger surface area for expansion , secretion , digestion , and some absorption.  Stomach Muscularis – has an additional oblique layer that: 1. Allows the stomach to churn, mix, and pummel food physically 2. Breaks down food into smaller fragments  The Stomach Major Functions of the Stomach 1. Storage of ingested food 2. Mechanical breakdown of ingested food 3. Disruption of chemical bonds in food material by acid and enzymes 4. Production of intrinsic factor, a glycoprotein required for absorption of vitamin B12 in small intestine Gastric Emptying and Mixing as a Result of Antral Peristaltic Contractions Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning  Gastric Secretory Cells Gastric secretion: 3 L/day, highly acidic (pH 1-2). 1. Mucous cells: secrete mucus and alkaline substances to help neutralize HCl in the gastric juice. 2. Parietal cells: secrete hydrochloric and (HCl) and "intrinsic factor" (which helps absorption of vitamin B12 in the intestines). 3. Chief cells: secrete pepsinogen (an inactive enzyme). 4. D cells: produce somatostatin which inhibit gastric acid secretion 5. G cells: secrete a hormone called gastrin , which stimulates the parietal cells and overall gastric secretion Gastric Cells Gastric Cells  HCl Secretion KEY 1 Diffusion Hydrogen ions (H+) are generated inside a parietal cell as the enzyme Parietal cell Carrier-mediated carbonic anhydrase converts CO2 transport and H2O to carbonic acid (H2CO3), CO2 + H2O which then dissociates. Active transport Carbonic Countertransport anhydrase 2 4 A countertransport mechanism The hydrogen ions are ejects the bicarbonate ions into H2CO3 actively transported into the the interstitial fluid and imports lumen of the gastric gland. chloride ions into the cell. HCO3− HCO3− + H+ H+ Interstitial fluid Cl− Cl− Cl− 3 Alkaline The chloride ions then diffuse tide across the cell and exit through Lumen of open chloride channels into the gastric Enters lumen of the gastric gland. gland bloodstream  HCl Functions Kills ingested bacteria. Aids protein digestion (activates pepsinogen into pepsin). Provides the optimum pH for pepsin action. Stimulates the secretion of hormones that promote the flow of bile and pancreatic juice. Protective Mechanisms of Stomach The stomach is exposed to the harshest conditions in the digestive tract To keep from digesting itself, the stomach has a mucosal barrier with: 1. Alkaline mucus contains HC03-. 2. Epithelial cells that are joined by tight junctions 3. Parietal and chief cells impermeable to HCl. Damaged epithelial cells are quickly replaced (3days). Chemical Digestion & Absorption in the Stomach -Carbohydrate digestion is continued with gastric amylase , resulting in disaccharides. -Protein digestion begins with pepsin (activation of pepsinogen by HCl) , resulting in peptides (small chains of protein). -Lipid digestion begins with gastric lipases which can only break down certain lipids such as butterfat , resulting in fatty acids. -Absorption in the stomach is limited, where only small and fat- soluble substances can be absorbed—water , alcohol, aspirin , and certain drugs. -The result of all these mixing , chemical digestion , secretion, and absorption is a yellowish paste called chyme , which will be passed on to the small intestine.  Phases Of Gastric Secretion 1) Cephalic phase: is activated by the thought, taste, smell and sight of food, and swallowing, mediated mostly by cholinergic/vagal mechanisms. 2) Gastric phase is due to the chemical effects of food and distension of the stomach. Gastrin appears to be the major mediator since the response to food is largely inhibited by blocking gastrin action at its receptors. 3) intestinal phase accounts for only a small proportion of the acid secretory response to a meal; Stomach: Neural & Hormonal Mechanisms  Regulation of Gastric Secretion Vagus Cephalic Intestinal Gastric G cells Gastrin Acidic Secretin ECL Fat CCK Histamine Parietal HCL D cells Somatostatin