BIOL 221 - Digestive System I Lecture Notes PDF

BIOL 221 – Digestive System I Cristofre Martin Department of Biochemistry St. George’s University  Learning objectives The student should be able to: Describe the oral cavity, with emphasis on the salivary glands, and the functions of the saliva Trace the food from the pharynx to the...

BIOL 221 – Digestive System I Cristofre Martin Department of Biochemistry St. George’s University  Learning objectives The student should be able to: Describe the oral cavity, with emphasis on the salivary glands, and the functions of the saliva Trace the food from the pharynx to the esophagus, identifying the structures protecting the nasopharynx and larynx, and the role of muscles in swallowing Describe the histology of the esophagus. List the regions of the stomach. Describe the muscular movements of the stomach and the layers of muscle responsible for these actions. List the various cells found in the stomach and their function. Describe how the gastric glands are stimulated to release their secretions. Discuss ulcers,identifying the main cause. How can aspirin cause ulcers?  Salivary The Digestive glands Mouth System is Composed of: Esophagus mouth salivary glands Gall- bladder Stomach oesophagus stomach Small liver Liver intestine gall-bladder Pancreas Large pancreas intestine small intestine Rectum Anus large intestine A schematic diagram of the rectum human digestive system anus The digestive system is consists of 4 layers: mucosa submucosa muscularis externa serosa or adventitia  Mucos a Epithelium Loose connective tissue (the lamina propria) Smooth muscle (muscularis mucosa) Functions: lumen Protection Secretion Absorption  Submuco sa Loose connective tissue Blood vessels Lymphatic vessels Nerves Functions: Nourish surrounding tissue Transport absorbed materials  Muscularis Externa Inner circular layer Outer longitudinal layer Function: Control movements of the tube.  Serosa or Serosa: outer epithelium and some underlying connective tissue. Adventitia Function: serosa secretes watery fluid, enabling abdominal organs to glide freely. Adventitia: connective tissue only. Adventitia is found around the esophagus and in retroperitoneal portions of intestines (behind abdominal cavity).  The GI Pathway; Entry Food enters through the mouth. Salivary glands secrete saliva into mouth. Saliva contains HCO3- which helps maintain the pH around 6.5-7.5. Saliva consists of : mucus secretion watery serous secretion containing amylase starch/ glycogen amylase  Parotid duct Parotid gland The salivary glands are a. Parotid gland: serous (contains amylase) b. Submandibular: primarily serous (contains amylase), some mucus c. Sublingual: mucus. Hard palate Soft palate Palatine tonsil Uvula Tongue Lingual frenulum Opening, submandibular gland duct  Nervous control of salivary glands: Stimulation: thought, smell, taste of “good” food Inhibition: unappetising food. Mumps (viral): inflamed parotid glands May affect pancreas, gonads, submandibular glands. What are gonads? Why do we care if they are inflamed?  Pharynx The pharynx connects nasal and oral cavities with larynx and oesophagus. Pharynx is divided into – Nasopharynx – Oropharynx – Laryngopharynx  Nasopharynx Oropharynx Epiglottis Laryngopharynx Glottis Larynx Inferior constrictors Esophagus Trachea  Food Tongue Epiglottis down Epiglottis up Glottis Larynx Esophagus Glottis up Inferior and covered constrictors contracted Inferior Trachea constrictors relaxed Food is chewed, moistened, passed to pharynx. Uvula is raised, protecting nasopharynx. Hyoid, larynx are raised, epiglottis shields glottis, protects trachea. Tongue presses against palate, protecting oral cavity. Longitudinal pharyngeal muscles contract, pull pharynx up. The oesophagus is ~25 cm long. It penetrates the diaphragm at the Tongue Salivary glands oesophageal hiatus, and is Esophagus continuous with the stomach. Liver The stomach’s cardiac Gall- sphincter impedes bladder Stomac regurgitation. Acid Pancreas h Small intestine reflux (gastric juice Large entering the oesophagus) intestine can cause oesophageal Rectum Anus inflammation (heartburn). Oesopha gus Only two areas of the digestive Submucosal tract have gland submucosal glands: oesophagus and the duodenum. Both secrete mucus. Why does the esophagus have Oesophageal muscle: upper 1/3 is skeletal middle 1/3 is mixed lowest 1/3 is smooth muscle  Stomach The stomach is a pouch-shaped organ located in the upper left quadrant of the abdominal cavity. The inner lining has thick folds (rugae) that disappear when it is distended. Capacity ~1 liter The stomach receives food from the oesophagus and: Rugum, a fold of the Mixes it with gastric stomach lining. juice Initiates protein digestion  Movements of the digestive tract A. propulsion: peristaltic movements occur throughout the GI tract. – A ring of constriction appears where the circular muscle is contracting. Just ahead of it, the muscular wall relaxes. The wave of contraction sweeps along, pushing the contents of the tube ahead of it. – The usual stimulus for peristalsis is food.  Movements of the stomach B. Mixing movements occur in the stomach, which has a third, oblique muscle layer. Waves of muscular contraction along the stomach wall mix the food with gastric secretions. Muscle layers of the stomach  Muscle contractions move stomach contents toward the pyloric sphincter. Small quantities of chyme slip through, but the sphincter quickly contracts, and most chyme is propelled backward. This back/forth motion continues. As more chyme enters the duodenum, it stretches the intestinal wall, stimulating the enterogastric reflex, slowing peristalsis, slowing the filling of the small intestine. If chyme has a high fat content, cholecystokinin is also released from the intestine, slowing peristalsis more. Very little absorption occurs in the stomach; only water, some drugs  Stomach regions Histologically, the stomach is divided into: – cardiac-portion around the oesophagus – fundus-pouch like area, in the superior stomach region – pylorus-region connecting to small intestine. Regions of the stomach are classified differently by anatomists and histologists C F P  Cells of the stomach: Gastric glands contain five types of cells: chief cells: found only in the fundus and produce pepsinogen, rennin, gastric lipase parietal cells: produce HCl and Intrinsic factor (functions in B12 absorption. B12 is required for erythropoiesis (making red blood cells). Lack of Intrinsic factor cause anaemia). mucous cells: mucus and produce some  Pyloric sphincter As the pylorus nears the small intestine, it narrows. The circular layer of muscle thickens, forming the powerful pyloric sphincter. Two infant abnormalities of the pyloric sphincter are: Pylorospasm: sphincter muscle does not relax. Only tiny quantities pass through, baby vomits frequently to get relief. Drugs relax the sphincter. Pyloric stenosis: abnormal enlargement of circular muscles narrows sphincter. This must be surgically corrected.  Note gastric pits containing mucous cells and tubular glands with additional cell types.  Pepsin Pepsin (from chief cells) is the most important gastric enzyme. It is secreted as the inactive pepsinogen; it is activated by increased acidity. Pepsin hydrolyses most peptide bonds, and as polypeptides accumulate the hormone gastrin will be secreted, stimulating more release of pepsinogen and HCl (from parietal cells).  Gastrin release: Increase: cephalic- stimulated by the brain gastric- stimulated by the presence of polypeptides intestinal-stimulated by the initial presence of chyme in the small intestine. Decrease: Stretching of small intestine: enterogastric reflex decreases gastrin secretion.  Ulcers Mucus lines the stomach, protecting it from HCl and pepsin, which can hydrolyse cellular proteins. If mucus release is suppressed by anti- inflammatory drugs, such as aspirin, HCl: a. makes a hole in the epithelium b. stimulates underlying mast cells to release histamine. Histamine stimulates HCl release, increasing the problem. This is a source of peptic ulcers.  The major source of ulcers is Helicobacter pylori. In 1982, two young Australian physicians, Barry J. Marshall and J. Robin Warren, proposed that a bacterium, Helicobacter pylori, is the prime cause of stomach ulcers.  The medical community believed ulcers were due to stress, and rejected their hypothesis....... so they drank a culture of the suspect bacteria, and developed gastritis. At last other experimenters took them seriously! (They found an antibiotic that killed h.pylori before swallowing the bacteria.) Further studies confirmed their hypothesis. Today a two week course of antibiotics can heal most ulcers.  Vomiting Vomiting-expulsion of stomach contents via mouth. The vomiting centre is in the medulla oblongata. Process: soft palate is raised; cardiac sphincter is relaxed, muscles of abdomen and diaphragm contract. Prolonged vomiting: disturbance of fluid and acid/base balances in the body.