Gastroenterology - Human Anatomy - Lecture Notes, Study notes of Anatomy

Download Gastroenterology - Human Anatomy - Lecture Notes and more Study notes Anatomy in PDF only on Docsity! A. Introduction 1. Digestion is the breaking down of larger food molecules into molecules that are small enough to enter body cells and absorption is the passage of these small molecules into blood and lymph; the organs that collectively perform these functions comprise the digestive system. 2. Gastroenterology is the medical specialty that deals with the structure, function, diagnosis, and treatment of diseases of the stomach and intestines; proctology is the medical specialty that deals with the diagnosis and treatment of disorders of the rectum and anus. B. Overview of the Digestive System 1. The organs of the digestive system are divided into two groups: i. gastrointestinal (GI) tract or alimentary canal, which is a tube that extends from the mouth to the anus through the ventral body cavity; it includes the following organs: a. mouth b. most of the pharynx c. esophagus d. stomach e. small intestine f. large intestine ii. accessory digestive organs, which include: a. teeth b. tongue c. salivary glands d. liver e. gallbladder f. pancreas 2. The digestive system performs six major activities: i. ingestion (eating) ii. secretion of water, acid, buffers, and digestive enzymes iii. mixing of food with secretions and propulsion of food along the GI tract, i.e., motility iv. digestion of food by both mechanical and chemical processes v. absorption of digested food molecules into the blood and lymph vi. defecation of wastes, indigestible substances, and bacteria in the form of feces 3. Mechanical digestion includes: i. chewing of food by the teeth before it is swallowed ii. churning of food by the smooth muscles of the stomach and small intestine so that it is thoroughly mixed with digestive enzymes Docsity.com 4. Chemical digestion is a series of catabolic reactions in which enzymes break down large food molecules, i.e., carbohydrates, lipids, protein, and nucleic acids, into smaller molecules that may be absorbed and used by body cells. C. Layers of the GI Tract 1. The wall of the GI tract, from the esophagus to the anal canal, consists of four major layers of tissue which, listed in sequence from innermost to outermost, are: i. mucosa, which is a mucous membrane that surrounds the lumen; it consists of three sublayers: a. inner lining layer of epithelium - nonkeratinized stratified squamous epithelium in the mouth, pharynx, esophagus, and anal canal provides protection - simple columnar epithelium throughout the rest of the GI tract functions in secretion and absorption; included are hormone-secreting enteroendocrine cells b. middle layer of areolar connective tissue called the lamina propria - contains blood vessels, lymphatic vessels, and scattered lymphatic nodules - contains most components of the mucosa-associated lymphoid tissue (MALT) c. outer layer of smooth muscle tissue called the muscularis mucosae - creates small folds in the stomach and intestinal mucosa that increase the surface area for digestion and absorption ii. submucosa, which consists of areolar connective tissue a. it is highly vascular b. it contains a portion of the submucosal plexus (plexus of Meissner) which is a component of the enteric nervous system (ENS) c. this plexus innervates the mucosa and submucosa and plays an important role in regulating: - movements of the mucosa and vasoconstriction of blood vessels - secretions by the GI tract d. it may also contain glands and lymphatic tissue iii. muscularis, which consists of muscle tissue a. in the mouth, pharynx, upper and middle parts of the esophagus, and the external anal sphincter, it consists of skeletal muscle tissue which produces voluntary swallowing and voluntary control of defecation b. throughout the rest of the GI tract, it consists of smooth muscle tissue that is generally divided into two sheets: - inner sheet of circular fibers - outer sheet of longitudinal fibers c. contractions help break down food physically, mix it with digestive secretions, and propel it along the tract d. it contains the myenteric plexus (plexus of Auerbach) which is a component of the enteric nervous system (ENS); this plexus primarily controls GI tract motility Docsity.com vi. Chemical stimulation (by food molecules) of receptors in the taste buds results in nerve impulses being transmitted to the superior and inferior salivatory nuclei in the brain stem; returning parasympathetic impulses in the facial (VII) and glossopharyngeal (IX) nerves stimulate secretion of saliva; the sight, smell, sound, or thought of food may also stimulate increased saliva secretion. 3. Tongue: i. The tongue and its associated muscles form the floor of the oral cavity a. it is composed of skeletal muscles covered by mucous membrane b. it is divided into symmetrical lateral halves by a median septum that is attached inferiorly to the hyoid bone, styloid process of the temporal bone, and mandible c. it consists of extrinsic and intrinsic muscles d. the extrinsic muscles originate outside the tongue and insert into it - they move the tongue for food manipulation during chewing and swallowing - they also form the floor of the mouth and hold the tongue in position e. the intrinsic muscles originate and insert within the tongue - they alter the shape and size of the tongue for speech and swallowing f. the lingual frenulum is a fold of mucous membrane that attaches the midline of the inferior surface of the tongue to the floor of the mouth - it limits movement of the tongue posteriorly - if the lingual frenulum is too short or rigid, ankyloglosia occurs (being “tongue-tied”) g. the superior and lateral surfaces of the tongue are covered with papillae, which are projections of the lamina propria covered with epithelium - fungiform papillae are mushroom-like projections distributed among the filiform papillae and are more numerous near the tip of the tongue - they appear red and most of them contain taste buds - vallate (circumvallate) papillae are arranged in the form of an inverted V on the posterior surface of the tongue - all of them contain taste buds - foliate papillae, on a tongue’s lateral margins, have few taste buds in adults - filiform papillae are cone-shaped projections distributed in parallel rows over the anterior two-thirds of the tongue - they appear whitish and they lack taste buds - they increase friction between the tongue and food, making it easier for the tongue to move food 4. Teeth (Dentes): i. The teeth are located in sockets of the alveolar processes of the mandible and maxillae; the alveolar processes are covered by the gingivae or gums, which extend slightly into each socket to form the gingival sulcus ii. The sockets are lined by the periodontal ligament or membrane which anchors the teeth in position and acts as a shock absorber during chewing. iii. A typical tooth consists of three major regions: a. crown is the visible portion located above the level of the gums b. one to three roots are embedded in the socket c. neck is the narrow junction line of the crown and root near the gum line iv. A tooth is composed of several substances: a. dentin forms most of the mass of a tooth - it consists of calcified connective tissue that is harder than bone tissue Docsity.com b. the dentin of the crown encloses a pulp cavity that is filled with pulp, which consists of connective tissue containing blood vessels, lymphatic vessels, and nerves - root canals are narrow extensions of the pulp cavity that travel through the root; at the end of each root canal is an opening called the apical foramen through which blood vessels, lymphatic vessels, and nerves enter the tooth c. the dentin of the crown is covered by enamel - it is composed of calcium salts and it is the hardest substance in the body - it protects the tooth against the wear of chewing and it is a barrier against acids that may easily dissolve the dentin d. the dentin of the root is covered by cementum - it is a bonelike substance that attaches the root to the periodontal ligament v. Blood supply to the teeth is provided by the anterior and posterior superior alveolar branches of the maxillary arteries and by the incisive and dental branches of the inferior alveolar arteries. vi. The teeth receive sensory fibers from branches of the maxillary and mandibular divisions of the trigeminal (V) nerves. vii. Endodontics is the branch of dentistry concerned with the prevention, diagnosis, and treatment of diseases that affect the pulp, root, periodontal ligament, and alveolar bone; orthodontics is the branch of dentistry concerned with the prevention and correction of abnormally aligned teeth; periodontics is the branch of dentistry concerned with the treatment of abnormal conditions of tissues immediately surrounding the teeth (e.g., gums) viii. Humans have two sets of teeth or dentitions: a. deciduous teeth, primary teeth, milk teeth, or baby teeth - begin to erupt at about 6 months of age, and one pair appears at about each month thereafter until all 20 teeth are present - each row contains: - 2 central incisors which are adapted to cut into food - each has one root - 2 lateral incisors which are also adapted to cut into food - each has one root - 2 cuspids (canines) which are adapted to tear and shred food -each has one root - 2 first molars which are adapted to crush and grind food -each has 4 cusps - upper ones have 3 roots; lower ones have 2 roots - 2 second molars which also crush and grind food -each has 4 cusps - upper ones have 3 roots; lower ones have 2 roots b. deciduous teeth are gradually lost between 6 and 12 years of age and are replaced by the permanent (secondary) teeth - the 32 permanent teeth appear between age 6 and adulthood - each row contains: - 2 central incisors - 2 lateral incisors - 2 canines - 2 first premolars (bicuspids) Docsity.com - replace the deciduous first molars - each has 2 cusps - upper ones have 2 roots; lower ones have 1 root - 2 second premolars (bicuspids) - replace the deciduous second molars - each has 2 cusps - each has 1 root - 2 first molars - 2 second molars - 2 third molars (wisdom teeth) ix. The following terms are used in dental terminology to describe the surface orientation of teeth (see Table 25.1): a. labial b. buccal c. lingual d. palatal e. mesial f. distal g. occlusal F. Pharynx 1. Mastication or chewing reduces the food to a soft, flexible mass called a bolus that is swallowed. 2. In deglutition or swallowing, the bolus of food first enters the pharynx, the funnel-shaped tube that extends from the internal nares to the esophagus posteriorly and the larynx anteriorly. 3. The pharynx is composed of skeletal muscle covered by mucous membrane. 4. Muscular contractions of the oropharynx and laryngopharynx help propel the bolus of food into the esophagus. G. Esophagus 1. The esophagus is a muscular, collapsible tube that travels from the laryngopharynx down through the mediastinum anterior to the spine and posterior to the trachea, through the esophageal hiatus in the diaphragm, and ends in the superior portion of the stomach; sometimes, a portion of the stomach protrudes through the esophageal hiatus resulting in a hiatal hernia. 2. Blood is supplied to the esophagus by the inferior thyroid, thoracic aorta, intercostal, phrenic, and left gastric arteries; blood drains into adjacent veins. 3. The esophagus is innervated by the recurrent laryngeal nerves, the cervical sympathetic chain, and the vagus (X) nerves. 4. Histology: The wall of the esophagus consists of the following layers which, listed in sequence from innermost to outermost, are: i. mucosa a. in addition to standard features of mucosa, it also contains mucous glands near the stomach ii. submucosa a. in addition to standard features of submucosa, it also contains mucous glands iii. muscularis a. upper third is skeletal muscle tissue b. middle third is skeletal and smooth muscle tissue Docsity.com 4. The pancreas receives its blood supply from the superior and inferior pancreaticoduodenal arteries and the splenic and superior mesenteric arteries; blood drains into veins of the same name and eventually into the hepatic portal vein. 5. The nerves to the pancreas are autonomic nerves derived from the celiac and superior mesenteric plexuses; included are preganglionic vagal, postganglionic sympathetic, and sensory fibers. 6. Histology: i. about 1% of the glandular epithelial cells are organized into clusters called pancreatic islets (islets of Langerhans) a. they form the endocrine portion of the pancreas and secrete the following hormones which were studied in Chapter 23: - glucagon - insulin - somatostatin - pancreatic polypeptide ii. the remaining 99% of the glandular epithelial cells are arranged in clusters called acini a. they form the exocrine portion of the pancreas and secrete a mixture of fluid and digestive enzymes called pancreatic juice 7. The major exocrine function of the pancreas is to secrete pancreatic juice which includes the following components: i. water ii. some salts iii sodium bicarbonate, which buffers acidic gastric juice in chyme, stops the action of pepsin, and provides the proper pH for the action of digestive enzymes in the small intestine iv. several digestive enzymes: a. pancreatic amylase, which digests carbohydrates b. trypsin, which digests proteins c. chymotrypsin, which also digests proteins d. carboxypeptidase, which also digests proteins e. elastase, which also digests protein f. pancreatic lipase, which digests triglycerides g. ribonuclease, which digests RNA h. deoxyribonuclease, which digests DNA Docsity.com J. Liver 1. The liver is the heaviest gland of the body and, after the skin, the second largest organ of the body; it is located under the diaphragm in the upper right portion of the abdominal cavity. 2. It is completely covered by a layer of dense irregular connective tissue that is, in turn, almost completely covered by visceral peritoneum. 3. The liver consists of two major lobes: i. large right lobe ii. smaller left lobe; associated with the left lobe are two smaller lobes: a. inferior quadrate lobe b. posterior caudate lobe - the right and left lobes are separated by the falciform ligament, which is a fold of the parietal peritoneum that extends from the undersurface of the diaphragm to the superior surface of the liver to be located between the two major lobes of the liver; in the free border of the falciform ligament is the ligamentum teres (round ligament), which extends from the liver to the umbilicus - the coronary ligaments suspend the liver from the diaphragm 4. Histology: i. the lobes consist of functional units called lobules that are composed of specialized epithelial cells called hepatocytes ii. the hepatocytes are arranged in irregular, branching, interconnected plates around a central vein that receives blood from sinusoids iii. the sinusoids are lined by endothelium and partly by phagocytic cells called stellate reticuloendothelial (Kuppfer’s) cells which destroy worn-out leukocytes, erythrocytes, bacteria, and other foreign matter iv. hepatocytes secrete bile, which is partially an excretory product and partially a digestive secretion; bile has several important components including: a. bile salts play a role in emulsification of fats and absorption of the products of fat digestion b. cholesterol c. bile pigments, notably bilirubin that is derived from heme; one of the breakdown products of bilirubin gives feces their normal brown color v. bile is secreted into bile canaliculi that empty into small bile ductules which in turn pass bile into bile ducts vi. these small ducts merge to eventually form the larger right and left hepatic ducts, which unite to form the common hepatic duct that exits the liver vii. the common hepatic duct merges with the cystic duct from the gallbladder to form the common bile duct; bile enters the cystic duct and is temporarily stored in the gallbladder 5. The liver receives blood from two sources, and its blood drains into one set of veins: i. the hepatic artery provides oxygenated blood ii. the hepatic portal vein delivers deoxygenated blood containing newly absorbed nutrients from the GI tract - branches of these two blood vessels carry blood to the sinusoids, where oxygen, most nutrients and certain toxins are absorbed by the hepatocytes - products manufactured by hepatocytes and nutrients needed by other cells are secreted back into the blood, which then drains into the central vein and eventually into a hepatic vein - branches of the hepatic portal vein, hepatic artery, and bile duct typically accompany each other and collectively form a portal triad Docsity.com 6. The nerve supply to the liver consists of parasympathetic innervation from the vagus (X) nerves and sympathetic innervation from the greater splanchnic nerves through the celiac ganglia. 7. The functions of the liver include: i. carbohydrate metabolism ii. lipid metabolism iii. protein metabolism iv. processing of drugs and hormones v. excretion of bilirubin vi. synthesis of bile salts vii. storage of numerous substances including glycogen, certain vitamins, and some minerals viii. phagocytosis ix. activation of vitamin D K. Gallbladder 1. The gallbladder is a pear-shaped sac that is located in a depression of the posterior surface of the right lobe of the liver; it consists of a broad fundus, a central body, and a tapering neck. 2. Histology: i. the mucosa consists of simple columnar epithelium arranged in rugae ii. it lacks a submucosa iii. the muscularis, composed of smooth muscle tissue, contracts upon hormonal stimulation to eject stored bile into the cystic duct iv. the outer layer is the visceral peritoneum 3. The functions of the gallbladder are to store and concentrate bile until it is needed in the small intestine. 4. When the small intestine is empty, the sphincter of the hepatopancreatic ampulla or sphincter of Oddi closes; this causes bile to accumulate in the common bile duct and enter the cystic duct and the gallbladder where it is stored. 5. When food enters the duodenum, the hormone cholecystokinin (CCK) stimulates contraction of the wall of the gallbladder and relaxation of the sphincter of the hepatopancreatic ampulla; this causes bile to flow into the duodenum. 6. The gallbladder receives its blood supply from the cystic artery, which usually arises from the right hepatic artery; blood drains into cystic veins. 7. The gallbladder is innervated by branches from the celiac plexus and the vagus (X) nerves. L. Small Intestine 1. The small intestine extends from the pyloric sphincter to the large intestine; it averages 2.5 cm in diameter and about 3 m in length in a living person, but is about 6.5 m long in a cadaver. 2. It consists of three regions: i. duodenum a. is about 25 cm long b. is retroperitoneal ii. jejunum a. is about 1 m long iii. ileum a. is about 2 m long b. meets the large intestine at the ileocecal sphincter 3. The small intestine receives blood from the superior mesenteric artery and the gastroduodenal artery; blood drains into the superior mesenteric vein. 4. The nerves to the small intestine are supplied by the superior mesenteric plexus; branches of the plexus contain postganglionic sympathetic fibers, preganglionic parasympathetic Docsity.com i. the sympathetic innervation is derived from the celiac, superior, and inferior mesenteric ganglia and superior and inferior mesenteric plexuses; the fibers reach the viscera via the thoracic and lumbar splanchnic nerves ii. the parasympathetic innervation is derived from the vagus (X) and pelvic splanchnic nerves 7. Histology: i. the mucosa contains: a. simple columnar epithelium whose main function is water reabsorption b. goblet cells which secrete mucus; note that no enzymes are secreted c. solitary lymphatic nodules ii. the submucosa has no unique features iii. the muscularis consists of two layers of smooth muscle tissue a. an inner circular layer and an outer longitudinal layer; the latter has 3 thickened longitudinal bands called teniae coli b. tonic contractions of these bands create a series of pouches called haustra in the colon iii. the serosa has epiploic appendages which are small pouches of visceral peritoneum that are filled with fat and attached to the teniae coli 8. Functions: i. ileal peristalsis moves chyme through the ileocecal sphincter into the cecum ii. within the large intestine several types of movements occur: a. haustral churning b. peristalsis c. mass peristalsis iii. bacteria that live in the lumen perform several activities: a. ferment any remaining carbohydrates and release gases that contribute to flatus in the colon b. convert remaining proteins into amino acids and simpler substances, some of which contribute to the odor of feces c. decompose bilirubin to simpler pigments that give feces their brown color d. produce several vitamins including some B vitamins and vitamin K iv. due to absorption of water by epithelial cells, the chyme is converted into feces v. distension of the rectum by feces initiates a defecation reflex which involves a number of events: a. contraction of longitudinal rectal muscles by parasympathetic nerves b. opening of the internal anal sphincter by parasympathetic nerves c. voluntary relaxation of the external anal sphincter d. voluntary contractions of the diaphragm and abdominal muscles - diarrhea is an increase in the frequency, volume and fluid content of the feces; constipation refers to infrequent or difficult defecation Note: provides a Summary of Organs of the Digestive System and Their Functions. N. Development of the Digestive System 1. Cells of the endoderm form the primitive gut; the splanchnic mesoderm becomes associated with this endoderm. 2. Therefore, the primitive gut has a double-layered wall: i. endodermal layer, which develops into the epithelial lining and glands of most of the Gl tract ii. mesodermal layer, which develops into smooth muscle and connective tissue of the Gl tract Docsity.com 3. The primitive gut elongates and differentiates into an anterior foregut, an intermediate midgut, and a posterior hindgut. 4. A depression of the ectoderm called the stomodeum develops into the oral cavity; the oropharyngeal membrane ruptures so that the foregut is continuous with the oral cavity. 5. Another depression of the ectoderm called the proctodeum develops into the anus; the cloacal membrane ruptures so that the hindgut is continuous with the anus. 6. The foregut develops into the following organs: i. pharynx ii. esophagus iii. stomach iv. portion of the duodenum 7. The midgut develops into: i. remainder of the duodenum ii. jejunum iii. ileum iv. portions of the large intestine 8. The hindgut develops into: i. remainder of the large intestine 9. The endoderm of the foregut develops into: i. salivary glands ii. liver iii. gallbladder iv. pancreas O. Aging and the Digestive System 1. The general effects of aging on the digestive system include: i. decreased secretory mechanisms ii. decreased motility of the digestive organs iii. loss of strength and tone of the muscular tissue and its supporting structures iv. changes in neurosensory feedback v. diminished response to pain and internal sensations 2. There are numerous specific changes that may occur in the Gl tract and associated digestive organs with aging. P. Key Medical Terms Associated with the Digestive System 1. Students should familiarize themselves with the glossary of key medical terms. Docsity.com