The Digestive System Ch 25 PDF

The Digestive System Ch 25 Intro A muscular tube called the digestive tract, alimentary canal, with several accessory organs make up the digestive system These organs work together to perform the following functions: Mechanical processing Secretion Excretion Compaction The lining of the digestive tr...

The Digestive System Ch 25 Intro A muscular tube called the digestive tract, alimentary canal, with several accessory organs make up the digestive system These organs work together to perform the following functions: Mechanical processing Secretion Excretion Compaction The lining of the digestive tract also plays an important role by protecting against The corrosive effects of digestive juices, mechanical stress, and pathogens Histological Organization The major layers of the digestive tract are the: Submucosa Muscular Layer Mucosa: A membrane that is the innermost lining of the digestive tract Many segments of the digestive tract have circular folds, circulares, that increase the surface area for absorption or secretion In most regions the border of the mucosa is a narrow band of smooth muscle This is called the mucosae Movement of these muscles changes the shape of the lumen and moves the epithelial plates and folds Submucosa: A layer of connective tissue Contains large blood and lymphatic vessels Some regions also contain glands Secrete buffers and enzymes into the lumen Muscular Layer: A double layer of smooth muscle deep to the submucosa Muscle fibers arranged into internal circular and external longitudinal layers Muscles mechanically process and move materials within the digestive tract At specific areas there are thickened areas of the circular muscle layer Forms the sphincters Prevent backflow of materials and/or restrict movement of material Serosa: Found in areas of the digestive tract withing the peritoneal cavity Composed of a serous membrane that covers the muscular layer Absent from the pharynx, esophagus, and rectum Muscles and Movement in the Digestive System Both the muscularis mucosae and muscular layer are composed of smooth muscle. These muscles are involuntary but do not receive ANS innervation Contain pacesetter cells which undergo spontaneous depolarization This depolarization spreads along the muscle as a wave that travels through the tissue Contractions within these muscles are called peristalsis During a peristaltic wave the circular muscles contract behind the digested material Pushes the material along the tract in one direction Most areas undergo segmentation These movements churn and fragment the material mixing it with digestive juices Peritoneum A serous membrane composed of 2 parts: Peritoneum Peritoneum Abdominal organs are associated with the peritoneum as: Intraperitoneal: organs lie within the peritoneal cavity Retroperitoneal: organs covered by peritoneum Secondarily Retroperitoneal: within the peritoneum but become retroperitoneal Mesenteries Within the peritoneal cavity, organs of the digestive system are suspended by sheets of serous membrane These mesentaries are fused sheets of peritoneal membrane They stabilize the position of the organs & prevent entanglement during digestion During development dorsal and ventral mesenteries suspend the digestive and accessory organs The ventral layer disappears except: On the ventral stomach and liver: The lesser omentum The second between the liver and anterior abdominal wall: The falciform ligament As the digestive trat elongates the dorsal mesentery of the stomach enlarges forming a pouch This is the greater omentum The Oral Cavity The mouth opens into the oral cavity The functions of the oral cavity include: Sensory analysis of food before swallowing Mechanical digestion (teeth, tongue, palatal surfaces) Lubrication with saliva and mucous Chemical digestion of carbohydrates (Amylase) The oral mucosa lines the oral cavity The mucosa is nonkeratinized stratified epithelium The cheeks forms the lateral wall and are continuous with the lips The space between the cheeks, lips, and teeth is the oral vestibule The ridges of the oral mucosa, or gingivae, surrounds the teeth The hard palate separates the nasal and oral cavity The soft palate separates the oral cavity from the nasopharynx Closes off the nasopharynx during swallowing The uvula dangles from the posterior end of the soft palate Prevents food from entering the pharynx too soon Tongue Has four primary functions: 1. 2. 3. 4. Mechanical digestion Manipulation to assist in chewing & swallowing Sensory analysis (touch, taste, temp) Secretion of mucus & lingual lipase The dorsum of the tongue contains the lingual papillae Taste buds are found along the edges of the papilla Along the inferior midline is the frenulum Connects the body of the tongue to the mucosa The tongue has two different groups of muscles: 1. Extrinsic Tongue Muscles Perform all gross movements of the tongue 2. intrisic Tongue Muscles Change the shape of the tongue Assist Extrinsic muscles during precise movements, like speech Salivary Glands Three pairs of salivary glands secrete into the oral cavity: 1. The Parotid Largest Found between zygomatic arch & sternocleidomastoid 2. The Sublingual Open along the sides of the frenulum 3. Submandibular Found long the floor of the mouth along the medial surface Teeth The teeth perform chewing, mastication, of food This breaks down connective tissues in meats and cellulose fibers in veggies The maxillary and mandibular arcades hole 4 types of teeth 1. 2. 3. 4. Incisors- Flat “clipping” teeth with a single root Cuspids- the canines, shearing or cutting teeth with a single root Biscuspids- the premolars have 2 prominent round cusps ans 1-2 roots Moloars- the crushing or grinding teeth have 4-5 cusps and 3 roots (upper) or 2 roots (lower) During development 2 sets of teeth form: The deciduous teeth appear first 4 incisors, 2 canines and 4 molars Replaced by the permanent teeth 2-3 sets of molars are added as the jaw expands in adulthood The third set, wisdom teeth, may not appear until age 21 Esophagus A hollow, muscular tube that transports food and liquids to the stomach Enters the peritoneal cavity at the esophageal hiatus Neither end of the esophagus has a well-defined sphincter muscle However, the region of attachment of the stomach the region called the cardiac sphincter is defined as it serves a similar purpose The wall of the esophagus has layers similar to those defined earlier The Stomach Functions to: Temporarily store ingested food Mechanically digest food Chemically digest food with acids and enzymes The mixture of food particles with these juices is referred to as chyme The stomach is intraperitoneal and J-shaped The anterior and posterior surfaces are smooth and rounded The shape and size varies greatly between people and between meals The stomach is divided into 4 regions: 1. The cardia The area closest to the heart at the intersection with the esophagus 2. The fundus Projects superior to the cardia and contacts the diaphragm 3. The body Largest region of the stomach Functions as the mixing tank for chyme 4. The pyloric end The terminal end attached to the duodenum of the small intestine The pyloric sphincter regulates the release of chyme The stomach lining contains extra layers of smooth muscle In addition to the typical circular and longitudinal layers the muscular layer has an inner oblique layer This strengthens the stomach wall and assists in mechanical digestion The Small Intestine Has 3 anatomical subdivisions: 1. The duodenum Shortest and widest segment A “mixing bowl” that receives chyme from the stomach and digestive enzymes from the liver and pancreas 2. The jejunum A sharp bend, the duodenojejunal flexure marks the transition The bulk of chemical digestion and absorption occurs here 3. The ileum The longest segment of the small intestine Ends at the ileocecal valve The mucosa of the small intestine form fingerlike projections These intestinal villi project into the lumen Each is covered by a simple columnar epithelium The apical surfaces are covered by microvilli The circular folds and microvilli increase the surface area by a factor of 600 The lamina propria contains numerous lymphatic cells and an extensive network of capillaries These absorb and carry nutrients to the hepatic portal circulation The Large Intestine Horseshoe-shaped, beginning at the junction of the ileum and ending at the anus Often referred to as the large bowel Divided into 3 sections 1. The cecum 2. The colon 3. The rectum Functions to: Absorb water and electrolytes while compacting the feces Absorbs bacterially derived vitamins Temporarily stores feces before egestion The colon: The wall forms a series of pouches, haustra, that allow for distension and elongation Three separate bands of smooth muscle, the tenia coli, run along the outer surface Tension in these muscles creates the haustra Subdivided into 4 regions: 1. 2. 3. 4. Ascending Colon: Transverse Colon Descending Colon Sigmoid Colon The rectum: The terminal end of the digestive tract Movement of the fecal material into this region generates the urge to defecate The anus is the exit to the anal canal, Here the epithelium is again karetinized Accessory Digestive Organs The Liver: The largest of the visceral organs Performs essential metabolic and synthetic functions: Metabolism: Regulates carbs, lipids, and AA’s in the blood Hematological: Largest blood reservoir and removes old and damages RBCs Also synthesizes plasma proteins Bile Production: Buffer the acids in chyme and make it possible for enzymes to digest foods The liver is intraperitoneal On its anterior surface, the ventral mesentery, or falciform ligament, divides the L/R lobes The shape of the liver conforms to the surrounding tissues: Its anterior, superior surface follows the curve of the body wall The posterior side has impressions from the stomach, small & large intestines, and kidney Classically the liver was divided into 4 lobes: The Left, Right, Caudate, and Quadrate New terminology divides it into 4 segments based on the major subdivisions of the hepatic artery, portal vein, and hepatic ducts The Gallbladder: A hollow, pear-shaped muscular sac Stores and concentrates bile before being released into the sm. Intestine Divided into 3 regions: The fundus, body, and neck The sphincter of ampulla opens and closes the bile duct Has 2 major functions: Stores and modifies the bile Bile is modified as water is removed, further concentrating the within The pancreas: Has three subdivisions: The head, body, and tail Is a mixed gland with both endocrine and exocrine functions The exocrine portion produces pancreatic juice Made of digestive enzymes and buffers The large pancreatic duct delivers exocrine secretions to the duodenal ampulla This is a chamber about ½ way along the duodenum Secretory units are called pancreatic acini