Chapter 23 Outline - Digestive System PDF

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CHAPTER TWENTY-THREE The Digestive System 23.1 Overview of the Digestive System A. The digestive system consists of a muscular tube called the gastrointestinal tract (or GI tract) and a variety of accessory organs. 1. Gastrointestinal tract – the muscular tube through which food passes. The GI tract, also known as the alimentary canal, begins with the mouth and continues through the oral cavity, pharynx, esophagus, stomach, small intestine, and large intestine which opens to the exterior through the anus. 2. Accessory organs – along the length of the GI tract, several accessory structures produce secretions containing water, enzymes, buffers and other components that assist in preparing organic and inorganic nutrients for absorption. The primary accessory organs include: salivary glands, liver, gallbladder, and pancreas. 3. The digestive system works with other systems to support tissues that have no direct connection to the outside environment and no other means to obtain nutrients. B. Histology - The organs of the GI tract have distinctive structural and functional characteristics, but all share an underlying pattern of histological organization. There are four basic layers of the muscular tube: 1. Mucosa – the innermost lining containing the mucous membrane of epithelial tissue supported by a basement membrane and thin layer of muscle tissue. a. Epithelium = the epithelium varies according to location within the GI tract i. Stratified squamous epithelium – found in the oral cavity, pharynx, the upper portions of the esophagus, and anus. ii. Simple columnar epithelium – found within the lower portions of the esophagus, the stomach, small intestine, and most of the large intestine. Possess numerous goblet cells (mucous producing cells). b. Lamina propria = a basement membrane composed of areolar connective tissue. Contains blood vessels, sensory nerve endings, lymphatic vessels, scattered areas of lymphoid tissue (recall Peyer’s patches and MALT), and mucous glands. c. Muscularis mucosae = two concentric layers of smooth muscle. The inner layer encircles the lumen (circular layer) and the outer layer runs parallel to the long axis of the GI tract (longitudinal layer). Contractions of the muscles within the muscularis mucosae create the folds that project from the internal surface of the GI tract such as rugae, plicae circularis, and villi. 2. Submucosa – a layer of dense irregular connective tissue surrounding the mucosa. Contains large blood vessels, lymphatic vessels, nerves, and in some regions, exocrine glands that secrete enzymes and buffers into the lumen of the GI tract. 3. Muscularis externa – dominated by smooth muscle oriented in two layers: an inner circular layer and an outer longitudinal layer. These layers are essential in the mechanical processing of food and the propulsion of food through the digestive tract. a. Smooth muscle cells are relatively long and slender, ranging from 5 to 10 m in diameter and 30 to 200 m in length. b. Although actin and myosin filaments are utilized in the contraction of smooth muscle, they arranged differently from that of skeletal and cardiac muscle. There are no

sarcomeres or myofibrils. As a result, there are no striations in smooth muscle and is called unstriated muscle. c. Thin fibers (actin) are attached to dense bodies rather than Z lines and thick filaments (myosin) have more heads per thick filament and are scattered throughout the sarcoplasm. d. Furthermore, there are no T-tubules and the sarcoplasmic reticulum (SR) forms a loose network throughout the sarcoplasm. e. Visceral smooth muscle cells have no direct contact with motor neurons but are connected to each other by gap junctions so whenever a contraction is stimulated, its electrical signal can spread from cell to cell. Pacesetter cells are present in areas where peristalsis, or rhythmic contraction, is necessary. C. Innervation. Intrinsic innervation of much of the alimentary canal is provided by the enteric nervous system, which are grouped into two plexuses: 1. The myenteric plexus (plexus of Auerbach) lies in the muscularis layer of the alimentary canal and is responsible for motility, especially the rhythm and force of the contractions of the muscularis. 2. The submucosal plexus (plexus of Meissner) lies in the submucosal layer and is responsible for regulating digestive secretions and reacting to the presence of food D. Blood supply. The blood vessels serving the digestive system have two functions. 1. They transport the protein and carbohydrate nutrients absorbed by mucosal cells after food is digested in the lumen. Lipids are absorbed via lacteals, tiny structures of the lymphatic system. 2. The blood vessels’ second function is to supply the organs of the alimentary canal with the nutrients and oxygen needed to drive their cellular processes. E. The Peritoneum. Along most portions of the organs in the peritoneal cavity, the muscularis externa is covered by a layer of visceral peritoneum called the serosa. There is no serosa covering the organs of the oral cavity, pharynx, or esophagus. Instead, a dense network of collagen fibers forms a sheath called the adventitia to anchor the organs to the surrounding tissues. In the abdominal cavity, the serosa often pulls away from the surface of the organs to create 5 major peritoneal folds: a. Greater omentum - apron-like structure that lies superficial to the small intestine and transverse colon; a site of fat deposition in people who are overweight b. Falciform ligament - anchors the liver to the anterior abdominal wall and inferior border of the diaphragm c. Lesser omentum - suspends the stomach from the inferior border of the liver; provides a pathway for structures connecting to the liver d. Mesentery - vertical band of tissue anterior to the lumbar vertebrae and anchoring all of the small intestine except the initial portion (the duodenum) e. Mesocolon - attaches two portions of the large intestine (the transverse and sigmoid colon) to the posterior abdominal wall 23.2 Digestive System Processes and Regulation: A. Digestive System Functions 1. Ingestion - occurs when foods and beverages enter the digestive tract via the mouth. 2. Mechanical processing - chewing, mixing of food with saliva, churning of food in the stomach and segmentation in the intestine. 3. Propulsion - movement of food through the alimentary canal by swallowing and peristalsis. 4. Chemical digestion - the chemical and enzymatic breakdown of foods into small organic molecules that can be absorbed such as carbohydrates, proteins and lipids. 5. Secretion - emission of water, enzyme, buffers, and acids into the lumen.

6. Absorption - movement of small organic end products and other important molecules (such as water, electrolytes, carbohydrates, proteins, lipids, nucleic acids, vitamins, and minerals like Fe or Ca) from the lumen of the GI tract into the interstitial fluids, blood, or lymph. 7. Compaction - the progressive dehydration of indigestible materials and organic wastes prior to elimination from the body. The compacted materials are called feces and the elimination of feces via the anus is called defecation. B. Digestive Processes 1. Neural control. The walls of the alimentary canal contain a variety of sensors that help regulate digestive functions. These include mechanoreceptors, chemoreceptors, and osmoreceptors, which are capable of detecting mechanical, chemical, and osmotic stimuli, respectively. 2. Hormonal control. A variety of hormones are involved in the digestive process. a. The main digestive hormone is gastrin - is secreted by the stomach in response to the presence of food. Gastrin stimulates the secretion of gastric acid by the parietal cells of the stomach mucosa. b. secretin - produced by the duodenum stimulates a watery secretion of bicarbonate by the pancreas c. cholecystokinin (CCK) - stimulates the secretion of pancreatic enzymes and bile from the liver and release of bile from the gallbladder d. gastric inhibitory peptide (GIP) - inhibits gastric secretion and slows gastric emptying and motility. 23.3 The Mouth, Pharynx and Esophagus (Upper GI tract) A. Mouth – lined with non-keratinized stratified squamous; produces the bolus 1. The anterior and lateral borders of the oral cavity is formed by: a. Labia - or lips, possess no sweat glands or sebaceous glands. Distinguished by the red margin and the orbicularis oris muscle. A superior and inferior labial frenulum attaches the lips to the gums. b. Cheeks - formed by the buccinator muscles. c. Vestibule - area bounded by the lips and cheeks externally and internally by the gingivae (gums) and teeth. 2. The superior boundary of the oral cavity (or roof) is formed by: a. Hard palate - composed of the palatine process of the maxilla and the palatine bones. b. Soft palate - the soft palate is composed of muscle tissue rather than bone. 3. The posterior boundary of the oral cavity is formed by: a. Uvula - composed of connective and reticular tissue; helps to prevent food from entering the oropharynx prematurely and houses WBCs. b. Palatine tonsils - lie on either side of the oral cavity in the archways called the fauces. c. Root of the tongue - the fixed portion of the tongue that enters into the oropharynx. Differentiated from the body of the tongue by the v-shaped circumvallate papillae. Possesses the lingual tonsils. 4. The inferior boundary (or floor) of the oral cavity is formed by: a. Body of the tongue - anterior, mobile portion of the tongue. b. Geniohyoid and mylohyoid muscles - support the body of the tongue. B. Tongue – composed of skeletal muscle to manipulate food, helps form words, and serves as a sensory organ for gustation. Anchored in the oral cavity by the lingual frenulum a. The superior surface of the tongue is covered in epithelial projections called lingual papillae. The human tongue possesses four primary types of lingual papillae with three of the four containing taste buds:

i.

Circumvallate papillae - 7 to 12 relatively large, round papillae shaped like the tip of a pencil eraser surrounded by deep epithelial folds. These are found on the posterior margin of the tongue in a V pattern. Each circumvallate papilla possesses about 100 taste buds along the sides of each papilla. ii. Fungiform papillae - mushroom-shaped papillae within shallow depressions scattered over the anterior 2/3 surface of the tongue. These typically possess only 5 taste buds located on the tops of each papilla. iii. Filiform papillae - hair-like papillae scattered all over the anterior 2/3 of the tongue. They do not have taste buds associated with them. These are designed to create friction on the surface of the tongue. iv. Foliate papillae - 4-5 vertical folds located on the lateral margins of the posterior region of the tongue. Each foliate papilla possesses many taste buds. b. Many of the papillae contain test receptors and specialized epithelial cells in sensory structures called taste buds. Although we have more than 10,000 taste buds when we are young, an adult possesses only about 5000 taste buds and by age 50, less than 1/3 of the taste buds still exist. C. Salivary glands - exocrine glands that produce and secrete saliva via ducts connected to the oral cavity. Saliva is a mixture of water, mucins, buffers, lysozymes, and enzymes. 1. Functions of the salivary glands: a. Moistens and lubricates the mouth and food; b. Cleanses food by defensins, lysozymes, and IgA antibodies; c. Dissolves food chemicals for gustation; d. Initiates chemical digestion of complex carbohydrates by salivary amylase and fats by lingual lipase. 2. Three types of salivary glands: a. Parotid glands - contains only serous cells for the production of salivary amylase, lysozymes, and IgA antibodies. Secrete into the parotid duct to the oral cavity. b. Sublingual glands - contains mostly mucous cells and produces a watery mucin to act as a buffer and lubricant. Secrete into the sublingual ducts. c. Submandibular glands - equal numbers of serous cells and mucous cells and therefore secrete a mixture of mucin and salivary amylase. Secrete into the submandibular ducts. d. The parotid glands are innervated by CN VII (facial nerve) while the sublingual and submandibular glands are innervated by CN IX (glossopharyngeal nerve). D. Teeth – breaks food into smaller pieces (mastication) thereby increasing the surface area for chemical digestion 1. Types of teeth a. Primary dentition or deciduous teeth (sometimes called milk teeth or baby teeth) 20 teeth erupt through the gums during embryonic development. b. Secondary dentition or permanent teeth fully replace the baby teeth by age 21. There are 32 permanent teeth in four general types: i. Incisors - blade-shaped teeth at the front of the mouth; for cutting and nipping; possess a single root; 8 total (4 on top/4 on bottom) described as either central or lateral incisors. ii. Canines - conical-shaped with a sharp ridgeline and pointed tip; for tearing and piercing; possess a single root; sometimes called cuspids or eyeteeth; 4 total (2 on top/2 on bottom) iii. Premolars - flattened crowns with prominent ridges; for grinding, mashing and crushing; possess one or two roots; sometimes called bicuspids; 8 total (4 on top/4 on bottom) described as either 1st or 2nd premolars.

iv.

Molars - flattened crowns with prominent ridges; for grinding and crushing; typically possess three or more roots; 12 total (6 on top/6 on bottom) described either as 1st, 2nd, or 3rd molars. The 3rd molars are sometimes called wisdom teeth. 2. Anatomy of a Tooth a. Crown - exposed part of the tooth that projects into the oral cavity. Covered by enamel over a highly mineralized (bone-like) material called dentin and a soft pulp located within the pulp cavity. b. Neck - narrow area that serves as the boundary between the crown and the root c. Root - below the gingiva (or gums) and sits into the sockets of the jawbone, called the alveoli, to form numerous gomphosis joints. Covered by cementum and anchored by the periodontal ligament. Holds the pulp cavity filled with pulp and extends down into the root canal and apical foramen. E. Pharynx – more commonly called the throat possesses skeletal muscle tissue for swallowing. The pharynx is divided into three regions: 1. Nasopharynx - the superior portion of the pharynx located between the soft palate and the internal nares. Lined with pseudostratified columnar epithelium and houses the pharyngeal tonsils. Transports AIR only. Food does not normally pass here. 2. Oropharynx - extends between the soft palate and the level of the hyoid bone. At the boundary between the nasopharynx and the oropharynx, the epithelial tissue changes from pseudostratified columnar to stratified squamous epithelium; accommodates the movement of food through this region and protects against abrasion. 3. Laryngopharynx - includes the portion of the pharynx between the hyoid bone and the entrance to the larynx and esophagus. Like the oropharynx, the laryngopharynx is lined with stratified squamous epithelium. F. Esophagus – a hollow muscular tube that functions to carry bolus from the pharynx to the stomach; passes through an opening in the diaphragm called the esophageal hiatus. Basic structure of the esophagus: 1. Upper third of the esophagus is composed of skeletal muscle for swallowing, the middle third is a mixture of skeletal and smooth muscle, and the lower two-third is made entirely of smooth muscle and undergoes peristalsis. 2. Upper portion is lined with stratified squamous epithelium while the lower portion near the stomach is lined with simple columnar epithelium. 3. Possesses many esophageal glands that produce mucus to lubricate the bolus as it moves to the stomach. 4. The upper esophageal sphincter prevents backflow of food into the oral cavity while the cardiac sphincter, also known as the gastroesophageal sphincter or the lower esophageal sphincter, prevents backflow of stomach contents into the esophagus. G. Deglutition (aka swallowing) - The movement of food from the oral cavity into the pharynx and then into the esophagus. Deglutition is divided into three phases: 1. buccal phase which is voluntary 2. pharyngeal phase which is involuntary controlled by the autonomic nervous system 3. esophageal phase which is involuntary. 23.4 The Stomach (Beginning of the Lower Gastrointestinal Tract) A. The organs of the lower GI are located within the peritoneal cavity and are lined by a serous membrane called the peritoneum and divided into a visceral peritoneum and parietal peritoneum. B. Stomach – a muscular, expandable, J-shaped organ that converts the bolus into chyme. 1. Regions of the stomach – divided into four distinct regions:

a. Cardiac region – also known as the cardia; area where the esophagus empties into the stomach; the cardiac sphincter serves as the junction between the esophagus and stomach and prevents backflow of stomach contents into the esophagus. b. Fundus – the dome-shaped portion at the top of the stomach and lies superior to the junction between the stomach and esophagus. c. Body – the largest region of the stomach; area between the fundus and the curve in the J; functions as a mixing bowl for ingested food and secretions from the walls. d. Pyloric region – forms the sharp curve of the J. As mixing movements occur during digestion, the pylorus frequently changes shape; terminates in a pyloric sphincter which regulates the flow of chyme out of the stomach and into the duodenum. 2. Gross anatomy of the stomach: a. Greater curvature – the lateral surface of the stomach to which the greater omentum is attached. The greater omentum forms an enormous pouch that drapes down over the anterior surface of the small intestine. Adipose tissue in the greater omentum conforms to the shape of the surrounding organs, providing padding and protection across the abdomen. b. Lesser curvature – the medial surface of the stomach to which the lesser omentum is attached; stabilizes the position of the stomach and provides an access route for blood vessels to enter or leave the liver. c. Rugae – longitudinal folds within the lumen of the stomach which aid in the stretch and expandability of the stomach. As the stomach fills, the rugae gradually flatten until they almost disappear. 1. Microscopic anatomy of the stomach: a. The mucosa of the stomach is simple columnar epithelium. The muscularis externa of the stomach is composed of a third, inner layer of smooth muscle oriented diagonal to the axis of the stomach (oblique layer). This extra layer provides the stomach with the ability to churn food for mixing with enzymes. b. Gastric pits – shallow depressions within the inner surface of the stomach. Each gastric pit communicates with several gastric glands that extend deep into the lamina propria. c. Gastric glands – located in the fundus and body; secrete most of the stomach juices used for gastric digestion. The gastric glands are dominated by several types of cells: i. Chief cells - secretes the inactive enzyme pepsinogen, which when activated to form pepsin, can begin the process of protein digestion. In newborn infants (but not adults), the chief cells also secrete gastric lipase and rennin which are essential in the digestion of milk. ii. Parietal cells - secrete hydrochloric acid for activating the pepsinogen and intrinsic factor which is important in the absorption of vitamin B 12. iii. G cells - enteroendocrine cells that produce a variety of hormones important in the digestive processes: a) Gastrin – increases stomach motility and churning and stimulates the release of HCl from parietal cells. b) Somatostatin – inhibits stomach motility and emptying. c) Ghrelin – regulates food intake stimulating hunger and satiety. 35.5 The Small and Large Intestines (Part of the Lower Gastrointestinal Tract) A. Small Intestine – specialized for the maximum absorption of nutrients. Receives chyme from the stomach and serves as the site for the majority of digestion and absorption of nutrients. 1. The small intestine performs peristalsis AND segmentation. 2. Gross anatomy of the small intestines:

a. Duodenum = upper region of the small intestine (about 10 – 12 inches in length); receives chyme from the stomach as well as digestive enzymes from the pancreas and bile from liver ...