LEC 6 || ANATOMY .pdf

Function of the digestive system The function of the digestive system is to change the complex organic nutrient molecules into simple organic and inorganic molecules that can then be absorbed into the blood or lymph to be transported to cells. Stages of Digestion: 1. Ingestion or introduction...

Function of the digestive system The function of the digestive system is to change the complex organic nutrient molecules into simple organic and inorganic molecules that can then be absorbed into the blood or lymph to be transported to cells. Stages of Digestion: 1. Ingestion or introduction of food and liquid into the oral cavity. 2. Mastication or chewing, which divides solid food into digestible pieces. 3. Motility includes muscular movements of materials through the tract. 4. Secretion of lubricating and protective mucus, digestive enzymes, acidic and alkaline fluids, and bile. 5. Hormone release for local control of motility and secretion. 6. Chemical digestion or enzymatic degradation of large macromolecules in food to smaller molecules and their subunits. 7. Absorption of the small molecules and water into the blood and lymph. 8. Elimination of indigestible, unabsorbed components of food.  Divisions of the Digestive System The digestive system divided into two parts:- 1. The alimentary canal (gastrointestinal tract, GIT): It extends from the mouth to the anus. It consists of oral cavity, pharynx, esophagus, stomach, small intestine, and large intestine. 2. The accessory organs of digestion: include salivary glands, liver, gallbladder, and pancreas.  Anatomy of the oral cavity The oral cavity (buccal cavity) consists of the teeth and tongue and the openings of the ducts of the salivary glands. The oral cavity has two parts:- 1. Vestibule of the mouth: the space lies outside the teeth and gums. Borders: Anteriorly and laterally: lips and cheeks. Posteriorly and medially: teeth and gums.  Anatomy of the oral cavity 2. Mouth cavity proper: Borders: Anteriorly and laterally: teeth and gums. Superiorly: hard and soft palates. Posteriorly: tonsils  Anatomy of The teeth Every tooth consists of three parts: the crown (the visible part of the tooth), neck (present between the crown and root), and root (It extends into the bone and holds the tooth in place). Every tooth made of several layers: the enamel, dentin, pulp and cementum. Enamel: The hardest, white outer part of the tooth. Dentin: A layer of a hard tissue underlying the enamel. Pulp: The softer, living inner structure of teeth. Blood vessels and nerves run through the pulp of the teeth. Cementum: A layer of connective tissue that binds the roots of the teeth firmly to the gums and jawbone.  Anatomy of The teeth The function of the teeth is chewing. An individual develops two sets of teeth: deciduous and permanent teeth. 1. The deciduous teeth begin to erupt through the gums at about 6 months of age, and the set of 20 teeth is usually complete by the age of 2 years. These teeth are gradually lost throughout childhood and replaced by permanent teeth. 2. The permanent teeth consists of 32 teeth; the types of teeth are eight incisors, four canines, eight premolars, and 12 molars. The wisdom teeth are the third molars on either side of each jawbone.  The tongue The tongue is a mass of striated muscle that is covered by mucosa. The muscle fibers are oriented in all directions, allowing a high level of mobility. The tongue is innervated by the hypoglossal nerves (12th cranial nerve of the cranial nerves). The functions of the tongue include chewing, speech, taste and swallowing. The tongue consists of two surfaces: The lower (Ventral) surface is smooth. The dorsal surface is irregular, having hundreds of small protruding papillae of various types on its anterior two-thirds and the massed lingual tonsils on the posterior third, or root of the tongue. The papillary and tonsillar areas of the lingual surface are separated by a V-shaped groove called the sulcus terminalis.  The surface of the tongue The dorsal surface The lower surface  Salivary glands Salivary gland: They are exocrine glands in the mouth produce saliva, which has digestive, lubricating, and protective functions. Collectively the salivary glands produce (1–1.5 L) of saliva each day, with a composition of 99.4% water, plus ions and enzymes.  Types of Salivary glands There are two types of salivary glands in the mouth: 1. The minor salivary glands: they located in oral mucosa which secrete about 10% of the total saliva volume. 2. The major salivary glands: are three pairs, with one of each pair on either side of the face. a. The parotid glands are just below and in front of the ears. b. The submandibular (submaxillary) glands are at the posterior corners of the mandible. c. The sublingual glands are below the floor of the mouth. The saliva in the mouth is a mixture of glandular secretions; about 70% of the saliva originates in the submandibular salivary glands, 25% from the parotid salivary glands, and 5% from the sublingual salivary glands. Each gland has at least one duct that takes saliva to the oral cavity. Salivary glands  General structure of the digestive tract The GIT is a hollow tube with a lumen of variable diameter and a wall made up of four main layers: 1. Mucosa. 2. Submucosa. 3. Muscularis externa 4. Serosa. Dr. Rawaa Salim Layers of GIT 1. The mucosa (mucous membrane) consists of: A. Epithelial layer. B. Lamina propria: is a loose connective tissue rich in blood vessels, lymphatics, lymphocytes, smooth muscle cells, and often containing small glands. C. Muscularis mucosae: is a thin layer of smooth muscle separating mucosa from submucosa and allowing local movements of the mucosa. 2. The submucosa contains dense connective tissue with larger blood and lymph vessels and the submucosal (Meissner) plexus of autonomic nerves. It may also contain glands and significant lymphoid tissue.  Layers of GIT 3. The thick muscularis (or muscularis externa) is composed of smooth muscle cells organized as two or more sublayers. A. Inner circular (closer to the lumen). B. Outer longitudinal. The connective tissue between the muscle sublayers contains blood and lymph vessels, as well as the myenteric (Auerbach) nerve plexus of autonomic nerves. Contractions of the muscularis, which mix and propel the luminal contents forward, are generated and coordinated by the myenteric plexus. The plexus of muscularis externa and the submucosa comprise the enteric nervous system of the digestive tract. 4. The serosa, a thin sheet of loose connective tissue, rich in blood vessels, lymphatics, and adipose tissue, and covered with a simple squamous covering epithelium or mesothelium, is the outermost layer of the digestive tract located within the abdominal cavity. The esophagus is not suspended in a cavity but bound directly to adjacent structures and therefore lacks a serosa, having instead a thick adventitia, a layer of connective tissue continuous with that of surrounding tissues.  The pharynx The pharynx is a muscular tube about 5 inch in length, located posterior to the nasal and oral cavities and anterior to the cervical vertebrae. The pharynx divided into three parts: Nasopharynx, Oropharynx and Laryngopharynx. The function of pharynx: No digestion takes place in the pharynx. Its only related function is swallowing, the mechanical movement of food.  Nasopharynx Nasopharynx is a passageway for air only. It is located behind the nasal cavities. The soft palate is elevated during swallowing to block the nasopharynx and prevent food or saliva from going up rather than down. On the posterior wall of the nasopharynx is the adenoid or pharyngeal tonsil, a lymph nodule that contains macrophages. Opening into the nasopharynx are the two Eustachian tubes, which extend to the middle ear cavities. The purpose of the Eustachian tubes is to permit air to enter or leave the middle ears, allowing the eardrums to vibrate properly. Oropharynx is located behind the mouth. On its lateral walls are the palatine tonsils. Together with the adenoid and the lingual tonsils on the base of the tongue, they form a ring of lymphatic tissue around the pharynx to destroy pathogens that penetrate the mucosa. Laryngopharynx is the most inferior portion of the pharynx. It opens anteriorly into the larynx and posteriorly into the esophagus. The oropharynx and laryngopharynx are food passageways connecting the oral cavity to the esophagus.  Anatomy of The esophagus The esophagus is a muscular tube about 25cm in length. it takes food from the pharynx to the stomach; no digestion takes place here. Peristalsis of the esophagus propels food in one direction and ensures that food gets to the stomach even if the body is horizontal or upside down. At the junction with the stomach, the lumen (cavity) of the esophagus is surrounded by the lower esophageal sphincter (LES or cardiac sphincter), a circular smooth muscle. The LES relaxes to permit food to enter the stomach, then contracts to prevent the backup of stomach contents. If the LES does not close completely, gastric juice may splash up into the esophagus; this is a painful condition we call heartburn, or gastroesophageal reflux disease (GERD).