Digestive System I - Introduction

DIGESTIVE SYSTEM I Mrs. Sichela OUTLINE Introduction to the Digestive System Divisions  Alimentary Canal  Accessory Organs Organs Location Functions INTRODUCTION The Digestive System is a group of organs working together to convert food into energy and basic nutrients to feed/nourish the body....

DIGESTIVE SYSTEM I Mrs. Sichela OUTLINE Introduction to the Digestive System Divisions  Alimentary Canal  Accessory Organs Organs Location Functions INTRODUCTION The Digestive System is a group of organs working together to convert food into energy and basic nutrients to feed/nourish the body. FIG: POSTERIOR VIEW OF THE DIGESTIVE SYSTEM DIVISIONS  The Digestive System consists of 2 parts: the Alimentary Canal, and their Accessory Organs.  The alimentary canal is also called the digestive tract OR gastrointestinal tract GIT  The alimentary canal is the long tube that runs from the mouth through to the anus. ALIMENTARY CANAL The main parts of the Alimentary Canal are: Mouth (Oral cavity) Pharynx (Throat) Oesophagus (also esophagus) Stomach Small intestine Large intestine Rectum and anal canal (anus).  ACCESSORY ORGANS Are not part of the long GI tube, but often develop as outgrowths from and are connected to the GI tract. The accessory digestive organs assist the GI tract in the digestion of material. These include: Teeth Tongue Salivary glands Liver Gallbladder Pancreas  DIGESTION  Digestion starts in the mouth, where food is chewed and mixed with saliva to form a bolus.  The bolus produced is then swallowed down the pharynx and oesophagus via peristaltic contractions and into the stomach.  In the stomach, it is mixed with gastric juice to form a semifluid substance called chyme, then moved to the duodenum (small intestine).  Most of the digestion takes place in the stomach and duodenum of the small intestine.  Water and some minerals are reabsorbed in the colon of the large intestine. The chyme is turned to faeces. The faeces are defecated from the anus via the rectum. FUNCTIONS Ingestion: introduction of food and liquid into the oral cavity. Mastication: chewing, which divides solid food into digestible pieces. Motility: muscular movements of materials through the tract. Secretion of lubricating and protective mucus, digestive enzymes, acidic and alkaline fluids, and bile. Hormone release for local control of motility and secretion. Chemical digestion: enzymatic degradation of large macromolecules in food to smaller molecules and their subunits. Absorption of the small molecules and water into the blood and lymph. Elimination of indigestible, unabsorbed components of food. LOCATION OF ORGANS Fig: Abdominopelvic Regions GENERAL STRUCTURE OF THE DIGESTIVE TRACT All regions in GIT have certain structures in common. Hollow tube with a lumen of variable diameters Wall made up of four layers  Mucosa  Submucosa  Muscularis  Serosa HISTOLOGY OF ALIMENTARY CANAL WALL SAME FOUR LAYERS FROM ESOPHAGUS TO ANAL CANAL 1. Mucosa 2. Submucosa 3. Muscularis externa 4. Serosa from lumen (inside) out  INNER LAYER: THE MUCOSA* (MUCOUS MEMBRANE) Three sub-layers 1. Lining epithelium 2. Lamina propria 3. Muscularis mucosae  MORE ABOUT THE MUCOSA Epithelium Absorbs nutrients, secretes mucus The epithelium in the mouth, pharynx, esophagus, and anal canal is mainly nonkeratinized stratified squamous epithelium that serves a protective function. Simple columnar epithelium, which functions in secretion and absorption, lines the stomach and intestines. Located among the absorptive epithelial cells are exocrine cells that secrete mucus and fluid into the lumen of the tract, and several types of endocrine cells, collectively called enteroendocrine cells, which secrete hormones. MORE ABOUT THE MUCOSA Lamina propria  Loose connective tissue rich in blood vessels, lymphatics, lymphocytes, smooth muscle cells, and often containing small glands;  Contains most of mucosa-associated lymphoid tissue (MALT) Muscularis mucosae  A thin layer of smooth muscle fibers called the muscularis mucosae causes the mucous membrane of the stomach and small intestine to form many small folds, increasing the surface area for digestion and absorption.  Produces local movements which ensure that all absorptive cells are fully exposed to the contents of the GI tract. TYPES OF MUCOSA  Four basic mucosal types are found lining the gastrointestinal tract and these can be classified according to their main function: Protective  This type is found in the oral cavity, pharynx, oesophagus and anal canal  The surface epithelium is of stratified squamous non keratinised which makes it well suited to potential frictional trauma, such as that associated with the passage of food during mastication and swallowing, or during the passage of faeces through the anal canal. Secretory.  Found only in the stomach  Has glands that produce various combinations of acid and digestive enzymes in order to facilitate digestion of food whilst also secreting mucus to protect the mucosa itself from injury. TYPES OF MUCOSA Absorptive. This mucosal form is typical of the entire small intestine The mucosa is arranged into finger-like projections called villi which serve to dramatically increase surface area of the mucosa, with intervening short glands called crypts. Absorptive/protective. This form lines the entire large intestine The mucosa is arranged into closely packed, straight tubular glands consisting of cells specialised for water absorption, as well as mucus-secreting goblet cells to lubricate the passage of faeces. MORE ABOUT THE MUCOSA The mucous membrane exhibits regional variations, reflecting the changing functions of the system from mouth to anus. At four points along the tract, the mucosa Gastroesophageal junction undergoes abrupt transition from one form to another: The gastrooesophageal junction, The gastroduodenal junction, The ileocaecal junction The rectoanal junction Recto-anal junction SECOND LAYER: THE SUBMUCOSA Connective tissue containing major blood and lymphatic vessels and nerves Many elastic fibers so gut can regain shape after food passes Submucosal (Meissner) Plexus MUSCULARIS EXTERNA Blood and lymph vessels Myenteric (Auerbach) nerve plexus of the autonomic neurons Enteric nervous system:- both the Meissners and Auerbachs plexuses Clinical application Hirschsprung disease Chaga’s disease  FOURTH LAYER: THE SEROSA Simple squamous epithelium (mesothelium)  Thin layer of areolar connective tissue underneath Exceptions:  Parts not in peritoneal cavity have adventitia, lack serosa  Some have both, e.g. retroperitoneal organs  Eosophagus has the adventitia  MOUTH (ORAL CAVITY) The oral cavity is divided into two parts. The vestibule: is the anterior slit like space bounded by the teeth and gums internally and the lips and cheeks externally. The oral cavity proper which is enclosed by the dental arches The oral cavity communicates with the oropharynx through oropharyngeal isthmus. MOUTH (ORAL CAVITY) The mouth or oral cavity is bounded by muscles and bones: Anteriorly – by the lips Posteriorly – it is continuous with the oropharynx (part of pharynx) Laterally – by the muscles of the cheeks Superiorly – by the bony hard palate and muscular soft palate Inferiorly – by the muscular tongue and the soft tissues of the floor of the mouth.  ORAL CAVITY Mouth = oral cavity Lining: thick stratified squamous epithelium Lips- orbicularis oris muscle Cheeks – buccinator muscle LIPS Both lips have three differently covered surfaces: The outer surface has thin skin, consisting of epidermal and dermal layers, sweat glands, and many hair follicles with sebaceous glands. Red vermillion border  Vascular, thin keratinized stratified squamous epithelium  Highly sensitive  No sweat glands nor sebaceous glands The internal mucous surface has lining mucosa with a thick, nonkeratinized epithelium and many minor labial salivary glands. ORAL CAVITY Labial frenulums (folds of mucosa): attach superior and inferior lips to the gingivae (gums) Palate – roof of oral cavity and acts as a barrier to separate it from the nasal cavity. Hard palate: anterior two thirds. Formed by the palatine processes of the maxillae and the horizontal plates of the palatine bones. Nonkeratinized stratified squamous epithelium Exhibits prominent transverse palatine folds, or friction ridges, that assist the tongue in manipulating ingested materials prior to swallowing.  ORAL CAVITY Soft palate: posterior one third Covered with nonkeratinized stratified squamous epithelium. Uvula: extends from the posterior part of the soft palate  When you swallow, the soft palate and the uvula elevate to close off the posterior entrance into the nasopharynx and prevent ingested materials from entering the nasal region. Lateral to the base of the uvula are two muscular folds that run down the lateral sides of the soft palate: Anteriorly, the palatoglossal arch extends to the side of the base of the tongue; Posteriorly, the palatopharyngeal arch extends to the side of the pharynx. The palatine tonsils are situated between the arches  TONGUE  The tongue is a mobile muscular organ located partly in the oral cavity and partly in the oropharynx.  The tongue consists of  A tip: directed forwards and it remains in contact with the incisor teeth when the mouth is closed  A root: lies in the floor of the mouth and is composed of genioglossus and hyoglossus muscles. It is attached by its base to the hyoid bone and mandible.  Inferior surface: covered with mucous membrane. A midline mucosal fold called frenulum linguae connects the inferior surface to the floor of the mouth.  Dorsal surface: most important  GROSS APPEARANCE OF DORSUM OF TONGUE  A groove called the terminal sulcus divides the tongue into two parts.  Anterior (oral): comprises two thirds of the tongue. Is covered by papillae (contains some taste buds).  Posterior (pharyngeal): comprises one third of the tongue. Contains few small glands and a large amount of lymphoid tissue, the lingual tonsil.  At the apex of the V-shaped sulcus is a small depression (the foramen cecum of tongue), which marks the site in the embryo where the epithelium invaginated to form the thyroid gland  TONGUE - PAPILAE Filiform Are conical in shape and cover most of the anterior two-thirds of dorsum. Have keratinsed stratified epithelium hence appear whitish Devoid of taste buds Fungiform Are found mainly on the margins of the tongue but a few are scattered on the dorsum. Are deep red in color due to absence of keratinized epithelium covering them.  They are mushroom-shaped with well- vascularized and innervated cores of lamina propria Have tastebuds TONGUE - PAPILAE Foliate Consist of several parallel ridges on each side of the tongue, anterior to the sulcus terminalis, Are rudimentary in humans, especially older individuals. Have taste buds. Circumvallate Are largest in size (1-3mm). These papillae are located just in front of the sulcus terminalis. They are set into the tongue surface and encircled by a deep cleft TONGUE - PAPILAE Aggregations of serous glands called Von Ebner glands open into the base of the circumvallate clefts, secreting a watery fluid which dissolves food constituents, thus facilitating taste reception. The stratified squamous epithelium lining the papillary wall of the cleft contains numerous taste buds  TASTE BUDS Piriform structure made up of modified epithelial cells Small cavity opening through Gustatory pore. TONGUE - TASTE SENSATION  TONGUE Functions Grip and reposition food Forms “bolus” of food (lump) Help in swallowing Speech – help form some consonants Note frenulum : can be too tight (ankyloglossia)  TEETH Also Called “dentition” Teeth live in sockets (alveoli) in the gum-covered margins of the mandible and maxilla Chewing: raising and lowering the mandible and moving it from side to side while tongue positions food between teeth Two sets Primary or deciduous: also called Baby or milk teeth Start at 6 months and all 20 are out by about 2 years Permanent: 32 total All but 3rd set of molars by end of adolescence 3rd set = “wisdom teeth” Variable Some can be “impacted” (imbedded in bone)  TEETH CLASSIFICATION Classified according to shape and function Incisors (8): chisel-shaped for chopping off pieces Canines (4): cone shaped to tear and pierce Premolars (bicuspids) are 8 and Molars (12) - broad crowns with 4-5 rounded cusps for grinding  ANATOMY OF TOOTH Two main regions 1. Crown (exposed) 2. Root (in socket) (Meet at neck) The crown is covered by very hard, acellular enamel The roots are covered by a bone-like tissue called cementum. These two coverings meet at the neck of the tooth. The tooth root is embedded in a bony ridge in the jaw called the alveolar ridge; the tooth socket is known as the alveolus. Cementum is connected to the bone of the socket by a thin fibrous layer called the periodontal ligament ANATOMY OF TOOTH The bulk of a tooth is composed of another calcified material, dentin, which surrounds an internal pulp cavity Dental pulp is highly vascular and well-innervated The pulp cavity narrows in each root as the root canal Each root canal has an opening at its base, the apical foramen, through which blood vessels, lymphatic vessels, and nerves extend.  SALIVARY GLANDS (TUBULOALVEOLAR GLANDS) Intrinsic salivary glands – within mucosa  Secrete saliva all the time to keep mouth moist Extrinsic salivary glands  Paired (2 each)  Parotid  Submandibular  Sublingual  External to mouth  Ducts to mouth  Mainly secrete in response to parasympathetic activity which is induced by physical, chemical and psychological stimuli.  PAROTID GLAND Is the largest Are located inferior and anterior to the ears, between the skin and the masseter muscle. Each secretes saliva into the oral cavity via a parotid (Stensen’s) duct that pierces the buccinators muscle to open into the vestibule opposite the second maxillary (upper) molar tooth. Facial nerve is divides into its 5 branches in the gland  EXTRINSIC SALIVARY GLANDS The submandibular glands are inferior to the body of the mandible Their ducts, the submandibular ducts (Wharton’s ducts) enter the oral cavity proper lateral to the lingual frenulum The sublingual salivary glands Are inferior to the tongue and internal to the oral cavity mucosa. Each gland extends multiple tiny sublingual ducts that open onto the inferior surface of the oral cavity, posterior to the submandibular duct papilla  HISTOLOGY OF SALIVARY GLANDS Compound tubulo-alveolar glands Serous or mucous cells Serous demilunes Parotid gland:- almost entirely serous Submandibular:- both serous and mucous Sublingual :- mostly mucous PAROTID GLAND SUBMANDIBULAR The large parotid gland consists entirely of serous acini with cells producing amylase and other proteins for storage in secretory GLAND The submandibular gland is a mixed granules. serous and mucous gland (serous cells predominate SUBLINGUAL GLANDS The sublingual gland is a mixed but largely mucous gland with a tubuloacinar arrangement of poorly stained mucous cells.  PHARYNX It connects to the oral cavity anteriorly, and is continuous with the oesophagus. Food passes from the oral cavity into the pharynx then to the oesophagus below it. The pharynx consists of three parts:  nasopharynx,  oropharynx, and the  laryngopharynx. It prevents food from entering the nasal cavity (by the soft palate) and the lower respiratory tract (by the epiglottis). PHARYNX The posterior apertures (choanae) of the nasal cavities open into the nasopharynx; The posterior opening of the oral cavity (oropharyngeal isthmus) opens into the oropharynx; The superior aperture of the larynx (laryngeal inlet) opens into the laryngopharynx. EPIGLOTTIS Figs: Actions of soft palate and Epiglottis in Pharynx during swallowing OESOPHAGUS Also called gullet or esophagus, it is an organ through which food passes from the pharynx to the stomach, aided by peristaltic contractions, of its musculature. It is about 25 cm long and 2 cm in diameter, and lies in the median plane (mediasternum) in the thorax, anterior to the spinal column, but posterior to the trachea. The main function of the esophagus is to transport food from the pharynx to the stomach CONSTRICTIONS OF THE OESOPHAGUS The oesophagus follows the curvature of the vertebral column. It also has 4 constrictions (narrowing), where adjacent structures produce impressions: 1. The junction of the esophagus with the pharynx in the neck (pharyngo-esophageal junction) 2. Where the esophagus is crossed by the arch of aorta 3. Where the esophagus is compressed by the left main bronchus; 4. At the esophageal hiatus in the diaphragm (T10) OESOPHAGUS (CONTD.) Immediately the oesophagus has passed through the esophageal hiatus (opening) of the diaphragm, it curves upwards before opening into the stomach. This sharp angle, as well as the sphincters at each end (e.g. cardiac sphincter), prevents the regurgitation (backflow) of gastric contents into the oesophagus. The oesophagus has thick walls consisting of the four layers/tunics common to the digestive tract: mucosa (innermost), submucosa, muscularis, and serosa/adventitia (outermost).  HISTOLOGY OF THE EOSOPHAGUS Epithelium: non-keratinized stratified squamous epithelium  At GE junction – thin simple columnar epithelium Mucus glands in wall Muscle (muscularis externa) changes as it goes down  Superior 1/3 of esophagus: skeletal muscle (like pharynx)  Middle 1/3 mixture of skeletal and smooth muscle  Inferior 1/3 smooth muscle (as in stomach and intestines) When empty, mucosa and submucosa lie in longitudinal folds Arterial Supply Venous Drainage Several arteries at different levels The cervical part of esophagus drains supply the esophagus through an into inferior thyroid veins. anastomotic chain on its surface. The thoracic part of esophagus drains The inferior thyroid arteries supply the into the azygos and hemiazygos veins. cervical part of esophagus. Abdominal part by left gastric vein The descending thoracic aorta and bronchial arteries supply the thoracic part of esophagus. The left gastric and left inferior phrenic arteries supply the abdominal esophagus.