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Biological Sciences
Kenneth Welch

The Digestive System Digestive System: Overview • The alimentary canal or gastrointestinal (GI) tract digests and absorbs food • Alimentary canal – mouth, pharynx, esophagus, stomach, small intestine, and large intestine • Accessory digestive organs – teeth, tongue, gallbladder, salivary glands, liver, and pancreas Digestive Process • The GI tract is a “disassembly” line • Nutrients become more available to the body in each step • There are six essential activities: • ingestion, propulsion, and mechanical digestion • chemical digestion, absorption, and defecation Essential Activities of Digestion • Ingestion – taking food into the digestive tract • Propulsion – swallowing and peristalsis • Peristalsis – waves of contraction and relaxation of muscles in the organ walls • Mechanical digestion – chewing, mixing, and churning food • Chemical digestion – catabolic breakdown of food • Absorption – movement of nutrients from the GI tract to the blood or lymph • Defecation – elimination of indigestible solid wastes GI Tract • External environment for the digestive process • Regulation of digestion involves: • Mechanical and chemical stimuli – stretch receptors, osmolarity, and presence of substrate in the lumen • Extrinsic control by CNS centers • Intrinsic control by local centers Receptors of the GI Tract • Mechano- and chemoreceptors respond to: • Stretch, osmolarity, and pH • Presence of substrate, and end products of digestion • They initiate reflexes that: • Activate or inhibit digestive glands • Mix lumen contents and move them along Nervous Control of the GI Tract • Intrinsic controls • Nerve plexuses near the GI tract initiate short reflexes • Short reflexes are mediated by local enteric plexuses (gut brain) • Extrinsic controls • Long reflexes arising within or outside the GI tract • Involve CNS centers and extrinsic autonomic nerves Digestive System Organs and Peritoneum • Peritoneum – serous membrane of the abdominal cavity • Visceral – covers external surface of most digestive organs • Parietal – lines the body wall • Peritoneal cavity • Lubricates digestive organs • Allows them to slide across one another • Mesentery – double layer of peritoneum that provides: • Vascular and nerve supplies to the viscera • A means to hold digestive organs in place and store fat • Retroperitoneal organs – organs outside the peritoneum • Peritoneal organs (intraperitoneal) – organs surrounded by peritoneum Homeostatic Imbalance • Peritonitis – inflammation of the peritoneum caused by a piercing wound, perforating ulcer, or burst appendix • Often, infected membranes tend to stick together localizing the infection • Generalized or widespread peritonitis is dangerous and often lethal • Treatment includes removing infectious debris and massive doses of antibiotics Blood Supply: Splanchnic Circulation • Arteries and the organs they serve include • The hepatic, splenic, and left gastric: spleen, liver, and stomach • Inferior and superior mesenteric: small and large intestines • Hepatic portal circulation: • Collects nutrient-rich venous blood from the digestive viscera • Delivers it to the liver for metabolic processing and storage Histology of the Alimentary Canal • From esophagus to the anal canal the walls of the GI tract have the same four tunics • From the lumen outward they are the mucosa, submucosa, muscularis externa, and serosa • Each tunic has a predominant tissue type and specific digestive function Mucosa • Moist epithelial layer that lines the lumen of the alimentary canal • Its three major functions are: • Secretion of mucus • Absorption of the end products of digestion • Protection against infectious disease • Consists of three layers: a lining epithelium, lamina propria, and muscularis mucosae Mucosa: Epithelial Lining • Consists of simple columnar epithelium and mucus-secreting goblet cells • The mucus secretions: • Protect digestive organs from digesting themselves • Ease food along the tract • Stomach and small intestine mucosa contain: • Enzyme-secreting cells and • Hormone-secreting cells (making them endocrine and digestive organs) Mucosa: Lamina Propria and Muscularis Mucosae • Lamina Propria • Loose areolar and reticular connective tissue • Nourish the epithelium and absorb nutrients • Contains lymph nodes (part of MALT) important in defense against bacteria • Muscularis mucosae– smooth muscle cells that produce local movements of mucosa Mucosa: Other Sublayers • Submucosa – dense connective tissue containing elastic fibers, blood and lymphatic vessels, lymph nodes, and nerves • Muscularis externa – responsible for segmentation and peristalsis • Serosa – the protective visceral peritoneum • Replaced by the fibrous adventitia in the esophagus • Retroperitoneal organs have both an adventitia and serosa Enteric Nervous System • Composed of two major intrinsic nerve plexuses • Submucosal nerve plexus – regulates glands and smooth muscle in the mucosa • Myenteric nerve plexus: • Major nerve supply that controls GI tract mobility • Segmentation and peristalsis are largely automatic involving local reflex arcs • Linked to the CNS via long autonomic reflex arc Mouth • Oral or buccal cavity: • Is bounded by lips, cheeks, palate, and tongue • Has the oral orifice as its anterior opening • Is continuous with the oropharynx posteriorly • To withstand abrasions: • The mouth is lined with stratified squamous epithelium • The gums, hard palate, and dorsum of the tongue are slightly keratinized Lips and Cheeks • Have a core of skeletal muscles • Lips: orbicularis oris • Cheeks: buccinators • Vestibule – bounded by the lips and cheeks externally and teeth and gums internally • Oral cavity proper– area that lies within the teeth and gums • Labial frenulum – median fold that joins the internal aspect of each lip to the gum Palate • Hard palate – underlain by palatine bones and palatine processes of the maxillae • Assists the tongue in chewing • Slightly corrugated on either side of the raphe (midline ridge) • Soft palate – mobile fold formed mostly of skeletal muscle • Closes off the nasopharynx during swallowing • Uvula projects downward from its free edge • Palatoglossal and palatopharyngeal arches form the borders of the fauces Tongue • Occupies the floor of the mouth and fills the oral cavity when mouth is closed • Functions include: • Gripping and repositioning food during chewing • Mixing food with saliva and forming the bolus • Initiation of swallowing, and speech • Intrinsic muscles change the shape of the tongue • Extrinsic muscles alter the tongue’s position • Lingual frenulum secures the tongue to the floor of the mouth Homeostatic Imbalance • Ankyloglossia – congenital situation where the lingual frenulum is extremely short • Commonly referred to as being “tongue-tied” • Corrected surgically by cutting the frenulum Tongue • Superior surface bears three types of papillae • Filiform – give the tongue roughness and provide friction • Fungiform – scattered widely over the tongue and give it a reddish hue • Circumvallate – V-shaped row in back of tongue • Sulcus terminalis – groove that separates the tongue into two areas: • Anterior 2/3 residing in the oral cavity • Posterior third residing in the oropharynx Salivary Glands • Produce and secrete saliva that: • Cleanses the mouth • Moistens and dissolves food chemicals • Aids in bolus formation • Contains enzymes that breakdown starch • Three pairs of extrinsic glands – parotid, submandibular, and sublingual • Intrinsic salivary glands (buccal glands) – scattered throughout the oral mucosa • Parotid – lies anterior to the ear between the masseter muscle and skin • Parotid duct – opens into the vestibule next to the second upper molar • Submandibular – lies along the medial aspect of the mandibular body • Its ducts open at the base of the lingual frenulum • Sublingual – lies anterior to the submandibular gland under the tongue • It opens via 10-12 ducts into the floor of the mouth Homeostatic Imbalance • Mumps – inflammation of the parotid glands caused by myxovirus • Signs and symptoms – moderate fever and pain in swallowing acidic foods • In adult males, mumps carries 25% risk that testes may become infected, leading to sterility Saliva: Source and Composition • Secreted from serous and mucous cells of salivary glands • A 97-99.5% water, hypo-osmotic, slightly acidic solution containing • Electrolytes – Na , K , Cl , PO , HCO– – 4 3 • Digestive enzyme – salivary amylase • Proteins – mucin, lysozyme, defensins, and IgA • Metabolic wastes – urea and uric acid Control of Salivation • Intrinsic glands keep the mouth moist • Extrinsic salivary glands secrete serous, enzyme-rich saliva in response to: • Ingested food which stimulates chemoreceptors and pressoreceptors • The thought of food • Strong sympathetic stimulation inhibits salivation and results in dry mouth Teeth • Primary and permanent dentitions have formed by age 21 • Primary – 20 deciduous teeth that erupt at intervals between 6 and 24 months • Permanent – enlarge and develop causing the root of deciduous teeth to be resorbed and fall out between the ages of 6 and 12 years • All but the third molars have erupted by the end of adolescence • There are usually 32 permanent teeth Classification of Teeth • Teeth are classified according to their shape and function • Incisors – chisel-shaped teeth adapted for cutting or nipping • Canines – conical or fanglike teeth that tear or pierce • Premolars (bicuspids) and molars – have broad crowns with rounded tips and are best suited for grinding or crushing • During chewing, upper and lower molars lock together generating crushing force Dental Formula • A shorthand way of indicating the number and relative position of teeth • Written as ratio of upper to lower teeth for the mouth • Primary: 2I (incisors), 1C (canine), 2M (molars) • Permanent: 2I, 1C, 2PM (premolars), 3M Tooth Structure • Two main regions – crown and the root • Crown – exposed part of the tooth above the gingiva (gum) • Enamel – acelluar, brittle material composed of calcium salts and hydroxyapatite crystals is the hardest substance in the body • Encapsules the crown of the tooth • Root – portion of the tooth embedded in the jawbone • Neck – constriction where the crown and root come together • Cementum – calcified connective tissue • Covers the root • Attaches it to the periodontal ligament • Periodontal ligament • Anchors the tooth in the alveolus of the jaw • Forms the fibrous joint called a gomaphosis • Gingival sulcus – depression where the gingival borders the tooth • Dentin – bonelike material deep to the enamel cap that forms the bulk of the tooth • Pulp cavity – cavity surrounded by dentin that contains pulp • Pulp – connective tissue, blood vessels, and nerves • Root canal – portion of the pulp cavity that extends into the root • Apical foramen – proximal opening to the root canal • Odontoblasts – secrete and maintain dentin throughout life Homeostatic Imbalance • Root canal therapy – blows to the teeth can cause swelling and consequently pinch off the blood supply to the tooth • The nerve dies and may become infected with bacteria • Then the cavity is sterilized and filled with an inert material • The tooth is then capped Tooth and Gum Disease • Dental caries – gradual demineralization of enamel and dentin by bacterial action • Dental plaque, a film of sugar, bacteria, and mouth debris, adheres to teeth • Acid produced by the bacteria in the plaque dissolves calcium salts • Without these salts, organic matter is digested by proteolytic enzymes • Daily flossing and brushing help prevent caries by removing forming plaque Tooth and Gum Disease: Periodontitis • Gingivitis – as plaque accumulates, it calcifies and forms calculus, or tartar • Accumulation of calculus: • Disrupts the seal between the gingivae and the teeth • Puts the gums at risk for infection • Periodontitis – serious gum disease resulting from an immune response • Attack of the immune system against intruders: • Also carves pockets around the teeth and • Dissolves bone away Pharynx • From the mouth, the oro- and laryngopharynx allow passage of: • Food and fluids to the esophagus • Air to the trachea • Lined with stratified squamous epithelium and mucus glands • Has two skeletal muscle layers • Inner longitudinal • Outer pharyngeal constrictors Esophagus • Muscular tube going from the laryngopharynx to the stomach • Travels through the mediastinum and pierces the diaphragm • Joins the stomach at the cardiac orifice Homeostatic Imbalance • Heartburn (gastroesophageal reflux disease or GERD) – burning, radiating substernal pain caused by acidic gastric juice regurgitated into the esophagus • Caused by excess eating or drinking, and conditions that force abdominal contents superiorly (e.g., extreme obesity, pregnancy, and running) • Hiatus hernia – structural abnormality in which the superior part of the stomach protrudes slightly above the diaphragm • Prolonged episodes can lead to esophagitis, ulcers, and cancer Esophageal Characteristics • Esophageal mucosa – nonkeratinized stratified squamous epithelium • The empty esophagus is folded longitudinally and flattens when food is present • Glands secrete mucus as a bolus moves through the esophagus • Muscularis changes from skeletal (superiorly) to smooth muscle (inferiorly) Digestive Processes in the Mouth • Food is ingested • Mechanical digestion begins (chewing) • Propulsion is initiated by swallowing • Salivary amylase begins chemical breakdown of starch • The pharynx and esophagus serve as conduits to pass food from the mouth to the stomach Deglutition (Swallowing) • Involves the coordinated activity of the tongue, soft palate, pharynx, esophagus and 22 separate muscle groups • Buccal phase – bolus is forced into the oropharynx • Pharyngeal-esophageal phase – controlled by the medulla and lower pons • All routes except into the digestive tract are sealed off • Peristalsis moves food through the pharynx to the esophagus Stomach • Chemical breakdown of proteins begins and food is converted to chyme • Cardiac region – surrounds the cardiac orifice • Fundus – dome-shaped region beneath the diaphragm • Body – midportion of the stomach • Pyloric region – made up of the antrum and canal which terminates at the pylorus • The pylorus is continuous with the duodenum through the pyloric sphincter • Greater curvature – entire extent of the convex lateral surface • Lesser curvature – concave medial surface • Lesser omentum – runs from the liver to the lesser curvature • Greater omentum – drapes inferiorly from the greater curvature to the small intestine • Nerve supply – sympathetic and parasympathetic fibers of the autonomic nervous system • Blood supply – celiac trunk, and corresponding veins (part of the hepatic portal system) Microscopic Anatomy of the Stomach • Muscularis – has an additional oblique layer that • Allows the stomach to churn, mix and pummel food physically • Breaks down food into smaller fragments • Epithelial lining is composed of: • Goblet cells that produce a coat of alkaline mucus • Gastric pits containing gastric glands that secrete: • Gastric juice • Mucus • Gastrin Glands of the Stomach Fundus and Body • Gastric glands of the fundus and body have a variety of secretory cells • Mucous neck cells – secrete acid mucus • Parietal (oxyntic) cells – secrete HCl and intrinsic factor • Chief (zymogenic) cells – produce pepsinogen • Pepsinogen is activated to pepsin by: • HCl in the stomach • Pepsin itself by a positive feedback mechanism • Enteroendocrine cells – secrete gastrin, histamine, endorphins, serotonin, cholecystokinin (CCK), and somatostatin into the lamina propria Stomach Lining • The stomach is exposed to the harshest conditions in the digestive tract • To keep from digesting itself, the stomach has a mucosal barrier with: • A thick coat of bicarbonate-rich mucus on the stomach wall • Epithelial cells that are joined by tight junctions • Gastric glands that have cells impermeable to HCl • Damaged epithelial cells are quickly replaced Digestion in the Stomach • The stomach: • Holds ingested food • Degrades it both physically and chemically • Delivers chyme to the small intestine • Enzymatically digests proteins with pepsin • Secretes intrinsic factor required for absorption of vitamin B 12 Regulation of Gastric Secretion • Neural and hormonal mechanisms regulate the release of gastric juice • Stimulatory and inhibitory events occur in three phases • Cephalic (reflex) phase: prior to food entry • Gastric phase: once food enters the stomach • Intestinal phase: as partially digested food enters the duodenum Cephalic Phase • Excitatory events include: • Sight or thought of food • Stimulation of taste or smell receptors • Inhibitory events include: • Loss of appetite or depression • Decrease in stimulation of the parasympathetic division Gastric Phase • Excitatory events include: • Stomach distension • Activation of stretch receptors (neural activation) • Activation of chemoreceptors by peptides, caffeine, and rising pH • Release of gastrin to the blood • Inhibitory events include: • A pH lower than 2 • Emotional upset which overrides the parasympathetic division Intestinal Phase • Excitatory phase – low pH and partially digested food enters the duodenum • Inhibitory phase – distension of duodenum, presence of fatty, acidic, or hypertonic chyme, and/or irritants in the duodenum • Initiate inhibition of local reflexes and vagal nuclei • Closes the pyloric sphincter • Releases enterogastrones that inhibit gastric secretion Regulation and Mechanism of HCl Secretion • HCl secr
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