Digestive Systempowerpoint.ppt
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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...
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 System: Overview Figure 23.1 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 Digestive Process Figure 23.2 Gastrointestinal Tract Activities • 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 Gastrointestinal Tract Activities • 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 Histology of the Alimentary Canal Figure 23.6 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 – Hormone-secreting cells (making them endocrine and digestive organs) 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 Anatomy of the Oral Cavity: Mouth Figure 23.7a 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 Oral Cavity and Pharynx: Anterior View Figure 23.7b 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) Palate • 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 Tongue Figure 23.8 Salivary Glands • Produce and secrete saliva that: – Cleanses the mouth – Moistens and dissolves food chemicals – Aids in bolus formation – Contains enzymes that break down starch • Three pairs of extrinsic glands – parotid, submandibular, and sublingual • Intrinsic salivary glands (buccal glands) – scattered throughout the oral mucosa Salivary Glands • 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 Salivary Glands Figure 23.9a 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–, PO42–, HCO3– – 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 Permanent Teeth Figure 23.10.2 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 Tooth Structure • 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 Tooth Structure Figure 23.11 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 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 Deglutition (Swallowing) Bolus of food Tongue Uvula Pharynx Bolus Epiglottis Epiglottis Glottis Esophagus Trachea (a) Upper esophageal sphincter contracted Bolus (c) Upper esophageal sphincter contracted (b) Upper esophageal sphincter relaxed Relaxed muscles Relaxed muscles Circular muscles contract, constricting passageway and pushing bolus down Bolus of food Gastroesophageal sphincter open Longitudinal muscles contract, shortening passageway ahead of bolus Gastroesophageal sphincter closed Stomach (d) (e) Figure 23.13 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 Stomach • 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 Stomach Figure 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 • The mucous surface layer traps a bicarbonaterich fluid beneath it • Gastric pits contain gastric glands that secrete gastric juice, mucus, and 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 cells – secrete HCl and intrinsic factor Glands of the Stomach Fundus and Body – Chief cells – produce pepsinogen • Pepsinogen is activated to pepsin by: – HCl in the stomach – Pepsin itself via 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 this food both physically and chemically – Delivers chyme to the small intestine – Enzymatically digests proteins with pepsin – Secretes intrinsic factor required for absorption of vitamin B12 Release of Gastric Juice Figure 23.16 Small Intestine: Gross Anatomy • Runs from pyloric sphincter to the ileocecal valve • Has three subdivisions: duodenum, jejunum, and ileum • The bile duct and main pancreatic duct: – Join the duodenum at the hepatopancreatic ampulla – Are controlled by the sphincter of Oddi • The jejunum extends from the duodenum to the ileum • The ileum joins the large intestine at the ileocecal valve Small Intestine: Microscopic Anatomy • Structural modifications of the small intestine wall increase surface area – Plicae circulares: deep circular folds of the mucosa and submucosa – Villi – fingerlike extensions of the mucosa – Microvilli – tiny projections of absorptive mucosal cells’ plasma membranes Small Intestine: Microscopic Anatomy Figure 23.21 Intestinal Juice • Secreted by intestinal glands in response to distension or irritation of the mucosa • Slightly alkaline and isotonic with blood plasma • Largely water, enzyme-poor, but contains mucus Liver • The largest gland in the body • Superficially has four lobes – right, left, caudate, and quadrate • The falciform ligament: – Separates the right and left lobes anteriorly – Suspends the liver from the diaphragm and anterior abdominal wall Liver: Associated Structures • The lesser omentum anchors the liver to the stomach • The hepatic blood vessels enter the liver at the porta hepatis • The gallbladder rests in a recess on the inferior surface of the right lobe Liver: Associated Structures • Bile leaves the liver via: – Bile ducts, which fuse into the common hepatic duct – The common hepatic duct, which fuses with the cystic duct • These two ducts form the bile duct Gallbladder and Associated Ducts Figure 23.20 Liver: Microscopic Anatomy • Hexagonal-shaped liver lobules are the structural and functional units of the liver – Composed of hepatocyte (liver cell) plates radiating outward from a central vein – Portal triads are found at each of the six corners of each liver lobule • Portal triads consist of a bile duct and – Hepatic artery – supplies oxygen-rich blood to the liver – Hepatic portal vein – carries venous blood with nutrients from digestive viscera Liver: Microscopic Anatomy • Liver sinusoids – enlarged, leaky capillaries located between hepatic plates • Kupffer cells – hepatic macrophages found in liver sinusoids Liver: Microscopic Anatomy • Hepatocytes’ functions include: – Production of bile – Processing bloodborne nutrients – Storage of fat-soluble vitamins – Detoxification • Secreted bile flows between hepatocytes toward the bile ducts in the portal triads Composition of Bile • A yellow-green, alkaline solution containing bile salts, bile pigments, cholesterol, neutral fats, phospholipids, and electrolytes • Bile salts are cholesterol derivatives that: – Emulsify fat – Facilitate fat and cholesterol absorption – Help solubilize cholesterol • Enterohepatic circulation recycles bile salts • The chief bile pigment is bilirubin, a waste product of heme The Gallbladder • Thin-walled, green muscular sac on the ventral surface of the liver • Stores and concentrates bile by absorbing its water and ions • Releases bile via the cystic duct, which flows into the bile duct The Gallbladder • Thin-walled, green muscular sac on the ventral surface of the liver • Stores and concentrates bile by absorbing its water and ions • Releases bile via the cystic duct, which flows into the bile duct Regulation of Bile Release • Cholecystokinin causes: – The gallbladder to contract – The hepatopancreatic sphincter to relax • As a result, bile enters the duodenum Regulation of Bile Release Figure 23.25 Pancreas • Location – Lies deep to the greater curvature of the stomach – The head is encircled by the duodenum and the tail abuts the spleen Pancreas • Exocrine function – Secretes pancreatic juice which breaks down all categories of foodstuff – Acini (clusters of secretory cells) contain zymogen granules with digestive enzymes • The pancreas also has an endocrine function – release of insulin and glucagon Acinus of the Pancreas Figure 23.26a Composition and Function of Pancreatic Juice • Water solution of enzymes and electrolytes (primarily HCO3–) – Neutralizes acid chyme – Provides optimal environment for pancreatic enzymes • Enzymes are released in inactive form and activated in the duodenum Composition and Function of Pancreatic Juice • Examples include – Trypsinogen is activated to trypsin – Procarboxypeptidase is activated to carboxypeptidase • Active enzymes secreted – Amylase, lipases, and nucleases – These enzymes require ions or bile for optimal activity Regulation of Pancreatic Secretion • Secretin and CCK are released when fatty or acidic chyme enters the duodenum • CCK and secretin enter the bloodstream • Upon reaching the pancreas: – CCK induces the secretion of enzyme-rich pancreatic juice – Secretin causes secretion of bicarbonate-rich pancreatic juice • Vagal stimulation also causes release of pancreatic juice Regulation of Pancreatic Secretion Figure 23.28 Digestion in the Small Intestine • As chyme enters the duodenum: – Carbohydrates and proteins are only partially digested – No fat digestion has taken place Digestion in the Small Intestine • Digestion continues in the small intestine – Chyme is released slowly into the duodenum – Because it is hypertonic and has low pH, mixing is required for proper digestion – Required substances needed are supplied by the liver – Virtually all nutrient absorption takes place in the small intestine Motility in the Small Intestine • The most common motion of the small intestine is segmentation – It is initiated by intrinsic pacemaker cells (Cajal cells) – Moves contents steadily toward the ileocecal valve • After nutrients have been absorbed: – Peristalsis begins with each wave starting distal to the previous – Meal remnants, bacteria, mucosal cells, and debris are moved into the large intestine Large Intestine • Has three unique features: – Teniae coli – three bands of longitudinal smooth muscle in its muscularis – Haustra – pocketlike sacs caused by the tone of the teniae coli – Epiploic appendages – fat-filled pouches of visceral peritoneum Large Intestine • Is subdivided into the cecum, appendix, colon, rectum, and anal canal • The saclike cecum: – Lies below the ileocecal valve in the right iliac fossa – Contains a wormlike vermiform appendix Large Intestine Figure 23.29a Colon • Has distinct regions: ascending colon, hepatic flexure, transverse colon, splenic flexure, descending colon, and sigmoid colon • The transverse and sigmoid portions are anchored via mesenteries called mesocolons • The sigmoid colon joins the rectum • The anal canal, the last segment of the large intestine, opens to the exterior at the anus Valves and Sphincters of the Rectum and Anus • Three valves of the rectum stop feces from being passed with gas • The anus has two sphincters: – Internal anal sphincter composed of smooth muscle – External anal sphincter composed of skeletal muscle • These sphincters are closed except during defecation Mesenteries of Digestive Organs Figure Large Intestine: Microscopic Anatomy • Colon mucosa is simple columnar epithelium except in the anal canal • Has numerous deep crypts lined with goblet cells • Anal canal mucosa is stratified squamous epithelium • Anal sinuses exude mucus and compress feces • Superficial venous plexuses are associated with the anal canal • Inflammation of these veins results in itchy varicosities called hemorrhoids Bacterial Flora • The bacterial flora of the large intestine consist of: – Bacteria surviving the small intestine that enter the cecum and – Those entering via the anus • These bacteria: – Colonize the colon – Ferment indigestible carbohydrates – Release irritating acids and gases (flatus) – Synthesize B complex vitamins and vitamin K Functions of the Large Intestine • Other than digestion of enteric bacteria, no further digestion takes place • Vitamins, water, and electrolytes are reclaimed • Its major function is propulsion of fecal material toward the anus • Though essential for comfort, the colon is not essential for life Defecation • Distension of rectal walls caused by feces: – Stimulates contraction of the rectal walls – Relaxes the internal anal sphincter • Voluntary signals stimulate relaxation of the external anal sphincter and defecation occurs Chemical Digestion: Carbohydrates • Absorption: via cotransport with Na+, and facilitated diffusion – Enter the capillary bed in the villi – Transported to the liver via the hepatic portal vein • Enzymes used: salivary amylase, pancreatic amylase, and brush border enzymes Chemical Digestion: Proteins • Absorption: similar to carbohydrates • Enzymes used: pepsin in the stomach • Enzymes acting in the small intestine – Pancreatic enzymes – trypsin, chymotrypsin, and carboxypeptidase – Brush border enzymes – aminopeptidases, carboxypeptidases, and dipeptidases Chemical Digestion: Proteins Figure 23.34 Chemical Digestion: Fats • Absorption: Diffusion into intestinal cells where they: – Combine with proteins and extrude chylomicrons – Enter lacteals and are transported to systemic circulation via lymph • Glycerol and short chain fatty acids are: – Absorbed into the capillary blood in villi – Transported via the hepatic portal vein • Enzymes/chemicals used: bile salts and pancreatic lipase Chemical Digestion: Fats Figure 23.35 Fatty Acid Absorption • Fatty acids and monoglycerides enter intestinal cells via diffusion • They are combined with proteins within the cells • Resulting chylomicrons are extruded • They enter lacteals and are transported to the circulation via lymph Fatty Acid Absorption Figure 23.36 Chemical Digestion: Nucleic Acids • Absorption: active transport via membrane carriers • Absorbed in villi and transported to liver via hepatic portal vein • Enzymes used: pancreatic ribonucleases and deoxyribonuclease in the small intestines Electrolyte Absorption • Most ions are actively absorbed along the length of small intestine – Na+ is coupled with absorption of glucose and amino acids – Ionic iron is transported into mucosal cells where it binds to ferritin • Anions passively follow the electrical potential established by Na+ Electrolyte Absorption • K+ diffuses across the intestinal mucosa in response to osmotic gradients • Ca2+ absorption: – Is related to blood levels of ionic calcium – Is regulated by vitamin D and parathyroid hormone (PTH) Water Absorption • 95% of water is absorbed in the small intestines by osmosis • Water moves in both directions across intestinal mucosa • Net osmosis occurs whenever a concentration gradient is established by active transport of solutes into the mucosal cells • Water uptake is coupled with solute uptake, and as water moves into mucosal cells, substances follow along their concentration gradients Cancer • Stomach and colon cancers rarely have early signs or symptoms • Metastasized colon cancers frequently cause secondary liver cancer • Prevention is by regular dental and medical examinations Cancer • Colon cancer is the 2nd largest cause of cancer deaths in males (lung cancer is 1st) • Forms from benign mucosal tumors called polyps whose formation increases with age • Regular colon examination should be done for all those over 50
