BIO 202 Digestive System Part I Overview and Mouth to Stomach

Transcription

1 1 BIO 202 Digestive System Part I Overview and Mouth to Stomach Function of the Digestive System Ingestion Taking food and water into the mouth Break down the food Mechanical digestion: chewing, mixing, and churning food Chemical digestion: digestive enzymes breakdown food Absorb nutrients Movement of nutrients from the GI tract to the blood or lymph Release of waste Elimination of indigestible solid wastes Organs Within the Digestive System Two groups of organs within the digestive system The digestive tract or gastrointestinal (GI) tract is a muscular tube that winds through the body and is responsible for the digestion and absorption of food Oral cavity, pharynx, esophagus, stomach, small intestine, large intestine, and anus The accessory digestive organs aid in the breakdown of foodstuffs Teeth, tongue, gallbladder, salivary glands, liver, and pancreas Tunics Three major functions: Secretion of mucus Absorption of end products of digestion Protection against infectious disease 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)

2 Tunics Mucosa Innermost layer that lines the lumen of the alimentary canal Consists of a mucous epithelium, a lamina propria, and a muscularis mucosae Submucosa Connective tissue layer containing the submucosal plexus (part of the enteric plexus), blood vessels, and small glands Muscularis Consists of an inner layer of circular smooth muscle and an outer layer of longitudinal smooth muscle The myenteric plexus is between the two muscle layers Serosa or adventitia Forms the outermost layer of the digestive tract 2 Enteric Nervous System Consists of nerve plexuses within the wall of the digestive tract Serves the digestive tract and regulates digestive activity. 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

3 3 Peritoneum The peritoneum is a serous membrane that lines the abdominopelvic cavity and organs Mesenteries are peritoneum that extend from the body wall to many of the abdominopelvic organs Retroperitoneal organs are located behind the peritoneum Oral Cavity Oral cavity or mouth Is bounded by lips, cheeks, palate, and tongue Has the oral orifice as its anterior opening Is continuous with the oropharynx posteriorly Divided into two regions Vestibule Space between the lips or cheeks and the alveolar processes, which contain the teeth Oral Cavity Proper Lies medial to the alveolar processes To withstand abrasions: The mouth is lined with stratified squamous epithelium Oral Cavity Lips and Cheeks Involved in facial expression, mastication, and speech Palate Forms the roof of the mouth. Consists of hard and soft areas Hard palate: assists the tongue in chewing Soft palate: mobile fold formed mostly of skeletal muscle. Projecting from the soft palate is the uvula Closes off the nasopharynx during swallowing The mouth contains accessory glands (salivary glands) and accessory organs (teeth and tongue) which begin the digestive process

4 4 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 Frenulum secures the tongue to the floor of the mouth Surface bears papillae Filiform whitish, give the tongue roughness and provide friction Fungiform reddish, scattered over the tongue Circumvallate (vallate) V shaped row in back of tongue These three house taste buds Foliate on the lateral aspects of the posterior tongue Teeth Tear and grind food Born with 20 deciduous teeth Erupt between about 6 months and 24 months of age Deciduous teeth are replaced by 32 permanent teeth Replacement starts at about 5 years and the process is complete by about 11 years The types of teeth are Incisors Canines Premolars Molars A tooth consists of a crown, a neck, and a root Crown Exposed part of the tooth above the gingiva Dentin covered by enamel Enamel: acellular material composed of calcium salts and hydroxyapatite crystals; the hardest substance in the body Root Portion of the tooth embedded in the jawbone Composed of dentin Within the dentin of the root is the pulp cavity, which is filled with pulp, blood vessels, and nerves Periodontal ligaments hold the teeth in the alveoli

5 5 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 Salivary glands produce serous and mucous secretions The three pairs of large salivary glands are the parotid, submandibular, and sublingual Saliva Produce ~1 liter of saliva/day Secreted from serous and mucous cells of salivary glands Serous: enzymes, ions, and mucin Mucous: produce mucous % water and slightly acidic solution containing Electrolytes Digestive enzyme: salivary amylase Proteins: mucin, lysozyme, defensins, and IgA Metabolic wastes: urea and uric acid Control of Salivation Primarily by the parasympathetic division of the autonomic nervous system 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 Pharynx and Esophagus Pharynx Consists of the nasopharynx, oropharynx, and laryngopharynx Food and fluids to the esophagus Air to the trachea Esophagus Connects the pharynx to the stomach The upper and lower esophageal sphincters regulate movement

6 6 Swallowing During the voluntary phase of swallowing, a bolus of food is moved by the tongue from the oral cavity to the pharynx The pharyngeal phase is a reflex caused by the stimulation of stretch receptors in the pharynx The soft palate closes the nasopharynx, and the epiglottis, vestibular folds, and vocal folds close the opening into the larynx Pharyngeal muscles move the bolus to the esophagus The esophageal phase is a reflex initiated by the stimulation of stretch receptors in the esophagus. A wave of contraction (peristalsis) moves the food to the stomach Digestive System Part II Stomach, Small Intestine, Liver Stomach Anatomy and Histology Temporary storage tank where chemical breakdown of proteins begins and food is converted from a bolus to chyme The openings of the stomach are the Gastroesophageal opening to the esophagus Pyloric orifice to the duodenum The major regions are the Cardiac part Fundus Body Pyloric part which is continuous with the duodenum through the pyloric sphincter The lateral sides of the stomach consist of greater and lesser curvatures

7 7 Stomach Anatomy and Histology The wall of the stomach consists of External serosa Muscle layer (longitudinal, circular, and oblique) Submucosa Simple columnar epithelium Surface mucous cells: produce an alkaline mucous with bicarbonate, which coats and protects the stomach lining An empty stomach has a volume of 50 ml but can hold about a gallon of food When empty one can see the folds called rugae (submucosa and mucosa) Stomach Anatomy and Histology There are gastric pits with glands that produce juice Glands include four secretory cells: Mucous neck: secrete acid mucus Parietal cells: secrete HCl and intrinsic factor Chief cells: produce pepsinogen Pepsinogen is activated to pepsin by: HCl in the stomach Pepsin itself via a positive feedback mechanism Endocrine cells: produce regulatory hormones Gastrin, histamine, endorphins, serotonin, cholecystokinin (CCK), and somatostatin into the lamina propria Stomach Secretions

8 8 Chyme is ingested food mixed with gastric juice Gastric juice Mucus protects the stomach lining Pepsinogen is converted to pepsin, which digests proteins Hydrochloric acid promotes pepsin activity and kills microorganisms Intrinsic factor is necessary for vitamin B12 absorption Gastrin and histamine regulate stomach secretions A proton pump (H + K + exchange pump) moves H + out of parietal cells Regulation of Stomach Secretion There are three phases of stomach secretion Cephalic phase Initiated by the sight, smell, taste, or thought of food Nerve impulses from the medulla stimulate hydrochloric acid, pepsinogen, gastrin, and histamine secretion Gastric phase Initiated by distention of the stomach, which stimulates gastrin secretion and activates CNS and local reflexes that promote secretion Gastrointestinal phase Acidic chyme, which enters the duodenum and stimulates neuronal reflexes and the secretion of hormones (secretin, cholecystokinin) that inhibit gastric secretions Small Intestine

9 9 The body s major digestive organ Digestion is completed and absorption occurs Runs from pyloric sphincter to the ileocecal valve Has three subdivisions: Duodenum ~ 10 long. The bile and pancreatic ducts empty here Jejunum ~ 8 long Ileum ~ 12 long Small Intestine Anatomy and Histology Structural modifications of the small intestine wall increase surface area about 600 fold Circular folds Deep folds of the mucosa and submucosa Villi Fingerlike extensions of the mucosa Microvilli Tiny projections of absorptive mucosal cells plasma membranes This forms a brush border The epithelium of the mucosa is simple columnar epithelium. Between the villi the mucosa contains pits called intestinal glands Small Intestine Anatomy and Histology The epithelium of the mucosa is made up of: Absorptive cells Produce digestive enzymes Goblet cells Produce a protective mucus Granular cells (Paneth cells) Protect the intestinal epithelium from bacteria Endocrine cells Produce regulatory hormones Peyer s patches (aggregated lymphoid follicles) are found in the submucosa

10 10 Small Intestine Secretions Mucus protects against digestive enzymes and gastric acids Digestive enzymes (disaccharidases and peptidases) are bound to the intestinal wall Chemical or tactile irritation, vagal stimulation, and secretin stimulate intestinal secretion Movement of the Small Intestine Segmental contractions mix intestinal contents Peristaltic contractions move materials distally Distension of the intestinal wall, local reflexes, and the parasympathetic nervous system stimulate contractions Distension of the cecum initiates a reflex that stimulates contraction of the ileocecal sphincter

11 11 Liver Anatomy and Histology The liver has four external lobes: right, left, caudate, and quadrate Internally, the liver is divided into eight segments Liver segments are divided into lobules 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

12 12 Liver Anatomy and Histology Portal triads consist of Hepatic duct: conduct bile toward the duodenum Hepatic artery: supplies oxygen rich blood to the liver Hepatic portal vein: carries venous blood with nutrients from digestive viscera The hepatic cords are composed of columns of hepatocytes separated by the bile canaliculi Sinusoids are enlarged spaces filled with blood and lined with endothelium and hepatic phagocytic cells Kupffer cells: hepatic macrophages found in liver sinusoids Functions of the Liver Produces bile, which contains bile salts that emulsify fats Stores and processes nutrients, produces new molecules, and detoxifies molecules Hepatic phagocytic cells phagocytize red blood cells, bacteria, and other debris Produces blood components Blood Flow Through the Liver Branches of the hepatic artery and the hepatic portal vein in the portal triads empty into hepatic sinusoids Hepatic sinusoids empty into central veins, which join to form the hepatic veins, which leave the liver

13 13 Digestive System Part III Bile Production, Pancrase, Large Intestine Gallbladder and Bile Gallbladder Bile A small sac on the inferior surface of the liver Stores and concentrates 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 Helps make cholesterol soluble The chief bile pigment is bilirubin, a waste product of heme. Bilirubin is metabolized by bacteria in the small intestines and urobilin is produced, which gives feces its dark color Regulation of Bile Secretion Acidic, fatty chyme causes the duodenum to release: Cholecystokinin (CCK) and secretin into the bloodstream Cholecystokinin causes: The gallbladder to contract and releases bile Secretin increases bile secretion (water and bicarbonate ions) Bile salts and secretin transported in blood stimulate the liver to produce bile Vagal stimulation causes weak contractions of the gallbladder

14 14 Pancreas Anatomy and Histology Location Lies deep to the greater curvature of the stomach The head is encircled by the duodenum and the tail sits against the spleen Exocrine function Secretes pancreatic juice which breaks down all categories of foodstuff The pancreas is divided into lobules that contain acini Clusters of secretory cells that contain zymogen granules with digestive enzymes Connect to a duct system that eventually forms the pancreatic duct The pancreas also has an endocrine function The islets of Langerhans release of insulin and glucagon

15 15 Pancreatic Secretions The aqueous component of pancreatic juice contains bicarbonate ions Water solution of enzymes and electrolytes (primarily HCO 3 ) Neutralizes acidic chyme Provides optimal environment for pancreatic enzymes The enzymatic component of pancreatic juice is produced by the acini and contains enzymes that digest carbohydrates, lipids, and proteins Enzymes are released in inactive form and activated in the duodenum Large Intestine Anatomy and Histology Extends from the ileocecal valve to the anus Absorbs water and eliminate the waste via feces Is subdivided into the cecum, appendix, colon, rectum, and anal canal The cecum forms a blind sac at the junction of the small and large intestines. The vermiform appendix is a blind tube off the cecum. The colon has distinct regions: ascending colon, transverse colon, descending colon, and sigmoid colon The sigmoid colon joins the rectum The anal canal, the last segment of the large intestine, opens to the exterior at the anus The anal canal contains two sphincters to allow the movement of feces Anatomy and Histology of the Large Intestine 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 The mucosal lining of the large intestine is simple columnar epithelium with mucus producing crypts

16 16 Large Intestine Secretions Mucus protects the intestinal lining The bacterial flora of the large intestine consist of: Bacteria surviving the small intestine that enter the cecum 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 Constitute about 30% of the dry weight of the feces Movement in the Large Intestine Haustra mix the contents and moves them slowly toward the anus Mass movements are strong peristaltic contractions that occur three or four times a day Defecation is the elimination of feces Distension of rectal walls caused by feces: Stimulates contraction of the rectal walls Relaxes the internal anal sphincter Reflex activity moves feces through the internal anal sphincter Voluntary activity regulates movement through the external anal sphincter Digestive System Part IV Digestion and Metabolism Digestion, Absorption, and Transport Digestion (mechanical and chemical) is the breakdown of organic molecules into their component parts Chemical digestion is the breaking of covalent chemical bonds in organic molecules by digestive enzymes Absorption is the uptake of digestive tract contents Transport is the distribution of nutrients throughout the body Chemical Digestion Carbohydrate digestion Begins in mouth with salivary amylase digesting starches Pancreatic amylase finishes the process in the small intestine. Simple sugars are also broken down here

17 17 Protein digestion Lipid digestion Broken down into amino acids Begins in the stomach with pepsin Trypsin and chymotrypsin are also important protein digesting enzymes. Nucleic acids Broken down by pancreatic nucleases in the small intestine Small intestine is the sole source of lipid digestion by lipases Enter lacteals and are transported to systemic circulation via lymph Carbohydrates Include starches, glycogen, sucrose, lactose, glucose, and fructose Polysaccharides are broken down into monosaccharides by a number of different enzymes Monosaccharides Taken up by intestinal epithelial cells by symport that is powered by a Na + gradient or by facilitated diffusion Carried to the liver, where the non glucose sugars are converted to glucose Glucose is transported to the cells that require energy Glucose enters the cells through facilitated diffusion Insulin influences the rate of glucose transport Lipids Include triglycerides, phospholipids, steroids, and fat soluble vitamins Emulsification is the transformation of large lipid droplets into smaller droplets and is accomplished by bile salts Lipase digests lipid molecules to form fatty acids and a monoglyceride Micelles form around lipid digestion products and move to epithelial cells of the small intestine, where the products pass into the cells by simple diffusion Lipids Within the epithelial cells, free fatty acids are combined with monoglycerides to form triglycerides Proteins coat triglycerides, phospholipids, and cholesterol to form chylomicrons Chylomicrons enter lacteals within intestinal villi and are carried through the lymphatic system to the bloodstream Triglycerides are stored in adipose tissue, converted into other molecules, or used as energy

18 18 Lipoproteins Lipoproteins include chylomicrons, VLDL, LDL, and HDL LDL transports cholesterol to cells, and HDL transports it from cells to the liver LDL are taken into cells by receptor mediated endocytosis, which is controlled by a negative feedback mechanism Proteins Pepsin in the stomach breaks proteins into smaller polypeptide chains Proteolytic enzymes from the pancreas produce small peptide chains Peptidases, bound to the microvilli of the small intestine, break down peptides Tripeptides, dipeptides, and amino acids are absorbed by symport that is powered by a Na gradient Amino acids are transported to the liver, where the amino acids can be modified or released into the bloodstream Amino acids are actively transported into cells under the stimulation of growth hormone and insulin Amino acids are used as building blocks or for energy Amino Acid Transport Across the Intestinal Epithelium Water and Ions About 9 liters of water enters the digestive tract each day Can move in either direction across the wall of the small intestine, depending on the osmotic gradients across the epithelium Epithelial cells actively transport Na, K, Ca 2+, and Mg 2+ from the intestine Chloride ions move passively through the wall of the duodenum and jejunum but are actively transported from the ileum

19 19 Stages of Metabolism Processing of nutrients Digestion, absorption and transport to tissues Cellular processing (in cytoplasm) Synthesis of lipids, proteins, and glycogen, or Catabolism (glycolysis) into intermediates Oxidative (mitochondrial) breakdown of intermediates into CO2, water, and ATP

20 20