The Digestive System PDF

The Digestive System Ryan R. Williams, M.D., Ph.D. Biology 122 California State University Dominguez Hills Overview of the Digestive System The gastrointestinal (GI) tract consists of different regions and accessory organs Function—to hydrolyze, or break down, the mac...

The Digestive System Ryan R. Williams, M.D., Ph.D. Biology 122 California State University Dominguez Hills Overview of the Digestive System The gastrointestinal (GI) tract consists of different regions and accessory organs Function—to hydrolyze, or break down, the macromolecules found in food The subunit molecules (monosaccharides, amino acids, fatty acids, and glycerol) are absorbed Cross plasma membranes of simple columnar epithelial cells using facilitated and active transport The nutrients are transported by the blood to our cells Overview of the Digestive System Ingestion—intake of food via the mouth Digestion—breaking down foods into their subunits Begins in the mouth, continues in the stomach, and is completed in the small intestine Mechanical digestion—chewing in the mouth and contractions of smooth muscles in the stomach Chemical digestion—digestive enzymes hydrolyze macromolecules into subunits Movement—food is passed from one organ to the next, normally by contractions of smooth muscle called peristalsis Absorption—movement of nutrients across the GI tract wall into the blood; they are then delivered to cells Elimination—removal of indigestible wastes Defecation  Overview of the Digestive System The digestive system consists of: The digestive tract Accessory organs of digestion Digestive tract Mouth Pharynx Esophagus Stomach Small intestine Large intestine © 2015 Pearson Education, Inc.  Overview of the Digestive System Accessory Organs of the Digestive Tract Teeth Tongue Salivary glands Pancreas Liver Gallbladder © 2015 Pearson Education, Inc.  Mouth (oral cavity) contains teeth and tongue Parotid gland (salivary gland) Sublingual gland (salivary gland) Submandibular gland (salivary gland) Pharynx Esophagus Stomach Liver Pancreas Duodenum (Sm intestine) Transverse colon Gallbladder (Lg Intestine) Jejunum (Sm intestine) Descending colon (Lg Intestine) Ascending colon (Lg intestine) Sigmoid colon Ileum (Sm intestine) (Lg intestine) Cecum (Lg intestine) Rectum Appendix Anal canal Anus (a) Right lateral view of head and neck and anterior view of trunk © 2015 Pearson Education, Inc. The Digestive Tract Wall of the digestive tract Lumen—open area of a hollow organ or vessel; in the GI tract, it contains food or feces Layers: Mucosa—innermost layer; produces mucus for protection; also produces digestive enzymes Submucosa—loose connective tissue; contains blood vessels, lymphatic vessels, and nerves Muscularis—made of 2 layers of smooth muscle (inner, circular layer, and an outer, longitudinal layer) that move food along the GI tract Serosa—outer lining; is part of the peritoneum  nerve lymphatic supply vessel Lumen central space containing food being digested Mucosa inner mucous membrane layer modified according to the digestive organ Submucosa broad band of loose connective tissue that contains nerves, blood, artery and lymphatic vessels vein Muscularis two layers of smooth muscle Serosa thin, outermost tissue that is the visceral peritoneum The Digestive Tract Diverticulosis A condition in which the mucosa of any part of the GI tract (usually the large intestine) pushes through the other layers and forms pouches where food collects Diverticulitis—when the pouches become infected This happens in 10–25% of people with diverticulosis Inflammatory bowel disease (IBD)—severe chronic diarrhea, abdominal pain, fever, and weight loss May have associated bleeding Ex: Chron’s disease and Ulcerative Colitis Irritable bowel syndrome (IBS)—contractions of the muscularis cause abdominal pain, constipation, and diarrhea The underlying cause is not known  Major Subdivisions of the Digestive Tract Mouth Oral Cavity Mechanical processing, moistening, Accessory Organs of mixing with salivary secretions the Digestive System Pharynx Salivary Glands Muscular propulsion of materials into Secretion of lubricating fluid the esophagus containing enzymes that break down carbohydrates Esophagus Transport of materials to the stomach Liver Secretion of bile (important Stomach for lipid digestion), storage of nutrients, many other Chemical breakdown of materials via acid vital functions and enzymes; mechanical processing through muscular contractions Gallbladder Small Intestine Storage and concentration Enzymatic digestion and absorption of of bile water, organic substrates, vitamins, and ions Pancreas Large Intestine Exocrine cells secrete buffers and digestive Enzymatic digestion and absorption of enzymes; endocrine cells water, organic substrates, vitamins, and ions secrete hormones Anus © 2015 Pearson Education, Inc. The Mouth (Oral Cavity) Receives food and begins mechanical and chemical digestion The roof of the mouth separates the nasal cavity from the oral cavity Two parts: a bony hard palate and a soft palate. The soft palate is made of muscle; ends in the uvula Tonsils—in the back of the mouth on either side of the tongue Lymphatic nodules; help protect from disease The pharyngeal tonsils in the nasopharynx, are commonly called the adenoids The Mouth (Oral Cavity) Three pairs of salivary glands secrete saliva, which contains: Salivary amylase—begins carbohydrate digestion Lingual lipase —begins lipid digestion Lysozyme—antibacterial enzyme Tongue Covered in taste buds Assists in mechanical breakdown, movement of food Forms a bolus (mass of chewed food) and moves it toward the pharynx The Mouth (Oral Cavity) Teeth Mechanically digest food 20 smaller deciduous (baby) teeth, 32 adult teeth Two main divisions: Crown—the part of the tooth above the gum line Root—the portion below Enamel—extremely hard outer layer Dentin—a thick layer of bonelike material Pulp—nerves and blood vessels Periodontal membranes—anchor the tooth to the jawbone Dental caries—tooth decay, cavities Bacteria metabolize sugar and produce acids, which erode the teeth Gingivitis—inflammation of the gums  enamel crown dentin pulp gum jawbone root canal root periodontal membrane The Pharynx and Esophagus The mouth and nasal passages lead to the pharynx In turn, the pharynx opens into both the food passage (esophagus) and air passage (trachea) These two tubes are parallel to each other; the trachea is in front (anterior) of the esophagus The esophagus is a muscular tube that leads to the stomach Swallowing Starts off voluntary (skeletal muscle) Once food or drink is pushed back into the pharynx, it becomes an involuntary (smooth muscle) reflex Food normally enters the esophagus because other possible avenues are blocked The soft palate moves back to close off the nasal passage, and the trachea moves up under the epiglottis to cover the glottis, the opening to the larynx  hard palate food bolus Soft palate closes nasopharynx off nasopharynx. soft palate epiglottis Esophagus glottis is closed Larynx rises and trachea epiglottis covers (airway) open glottis. esophagus opens Swallowing The Pharynx and Esophagus Peristalsis—contractions that push food through the digestive tract Sphincter—a ring of muscle that acts as a valve When it contracts, it stops food from moving through; when it relaxes, it allows food through The lower esophageal sphincter is between the esophagus and the stomach Heartburn—failure of this sphincter; stomach contents move from the stomach into the esophagus Gastroesophageal reflux disease (GERD)—chronic heartburn Vomiting—when strong contractions of the abdominal muscles and the diaphragm force the contents up and out  peristaltic bolus wave Lower gastroesophageal esophagus sphincter relaxes and stomach food enters stomach. Peristalsis The Stomach Stores food, starts digestion of proteins, and of controls movement food into the small intestine Does not absorb nutrients However, it does absorb alcohol, because alcohol is fat- soluble and can pass through membranes easily There are three layers of muscle in the muscularis layer (instead of two) to help in mechanical digestion Circular, longitudinal, and third oblique layer The Stomach The mucosa has deep folds called rugae The mucosa also has gastric pits, which contain gastric glands The gastric glands produce gastric juice, which contains: Pepsin—digests proteins Hydrochloric acid (HCl)—pH of 2 Kills bacteria and activates pepsin Mucus Normally, the stomach empties in 2–6 hours Chyme—a mixture of food and gastric juice Pyloric sphincter—allows only a small amount of chyme to enter the small intestine at a time  esophagus lower gastroesophageal sphincter pyloric Muscularis layer sphincter has three layers of muscle. Mucosa layer has rugae. Gastric pits in mucosa gastric pit SEM 3,260× lower gastroesophageal sphincter gastric pit gastric gland cells that secrete gastric juice pyloric sphincter Gastric glands How the stomach empties  The Small Intestine Approximately 20 feet in length / 1.5–2.5 inches in diameter Consists of three regions: Duodenum Receives chyme from the stomach 10 inches long; receives digestive enzymes from the pancreas, bile from the liver and gallbladder Jejunum 8 feet long; most of the digestion and absorption occurs in the jejunum Ileum 12 feet long Empties into the cecum of the large intestine © 2015 Pearson Education, Inc. Figure 25.13 Regions of the Small Intestine Transverse colon Regions of the Small Intestine Duodenum Jejunum Ileum Ascending colon Cecum Descending colon Sigmoid colon Rectum © 2015 Pearson Education, Inc. The Small Intestine Pancreas—secretes digestive enzymes through a duct into the duodenum, the first portion of the small intestine Another duct that passes through the head of the pancreas delivers bile into the duodenum Bile is made in the liver and stored in the gallbladder Bile emulsifies fat Mechanical digestion; causes fat droplets to disperse in water, exposing fat to enzymes Bile also contains waste from the liver that is eventually excreted in the feces The Small Intestine Pancreatic enzymes Pancreatic lipase Hydrolyzes fats into glycerol and fatty acids Pancreatic amylase—produced by the pancreas, secreted into the duodenum Digests carbohydrates Trypsin—produced by the pancreas, secreted into the duodenum Digests proteins, so is a protease—a class of enzymes that digest proteins Pancreatic juice contains sodium bicarbonate, which neutralizes acidic chime The Small Intestine The mucosa contains fingerlike projections called villi (singular, villus) The cells that cover the villi have microvilli Give the villi a fuzzy “brush border” Contain brush border enzymes that complete digestion These two structures greatly increase the surface area of the small intestine for absorption of nutrients Nutrients are absorbed into the villi, which contain blood capillaries and a small lymphatic capillary called a lacteal Monosaccharides and amino acids enter the blood capillaries of a villus Glycerol and fatty acids form lipoprotein droplets called chylomicrons, which then enter a lacteal After nutrients are absorbed, they are eventually carried to all the cells of the body by the blood Small intestine Section of intestinal wall villus lumen lacteal blood villus microvilli capillaries goblet cell Intestinal lining lymph nodule venule lymphatic vessel Villi arteriole 100× Structure of the villi  carbohydrate protein pancreatic amylase bile salts fat globules trypsin emulsification peptides droplets maltase peptidase lipase cell of glucose intestinal monoglycerides villus and free amino acids fatty acids chylomicron pH = basic pH = basic pH = basic lymphatic blood capillary blood capillary capillary Carbohydrate digestion Protein digestion Fat digestion  Table 9.1a Major Digestive Enzymes: Carbohydrate Digestion. Enzyme Produced By Site of Action Optimum pH Digestion Salivary amylase Salivary glands Mouth Neutral Starch + H2O → maltose Pancreatic amylase Pancreas Small intestine Basic Starch + H2O → maltose Maltase Small intestine Small intestine Basic Maltose + H2O → glucose + glucose Lactase Small intestine Small intestine Basic Lactose + H2O →"glucose + galactose Table 9.1b Major Digestive Enzymes: Protein Digestion. Enzyme Produced By Site of Action Optimum pH Digestion Pepsin Gastric glands Stomach Acidic Protein + H2O → peptides Trypsin Pancreas Small intestine Basic Protein + H2O → peptides Peptidases Small intestine Small intestine Basic Protein + H2O → amino acids  Table 9.1c Major Digestive Enzymes: Nucleic Acid Digestion. Enzyme Produced By Site of Action Optimum pH Digestion Nuclease Pancreas Small intestine Basic RNA and DNA + H2O → nucleotides Nucleosidases Small intestine Small intestine Basic Nucleotide + H2O→ base + sugar + phosphate Table 9.1d Major Digestive Enzymes: Fat Digestion. Enzyme Produced By Site of Action Optimum pH Digestion Lingual Salivary Mouth Neutral Fat droplet + H2O → monoglycerides Lipase glands + fatty acids Lipase Pancreas Small intestine Basic Fat droplet + H2O → monoglycerides + fatty acids The Small Intestine Secretion of digestive juices is controlled by the nervous system and digestive hormones When you look at or smell food, the parasympathetic nervous system stimulates gastric secretion A meal rich in protein causes the stomach to produce the hormone gastrin Increases the secretory activity of gastric glands Secretin—secreted by the duodenum Release is stimulated by HCl (present in chyme) Cholecystokinin (CCK)—released by the duodenum when proteins and fat are present Causes the liver to increase production of bile and causes the gallbladder to contract and release stored bile These hormones increase the production of pancreatic juice gallbladder liver stomach pancreas duodenum blood vessel secretin CCK gastrin The Small Intestine Lactose intolerance Lactose is the primary sugar in milk Lactase—the brush border enzyme that digests lactose If the enzyme does not function very well it causes diarrhea, gas, bloating, and abdominal cramps after ingesting milk and other dairy products Diarrhea occurs because the undigested lactose causes fluid retention in the small intestine (due to increased osmotic pressure in the lumen) Gas, bloating, and cramps occur when bacteria break down the lactose anaerobically The Small Intestine Celiac Disease Autoimmune response against a protein called gluten, which is naturally found in grains such as wheat, barley, and rye The presence of gluten in the small intestine results in an inflammatory response, which damages the villi and microvilli of the small intestine Can lead to abdominal pain, diarrhea, and malnutrition The Large Intestine Large intestine—includes the cecum, the colon, the rectum, and the anal canal Larger in diameter than the small intestine, but it is shorter in length Cecum The first portion of the large intestine Where contents from the small intestine (ileum) empty Appendix—projection off of the cecum Fights infections Appendicitis—inflamed appendix; can cause peritonitis, a life-threatening infection of the peritoneum The Large Intestine Colon From the cecum, the ascending colon goes up the right side of the body The transverse colon crosses the abdominal cavity The descending colon passes down the left side and becomes the sigmoid colon, which enters the rectum, the last portion of the large intestine The rectum opens at the anus, where defecation, the expulsion of feces, occurs  transverse colon ascending colon descending small intestine colon cecum orifice of appendix vermiform appendix rectum sigmoid colon internal anal sphincter external anal sphincter anal canal anus The Large Intestine Functions of the large intestine Does not produce digestive enzymes Does not absorb nutrients except certain vitamins Absorbs water from feces to prevent dehydration Absorbs vitamins produced by intestinal flora, the bacteria that inhabit the intestine Bacteria break down indigestible material and produce B- complex vitamins and vitamin K Forms feces which contains water, bacteria and dietary fiber (indigestible remains) Defecation—ridding the body of feces Internal anal sphincter (smooth muscle) is under involuntary control External anal sphincter (skeletal muscle) is under voluntary control The Large Intestine Diarrhea—increased peristalsis and failure to absorb water from feces, due to either an infection or nervous stimulation Constipation—dry, hard feces; may be controlled with water and fiber Hemorrhoids—enlarged, inflamed blood vessels of the anus due to chronic constipation, pregnancy, aging, anal intercourse Polyps and cancer Small growths in the colon lining Can be benign or cancerous Increased dietary fat raises the risk of colon cancer Fiber in the diet inhibits colon cancer The Pancreas Located behind the stomach An exocrine gland that produces pancreatic juice, which enters the duodenum via the pancreatic duct Contains sodium bicarbonate and digestive enzymes Also an endocrine gland secretes the hormone insulin when blood glucose levels rise Type 1 diabetes Not enough insulin Normally diagnosed in childhood Type 2 diabetes The body’s cells are insulin-resistant Normally occurs in adulthood Risk factors: obesity, inactivity, family history The Gallbladder Pear-shaped organ just below the liver Stores bile Stimulation by CCK causes contraction Gallstones—made of a stone-like material The Liver The largest organ in the body Lies mainly in the upper right abdominal cavity, under the diaphragm Lobules—structural and functional units The hepatic portal vein brings blood to the liver from the GI tract The lobules filter blood, removing poisonous substances The Liver Major metabolic organ Produces bile, which contains bile salts, water, cholesterol, and bicarbonate Contains bilirubin, a green pigment formed from the breakdown of hemoglobin Stores vitamins Involved in blood glucose homeostasis Stores glucose as glycogen; when blood glucose is low, it releases glucose by breaking down glycogen Converts glycerol and amino acids to glucose As amino acids are converted to glucose, urea is formed Produces plasma proteins Regulates blood cholesterol levels  bile canals branch of hepatic bile central artery vein common hepatic duct pancreas pancreatic duct pancreatic juice branch of gallbladder bile duct hepatic common bile duct Structure of liver portal vein duodenum Location of accessory glands  Table 9.2 Functions of the Liver. 1. Destroys old red blood cells; excretes bilirubin, a breakdown product of hemoglobin in bile, a liver product 2. Detoxifies blood by removing and metabolizing poisonous substances 3. Stores iron (Fe2+), the water-soluble vitamin B12, and the fat-soluble vitamins A, D, E, and K 4. Makes plasma proteins, such as albumins and fibrinogen, from amino acids 5. Stores glucose as glycogen after a meal; breaks down glycogen to glucose to maintain the glucose concentration of blood between eating periods 6. Produces urea after breaking down amino acids 7. Helps regulate blood cholesterol level, converting some to bile salts The Liver Liver disease can cause bile pigments to leak into the blood, causing jaundice Yellowish tint to the whites of the eyes and the skin Hepatitis Inflammation of the liver Hepatitis A—usually acquired from sewage-contaminated drinking water and food Hepatitis B—usually from sexual contact, but also from blood transfusions or contaminated needles More contagious than the AIDS virus and is spread in the same way Vaccines are available for hepatitis A and B Hepatitis C—usually acquired by infected blood Can lead to chronic hepatitis, liver cancer, and death The Liver Cirrhosis Chronic disease; first, liver becomes fatty, and then filled with fibrous scar tissue Often seen in alcoholics due to malnutrition and the excessive alcohol (a toxin) the liver is forced to break down The liver can regenerate and recover if the rate of regeneration exceeds the rate of damage During liver failure there may not be enough time to let the liver heal so will need a liver transplantation