10/24/2016. Introduction

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1 Introduction Chapter 17 Lecture Outline See separate PowerPoint slides for all figures and tables preinserted into PowerPoint without notes. Main portion of the digestive system is a long tube extending through the body The inside of the digestive tube is part of the outside world, not the internal environment The ph, electrolyte composition and water volume of the internal environment are carefully regulated, but these change in the digestive tube, according to the food we eat Food materials that enter the digestive tube are broken down into their building blocks (sugars, amino acids, nucleotides, fatty acids, glycerol) Building blocks are absorbed across the wall of the tube Once absorbed, building blocks are used to create the molecules needed by the body Copyright McGraw-Hill Education. Permission required for reproduction or display Overview of the Digestive System Digestion: The mechanical and chemical breakdown of foods into forms that cell membranes can absorb: Mechanical digestion breaks down large particles into smaller ones, but does not change chemical composition Chemical digestion breaks down food particles by changing them into simpler chemicals Digestive System: Organs of the digestive system carry out mechanical & chemical digestion, as well as ingestion, propulsion, absorption and defecation The digestive system consists of the alimentary canal and accessory organs 3 Organs of the Digestive System The digestive system consists of 2 portions: Alimentary canal: Consists of organs that extend from the mouth to the anus; the food passageway Accessory organs: Consists of organs that empty secretions into the alimentary canal; food does not pass through them 4 General Characteristics of the Alimentary Canal The alimentary canal is a muscular tube about 8 m long Passes through the thoracic & abdominopelvic cavities Structure of the Wall 5 Wall of alimentary canal is composed of 4 layers; from innermost to outermost, the layers are: Mucosa: innermost layer, mucous membrane Submucosa: nourishes cells, transports absorbed food molecules Muscularis: muscle tissue, moves tube and food materials Serosa: outermost layer; serous fluid eliminates friction 6 1

2 Structure of the Wall Movements of the Tube The wall of the small intestine shows the 4 layers of the wall of the alimentary canal: mucosa, submucosa, muscularis, serosa 7 2 types of movement in the: alimentary canal: Mixing movements: Muscle in small sections contracts rhythmically Does not move materials in one direction Example: segmentation Propelling movements: Moves materials in one direction Peristalsis: ring of contraction progresses down tube; propels food particles down the tract 8 Innervation of the Tube 17.2: Mouth Branches of the sympathetic and parasympathetic divisions of the autonomic nervous system extensively innervate the alimentary canal: Submucosal plexus: controls secretions Myenteric plexus: controls gastrointestinal motility Autonomic control of digestive activity: Parasympathetic impulses: increase activities of digestive system (secretion and motility) Sympathetic impulses: inhibit digestive actions (secretion and motility) The mouth: First part of alimentary canal Ingests food Mastication: mechanical breakdown of solid particles, mixes them with saliva Functions as an organ of speech and sensory reception Surrounded by lips, cheeks, tongue, palate Includes oral cavity and vestibule 9 10 Cheek and Lips Cheeks: Form the lateral walls of the mouth Contain muscles for facial expression and chewing Have an inner lining of stratified squamous epithelium (moist) Lips: Highly mobile structures that surround the mouth opening Sensory receptors judge temperature and texture of food Boundary between skin and mucous membrane inside mouth Tongue: Thick, muscular organ that occupies the floor of the mouth, and nearly fills the oral cavity when the mouth is closed Lingual frenulum connects tongue to floor of mouth Papillae: projections that move food, contain taste buds Lingual tonsils: lymphatic tissue masses on root of tongue Tongue

3 Palate Teeth Palate: Forms the roof of the oral cavity Consists of a hard (bony) anterior part and a soft (muscular) posterior part The uvula extends from the soft palate Palatine tonsils: lymphatic masses on sides of tongue Pharyngeal tonsils (adenoids): masses of lymphatic tissue in posterior wall of pharynx Teeth: Hardest structures in the body 20 primary (deciduous) teeth 32 secondary (permanent) teeth Clinical Application 17.1 Mouth Parts and Their Functions Dental Caries Dental Caries: Cavities within enamel of a tooth Formed when sticky foods lodge between teeth or in crevices of molars Bacteria on teeth metabolize sugars Acidic by-products destroy enamel and dentin Prevention: Brush and floss Dental exams and cleanings Fluoride treatments Sealants

4 17.3: Salivary Glands Salivary glands secrete saliva: Moistens food particles and binds them together Dissolves food so it can be tasted Saliva contains enzymes (begin chemical digestion of carbohydrates) and bicarbonate ions (keep ph favorable for enzyme activity and protect teeth from acidic foods) 3 pairs of major salivary glands: Parotid glands Submandibular glands Sublingual glands There are many minor glands scattered throughout the mucosa of the tongue, palate, and cheeks; keep lining of mouth moist Salivary Secretions The different pairs of salivary glands have varying proportions of 2 types of secretory cells (serous cells and mucous cells): Serous cells produce a watery fluid, containing a digestive enzyme called salivary amylase, which splits starch and glycogen into disaccharides Mucous cells secrete mucus, which binds food particles and lubricates food while swallowing Major Salivary Glands Major Salivary Glands 3 Major Pairs of Salivary Glands: Parotid glands: Largest in body Anterior/inferior to ear Serous glands Saliva contains amylase Submandibular glands: Serous + mucous glands Floor of mouth Sublingual glands: Under tongue Mainly mucous glands Major Salivary Glands 17.4: Pharynx and Esophagus The pharynx is a cavity posterior to the mouth Pharynx extends from nasal cavity to esophagus The esophagus is the tubular organ that extends from the pharynx to the stomach Pharynx and esophagus do not help in food digestion Both the pharynx and esophagus have muscular walls, which function in swallowing Parotid gland Submandibular gland Sublingual gland (serous) (mixed: serous + mucous) (mainly mucous)

5 Structure of the Pharynx The pharynx connects nasal and oral cavities, and can be divided into the following parts: Nasopharynx Oropharynx Laryngopharynx 25 Swallowing Mechanism Swallowing can be divided into 3 stages: First stage: Voluntary stage, in which saliva is mixed with chewed food, forming a bolus Second stage: Begins as food reaches oropharynx, stimulates sensory receptors, and triggers swallowing reflex: The soft palate and uvula raise, to protect nasal cavity from food The hyoid bone and larynx elevate The epiglottis closes off top of the larynx, to protect trachea The longitudinal muscles of pharynx contract The inferior constrictor muscles relax and the esophagus opens Peristaltic waves force food into esophagus Third stage: Peristalsis transports food in the esophagus to the stomach 26 Esophagus Esophagus: A muscular food passageway from the pharynx to the stomach (25 cm) Penetrates the diaphragm through the esophageal hiatus Contains mucous glands in submucosa Lower esophageal (cardiac) sphincter regulates food passage into stomach : Stomach Parts of the Stomach Stomach: J-shaped, pouch-like organ, about centimeters long Inferior to the diaphragm, in the upper-left portion of the abdominal cavity Rugae are folds of mucosa and submucosa that allow for distention The stomach: receives food from the esophagus mixes food with gastric juice initiates protein digestion has limited absorption moves food into small intestine The stomach contains the following layers of smooth muscle: An inner circular layer An outer longitudinal layer Some portions have third (innermost) layer of oblique fibers 29 4 parts of the stomach: Cardia, fundus, body, and pylorus 30 5

6 Parts of the Stomach Radiograph of the stomach 31 Gastric Secretions Gastric glands contain 3 types of secretory cells, which produce a mixture called gastric juice. Components of gastric juice: Pepsinogen: Inactive form of pepsin; secreted by chief cells Pepsin: Active enzyme that beaks down proteins into polypeptides; forms from pepsinogen in presence of hydrochloric acid Gastric lipase: Fat-splitting enzyme, found in small quantities; action inhibited by low ph Hydrochloric acid: Produced by parietal cells; converts pepsinogen into pepsin Mucus: Secreted by mucous cells; provides lubrication and protects stomach lining Intrinsic factor: Produced by parietal cells; required for absorption of vitamin B Lining of the Stomach Components of Gastric Juice Regulation of Gastric Secretions Regulation of Gastric Secretions Gastric juice is produced continuously Rate of production varies, and is under neural and hormonal control Neural regulation: Sympathetic impulses decrease gastric activity Parasympathetic impulses increase gastric activity; promote release of histamine, which stimulates gastric secretion Hormonal regulation: Somatostatin: inhibits hydrochloric acid secretion Gastrin: increases gastric juice secretion Cholecystokinin (CCK): released by small intestine cells when proteins and fat enter the small intestine; decreases gastric motility

7 Phases of Gastric Secretion Gastric Absorption Gastric enzyme, pepsin, begins breaking down proteins Wall of stomach is not well-adapted to absorb digestive products The stomach does absorb: Some water Certain salts Certain lipid-soluble drugs Some alcohol Mixing and Emptying Actions The mixing of food in the stomach with gastric juice produces a semifluid paste called chyme. Peristaltic waves push chyme toward pylorus of stomach Small amount of chyme is transported through pyloric sphincter at a time Mixing and Emptying Actions When chyme starts to fill the duodenum, stretch receptors initiate the enterogastric reflex This slows stomach emptying and intestinal filling Clinical Application 17.2 A Common Problem: Heartburn Stomachache results from eating a lot of food too quickly It takes up to 20 minutes for hypothalamus to sense full stomach Excess fullness leads to abdominal pain and gastric reflux, as stomach contents enter esophagus Stomach contents in esophagus cause inflammation, called esophagitis; feels like the pain is derived from the heart, so it is called heartburn Antacids can provide some relief Prevention: eat small meals, eat slowly, stay upright after eating, and avoid caffeine, nicotine, and alcohol 17.6: Pancreas The pancreas is an endocrine gland, secreting insulin and glucagon to regulate blood glucose Pancreas is also an exocrine gland of the digestive system In its exocrine function, it secretes digestive fluid called pancreatic juice

8 Structure of the Pancreas Structure of the Pancreas Pancreatic acinar cells make up most of pancreas, and release pancreatic juice into tiny ducts which lead to the pancreatic duct Pancreatic duct (along with the common bile duct from the liver and gall bladder) empties into the duodenum of the small intestine Pancreatic duct and Common bile duct join at dilated tube called the hepatopancreatic ampulla (of Vater) Hepatopancreatic sphincter (of Oddi) surrounds ampulla; controls movement of bile and pancreatic juice into duodenum 43 The pancreas has a large head, that fits into the curvature of the duodenum. The tail of the pancreas lies against the spleen. 44 Pancreatic Juice Regulation of Pancreatic Secretion Pancreatic juice contains enzymes that digest carbohydrates, fats, proteins, and nucleic acids Components of pancreatic juice: Pancreatic amylase: splits starch and glycogen into disaccharides Pancreatic lipase: breaks down triglycerides Trypsin: digests proteins; released as inactive trypsinogen, which is activated by enterokinase in small intestine Chymotrypsin: digest proteins; released as inactive, activated by trypsin Carboxypeptidase: digests proteins; released as inactive, activated by trypsin Nucleases: digest nucleic acids Bicarbonate ions: make pancreatic juice alkaline; buffer stomach acid Regulation of Pancreatic Secretion Hormones that help regulate the release of pancreatic juice: Secretin: stimulates pancreas to release pancreatic juice high in bicarbonate ions CCK: stimulates pancreas to release pancreatic juice high in enzymes 17.7: Liver Liver: Largest internal organ Located in the upper-right abdominal quadrant, just beneath the diaphragm Reddish-brown organ Well-supplied with blood vessels

9 Liver Structure Liver has 4 lobes: Right lobe: largest Left lobe: smaller than right lobe Quadrate lobe: minor lobe, near gallbladder Caudate lobe: minor lobe, near inferior vena cava 49 Liver lobes are divided into hepatic lobules; these are the functional units of the liver. Lobules consist of plates of hepatic cells radiating out from a central vein. Hepatic sinusoids run between plates of cells. 50 Liver Functions Oxygen-poor blood from the digestive tract is transported to the liver through the hepatic portal vein. It mixes with oxygen-rich blood from the hepatic artery. This blood filters through the hepatic sinusoids. Hepatocytes adjust nutrient concentrations, as the nutrients from the digestive tract flow by. Bile produced by hepatocytes leaves the liver through bile ducts. 51 The liver carries on many important activities: Produces glycogen from glucose Breaks down glycogen into glucose Converts non-carbohydrates to glucose Oxidizes fatty acids Synthesizes lipoproteins, phospholipids, and cholesterol Converts excess carbohydrates and proteins into fats Deaminates amino acids Forms urea Synthesizes plasma proteins Converts some amino acids to other amino acids Stores glycogen, iron, and vitamins A, D, and B 12 Phagocytizes damaged RBCs and foreign substances/antigens Removes toxins, such as alcohol and certain drugs from the blood Acts as blood reservoir Role in digestion is to secrete bile 52 Major Functions of the Liver From Science to Technology 17.1 Replacing the Liver 53 The liver is required to maintain life Liver is capable of regeneration if 25-30% of it is healthy If cancer spreads to liver, life can continue only weeks months Donor can donate part of the liver, as regeneration can occur There is a shortage of donor livers for transplant Extracorporeal liver assist device (ELAD) can temporarily perform bloodcleansing functions of the liver, until donor liver becomes available ELAD is bioartificial, since it contains artificial portion (cartridges) and biological portion (human liver cells) Patient s blood plasma is passed through the ELAD, where toxins are removed and liver secretions are added; then plasma is returned to patient 54 9

10 Composition of Bile Bile is a yellowish-green liquid that hepatic cells continuously secrete Components of Bile: Water Bile salts: Produced from cholesterol Emulsify fats Only bile component that have a digestive function (act as fat emulsifying agents) Bile pigments: bilirubin and biliverdin, derived from hemoglobin breakdown Cholesterol Electrolytes Clinical Application 17.3 Hepatitis Hepatitis is an inflammation of the liver Many types, chronic or acute, types vary in severity Most common cause of hepatitis is one of several types of viruses Some people have symptoms, and some do not (they can still be carriers) Some forms are blood-borne Some forms are transmitted by contact with food, body fluids, or objects contaminated with feces that contain the virus Antibiotics are not effective against viral hepatitis Hepatitis C accounts for about half of all known cases of hepatitis Gallbladder Gall bladder is a pear-shaped sac on inferior surface of liver It stores and concentrates bile Cystic duct from gall bladder joins with the common hepatic duct from liver to form common bile duct Bile duct empties into the duodenum, via the hepatopancreatic ampulla Hepatopancreatic sphincter regulates release of bile into the duodenum Gallbladder Gallstones: Gallbladder normally concentrates bile salts, bile pigments, cholesterol Sometimes cholesterol precipitates and forms solid crystals Crystals can enlarge, forming gallstones Causes: excess bile concentration, too much cholesterol secretion by liver, or inflammation of the gallbladder Regulation of Bile Release Hormones of the Digestive Tract The hormone CCK, cholecystokinin, causes the gallbladder to contract, in response to fats entering the duodenum. The bile is then released into the duodenum

11 Functions of Bile Salts Bile salts aid digestive enzymes through emulsification: - Break up of large fat globules into smaller droplets (like soap or detergent) - Aid in fat digestion by increasing surface area accessible to the enzyme lipase Bile salts also enhance absorption of fatty acids and cholesterol by forming micelles They help absorb fat-soluble vitamins A, D, E and K Almost all bile salts are recycled Clinical Application 17.4 Gallbladder Disease Symptoms of gallbladder disease: Pain in right upper abdominal quadrant, back and right shoulder; perhaps nausea and sweating Tests to observe the gallbladder: Ultrasonography of gallbladder, or an X-ray called a cholecystogram Gallstones can be found in the gallbladder, cystic duct, hepatic ducts, or the common bile duct Treatments: - Cholecystectomy: surgical removal of the gallbladder - Endoscopy to find stones or remove them from certain areas : Small Intestine The small intestine is a tubular organ that extends from the pyloric sphincter to the beginning of the large intestine Fills most of abdominal cavity Receives chyme from stomach, and liver and pancreatic secretions Completes digestion of the nutrients in chyme Absorbs products of digestion Transports the remaining residue to the large intestine Parts of the Small Intestine Small intestine consists of 3 parts: Duodenum, jejunum, ileum Duodenum: shortest and most fixed portion of small intestine Jejunum: middle portion, thicker and more active than ileum Ileum: distal portion; contains Peyer s patches (lymph nodules) The jejunum and ileum are suspended by a double-layered fold of peritoneum called a mesentery, which transports blood & lymphatic vessels and nerves to wall of intestine A double fold of peritoneum, called the greater omentum, drapes down from stomach, over the large intestine and folds of small intestine Parts of the Small Intestine Mesentery & Greater Omentum

12 Structure of the Small Intestinal Wall Tiny projections of the mucosa, intestinal villi, greatly increase the surface area for absorption of digestive end products Each villus consists of simple columnar epithelium with a core of connective tissue, with blood vessels, a lacteal (lymphatic capillary), and nerve fibers Free surfaces of epithelial cells contain microvilli, which also increase surface area for absorption Intestinal glands/crypts of Lieberkühn are located between bases of villi Plicae circulares are circular folds of mucosa, which also increase surface area Structure of the Small Intestinal Wall Intestinal Epithelium Plicae circulares Secretions of the Small Intestine In addition to mucous-secreting goblet cells, there are many specialized mucus-secreting glands, Brunner s glands, that secrete a thick, alkaline mucus in response to certain stimuli Intestinal glands secrete a watery fluid, that picks up digestion products, and transports them into villi; contains no digestive enzymes Enzymes in the membranes of the microvilli: Peptidases: break down peptides into amino acids Sucrase, maltase, lactase: break down disaccharides into monosaccharides Lipase: breaks down fats into fatty acids and glycerol

13 Regulation of Small Intestinal Secretions Regulation of small intestine secretion occurs by these methods: Mucus secretion occurs in response to mechanical stimulation, or presence of irritants such as gastric juice Contact with chyme stimulates goblet cells to secrete mucus, and intestinal glands to secrete watery fluid Distension of the intestinal wall activates nerve plexuses in the wall of the small intestine Distension also stimulates parasympathetic reflexes that trigger the release of intestinal enzymes Absorption in the Small Intestine Villi increase the surface area for absorption Small intestine is most important absorbing organ of alimentary canal Small intestine absorption is so effective that very little absorbable material reaches the organ s distal end Carbohydrate digestion and absorption: Salivary and pancreatic amylase break down starch and glycogen into disaccharides Intestinal enzymes break down disaccharides into monosaccharides Monosaccharides are absorbed by facilitated diffusion and active transport, into the blood vessels in villi Absorption in the Small Intestine Protein digestion and absorption: Pepsin in the stomach breaks down proteins into polypeptides Pancreatic proteases (trypsin, chymotrypsin, etc.) break down proteins and polypeptides into smaller peptides Intestinal peptidases break peptides into amino acids Amino acids are absorbed by active transport into blood vessels of villi Absorption in the Small Intestine Fat digestion and absorption: Fats are emulsified by bile salts Fats are digested mainly by enzymes from pancreas and small intestine Fats are digested into glycerol and fatty acids Fatty acids and glycerol are absorbed by a process involving several steps; absorbed into blood or lymphatic capillaries (lacteals) Fat Absorption

14 Movements of the Small Intestine The small intestine carries on the following movements: Peristalsis: wave-like pushing movements that propel chyme in proper direction down the small intestine Segmentation: ring-like contractions that move chyme back and forth (mixing movement) Parasympathetic impulses stimulate both mixing and peristaltic movements, and sympathetic impulses inhibit them 17.9: Large Intestine Large intestine: Named because diameter is greater than that of small intestine 1.5 m long At distal end, opens to outside of body through the anus Absorbs some water and electrolytes Reabsorbs and recycles water and digestive secretions Forms and stores feces If small intestine becomes over distended or irritated, a strong peristaltic rush may sweep contents quickly into large intestine, resulting in diarrhea Ileocecal sphincter joins ileum of small intestine to cecum of large intestine, and helps regulate flow of chyme Parts of the Large Intestine Parts of the Large Intestine Large intestine: Consists of cecum, colon, rectum, and anal canal: Cecum: - Pouch, forms beginning of large intestine - Appendix is attached to cecum; contains lymphatic tissue Colon: - Ascending, transverse, descending and sigmoid portions Rectum: Extends from sigmoid colon to anal canal Anal canal: - Last cm of large intestine; opens to outside as anus - Internal and external anal sphincters guard anus Structure of the Large Intestine Structure of the Large Intestinal Wall Wall of large intestine has same 4 layers as other tubular organs of the alimentary canal Wall of large intestine lacks villi and plicae circulares Longitudinal muscle is organized into 3 bands, taeniae coli, that run down entire length of colon Bands create pouches called haustra, which help form feces

15 Functions of the Large Intestine Large intestine: Has little or no digestive function Contains tubular glands containing goblet cells; secrete mucus, the only significant secretion of the large intestine Absorbs water (about 90% of water that enters it) and electrolytes Houses intestinal flora, which break down contents such as cellulose, and produce vitamins K, B 12, thiamine Forms feces, and carries out defecation Movements of the Large Intestine Movements of the large intestine are similar to those of the small intestine They are slower and less frequent than those of the small intestine Movements: Mixing movements Peristalsis: 2-3 times/day Peristaltic waves produce strong mass movements, which usually follow meals The defecation reflex relaxes the internal anal sphincter and then the external anal sphincter Feces Feces are composed of materials not digested or absorbed, and also contain: Water (~75% of feces) Electrolytes Mucus Bacteria Bile pigments, which provide the color, after bacterial alteration The pungent odor is produced by bacterial compounds, including: Phenol Hydrogen sulfide Indole Skatole Ammonia 87 Clinical Application 17.5 Disorders of the Large Intestine Diverticulosis: Weakening of intestinal wall leads to protrusion of mucous membrane; forms outpouchings that can become inflamed and infected Inflammatory bowel disease: Group of disorders, including: Ulcerative colitis: affects mucosa & submucosa of large intestine; causes bloody diarrhea, cramps Crohn s disease: more serious; affects all layers, and occurs in both small and large intestines; causes diarrhea and pain Colorectal cancer: Cancer of large intestine or rectum Fourth most common cancer in US Screening tests include fecal occult blood test and colonoscopy : Life-Span Changes Changes to the digestive system are slow and slight, and eventually include the following: Tooth enamel thins; teeth may become sensitive Gums may recede Teeth may loosen, break or fall out GI tract becomes less efficient Slowing peristalsis may lead to heartburn Gastric secretion slows Constipation may become more frequent Nutrient absorption decreases, including fat-soluble vitamins Incidence of lactose intolerance increases Accessory organs age, but typically not necessarily in ways that affect health 89 15