Chapter 25 The Digestive System PDF

Chapter 25 The Digestive System Digestive System: Overview Alimentary canal – mouth, pharynx, esophagus, stomach, small intestine, and large intestine – Function: Digestion – a disassembly of foods to make nutrients more available for absorption Accessory digestive organs – teeth, tongue, gallbladde...

Chapter 25 The Digestive System Digestive System: Overview Alimentary canal – mouth, pharynx, esophagus, stomach, small intestine, and large intestine – Function: Digestion – a disassembly of foods to make nutrients more available for absorption Accessory digestive organs – teeth, tongue, gallbladder, salivary glands, liver, and pancreas – Function: produce secretions that assist in preparing organic and inorganic nutrients for absorption Digestive System Digestive System Peritoneum Digestive Functions Digestion in Mouth, Pharynx, Esophagus Mouth (oral or buccal cavity) Functions: – a. Ingestion – b. Begins the mechanical and chemical digestion processes – c. Starts the propulsive process of swallowing Mastication: chewing – mechanical digestion Chemical digestion: a series of metabolic steps in which complex food molecules are broken down into chemical building blocks by enzymes secreted into the lumen of the GI tract. The Mouth The Tongue Teeth Anatomy of a Tooth Oral Cavity and Pharynx Digestion in Mouth, Pharynx, Esophagus Salivary glands: secrete saliva Parotid glands Sublingual glands Submandibular Functions – a. Dissolves food chemical so they can be tasted – b. contains enzymes that breaks down starchy foods – c. moistens food and aids in compacting into bolus Secretory cells – a. Serous cells- produce watery secretions containing enzymes, ions, and a tiny bit of mucin – b. Mucous cells- produce mucus - a stringy, viscous solution The salivary glands Ducts of sublingual gland Saliva Composition of saliva – – – – – a. 97 – 99.5% water b. slightly acidic (pH 6.75 – 7.00) c. electrolytes d. salivary amylase: digestive enzyme e. mucin: forms mucus for lubrication and hydration of food stuffs – f. lysozyme Bacteriostatic: inhibits bacterial growth and tooth decay – g. IgA antibodies – h. defensins: local antibiotics Saliva Control of salivation: controlled primarily by the parasympathetic division of the ANS – Chemoreceptors: activated by acidic substances like vinegar and citrus juices – Mechanoreceptors: activated by any mechanical stimulus in the mouth Sympathetic division: causes the release of thick mucin-rich saliva, strong activation inhibits saliva release causing dry mouth – Dehydration- also inhibits saliva release because of low blood volume Digestion in Mouth, Pharynx, Esophagus Deglutition: swallowing – Buccal phase- occurs in the mouth and is voluntary. – Pharyngeal-esophageal phase- controlled by the swallowing center of the brain stem Pharynx- once the food enters the pharynx and touches the tactile receptors, swallowing goes from voluntary to involuntary control Esophagus- food from pharynx is moved by wavelike peristaltic contractions down the esophagus – Peristalsis: involuntary, major means of propulsion – Lower Esophageal sphincter- relaxes allowing food to enter stomach – Bolus: rounded mass of food Esophagus Deglutition Peristalsis Stomach Temporary storage tank Chemical digestion: gastric juices are released and chemical breakdown of proteins begins Mechanical digestion- churning in stomach Bolus from mouth mixes with gastric juices and is now called chyme – Chyme: creamy paste consisting of partially digested food and gastric juices Stomach Regulation of Gastric Secretion Neural and hormonal mechanisms regulate the release of gastric juice – Parasympathetic: increases secretory activity – Sympathetic: decreases secretory activity Events occur in three phases 1. Cephalic (reflex) phase 2. Gastric phase 3. Intestinal phase Cephalic Phase Occurs before food enters the stomach Triggered by: – – – – Aroma Taste Sight Thought of food Olfactory receptors and taste buds relay to the hypothalamus Hypothalamus stimulates the vagal nuclei of the medulla and ends up stimulating stomach glands Gastric Phase When food enters the stomach – Lasts 3 -4 hours – Provides about two-thirds of the gastric juice released Triggered by: stretch receptors Distention: stimulates more gastric juice release Peptides & low acidity: lead to more HCl release Gastrin: – stimulates contraction of stomach and intestinal muscles – causes more release of HCl and enzymes and High acidity: inhibits gastrin secretion Gastric Contractions Contractions mix food with gastric juices to form chyme Peristaltic wave reaches pyloric sphincter Contractions also close sphincter Contractions cause stomach to empty contents into duodenum Intestinal Phase Enterogastric reflex – Caused by distention of the duodenum when chyme enters – Inhibits gastrin production and gastric contractions – Stimulates contraction of the pyloric sphincter to limit chyme release from the stomach – Chyme limiting helps protect duodenum from arriving acid and enzymes Cells of the Stomach Mucous neck cells: secrete an acidic mucus Parietal cells: secrete hydrochloric acid and intrinsic factor Hydrochloric acid: (pH 1.5 – 3.5) Breaks down proteins Breaks down cell walls of plant foods Kills many ingested bacteria Intrinsic factor: Required for B₁₂ absorption in the small intestine Cells of the Stomach Chief cells: – Produce pepsinogen – Pepsinogen is activated by HCl to form pepsin – Then pepsin also activates pepsinogen Pepsin: protein digesting enzyme Rennin: an enzyme that acts on milk proteins (not produced in adults) Histology of the Stomach Gastric Emptying Usually occurs four hours after meal Large meals with more liquid- causes the stomach to empty quicker Stretch & chemical signals in duodenum – affect the rate of gastric emptying Carbohydrate-rich meals pass quickly Fats form an oily layer on top of the chyme and are digested more slowly and may remain in the stomach up to six hours Small Intestine Segmentation: rhythmic local contractions of small intestines Absorption: the passage of digested food end products to blood or lymph. Longest part of the canal – 7 – 13 ft long during life – 20 ft long in a cadaver Three divisions – Duodenum: 10 inches in length receives digestive secretions from the liver and pancreas – Jejunum: 8.2 ft long Supported by sheet of mesentery Most chemical digestion and nutrient absorption occurs here – Ileum: 11.5 ft long Final segment of small intestines Small Intestine Segmentation Small Intestine: Microscopic Anatomy Small Intestine Cells of the epithelium Mainly columnar: – absorptive cells bound by tight junctions Goblet cells: – secrete mucus Enteroendocrine cells: – Source of entergastrones Secretin Cholecystokinin (CCK) Liver and Gallbladder Liver and Gallbladder Liver: produces bile for export to the duodenum – Bile is a fat emulsifier Gallbladder: – Stores and concentrates bile – Bile enters cystic duct, then common bile duct Liver and Gallbladder Liver and Gallbladder Relationship of the liver, gallbladder, and pancreas to the duodenum. Liver and Gallbladder Cholecystokinin (CCK) : – Released from the entroendocrine cells of the duodenum when fatty chyme is emptied from stomach – Causes contraction of the gallbladder in order to expel bile into the cystic duct and then flows into the bile duct and empties into the duodenum Bile Bile: Alkaline solution containing bile salts, bile pigments, cholesterol, triglycerides, phospholipids and a variety of electrolytes Bile salts: cholesterol derivatives – – – – Emulsify fats Facilitate fat and cholesterol absorption Help solubilize cholesterol Keep cholesterol dissolved in bile Absorption of Fat Bile Pigment Bilirubin: – Waste product of heme from hemoglobin – Chief bile pigment – Liver absorbs and excretes in bile – Metabolized in the small intestines – Breaks down to stercobilin and this give feces it brown color Digestive Processes Most substances required for chemical digestion come from liver and pancreas Delivery of chyme must be slow from the stomach Mixes digestive juices by segmentation Pancreas Secretions breakdown all categories of foodstuff Acini cells: (exocrine) – Secrete pancreatic juice with HCO₃⁻ which neutralizes the acidic chyme Islets of Langerhans (pancreatic islets): (endocrine) – α-cells: secrete glucagon – β-cells: secrete insulin Pancreatic Juice Mainly water, enzymes, electrolytes – Protein-digesting enzymes are released in inactive form Trypsinogen activated to become trypsin – Trypsin cleaves proteins into smaller peptides Chymotrypsin: same as trypsin Carboxypeptidase: splits off one amino acid at a time off of polypeptide (protein) chains Pancreatic Juice Pancreatic amylase: breaks down starchy foods Lipases: lipid digestion Nucleases: breaks down nucleic acids (DNA/RNA) Bicarbonate ions (HCO₃¯): neutralizes the acidic chyme Chemical Digestion Carbohydrate Digestion Protein Digestion Large Intestine Subdivisions – Cecum – Vermiform appendix – Colon – Rectum – Anal canal Colon Colon Histology Colon Anatomy Bacterial Flora Bacterial flora: normal bacteria in the GI tract Causes the release a mixture of gases 500 mL of flatus/day Bacteria makes vitamin K and B vitamins Large Intestine Contents remain 12 to 24 hours – Absorbs most of the remaining water from indigestible food residue – Stores the residue temporarily Defecation Feces contains – 75% water – 5% bacteria – Indigestible materials – Inorganic matter – Remains of epithelial cells Defecation Anal canal leads to anus Internal anal sphincter: – Smooth muscle – Involuntary control External anal sphincter: – Skeletal muscle – Voluntary control Defecation Reflex Stretching of rectal wall starts defecation reflex – Sigmoid colon and rectum contract – Anal sphincter relaxes – Signals to brain – Is this a good time? If not: reflex contractions ends within a few seconds If yes: rectum muscles contract – Diaphragm and abdominal muscle contract – Levator ani muscle lifts the anal canal leaving the feces below the anus