BSC 216 Digestion Notes 10/7 and 10/11
BSC 216 Digestion Notes 10/7 and 10/11 BSC 216
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This 19 page Class Notes was uploaded by Vanessa Notetaker on Sunday October 16, 2016. The Class Notes belongs to BSC 216 at University of Alabama - Tuscaloosa taught by Austin Hicks in Fall 2016. Since its upload, it has received 4 views. For similar materials see Anatomy & Physiology II in Biology at University of Alabama - Tuscaloosa.
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BSC 216 Chapter 25 Digestive System Introduction Most nutrients must be broken down into smaller components so that the body could use them for energy The digestive system is essentially a disassembly line o As nutrients go through the system they are broken down into smaller and smaller components and they are absorbed so they can be distributed to the tissues Gastroenterology o The study of the digestive tract and the diagnosis and treatment of its disorders Digestive Function Five stages of digestion 1. Ingestion: selective intake of food a. Confined to oral cavity 2. Secretion: Endocrine and exocrine glands secrete substances that aid in our digestive process a. Each gland has its own kind of secretion specialized to necessary function 3. Propulsion: ingested food is moved from one organ to the next via peristalsis 4. Digestion: mechanical and chemical breakdown of food into a form useable to the body 5. Absorption: uptake of nutrient molecules into the epithelial cells of the digestive tract and then into the blood and lymph 6. Defecation: elimination of feces Mechanical digestion is the physical breakdown of food into tiny particles o Cutting and grinding action of teeth o Churning of the stomach and small intestine o Increases exposed surface area of food to speed enzyme action Chemical digestion is a series of hydrolysis reactions that break down macronutrients into their respective monomers o Carried out by digestive enzymes secreted by salivary glands, stomach, small intestine and pancreas o Carbohydrates break down into monosaccharides o Fats break down into 1 glycerol and 3 fatty acids o Proteins break down into amino acids o Nucleic acids break down into nucleotides Some nutrients are ingested in useable form and do not need digestion o Vitamins, free amino acids, minerals, cholesterol, water and electrolytes General Anatomy 1. Digestive tract/Alimentary Canal a. 30 ft long muscular tube extending from mouth to anus b. Mouth, pharynx, esophagus, stomach, small intestine, large intestine c. Gastrointestinal tract includes the stomach and the intestines 2. Accessory Organs a. Teeth, tongue, salivary glands, liver, gall bladder and pancreas General Anatomy The digestive tract is open to the environment at both ends o Considered outside of the body until absorption occurs in enterocyte3 o Defecated food residue was never technically in the body Basic Structural Plan of Digestive Tract The general structure of the digestive tract wall from the inner to outer surface includes o Mucosa Produces secretions to enter lumen Epithelium Lamina propria Muscularis mucosae Muscle squeezes to push secretions out o Submucosa o Muscularis externa Contractions aid in food churning and peristalsis Inner circular layer Outer longitudinal layer o Serosa Areolar tissue Mesothelium Enteric nervous system: nervous network in the esophagus, stomach and intestines that regulate digestive tract motility, secretion, and blood flow o Estimated 100 million neurons o More than spinal cord o Functions independently of the central nervous system but, the CNS can influence it Enteric nervous system contains sensory neurons that monitor tension in gut wall and conditions in lumen Composed of two networks of neurons o Submucosal (Meissner) plexus: in submucosa Just beneath epithelial cells Controls glandular secretion of mucosa o Myenteric (Auerbach) plexus: parasympathetic ganglia and nerve fibers between the two layers of the muscularis interna Controls peristalsis and other contractions of muscularis externa Relationship to the Peritoneum Mesenteries: connective tissue sheets that loosely suspend the stomach and intestines from the abdominal wall o Allow stomach and intestines to undergo strenuous contractions o Allow freedom of movement in the abdominal cavity o Hold abdominal viscera in proper relationship to each other o Prevent the intestines from becoming twisted and tangled by changes in the body position and its own contractions o Provide passage of blood vessels and nerves that supply the digestive system o Contain many lymph nodes and lymphatic vessels Serous Membranes Mesentery of small intestine holds many blood vessels Mesocolon anchors colon to posterior body wall Lack MALT tissue Lymphatic tissue nearby helps assist in protection from antigens Regulation of the Digestive Tract Motility and secretion of the digestive tract are controlled by paracrine, endocrine and neural mechanisms Neural control o Short (myenteric) reflexes: stretch or chemical stimulation acts through myenteric plexus Stimulates peristaltic contractions of swallowing o Long (vagovagal) reflexes: parasympathetic stimulation of digestive motility and secretion Longer range and wider effect Hormones o Chemical messengers secreted into the bloodstream and stimulate distant parts of the digestive tract o Gastrin and secretin Paracrine secretions o Chemical messengers that diffuse through tissue fluids to stimulate nearby target cells The Oral Cavity Serves as the start point of the digestive system o Ingestion, secretion, chemical and mechanical digestions and propulsion all occur in the oral cavity Houses accessory organs o Teeth o Tongue o Salivary glands The oral cavity and its accessory organs work together to turn ingested food into a bolus and move it to the posterior of the cavity to be swallowed Mastication Chewing breaks food down into smaller pieces to be swallowed and exposes more surface to the action of digestive enzymes o First step in mechanical digestion o Food stimulates oral receptors that trigger an involuntary chewing reflex The Teeth Key organs of mechanical digestion Located in the bony socket found in the maxilla and mandible and held in place by the periodontal ligament Three types o Incisors: central teeth that are broad and flat with narrow crown specialized for cutting off pieces of food o Canines/Cuspids: located on either sides of the incisors. Have pointed crowns that are specialized for ripping and tearing o Molars: Posterior and lateral to the canines. Contain broad crowns and are specialized for grinding Saliva and the Salivary Glands Functions of saliva o Moisten mouth o Begin starch and fat digestion o Cleanse teeth o Inhibit bacterial growth o Dissolve molecules so they can stimulate the taste buds o Moisten food and bind it together into bolus and aid swallowing Hypotonic solution of 97% to 99.5% water and the following solutes o Salivary amylase: begins starch digestion in mouth o Lingual lipase: enzyme that is activated by stomach acid and digests the fat after it is swallowed o Mucus: binds and lubricates the mass of food and aids in swallowing o Lysozyme: enzyme that kills bacteria Part of innate immunity o Immunoglobulin A (IgA): antibody that inhibits bacterial growth o Electrolytes: Sodium, potassium, chloride, phosphate and bicarbonate pH 6.8 to 7.0 Intrinsic salivary glands- small glands dispersed among other tissues o Lingual glands: in the tongue produce lipase o Labial glands: inside of the lips o Buccal glands: inside of the cheek o All secrete saliva in small amounts at a fairly constant rate Extrinsic salivary glands: three pairs connected to oral cavity via ducts o Parotid: located beneath the skin anterior to the earlobe Mumps is an inflammation and swelling of the gland to by a virus o Submandibular located halfway along the body of the mandible Duct empties at the side of the lingual frenulum near the lower central incisors o Sublingual glands located in the floor of the mouth Has multiple ducts that empty posterior to the papilla of the submandibular duct Salivation Extrinsic salivary glands secrete 1-1.5L of saliva a day Cells of acini filter water and electrolytes from the blood and add amylase, mucin and lysozyme Salivary nuclei in the medulla oblongata and pons respond to signals generated by the presence of food o Tactile, pressure and taste receptors o Salivary nuclei receive input from higher brain centers as well Smells, sights and thoughts of food stimulate salivation o Send signals by way of autonomic fibers in the facial and glossopharyngeal nerves to the glands Parasympathetics stimulate the glands to produce an abundance of thin, enzyme rich saliva Rest and digest Sympathetic stimulation stimulates the glands to produce less quantity and thicker saliva with more mucus Dry or sticky under stress and dehydration Bolus is a mass swallowed as a result of saliva binding food particles into a soft, slippery, easily swallowed mass The Pharynx Pharyngeal constrictors (superior, middle and inferior) circular muscles that force food downward during swallowing When not swallowing the inferior constrictor remains contracted to exclude air from the esophagus This constriction is considered the upper esophageal sphincter although it is not an anatomical feature Disappears at the time of death when the muscles relax so it is a physiological sphincter not an anatomical feature The Esophagus A straight muscular tube 25 to 30 cm long Begins at level between C6 and the cricoid cartilage Extends from pharynx to cardiac orifice or stomach passing through the esophageal hiatus in the diaphragm Esophageal hiatus is a hole in the diaphragm that accomodates the esophagus Lower esophageal sphincter where food pauses because of this constriction Prevents stomach contents from regurgitating into the esophagus and destroying tissue with HCl from stomach Heartburn- burning sensation produced by acid reflux into the esophagus Swallowing/Deglutition A complex action involving over 22 muscles in the mouth, pharynx and esophagus Swallowing center is a pair of nuclei in medulla oblongata that coordinate swallowing Communicates with muscles of the pharynx and esophagus by the way of trigeminal, facial, glossopharyngeal, and hypoglossal nerves Occurs in 3 phases Voluntary phase Tongue collects the food, presses it against the palate forming a bolus and pushes it posteriorly Food accumulates in the oropharynx in front of the “blade” of the epiglottis Epiglottis tips posteriorly and the bolus slides around it through the laryngeal opening Bolus enters laryngopharynx and stimulates tactile receptors and activates next stage Pharyngeal phase: involuntary Three actions prevent food and drink from entering the nasal cavity or larynx or reentering the mouth o Root of tongue blocks oral cavity o Soft palate rises and blocks the laryngopharynx o Infrahyoid muscles pull the larynx up to meet the epiglottis while laryngeal folds close the airway Food bolus is driven downward by constriction of the upper, then middle and finally the lower pharyngeal constrictors Bolus enters esophagus, stretches it and stimulates peristalsis Esophageal phase: involuntary Bolus enters esophagus stretches it and begins peristalsis Travels down the esophagus until it reaches the stomach Peristalsis is the wave of muscular contraction that pushes the bolus ahead of it Entirely involuntary reflex When standing or sitting upright the food and liquid drops through the esophagus by gravity faster than peristalsis can keep up with it Peristalsis ensures you can swallow regardless of body position Liquid reaches the stomach in 1-2 seconds Food bolus reaches in 4 to 8 seconds When it reaches the lower end of the esophagus the lower esophageal sphincter relaxes to let food pass into the stomach The Stomach A muscular sac in the upper left abdominal cavity immediately inferior to the diaphragm o Primarily functions as a food storage organ Internal volume of 50 mL when empty 1-1.5 L after a typical meal 4L when extremely full and can extend as far as pelvis Mechanically breaks up food particles ,liquefies food, and begins chemical digestion of protein and fat o Chime: soupy or pasty mixture of semidigested food in the stomach with stomach secretions mixed in (HCl, pepsin, gastric lipase, etc) o Most digestion occurs after the chime passes on into the small intestine Stomach Anatomy Outer longitudinal layer of muscularis externa runs the length of the stomach from the cardia to the pylorus Middle circular layer runs around the stomach Inner oblique layer runs diagonal and is a unique feature and layer of the stomach Rugae are the folds and bunches of the stomach Stomach Microscopic Anatomy Gastric pits are depressions in gastric mucosa o Lined with simple columnar epithelium o Two or three tubular glands open into the bottom of each gastric pit Cardiac glands in the cardiac region First region after the esophagus empties Pyloric glands in the pyloric regions Inferior portion before the small intestine Gastric glands in the rest of the stomach Mucous cells secrete mucus o Predominate in the cardiac and pyloric glands o In gastric glands are called mucous neck cells and are concentrated at the neck of the gland (top) Regenerative stem cells found in the base of the pit and in the neck of the gland o Divide rapidly and produce a continual supply of new cells to replace the ones that die and keep the stomach lining intact from constant acidic conditions o When stem cells divide they divide into one stem cell and another specialized cell (stomach cell) Parietal cells found mostly in the upper half of the gland o Secrete hydrochloric acid, intrinsic factor and the hunger hormone ghrelin Chief cells are the most numerous o Secrete gastric lipase and pepsinogen o Dominate lower half of gastric glands o Absent in pyloric and cardiac glands Enteroendocrine cells concentrated in the lower end of the gland o Secrete hormones and paracrine messengers that regulate digestion Gastric Secretions Gastric juice- 2-3L per day produced by the gastric glands o Mainly a mixture of water, HCl and pepsin Five main secretions of gastric glands o Hydrochloric acid/ HCl o Pepsin for breakdown of protein o Gastric lipase for breakdown of fat o Intrinsic factor o Chemical messengers 1. Hydrochloric Acid a. pH can be as low as .8 in the stomach because of it b. Parietal cells produce it and contain carbonic anhydrase i. CO 2H O2H CO 2HC3 +H 3- + ii. Hydrogen ions are pumped into the gastric gland lumen by hydrogen-potassium ATP-ase pump 1. Antiporter uses ATP to pump hydrogen out and potassium in iii. Bicarbonate exchanged from chloride in the chloride shift from blood plasma 1. Chloride ion pumped into the lumen of gastric gland to join hydrogen and form HCl 2. Elevated bicarbonate in blood causes alkaline tide increasing the blood pH (short spike) c. HCl activates pepsin and lingual lipase d. Breaks up connective tissues in plant cell walls i. Helps liquefy food to form chime e. Converts ingested ferric ions to ferrous ions to be absorbed and used for hemoglobin synthesis f. Contributes to nonspecific disease resistance by destroying ingested pathogens 2. Pepsin a. Zymogens are digestive enzymes produced in their inactive forms i. Converted into active forms by other enzymes ii. Usually by amino acid cleavage b. Pepsinogen is zymogen of pepsin secreted by chief cells i. HCl removes some of the amino acids to form pepsin ii. Autocatalytic effect: as some pepsin is formed it converts more pepsinogen into more pepsin c. Pepsin digests dietary proteins into shorter peptide chains i. Protein digestion is completed in the small intestine 3. Gastric lipase a. Produced by chief cells b. Plays minor role along with lingual lipase to digest fats i. 10-15% of fats are digested in stomach ii. Small intestine is where the rest of the digestion occurs 4. Intrinsic Factor a. Glycoprotein secreted by parietal cells b. Essential for absorption of vitamin b12 in the small intestine i. Binds b12 so intestinal walls absorb the complex by receptor mediated endocytosis c. Needed for hemoglobin synthesis d. Prevents pernicious anemia 5. Chemical Messengers a. Gastric and pyloric glands have various kinds of enteroendocrine(G) cells that produce as many as 20 different chemical messengers i. Most are hormones that enter the blood and stimulate distant cells ii. Others are paracrine secretions that stimulate neighboring cells iii. Several are peptides produced in both the digestive tract and the central nervous system: gut-brain peptides 1. Secretin and cholecystokinin Gastric Motility Swallowing center of medulla oblongata signals stomach to relax o Gets ready to accept food Food stretches stomach activating receptive-relaxation response o Resists stretching briefly but, relaxes to hold more food Soon stomach shows a rhythm of peristaltic contractions controlled by pacemaker cells in longitudinal layer of muscularis externa o Gentle ripple of contraction every 20 seconds churns and mixes food with gastric juice o Becomes stronger contraction at the pyloric region o After 30 minutes or so these contractions become quite strong They churn the food, mix it with gastric juice and promote its physical breakup and chemical digestion o Antrum holds 30 mL of chime o As a peristaltic wave passes the antrum it moves 3 mL of chime into the duodenum at a time This enables time for neutralization of stomach acid and digestion of nutrients little by little o If duodenum is overfilled it inhibits gastric mobility Duodenum needs time to digest and absorb entering chime o Typical meal is emptied from stomach in 4 hours less if the meal is more liquid A high fat meal can take up to 6 hours Digestion and Absorption in the Stomach Digestion- salivary and gastric enzymes partially digest protein and lesser amounts of starch and fat in the stomach Stomach not involved in absorption of nutrients Most digestion and NEARLY ALL absorption occurs in the small intestine Digestion and Absorption Stomach does not absorb any significant number of nutrients o Aspirin o Lipid soluble drugs Alcohol is absorbed mainly by the small intestine o Intoxicating effects depend partly on how fast the small intestine is emptied Protection of the Stomach The lining of the stomach is protected in three ways from the harsh acidic enzymatic environment it creates o Mucous coat: thick highly alkaline mucus resists action of acid and enzymes o Tight junctions: between epithelial cells prevent gastric juice from seeping between them and damaging the connective tissue of the lamina propria and beyond o Epithelial cell replacement: stomach epithelial cells live only 3 to 6 days Sloughed off into the chime and digested with the food Replaced rapidly by cell division in the gastric pits Breakdown of these protective measures can result in inflammation and peptic ulcer Peptic Ulcer Gastritis, inflammation of the stomach, can lead to a peptic ulcer as pepsin and hydrochloric acid erode the stomach wall Most ulcers are caused by acid resistant bacteria Helicobacter pylori that can be treated by antibiotics and Pepto-bismol Regulation of Gastric Function Nervous and endocrine systems collaborate o Increases gastric secretions and motility when food is eaten o Suppresses them when the stomach empties Gastric activity is divided into three phases o Cephalic phase where stomach is being controlled by brain Cephalic means head o Gastric phase when stomach controls itself o Intestinal phase when stomach is being controlled by small intestine Phases overlap and can occur simultaneously 1. Cephalic Phase a. Stomach responds to taste, smell, sight or thought of food i. G cells stimulated by the vagus nerve that increase histamine and gastrin that promotes the secretion from other cells b. Sensory and mental inputs converge at the hypothalamus i. Relays signal to medulla oblongata c. Vagus nerve fibers from medulla oblongata stimulate the enteric nervous system of the stomach i. Stimulate gastric secretion 2. Gastric phase a. Period in which swallowed food and semi-digested protein activate gastric activity i. Two thirds of gastric secretion occurs in this phase b. Ingested food stimulates gastric activity by i. Stretching the stomach 1. Activating short reflex mediated through myenteric nervous system 2. Activating long reflex mediated through the vagus nerves and brainstem ii. By increasing the ph of its contents c. Gastric secretion is stimulated by i. Acetylcholine (ACh) secreted by both parasympathetic nerve fibers of both reflexes 1. Neurotransmitter ii. Histamine a paracrine secretion from enteroendocrine cells in the gastric glands iii. Gastrin: a hormone produced by the enteroendocrine G cells in pyloric glands 3. Intestinal Phase a. Stage where the duodenum responds to arriving chime and moderates gastric activity through hormones and nervous reflexes b. Duodenum initially enhances gastric secretion but soon inhibits it i. Enhancing mechanisms 1. Stretching of the duodenum accentuates vagovagal reflex that stimulates the stomach 2. Peptides and amino acids in chime stimulate G cells of the duodenum to secrete more gastrin which further stimulates the stomach ii. Inhibitory mechanisms 1. Enterogastric reflex- duodenum sends inhibitory signals to the stomach by way of the enteric nervous system and signals to the medulla oblongata triggered by acid and semi digested fats in the duodenum a. Inhibits vagal nuclei: reducing vagal stimulation of the stomach b. Stimulate sympathetic neurons to send inhibitory signals to the stomach 2. Chyme also stimulates duodenal enteroendocrine (G) cells to release secretin and cholecystokinin a. They stimulate the pancreas and gallbladder b. Suppress gastrin secretion The Liver, Gallbladder and Pancreas Small intestine receives chime from stomach Also secretions from liver and pancreas o Enter digestive tract near the junction of stomach and small intestine Secretiuons are vtial to the digestive process of the small intestine The Liver Reddish brown gland located immediately inferior to the diaphragm on the right side The back contains the caudate and quadrate lobe The body’s largest gland Variety of functions o Secretes bile which is essential in lipid digestion The hepatic vein takes blood away from the liver Microscopic Anatomy After a meal, the hepatocytes (liver cells) absorb from the blood- glucose, amino acids, iron, vitamins, and other nutrients for metabolism and storage Removes and degrades o Hormones, toxins, bile pigments, and drugs Secretes into the blood o Albumin, lipoproteins, clotting factors, angiotensinogen (maintains blood pressure), and other products Between meals, hepatocytes break down stored glycogen and release glucose into the blood The Gallbladder and Bile Gallbladder- a pear shaped sac on the underside of the liver o Serves to store and concentrate bile by a factor of 20 by absorbing water and electrolytes o DOES NOT PRODUCE BILE o 10 cm long o Connected to liver and small intestine via ducts Hepatic duct from liver Common bile duct joins pancreatic duct at duodenum Bile- yellow green fluid containing minerals, cholesterol, neutral fats, phospholipids, bile pigments and bile acids o Bilirubin: principal pigment derived from the decomposition of hemoglobin o Bacteria in large intestine metabolize bilirubin to urobilinogen Responsible for the brown color of feces o Bile acids/bile salts are steroids synthesized from cholesterol Aid in fat digestion and absorption o Gallstones may form if bile becomes excessively concentrated o Bile gets to the gallbladder by first filling the bile duct and then overflowing into the gallbladder o Liver secretes 500mL to 1000mL of bile a day o 80% of bile acids are reabsorbed in the ileum and returned to the liver Hepatocytes absorb and resecrete them Enterohepatic circulation- route of secretion, reabsorption, and resecretion of bile acids two or more times during digestion of an average meal 20% of the bile acids are secreted in the feces Body’s only way of eliminating cholesterol Liver synthesizes new bile acids from cholesterol to replace their loss in feces 1. Duodenal cells secrete cholecystokinin and secretin into the blood 2. CCK triggers contraction of the gallbladder which releases bile 3. Bile salts and secretin trigger the release of more bile an example of positive feedback The Pancreas Both an endocrine and exocrine gland o Endocrine portion has pancreatic islets that produce insulin and glucagon o Exocrine portion- 99% of pancreas that secretes 1200-1500 mL of pancreatic juice a day Secretory acini release their secretion into small ducts that converge on the main pancreatic duct Pancreatic juice: alkaline mixture of water, enzymes, zymogens, sodium bicarbonate, and other electrolytes o Acini secrete the enzymes and zymogens o Ducts secrete bicarbonate Bicarbonate buffers HCl arriving from the stomach Pancreatic duct runs lengthwise through the middle of the gland Joins the bile duct at the hepatopancreatic ampulla Hepatopancreatic sphincter controls the release of both bile and pancreatic juice into the duodenum Accessory pancreatic duct is a smaller duct that branches from the main pancreatic duct Opens independently into the duodenum Bypasses the sphincter and allows pancreatic juice to be released into the duodenum even when bile is not Pancreatic zymogens are o Trypsinogen Secreted into the intestinal lumen Converted to trypsin by enterokinase, an enzyme secreted by mucosa of small intestine Trypsin is autocatalytic and converts trypsinogen into more trypsin o Chymotrypsinogen: converted to chymotrypsin by trypsin o Procarboxypeptidase converted to carboxypeptidase Other pancreatic enzymes o Pancreatic amylase digests starch o Pancreatic lipase digests fats o Ribonuclease and deoxyribonuclease digest RNA and DNA Regulation of Secretion Three stimuli are chiefly responsible for the secretion of pancreatic juice and bile o Acetylcholine from vagus and enteric nerve Stimulates acini to secrete their enzymes during the cephalic phase of gastric control even before food is swallowed Enzymes remain in acini and ducts until chime enters the duodenum o Cholecystokinin secreted by mucosa of duodenum in response to arrival of fats in duodenum Stimulates pancreatic acini to secrete enzymes Strongly stimulates gallbladder Includes contractions of the gallbladder and relaxation of hepatopancreatic sphincter causing discharge of bile into the duodenum o Secretin released from duodenum in response to scidic chime arriving from the stomach Stimulates ducts of both liver and pancreas to secrete more sodium bicarbonate (buffering action) Raising ph to level pancreatic and intestinal digestive enzymes require 1. Duodenal cells release secretin and cholecystokinin into the blood 2. CCK binds to receptors on acinar cells and secretin binds duct cells 3. This binding triggers secretion of enzymes and bicarbonate ions from the pancreas into the duodenum The Small Intestine Nearly all chemical digestion and nutrient absorption occurs in the small intestine The longest part of the digestive tract o 2.7-4.5 m long in a living person o 4-8m long in a cadaver with no muscle tone Small intestine refers to the diameter not length o Longer and thinner than large intestine Gross Anatomy 1. Duodenum a. First 25 cm b. Begins at pyloric valve i. Receives major and minor pancreatic ducts c. Receives stomach contents, pancreatic juice and bile d. Stomach acid is neutralized here e. Fats are physically broken up by bile (emulsified) here f. Pepsin is inactivated by increased ph g. Pancreatic enzymes take over the job of chemical digestion h. Duodenal glands in submucosa of duodenum i. Secrete an abundance of bicarbonate rich mucus ii. Neutralize stomach acid and shield the mucosa from its erosive effects 2. Jejunum a. First 40% of small intestine beyond duodenum b. Roughly 1-1.7 m in a living person c. Especially rich in blood supply giving it a red color d. Most digestion and nutrient absorption occurs here 3. Ileum a. Last 60% of the post duodenal small intestine b. About 1.6-2.7 m c. Thinner, less muscular, less vascular and paler pink color d. Peyer’s patches- prominent lymphatic nodules in clusters on the side opposite the mesenteric attachment i. Immune surveillance of pathogens in the small intestine Microscopic Anatomy Tissue layers have modifications for nutrient digestion and absorption o Large internal surface area for effective digestion and absorption by great length and three types of internal folds or projections Circular folds (plicae circulares)- increase surface area by a factor of 2 or 3 Villi- increase surface area by a factor of 10 Microvilli- increase surface area by a factor of 20 1. Circular folds (plicae circulares) largest folds of intestinal wall a. Up to 10 mm high b. Involve only mucosa and submucosa c. Occur from the duodenum to the middle of the ileum d. Cause chime flow in a spiral path causing more contact with mucosa e. Promotes more thorough mixing and nutrient absorption f. Relatively small and sparse in ileum and not found in distal half i. Most nutrient absorption is completed by this point 2. Villi- fingerlike projections 0.5 to 1 mm tall a. Make mucosa look fuzzy b. Villus covered with two types of epithelial cells i. Absorptive cells (enterocytes) ii. Goblet cells- secrete mucus c. Epithelia joined by tight junctions that prevent digestive enzymes from seeping between them 3. Microvilli- fuzzy border on apical surface of each absorptive cell a. About 1 micrometer high b. Brush border increases absorptive surface area Brush border enzymes- contained in the plasma membrane of microvilli o Carry out some of the final stages of enzymatic digestion o Not released into the lumen o Contact digestion: chime must contact the brush border for digestion to occur o Intestinal churning of chime ensures contact with the mucosa Intestinal Secretion Intestinal crypts secrete 1 to 2 L of intestinal juice per day o Similar to gastric glands o Response to acid, hypertonic chime and distention of the intestines o pH of 7.4-7.8 o Contains water mucus and a little enzyme Most enzymes that function in the small intestine are found in the brush border and pancreatic juice Intestinal Motility Contractions of small intestine serve three functions o Mix chime with intestinal juice, bile and pancreatic juice To neutralize acid Digest nutrients more effectively o To churn chime and bring it in contact with the mucosa for contact digestion and nutrient absorption o To move digestive residue toward large intestine Segmentation- movement in which stationary ring-like constrictions appear in several places along the intestine o Most common kind of intestinal contraction o Pacemaker cells in muscularis externa set rhythm of segmentation When most nutrients have been absorbed and little remains but undigested residue, segmentation declines and peristalsis begins o Purpose of segmentation is to mic and churn not to move material along as in peristalsis Peristalsis- gradual wave like movement of contents towards colon o Peristaltic wave begins in duodenum and travels 10-70 cm and dies out o Followed by another wave starting further down the tract o Migrating motor complex- successive, overlapping waves of contraction o Milk chime toward colon over a period of two hours Carbohydrate Digestion and Absorption 1. Polysaccharides are broken into oligo- and disaccharides in reactions catalyzed by pancreatic amylase 2. Brush border enzymes catalyze the breakdown of disaccharides into monosaccharides 3. The sodium potassium pump creates a gradient for sodium absorption from the fluid in the lumen 4. This gradient drives the secondary active transport of glucose and galactose via the sodium glucose transporter 5. Fructose is absorbed by facilitated diffusion 6. All three monosaccharides cross the basal side of the enterocyte membrane and then diffuse into blood Amino Acid Digestion and Absorption 1. Oligopeptides are broken down into free amino acids in reactions catalyzed by pancreatic and brush border enzymes 2. The sodium potassium pump creates a sodium gradient 3. This gradient drives the secondary active transport of certain amino acids into the enterocyte 4. Amino acids cross the basal enterocyte by facilitated diffusion and enter the blood Lipid Digestion 1. Lipids are broken apart by stomach churning and broken down in reactions catalyzed by gastric lipase 2. Lipids enter the small intestine and are emulsified by bile salts 3. Pancreatic lipase catalyzes reactions that digest the lipids into free fatty acids and monoglycerides 4. Bile salts remain associated with the digested lipids to form micelles Lipid Absorption 1. Micelles escort lipids to the enterocyte plasma membrane 2. Lipids diffuse through the phospholipid bilayer and enter the cytosol 3. Lipids are reassembled into triglycerides and packaged into chylomicrons 4. Chylomicrons are released into the interstitial fluid by exocytosis and then enter a lacteal Water Digestive system is one of several systems involved in water balance o Urinary, lymphatic, circulatory Digestive tract receives about 9 L water/day o .7L in food, 1.6 L in drink, 6.7 L in gastrointestinal secretions o 8L is absorbed by small intestine and .8L by large intestine o .2 L voided in daily fecal output Water is absorbed by osmosis following the absorption of salts and organic nutrients Diarrhea-occurs when large intestine absorbs too little water o Feces passes through too quickly when the intestine is irritated o Feces contains high levels of solute/ lactose Constipation- occurs when fecal movement is too slow and too much water gets reabsorbed and feces becomes hardened Gross Anatomy of Large Intestine Large Intestine receives about 500 mL of indigestible residue per day o Reduces it to about 150 mL of feces by absorbing water and salts o Measures 1.5 m long and 6.5 cm in diameter in cadaver o Eliminates feces by defecation o Called large because it has a much larger diameter than small intestine o Cecum Ascending colon transverse colon sigmoid colon Bacterial Flora and Intestinal Gas Bacterial flora populate large intestine o About 800 species of bacteria o Digest cellulose and other undigested carbohydrates Body absorbs resulting sugars o Help in synthesis of vitamins B and K Flatus- intestinal gas o Average person produces 500 mL per day from 7-10L of gas present but reabsorbed o Most is swallowed air but, hydrogen sulfide, indole, and skatole produce odor Hydrogen gas may explode during electrical cauterization used in surgery Absorption and Motility Large intestine takes about 12-24 hours to reduce the residue of a meal to feces o Does not chemically change the residue o Reabsorbs water and electrolytes Feces contains 75% water and 25% solids o Solids consist of 30% bacteria, 30% undigested fiber, 10-20% fat, small amounts of mucus and sloughed epithelial cells Haustral contractions occur every 30 minutes o This kind of colonic motility is a form of segmentation o Distention of a haustrum stimulates it to contract Mass movements occur one to three times a day o Triggered by gastrocolic and duodenocolic reflexes Filling of the stomach and duodenum stimulates motility of colon Moves residue for several centimeters with each contraction Neural Control of Defecation 1. Filling of the rectum 2. Reflex contraction of rectum and relaxation of internal anal sphincter 3. Voluntary relaxation of external sphincter
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