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The Digestive System Part 4: Regulation & Maintenance The Digestive System Digestive System: The system whose function it is to break down foods into molecules small enough to enter body cells. Allows the body to ingest & digest proteins, fats & carbohydrates & absorb them into the bloodstream & lymph to be taken to body cells for metabolism & conversion to ATP. The Digestive System Gastrointestinal (GI) Tract aka Alimentary Canal: A continuous tube that extends from the mouth to the anus. Tonus: The sustained muscular contraction of the GI tract walls that helps to move food along. Food travels through… The oral cavity (mouth) Part of the pharynx The esophagus The stomach The small intestine The large intestine The Digestive System Accessory Organs: Organs not a part of the GI tract that aid in the digestive process. Produce or store secretions that flow into the GI tract through ducts to aid in the chemical breakdown of food. Aside from the teeth & tongue, they do not come in direct contact with food. Accessory organs are… The teeth The tongue The salivary glands The pancreas The liver The gallbladder GERD Gastroesophageal Reflux Disease (GERD): Occurs when the lower esophageal sphincter fails to close adequately after food has entered the stomach. Allows the stomach contents to back up into the inferior portion of the esophagus. Hydrochloric acid from the stomach irritates the esophageal wall & causes “heartburn”. Can be controlled with medication & avoiding certain foods that can strongly stimulate stomach acid production. 6 Digestive System Functions Ingestion (Eating): Taking food or liquids into the body. Secretion: Cells within the walls of the GI tract & accessory organs secrete roughly 7L of water, acid, buffers, & enzymes into the lumen of the tract daily. Propulsion & Mixing (Motility): Moving food through the GI tract by peristalsis. Digestion: Occurs mechanically or chemically. Mechanical digestion - chewing to break food into smaller pieces, and churning by the stomach & small intestine to dissolve & mix the food with digestive enzymes. Chemical digestion – hydrolysis splits the large carbohydrates, lipids, proteins, and nucleic acid molecules into smaller molecules, while water, vitamins, ions, minerals, amino acids, glucose, & cholesterol can be absorbed without chemical digestion. Absorption: Epithelial lining of the lumen of the GI tract absorbs end products of digestion & moves them into the blood or lymph – most absorption occurs in the small intestine. Defecation: The elimination of feces (composed of indigestible materials, bacteria, sloughed off cells, & digested materials that were not absorbed) via the anus. GI Tract Made up of 4 basic tissue layers: Mucosa (Mucous Membrane): The innermost layer closest to the lumen. Protects against pathogens, absorb nutrients, & secretes mucous & hormones. Epithelium: Comes in direct contact with the GI tract contents – made up of nonkeratinized stratified squamous epithelium that serves a protective function, with simple columnar epithelium functions in secretion & absorption & lines the stomach & intestines. Lamina Propia: Layer of connective tissue underlying the mucosa & containing the blood vessels, lymph vessels, & lymph nodes – made of areolar connective tissue & supplies the GI tract with a route to export nutrients. Mucosa-Associated Lymphatic Tissue (MALT): Lymphatic nodules containing immune system cells located along the entire GI tract – provide defense against disease. Muscularis Mucosae: Smooth muscle layer that creates folds in the mucous membrane to increase surface area for digestion & absorption – movement ensures that the absorptive cells are fully exposed to the GI contents. GI Tract Submucosa: Consists of areolar connective tissue that binds the mucosa to the muscularis. Contains many of the blood & lymphatic vessels that receive absorbed food molecules – may also contain glands & lymphatic tissue. Submucosal Plexus: Network of neurons that form the enteric nervous system (ENS) located in the submucosa – controls the motility & secretions of the GI tract. GI Tract Muscularis: The muscular layer outside the submucousa. Inner layer is circular & outer layer is longitudinal. Segmentation: The mixing of food via muscular contractions. Peristalsis: The propulsion of food through the GI tract via muscular contractions. Thickens in some areas to form sphincters, which act as valves to control food passage. Contains the myenteric (Auerbach’s) plexus, which controls GI motility. GI Tract Serosa: The outermost layer of the portions suspended in the abdominopelvic cavity. Inferior to the diaphragm. Known as the visceral peritoneum. Adventitia: The external covering of the esophagus that binds the esophagus to surrounding structures. Neural Innervation of the GI Tract Enteric Nervous System (ENS): Around 100 million neurons extending from the esophagus to the anus. Arranged in 2 plexuses: Myenteric Plexus aka Plexus of Auerbach: Found between the longitudinal and circular smooth muscle layers of the muscularis. Mostly controls the GI tract motility, particularly the frequency & strength of the muscle contractions of the musclaris. Submucosal Plexus aka Plexus of Meissner: Found within the submucosa. Motor neurons in this plexus supply the secretory cells of the mucosal epithelium, controlling the secretions of the GI accessory organs. Neural Innervation of the GI Tract Motor Neurons: Supply the muscularis layers via the myenteric plexus. Interneurons: Connect the myenteric & submucosal plexus neurons. Sensory Neurons: Act as chemoreceptors and/or stretch receptors. Chemoreceptors: Activated by the presence of certain chemicals in food. Stretch Receptors: Activated when food distends the wall of a GI organ. Neural Innervation of the GI Tract Autonomic Nervous System: The portion of the nervous system that controls the ENS. Vagus Nerve (Cranial Nerve X): Supplies parasympathetic fibers to most parts of the GI tract, except the last half of the large intestine. Stimulation of the parasympathetic nerves causes an increase in the GI secretion & motility by increasing the activity of the ENS neurons. Stimulation of the sympathetic nerves causes a decrease in GI secretion & motility by inhibiting the ENS neurons. Strong emotions can slow down digestion due to stimulation of the sympathetic nervous system. Mouth (Oral or Buccal Cavity) Mouth: The cheeks (forming the lateral walls), soft and hard palates, & the tongue. Externally covered by skin & internally covered by a mucous membrane. Buccinator Muscles & connective tissue lie between the skin & mucous membranes of the cheeks. Mouth (Oral or Buccal Cavity) Lips aka Labia: Fleshy folds that surround the opening of the mouth. Contain the.. Obrbicularis Oris: Muscles covered externally by skin & internally by mucous membranes. Labia Frenulum: The medial fold of mucous membrane between the inner surface of the lips & the gums. “Red Area” gets its color from the tall dermal papillae & the proximity of blood vessels to the surface. Cutaneous area appears as roughly the same color as the skin tone. Buccinator & orbicularis oris muscles contract during chewing to keep food between the teeth & aid in speech. Mouth (Oral or Buccal Cavity) Vestibule: The mouth area bounded externally by the cheeks & lips and internally by the gums & teeth. Oral Cavity Proper: The space extending from the gums & teeth to the fauces. Fauces: The opening between the oral cavity & the pharynx. Mouth (Oral or Buccal Cavity) Palate: Separates the oral cavity from the nasal cavity and makes it possible to breath while chewing. Hard Palate: The anterior portion of the roof of the mouth – supported by the palatine process of the maxilla & the palatine bones & is covered by a mucous membrane. Soft Palate: The posterior portion of the roof of the mouth – arch-shaped muscular portion between the oropharynx & nasopharynx that is lined with a mucous membrane. Mouth (Oral or Buccal Cavity) Uvula: The soft, fleshy mass that descends from the soft palate at the back of the mouth. When swallowing, the soft palate & uvula draw superiorly to close off the nasopharynx and prevent swallowed foods & liquids from entering the nasal cavity. Palatoglossal Arch: Extends to the side of the base of the tongue. Palatopharyngeal Arch: Extends to the side of the pharynx. Palatine Tonsils: Located between the two arches. Lingual Tonsils: Located at the base of the tongue. The Pharynx Pharynx: The tube extending from the internal nerves to the esophagus posteriorly & to the larynx anteriorly. Composed of skeletal muscles lined by mucous membrane. Divided into 3 parts: Nasopharynx: Only functions in respiration. Oropharynx: Has digestive & respiratory functions – swallowed food passes from the mouth into the oropharynx. Laryngopharynx: Has digestive & respiratory functions – food passes from the oropharynx to the laryngopharynx. Muscular contractions in the oropharynx & laryngopharynx help propel food into the esophagus. Saliva Salivary Gland: Any cell or organ that secretes saliva into the mouth. The mucous membrane of the mouth & tongue contain many small salivary glands that open to the mouth via short ducts. Labial, buccal & palatal glands are located in the lips, cheeks, & palate respoectively. Lingual glands are located in the tongue. Saliva Major Salivary Glands: Located beyond the oral mucosa & empty their secretions into ducts that lead into the mouth – 3 pairs. Parotid Glands: Inferior & anterior to the ears, between the skin & the masseter muscle. Submandibular Glands: Located in the floor of the mouth, medial & partially inferior to the body of the mandible. Secretes saliva into the mouth via the parotid duct. Secretes serous liquid that contains salivary amylase (enzymes). Secrete a mucousy fluid that contains amylase via the submandibular ducts entering lateral to the lingual freulum. Sublingual Glands: Located beneath the tongue & superior to the submandibular glands. Secrete an even thicker fluid containing a small amount of salivary amylase. Saliva Saliva: Fluid produced by the salivary glands that lubricates, dissolves, and begins chemical digestion of food. water – allows for particles to be dissolved so they can be tasted by the gustatory receptors. 0.5% solutes: 99.5% Chloride ions (activate salivary amylase to start the breakdown of starch). Bicarbonate & phosphate ions that buffer acidic foods Organic substances such as urea & uric acid Mucous that lubricates food so it can be swallowed. Immunoglobin A, which prevents the attachment of microbes to the epithelium. Lysozyme (enzymes that kill some bacteria) Salivary amylase – a digestive enxyme that acts on starch. Saliva Salivation: The secretion of saliva into the mouth increases when food enters the mouth. Chemicals in food stimulate the taste bud receptors which send impulses to the superior & inferior salivary nuclei in the brain stem. The sight, sound, smell, taste, or thought of food can increase salivation. Controlled by the autonomic nervous system. Average daily saliva secretion is 1,000 – 1,500 mL. Parasympathetic stimulation maintains moderate secretion to keep membranes moist & lubricate the movement of the tongue during speech. When swallowed, aids in moistening the esophagus. Most saliva components are reabsorbed to prevent fluid loss. Salivation ceases in dehydration to conserve water & create the sensation of thirst. The Tongue The Tongue: The accessory digestive organ composed of skeletal muscle & covered in a mucous membrane. Forms the floor of the oral cavity in conjunction with its associated muscles. Performs the sensory task of tasting our food. Median Septum: Divides the tongue in half. Lingual Frenulum: Connects the tongue to the floor of the mouth. Extrinsic Muscles: Originate outside the tongue & function to change the tongue’s position. Intrinsic Muscles: Originate within the tongue & function to change the tongue’s shape. The Tongue Bolus: A compact mass of food & saliva mixed by movements of the tongue. Papillae: The projections along the superior surface of the tongue that provide friction, helps to manipulate soft foods, & contain taste buds. Lingual Glands: Secrete mucus & a watery serous fluid containing lingual lipase (an enzyme that acts on triglycerides). Located in the lamina propria of the tongue. Teeth Dentes aka Teeth: Function in mastication (chewing) of food. Part of mechanical digestion. Primary Dentition aka Deciduous Teeth: “Baby teeth” that first appear around 6 months, with all 20 present by 24 months. Permanent Teeth: “Adult teeth” that replace the baby teeth with a full set of 32: 8 incisors for cutting 4 canines for puncturing & tearing 8 premolars for crushing & grinding 12 molars for crushing & grinding Teeth Two regions to each tooth: Crown: The exposed portion above the gum/gingiva – covered with enamel (the hardest substance in the body!). Root: The portion of the tooth embedded in the bone – covered with cementum. Periodontal Ligaments aka Periodontal Membranes: sAttach the cementum of the tooth to the bony socket. Dentin: The material underneath the cementum & enamel forming the bulk of the tooth. The secondhardest material in the body. Pulp Cavity: Located within the dentin & containing the pulp. Pulp: Connective tissue containing the blood vessels, nerves, & lymphatic vessels supplying each tooth. Root Canal: The cavity extending into the root portion. Apical Foramen: The place where vessels & nerves enter & exit – located at the proximal end of the root canal. Teeth Dental Plaque: The accumulation of bacteria, sugar, & debris upon the teeth. Can cause the enamel & dentin to decay, causing cavities aka dental caries. Gingivitis: An inflammation or infection of the gums due to the presence of dental plaque. The Esophagus Esophagus: The collapsible “food tube: connecting the pharynx to the stomach. Extends from the larynx to the stomach at the cardiac orafice. Gastroesophageal (Cardiac) Sphincter aka Lower Esophageal Sphincter: Regulates the movement of food from the esophagus into the stomach. Secretes mucus & transports food into the stomach. The Esophagus 4 Basic Tissue Layers: Mucosa: Nonkeratinized stratified squamous epithelium, lamina propria, & muscularis mucosae. Also contains mucous glands near the stomach. Submucosa: Contains the mucous glands & consists of areolar connective tissue & blood vessels. Muscularis: The upper 1/3 is skeletal muscle, the lower 1/3 is smooth muscle, & the middle is a mixture of both. Becomes slightly more prominent & forms sphincters at either end of the esophagus. Upper Esophageal Sphincter (UES): Skeletal muscle – regulates the movement of food from the pharynx into the esophagus. Lower Esophageal Sphincter (LES): Smooth muscle – regulates the movement of food from the esophagus into the stomach. Adventitia: The superficial layer that attaches the esophagus to the surrounding structures. Deglutition (Swallowing) Deglutition aka Swallowing: Follows the ingestion of food & mastication – facilitated by the secretion of saliva & mucous. 3 phases of Swallowing: Voluntary Phase: Food bolus is passed into the oropharynx, causing… Pharyngoesophageal Phase: The involuntary passage of the bolus through the pharynx into the esophagus. Esophageal Phase: The involuntary passage of the bolus through the esophagus & into the stomach. This stimulates receptors in the oropharynx which send impulses to the deglutition center of the medulla oblongata & lower pons of the brain stem. Returning impulses cause the soft palate & uvula to elevate & close of the nasopharynx (preventing the bolus from entering the nasal cavity). Epiglottis closes off the opening to the larynx to prevent the bolus from entering the respiratory tract. The UES relaxes & the bolus moves through the oropharynx & laryngopharynx to the esophagus. Peristalsis: The wavelike progression of contractions & relaxations of the circular & longitudinal layers of the muscularis – pushes the bolus toward the stomach. The LES relaxes & the bolus moves into the stomach. Liquid reaches the stomach in 1-2 seconds. A food bolus reaches the stomach in 4-8 seconds. The Peritoneum Peritoneum: The largest serous membrane of the body. Divided into… Peritoneal Cavity: The slim space containing serous fluid & located between the parietal & visceral peritoneum. Parietal Peritoneum: The portion of the peritoneum lining the wall of the abdominopelvic cavity. Visceral Peritoneum: The portion of the peritoneum covering some of the organs in the abdominopelvic cavity & serving as their serosa. Ascites: A condition where the peritoneal cavity becomes distended by the accumulation of fluid. Retroperitoneal: The term for organs who only have peritoneal covering on the anterior surface, e.g. the kidneys & pancreas. The Peritoneum Two folds of peritoneum hold the intestines loosely in place. Allows for movement while muscular contractions mix & move their contents along the GI tract. Mesentary: The fan-shaped fold of peritoneum that binds the small intestine to the posterior abdominal wall. Mesocolon: The fold of the peritoneum that binds the large intestine to the posterior abdominal wall. The Stomach Stomach: A muscular sac that stores the ingested food while being mechanically & chemically digested. Located in the upper left quadrant of the abdominal cavity, nearly hidden by the liver. J-shaped and collapsible with temporary rugae folds & a volume of 50 mL when empty & 4 L when full. Mixing Waves: Muscular contractions that pass through the stomach every 15-20 seconds, causing the stomach to mechanically break up food particles, liquefy food, & begin chemical digestion of proteins & a small amount of fat. Chyme: A semidigested mixture of food produced by mixing waves in the stomach. Gastric Emptying: The movement of around 3 mL of chyme into the duodenum through the pyloric sphincter – caused by increasingly strong mixing waves as the food moves down the GI tract. The Stomach Greater Curvature: The convex lateral surface of the stomach. Lesser Curvature: The concave medial surface of the stomach. Greater Omentum: The peritoneal fold attached to the stomach & colon & hanging over the small intestine – anchors at the greater curvature & hangs from the stomach to interconnect the stomach, duodenum, & large intestine. Lesser Omentum: Connects the stomach to the liver – attached to the stomach, part of the duodenum, & part of the liver & supporting hepatic vessels. 4 Major Regions: Cardia: Where the food enters from the esophagus – surrounds the cardiac orifice. Fundis: The dome shaped portion adjacent to the cardia. Body: The large midportion. Pyloris: The funnel-shaped bottom portion that connects with the duodenum via the pyloric sphincter. Divides into the.. Pyloric Antrum: Connects to the body of the stomach. Pyloric Canal: Leads to the duodenum – terminates at the pyloris. Stomach The Stomach 4 Layers of the Stomach: Mucosa: Surface Mucous Cells: Simple columnar epithelial cells along the mucosa’s surface. Gastric Glands: Epithelial cells that extend down into the lamina propria & forming columns of secretary cells. Gastric Pits: Channels lined by gastric glands & containing secretions from those glands. Exocrine Gland Cells: 3 types found within the gastric glands; form gastric juice. Mucous Neck Cells & Mucous Cells: Secrete mucous. Parietal Cells: Produce intrinsic factor & hydrochloric acid. Chief Cells: Secrete pepsinogen & gastic lipase, which split short-chain triglycerides in fat molecules into fatty acids & monoglycerides. Submucosa: Composed of areolar connective tissue. Muscularis: Composed of an outer longitudinal layer, a middle circular layer, and an inner oblique layer. Serosa: Simple squamous epithelium & areolar connective tissue. Phases of Gastric Digestion Cephalic Phase: Triggered by the smell, taste, thought & sight of food – prepares the mouth & stomach for food. Facial & glossopharyngeal nerves stimulate salivary glands to secrete saliva. Vagus nerve stimulates the secretion of gastric juices. Phases of Gastric Digestion Gastric Phase: Stimulated by the presence of food in the stomach – accounts for 2/3 of gastric juice secretion. Stomach distension activates stretch receptors. Stretch receptors initiate reflexes that increase the neural stimulation of gastric glands. Partially digested proteins, caffeine, & increased pH stimulate enteroendocrine cells (G cells) to secrete gastrin into the bloodstream. Gastrin causes mucous cells to secrete mucous, chief cells to secrete pepsinogen, and parietal cells to secrete HCl & intrinsic factor. Also increases gastric motility (the muscle activity & gastric emptying). Phases of Gastric Digestion Intestinal Phase: Divides into two aspects. Excitatory Aspect: Chyme reaches the duodenum & stimulates the release of gastrin. Inhibitory Aspect (Enterogastric Reflex): Caused by the intestinal distension & an accumulation of chyme within the duodenum. Causes an increase in sympathetic stimulation to the stomach. Secretions decrease, motility decreases, & the pyloric sphincter tightens to limit the movement of more chyme – this gives the duodenum time to work on the chyme already present. Arrival of chyme spurs the release of intestinal hormones, which decrease gastric activity. Cholecystokinin & secretin hormones are released. Vomiting Vomiting: Reflexive contraction of the diaphragm & abdominal wall muscles, relaxation of the cardiac sphincter & rise of the soft palate, which results in the movement of the stomach contents back into the oral cavity. Can be triggered by… Extreme stretching in the stomach or intestines Irritants present will trigger the emetic center of the medulla, which sends signals to the diaphragm & the abdominal wall muscles. Small Intestine Small Intestine: The major organ of digestion & absorption of nutrients – a convoluted tube extending from the pyloric sphincter to the ileocecal valve. Receives chyme from the stomach & secretions from the liver & pancreas. Divides into the… Duodenum: The 10” long section that curves around the head of the pancreas – starts at the pyloric sphincter of the stomach & extends ~25 cm until it merges with the jejunum. Common Bile Duct: Delivers bile from the liver & the gallbladder. Main Pancreatic Duct: Delivers pancreatic juice from the pancreas. Hepatopancreatic Ampulla: The uniting of the common bile & main pancreatic ducts in the wall of the duodenum. Hepatopancreatic Sphincter: Muscle that controls the entry of bile & pancreatic juice into the duodenum. Jejunum: Connects the duodenum & ileum. Ileum: Extends from the jejunum to the ileocecal valve, joining the large intestine via the ileocecal sphincter. Small Intestine 4 Tissue Layers in the Small Intestine: Mucosa: Simple columnar absorptive cells interspersed with goblet cells (secrete mucous) make up the epithelium. Absorptive Cells: Digest & absorb nutrients in the small intestinal chyme. T lymphocytes: Interspersed among the absorptive cells to provide immune defense. Deep crevices lined with glandular epithelium… Intestinal Glands aka Crypts of Lieberkuhn: Pits located between the villi that secrete intestinal juice. Paneth Cells: Located in the intestinal glands & secrete lysozyme – capable of phagocytosis. Enteroendocrine Cells: Include S cells, CCK cells, & K cells – secrete the hormones secretin, cholecystokinin (CCK) & glucose-dependant insulinotropic peptide (GIP). Lamina Propria: Contains areolar connective tissue & an abundance of mucosa-associated lymphoid tissue (MALT). Submucosa: Contains the proximal duodenum (houses Brunner’s glands which secrete alkaline mucus to help neutralize acidic chyme) & terminal ileum. Peyer;s Patches: Clisters of lymphatic nodules that help prevent colonic bacteria from entering the small intestine. Muscularis: The muscular layer composed of inner circular & outer longitudinal layers of smooth muscle. Except for a major part of the duodenum, most of the small intestine is also covered by a serosa. Small Intestine Small Intestine The small intestine has lots of surface area to aid digestion & absorption. 3 Main surface-area enhancing adaptations: Circular Folds: Large, deep, permanent folds of the mucosa & submucosa to increase the surface area. Villi: Finger-like projections of the mucosa that increase surface area for digestion & absorption. Slow the movement of chyme to allow more time for digestion & absorption. Core contains a capillary bed & lacteal (lymphatic capillary) for the transport of absorbed nutrients. Microvilli: Tiny projections of the apical membrane of the absorptive cells. Known as the brush border due to its appearance. Further increases the surface area. Secretes membrane-bound enzymes that are involved in digestion. Small Intestine Intestinal Juice: A clear-yellow fluid containing water & mucous, which is slightly alkaline. 1-2 liters secreted daily. Pancreatic & intestinal juice aid in the absorption of nutrients from the chyme. Absorptive Cells: Cells in the small intestine that synthesize several digestive enzymes (brush border enzymes) & insert them into the plasma membrane of the microvilli. Release enzymes that help digest nutrients in the chyme as they slough off & break apart. Mechanical Digestion Myenteric Nerve Plexus controls the two types of movements in the small intestine. Segmentation: Common movement of the small intestine, where ring-like constrictions form at several places along the intestine then relax while constrictions occur elsewhere. Results in churning of the intestinal contents. When most nutrients have been absorbed, only residue remains, & segmentation tapers off and peristalsis begins. Migrating Motility Complex (MMC): The type of peristalsis that occurs in the small intestine. Begins in the lower part of the stomach & pushes chyme forward along a short stretch of small intestine before stopping. Slowly migrates down the small intestine until it reaches the end of the ileum. Mechanical Digestion Contractions in the small intestine…. Mix chyme with intestinal juice, bile, & pancreatic juice. Churn chyme & bring it in contact with the mucosa for digestion & absorption. Moves residue toward the large intestine. The Liver Liver: An accessory organ of digestion. Has both digestive & nondigestive functions. Primary digestive function is the synthesis & secretion of bile (emulsifies fats). Largest gland in the body! Located beneath the diaphragm & runs across the entire body. The Liver 4 Lobes of the Liver: Right & Left Lobes: The larger lobes in the liver. Falciform Ligament: A fold of ligament that attaches the liver to the abdominal wall & diaphragm & separates the right & left lobes. Ligamentum Teres (Round Ligament): The free border of the falciform ligament – a remnant of the umbilical vein of the fetus, which extends from the liver to the umbilicus. Right & Left Coronary Ligaments: Suspend the liver fro the diaphragm. Inferior Quadrate Lobe & Posterior Cuadate Lobe: Smaller lobes associated with the right lobe of the liver. Functions of the Liver Carbohydrate Metabolism: Glycogen is broken down into glucose & released into the blood stream if blood glucose is low, & glucose is converted into glycogen & triglycerides for storage if blood glucose is high. Lipid Metabolism: Hepatocytes store some triglycerides, break down fatty acids for ATP generation, synthesizes lipoproteins, synthesize cholesterol, & uses cholesterol to make bile salts. Protein Metabolism: Hepatocytes deaminate (remove amino group NH2) amino acids to be used for ATP production or converted to carbohydrates or fats. Results in toxic ammonia (NH3) which is converted into urea & excreted via the urine. Also synthesize most plasma proteins. Processing of Drugs & Hormones: Detoxifies alcohol & drugs, chemically alters & excretes hormones. Excretion of Bilirubin into Bile: Bilirubin (derived from the heme in aged RBCs) in bile is metabolized by the bacteria in the small intestine & is eliminated in feces. Synthesis of Bile Salts: Used for the emulsification & absorption of lipids in the small intestine. Storage: Stores glycogen, certain vitamins & minerals which are released from the liver when they are needed elsewhere in the body. Phagocytosis: Stellate reticuloendothelial (Kupffer) cells of the liver phagocytize aged RBCs, WBCs, & some bacteria. Activation of Vitamin D: Skin, liver, & kidneys synthesize the active form of Vitamin D. Histology of the Liver Lobule: The structural & functional units of the liver – a hexagonal (6-sided) structure made up of hepatocytes (liver cells) that surround a central vein. Hepatocytes: Liver cells that exchange oxygen for carbon dioxide & take up nutrients for processing & storage. Also take up fat-soluble vitamins (A, D, E, K) for storage & toxins for detoxification. Produce bile, which flows through the lobule in the opposite direction of the blood. Bile Canaliculi: Small channels that hepatocytes secrete bile into – run into bile ductules which pass into bile ducts at the periphery of the lobules. Portal Triads: Found at each of the 6 corners of the lobule. Includes a branch of the hepatic artery, hepatic portal vein, & bile duct. Liver Sinusoids: Connect the hepatic artery (where blood enters the lobule) & the hepatic vein with the central vein. Stellate Reticuloendothelial (Kupffer) Cells: Fixed phagocytes contained within the sinusoids. Hepatic Vein: The collection of all the central veins from all the lobules. Right & Left Hepatic Ducts: Merging of the bile ducts which unit & exit the liver as the common hepatic duct. Common Bile Duct: Fusing of the common hepatic duct & the cystic duct from the gallbladder, which then empties into the duodenum. The Liver Oxygenated blood enters the liver from the hepatic artery. Deoxygenated blood enters the liver from the hepatic portal vein. The veins branch into the liver sinusoids where oxygen, nutrients, & certain toxic substances are taken up by hepatocytes. Products produced by the hepatocytes & nutrients needed by other cells are secreted back into the blood, which drains into the central vein & eventually enters the hepatic vein. The Liver Bile: A yellow-green, alkaline solution that contains bile salts, bile pigments, cholesterol, neutral fats, phospholipids, & electrolytes. Phospholipids & bile acids aid in fat digestion & absorption - are reabsorbed in the ileum & returned to the liver. Bile is continuously made by the liver & stored & modified in the gallbladder until it is needed. Phagocytosis of aged RBCs release iron, globin, & bilirubin. Iron & globin are recycled while bilirubin is secreted into the bile to be broken down in the intestine. Bilirubin: The main bile pigment, derived from the decomposition of hemoglobin in the RBCs. Bile Salts: Cholesterol derivatives whose chief function is the emulsification of fats. Nonpolar – causes ingested fats to clump together in the watery environs of the digestive tract which decreases the surface area for lipases (fatdigesting enzymes). Emulsify fats by separating them into smaller pieces to increase the surface area for digestive enzymes. The Gallbladder Gallbladder: The thin-walled, pear-shaped muscular sac responsible for storing & concentrating bile until it is needed by the small intestine. Located on the inferior surface of the right lobe of the liver, within a shallow fossa on the ventral surface of the liver. Consists of… A fundus projecting inferiorly beyond the inferior border of the liver. A body, which is the center portion. A neck, which is the tapered portion. Covered by visceral peritoneum. Concentrates the bile by absorbing water & ions. The Gallbladder Chyme, consisting of fat & protein, enters the duodenum, causing enteroendocrine cells to secrete cholecystokinin (CCK) into the bloodstream. CCK causes the gallbladder to contract & the hepatopancreatic sphincter to relax & the pancreas to secrete pancreatic juice. Muscular contractions cause bile to be expelled into the cystic duct, common bile duct, & the duodenum. The Gallbladder Gallstones: Caused by cholesterol & bile salts crystalizing within the gallbladder. Obstruct the flow of bile, causing severe pain. The Pancreas Pancreas: A soft, tadpole-shaped organ that functions as both an endocrine & exocrine gland. Head: The expanded portion located near the curve of the duodenum & is encircled by the duodenum. Body: Superior to and to the left of the head. Tail: Tapers off & abuts the spleen. The Pancreas The Endocrine Portion of the Pancreas: Islets of Langerhans: Patches of hormonesecreting cells responsible for the secretion of insulin & glucagon (regulate blood glucose levels via negative feedback). Beta Cells: Release insulin in response to high blood glucose levels. Alpha Cells: Release glucagon in response to low blood glucose levels. The Pancreas The Exocrine Portion of the Pancreas: Main Pancreatic Duct (Duct of Wirsung): Empties into the duodenum & receives secretions form multiple, smaller ducts from within the pancreas. Pancreatic Acini: Secretory cells that secrete a mixture of fluid & digestive enzymes called pancreatic juice - located at the end of the smaller ducts. Cells lining the ducts secrete an alkaline solution. Secrete proteases, pancreatic amylase, lipase, & nuclease enzymes. Zymogens: Inactive proteins that are activated by brush border enzymes within the duodenum – many enzymes secreted this way, particularly pepsinogen. Enzyme release triggered by the CCK produced in the duodenum in response to chyme. Secretin: A hormone that stimulates pancreatic duct cells to release alkaline fluid in response to an acidic chyme. Functions to secrete digestive enzymes and bicarbonate-rich fluid. Large Intestine Large Intestine: Receives indigestible material from the small intestine, reabsorbs water from the food residue, & eliminates the rest as feces. 0.5-1.0 liters of water enter the large intestine daily, and all but 100-200 mL is reabsorbed through osmosis. Some vitamins & ions (including sodium & chloride) are reabsorbed in the large intestine as well. Frames the small intestine on 3 sides: Extends from the ileocecal valve to the anal canal. Larger in diameter than the small intestine, but much shorter. Histology of the Large Intestine Mucosa: Simple columnar epithelium, lamina propria, & musclaris mucosae. Contains mostly absorptive cells that function in water absorption & globlet cells that secrete mucous to lubricate the passage of the contents. Muscularis: The muscle layer – once again, there is an external longitudinal layer & internal circular layer. Taenia Coli: 3 bands of smooth muscle in the longitudinal layer. Tonic contractions in these bands gather the colon together into haustra (pocket-like sacs or pouches). Epiploic Appendages: Fat-filled bags of visceral peritoneum attached to the taenia coli. Submucosa: Similar to that in the rest of the GI tract. Serosa: In the large intestine, this is part of the visceral peritoneum. Large Intestine Large Intestine Divided into 4 regions: Cecum: The sac-like area located below the iliocecal valve – the first part of the large intestine. Ileocecal Sphincter (Valve): The opening from the ileum to the large intestine – allows material from the small intestine to pass into the large intestine. Appendix (Vermiform Appendix): The twisted, coiled tube attached to the cecum – bacteria often accumulates here and induces appendicitis. Colon: The long tube merging from the open end of the cecum. Ascending Colon: The portion that travels up the right side of the abdominal cavity & turns medially. Transverse Colon: The portion that travels across the abdominal cavity. Descending Colon: The portion that turns downward at the spleen & descends on the left side of the abdominal cavity. Sigmoid Colon: The final portion of the colon that enters the pelvis inferiorly & merges with the rectum. Large Intestine Anterior to the sacrum & coccyx – connects the colon & anal canal. Anal Canal: The last segment of the large intestine – begins where the rectum penetrates the muscles of the pelvic floor & continues to the outside of the body. Rectum: Anus: The external opening controlled by the sphincter muscles. Internal Anal Sphincter: Smooth & involuntary muscle band. External Anal Sphincter: Skeletal muscle that allows control for keeping the anus closed except during elimination of feces. Anal Columns: Longitudinal folds in the mucous membranes of the anal canal containing a network of arteries & veins. Types of Digestion Mechanical Digestion in the Large Intestine: The breakdown of food through physical means. Gastroileal Reflex: Intensifies peristalsis after a meal to force chyme into the cecum. Gastrin relaxes the sphincter allow movement of the chyme. Contraction of the ileocecal sphincter intesifieds when the cecum is distended. When chyme passes the ileocecal sphincter & accumulate in the ascending colon, colon movements begin. Distension of the haustra triggers haustric churning to squeeze contents forward. Peristalsis slows down temporarily. Gastrocolic Reflex: The presence of food in the stomach initiates mass peristalsis 3-4 times a day. Mass peristalsis: The final movement of a strong peristaltic wave to push the contents of the colon into the rectum. Types of Digestion Chemical Digestion: The breakdown of food through enzymes. final stage of digestion occurs in the colon – no enzymes are secreted, but bacteria will ferment the remaining carbohydrates to release hydrogen, carbon dioxide, & methane gases (causes flatulence when excessive). Bacteria convert proteins into simpler substances, including indole & skatole (eliminated in feces & contribute to the odor). Bacteria also decompose bilirubin to simpler pigments, including stercobilin (gives feces its brown color). Bacteria may produce vitamins B & K to be absorbed in the colon. The Types of Digestion Feces: Chyme that has remained in the large intestine for 3-10 hours and has become solid or semisolid. Composed of 75% water & 25% solid matter (inorganic salts, sloughed off epithelial cells from the GI tact, bacteria (30% of the solid matter), products from bacterial decomposition, unabsorbed digested materials, & indigestible parts of food (30% of solid matter)). Defecation Reflex: Initiated when feces enter & stretch the rectal wall, resulting in sigmoid colon & rectal contractions, & relaxation of the anal sphincter muscles. Types of Digestion Diarrhea: An increase in frequency, volume, & fluid content of the feces. Caused by increase motility of & decreased absorption by the intestines. Prolonged diarrhea may lead to dehydration, acidosis, & electrolyte imbalances. Constipation: Infrequent or difficult defecation. Caused by decreased motility of the intestines. The prolonging of the time in the colon leads to excessive water absorption & drying out of the fecal matter. Nutrient Digestion & Absorption Absorption: The passage of digested nutrients from the gastrointestinal tract into the blood or lymph. Occurs via diffusion, facilitated diffusion, osmosis, and active transport. 90% of absorption occurs in the small intestine, the other 10% in the stomach & large intestine. Undigested & unabsorbed material remaining in the small intestine passes on to the large intestine. Carbohydrate Digestion & Absorption Most ingested carbohydrates: Monosaccharides: Simple sugars, including glucose, fructose, & galactose. Readily absorbed & need no further digestion. Disaccharides: Formed by 2 monosaccharides chemically bonded together, including sucrose & lactose. Must be broken down into monosaccharides to be absorbed. Polysaccharides: Large polymers of monosaccharides, including plant starch or glycogen (animal starch). Must be broken down into monosaccharides to be absorbed. Carbohydrate Digestion & Absorption Salivary Amylase: Secreted into the mouth to begin the breakdown of startch into oligosaccharides (2-8 monosaccharides bonded together). Pancreatic Amylase: Continues the chemical digestion of starch in the small intestine. Brush Border Enzymes: Include sucrase & lactase break down oligosaccharides & disaccharides. Carbohydrate Digestion & Absorption Absorption of Monosaccharides: Cotransport with Sodium: Allows glucose & galactose to be absorbed in the intestinal epithelial cells along with sodium ions. Facilitated Diffusion: Diffusion using a facilitating molecule. All monosaccharides use this to move out of the epithelial cells through their basolateral surfaces & enter the villus capillaries to be transported to the liver via the hepatic portal vein. Protein Digestion & Absorption Protein: Polymers of amino acids. Digestion begins in the stomach with pepsin. Digestion continues in the small intestine with trypsin, chymotrypsin, carboxypeptidase, & elastase (enzymes in pancreatic juice). Digestion is completed in the intestinal brush border by two peptidases.. Aminopeptidase: Cleaves off the terminal amino acid at the amino end of the peptide. Dipeptidase: Splits dipeptides into single amino acids. Free amino acids are the end result of protein digestion. Protein Digestion & Absorption Protein Absorption: Cotransport with Sodium: Helps amino acids to enter the intestinal epithelial cells to move into the villus capillaries & travel to the liver via the hepatic portal vein. Most proteins absorbed as amino acids via active transport (the use of ATP to move molecules across the membrane). Lipid Digestion & Absorption Lipid: Fatty acid molecules – most abundant type is the triglyceride (a molecule of glycerol bonded to 3 fatty acid molecules). Lipases: Enzymes that split triglycerides & phospholipids. 3 types participate in lipid digestion… Lingual Lipase: Aid in digestion in the stomach. Gastric Lipase: Aid in digestion in the stomach. Pancreatic Lipase: Aid in digestion in the small intestine – break down the lipids into fatty acids & monoglycerides. Emulsification: The process of breaking down large lipid globules into several small globules. Bile salts are ideally suited for emulsification due to their amphipathic nature (have hydrophobic & hydrophilic regions). Lipid Digestion & Absorption Lipid Absorption: All lipids are absorbed via simple diffusion – molecules move from an area of low concentration to areas of high concentration. Micelles: Tiny spheres formed by the bile salts around long-chain fatty acids & monoglycerides in the intestinal chyme. Long-chain fatty acids & monoglycerides recombine to form triglycerides inside the absorptive cells. Chylomicrons: Masses of triglycerides coated with proteins – leave the absorptive cells by exocytosis & enter the lymph vessels & then the blood through the left subclavian vein. Lipoprotein Lipase: An enzyme attached to the apical surface of the capillary endothelial cells – helps to move chylomicrons from the blood. Move from the interior of the small intestinal lumen to the brush border of absorptive cells, allowing long-chain fatty acids & monoglycerides to diffuse into the absorptive cells leaving the micelles behind in the chyme. Breaks down triglycerides in the chylomicorns & other lipoproteins into fatty acids & glycerol. Enterohepatic Circulation: The cycle of bile salt secretion by hepatocytes in bile, reabsorption by the ileum (small intestine), & resecretion into the bile. Nucleic Acid Digestion & Absorption Nucleic Acids: Pancreatic juices contain two nucleases to break down nucleic acid Ribonuclease: Digests RNA Deoxyribonuclease: Digests DNA. Nucletides resulting from nucleic acid digestion are further digested by the brush border enzymes. The final products of digestion are absorbed via active transport. Absorption Of Electrolytes Many of the electrolytes absorbed by the small intestine are from gastrointestinal secretions – others are part of ingested foods & liquids. Sodium ions move into absorptive cells via diffusion & secondary active transport. Are Negatively charged ions can be actively transported. E.g, actively transported out of the absorptive cells. bicarbonate, chloride, iodide, & nitrate. Calcium Ions are absorbed actively, along with iron, potassium, magnesium, & phosphate. Absorption of Vitamins Water-Soluble Vitamins: Vitamins that are soluble in water and are absorbed via simple diffusion. E.g. B vitamins, Vitamin C. Exception: B12 must bind with an intrinsic factor produced by the stomach lining & the combination is absorbed into the ileum via active transport. Absorption by Diffusion Absorption of Water All water absorption in the GI tract occurs via osmosis from the lumen of the intestines through the absorptive cells & into blood capillaries. Absorption of water depends on the absorption of electrolytes & nutrients to maintain the osmotic balance of the blood Absorbed electrolytes, monosaccharides, & amino acids establish a concentration gradient for water that promotes water absorption. Approximately 9.3 liters per day enters the small intestine – 2.3 liters from ingestion, and 7.0 liters from GI secretions. Around 8.3 liters is absorbed by the small intestine. Only 0.1 liters (100mL) is secreted via feces each day. Absorption by Osmosis