Digestive System II: Digestive ActivitiesDigestive Activities in the Mouth, SwallowingDigestive Activities in the StomachGastric juiceHormones that stimulate stomach acid and enzyme secretionContributions of the Liver in DigestionDigestion in the Small IntestinePancreatic juiceHormonal stimulation of pancreasMethods employed to digest and absorb carbohydrates, proteins, lipids, and nucleic acidsAbsorption in the Large Intestine and Feces Formation

Salivary GlandsExtrinsic salivary glands (parotid, submandibular, and sublingual) Intrinsic (buccal) salivary glands are scattered in the oral mucosaSecretion (saliva) Cleanses the mouthMoistens and dissolves food chemicals Aids in bolus formationContains enzymes that begin the breakdown of starch

Composition 9799.5% water, slightly acidic solution containing: ElectrolytesNa+, K+, Cl, PO4 2, HCO3Salivary amylase and lingual lipase; mucin; Metabolic wastesurea and uric acid; lysozyme, IgA, defensins, and a cyanide compound protect against microorganisms

Control of Digestive Activity and MouthMostly controlled by reflexes via the parasympathetic divisionEnteric Nervous System (ENS) Intrinsic nerve supply of the alimentary canalSympathetic impulses inhibit secretion and motilityParasympathetic impulses stimulateChemical and mechanical receptors are located in organ walls that trigger reflexes

Digestive System II: Digestive ActivitiesDigestive Activities in the Mouth, SwallowingDigestive Activities in the StomachGastric juiceHormones that stimulate stomach acid and enzyme secretionContributions of the Liver in DigestionDigestion in the Small IntestinePancreatic juiceHormonal stimulation of pancreasMethods employed to digest and absorb carbohydrates, proteins, lipids, and nucleic acidsAbsorption in the Large Intestine and Feces Formation

Food Breakdown in the StomachGastric juice is regulated by neural and hormonal factorsPresence of food or falling pH causes the release of gastrinGastrin causes stomach glands (chief cells) to produce protein-digesting enzymes (pepsinogen)Hydrocholoric acid from parietal cells makes the stomach contents very acidic, kills bacteriaHeartburn from Acid RefluxAcid reflux animation onlineUlcer formation movie onlineChief cellpepsinogen

Digestion and Absorption in the StomachProtein digestion enzymes (proteases)Stomach proteins are exported into the stomach as zymogens (extra long, inactive polypeptide chains)Pepsin an active protein digesting enzyme (converted from pepsinogen through action of HCl and active pepsin)Rennin (chymosin) works on digesting milk casein protein in infantsThe only absorption that occurs in the stomach is of alcohol and aspirinConversion of pepsinogen to pepsinThe action of chymosin on casein

Figure 23.17 Presence of lowpH, partially digested foods, fats, or hypertonic solution in duodenum when stomach begins to empty Distension;presence offatty, acidic,partiallydigested foodin theduodenumBriefeffectIntestinal(enteric)gastrinreleaseto bloodEntero-gastricreflexRelease of intestinalhormones (secretin,cholecystokinin, vasoactiveintestinal peptide)LocalreflexesVagalnucleiin medullaPyloricsphincterStimulateInhibit112Stomachsecretoryactivity Sight and thoughtof food Stomachdistensionactivatesstretchreceptors Stimulation oftaste and smellreceptors Food chemicals(especially peptides and caffeine) and rising pHactivate chemoreceptors Loss ofappetite,depression EmotionalupsetLack ofstimulatoryimpulses toparasym-patheticcenterCerebralcortexCerebral cortexConditioned reflexVagovagalreflexesLocalreflexesMedullaG cellsHypothalamusand medullaoblongataVagusnerveVagusnerveGastrinreleaseto bloodGastrinsecretiondeclinesG cellsOverridesparasym-patheticcontrolsSympatheticnervoussystemactivation1111222Stimulatory eventsInhibitory eventsCephalicphaseGastricphaseIntestinalphase Excessiveacidity (pH

Digestive System II: Digestive ActivitiesDigestive Activities in the Mouth, SwallowingDigestive Activities in the StomachGastric juiceHormones that stimulate stomach acid and enzyme secretionContributions of the Liver in DigestionDigestion in the Small IntestinePancreatic juiceHormonal stimulation of pancreasMethods employed to digest and absorb carbohydrates, proteins, lipids, and nucleic acidsAbsorption in the Large Intestine and Feces Formation

LiverLargest gland in the bodyConnected to the gall bladder via the common hepatic ductBile Production in the LiverCompositionBile salts, bile pigment (broken down hemoglobin, cholesterol, phospholipidsBile is introduced into the duodenum in the presence of fatty food to cause emulsificationEmulsification solubilizes lipids so that they can come in contact with water-soluble lipases and be broken down into fatty acids and glycerol.

Role of the Liver in MetabolismProduces blood proteins (albumin, clotting proteins) and lipoproteins Degrades hormones

Figure 23.25a, b(a)(b)LobuleCentral veinConnectivetissue septumThe Liver Processes/Detoxifies Absorbed Food in Lobules

Digestive System II: Digestive ActivitiesDigestive Activities in the Mouth, SwallowingDigestive Activities in the StomachGastric juiceHormones that stimulate stomach acid and enzyme secretionContributions of the Liver in DigestionDigestion in the Small IntestinePancreatic juiceHormonal stimulation of pancreasMethods employed to digest and absorb carbohydrates, proteins, lipids, and nucleic acidsAbsorption in the Large Intestine and Feces Formation

Digestion in the Small IntestineEnzymes from the brush borderBreak double sugars into simple sugars (carbohydrases)Complete some protein digestion (proteases)Pancreatic enzymes play the major digestive function; usually zymogensHelp complete digestion of starch (pancreatic amylase)Carry out about half of all protein digestion (trypsin, etc.)Responsible for fat digestion (lipase)Digest nucleic acids (nucleases)Bicarbonate ion neutralizes acidic chyme, raises pH to 8

Chemical Digestion : PancreasFigure 14.6Produces a wide spectrum of digestive enzymes that break down all categories of foodAcinar cells produce enzymes that are secreted into the duodenumAlkaline fluid introduced with enzymes neutralizes acidic chyme

Stimulation of the Release of Pancreatic JuiceVagus nerve: secretion of enzymes, weak release of bileLocal hormones from duodenal mucosaSecretin (enzyme release, bile production, gastrin & stomach activities)Cholecystokinin (enzyme and bicarbonate release, bile release)Liver +

Digestive System II: Digestive ActivitiesDigestive Activities in the Mouth, SwallowingDigestive Activities in the StomachGastric juiceHormones that stimulate stomach acid and enzyme secretionContributions of the Liver in DigestionDigestion in the Small IntestinePancreatic juiceHormonal stimulation of pancreasMethods employed to digest and absorb carbohydrates, proteins, lipids, and nucleic acidsAbsorption in the Large Intestine and Feces Formation

Other Activities in the Small IntestineWater absorptionNutrient absorption by mucosal cells via active or passive transportLipids are absorbed by diffusion into lactealsSubstances are transported to the liver by the hepatic portal vein or lymph (lacteal)

Figure 23.32 (1 of 4)Carbohydrate digestion Glucose and galactose are absorbed via cotransport with sodium ions. Fructose passes via facilitated diffusion. All monosaccharides leave the epithelial cells via facilitated diffusion, enter the capillary blood in the villi, and are transported to the liver via the hepatic portal vein.Starch and disaccharidesOligosaccharidesand disaccharidesLactoseMaltoseSucroseGlucoseFructoseSalivaryamylase MouthPancreaticamylase Brush borderenzymes in small intestine(dextrinase, gluco-amylase, lactase, maltase, and sucrase)Smallintestine Smallintestine FoodstuffGalactosePath of absorptionEnzyme(s)and sourceSite ofactionSummary of Carbohydrate Digestion(monosaccharides)

Figure 23.32 (2 of 4)Protein digestion Amino acids are absorbed by cotransport with sodium ions. Some dipeptides and tripeptides are absorbed via cotransport with H+ and hydrolyzed to amino acids within the cells.+ Amino acids leave the epithelial cells by facilitated diffusion, enter the capillary blood in the villi, and are transported to the liver via the hepatic portal vein.Smallintestine Smallintestine StomachFoodstuffProteinLarge polypeptidesPepsin(stomach glands)in presence of HClSmall polypeptides,small peptidesPancreaticenzymes (trypsin, chymotrypsin,carboxypeptidase)Amino acids(some dipeptidesand tripeptides)Brush border enzymes(aminopeptidase,carboxypeptidase,and dipeptidase)Path of absorptionEnzyme(s)and sourceSite ofactionSummary of Protein Digestion

Figure 23.34Epithelialcells ofsmallintestineFat dropletscoated withbile saltsFat globuleLactealBile saltsMicelles made up of fatty acids, monoglycerides,and bile salts1 Large fat globules are emulsified (physically broken up into smaller fat droplets) by bile salts in the duodenum. 2 Digestion of fat by the pancreatic enzyme lipase yields free fatty acids and monoglycerides. These then associate with bile salts to form micelles which ferry them to the intestinal mucosa.3 Fatty acids and monoglycerides leave micelles and diffuse into epithelial cells. There they are recombined and packaged with other lipoid substances and proteins to form chylomicrons.4 Chylomicrons are extruded from the epithelial cells by exocytosis. The chylomicrons enter lacteals. They are carried away from the intestine by lymph.Lipid Digestion and Fatty Acid/Glycerol Import

Figure 23.32 (3 of 4)Fat digestionSmall intestineSmall intestineFoodstuffUnemulsifiedfatsEmulsification by the detergent action of bile salts ductedin from the liverPancreatic lipasesMonoglyceridesand fatty acidsGlycerolandfatty acids Path of absorptionEnzyme(s)and sourceSite ofaction Fatty acids and monoglycerides enter the intestinal cells via diffusion. Fatty acids and monoglycerides are recombined to form triglycerides and then combined with other lipids and proteins within the cells, and the resulting chylomicrons are extruded by exocytosis. The chylomicrons enter the lacteals of the villi and are transported to the systemic circulation via the lymph in the thoracic duct. Some short-chain fatty acids are absorbed, move into the capillary blood in the villi by diffusion, and are transported to the liver via the hepatic portal vein.Summary of Lipid Digestion

Figure 23.32 (4 of 4)Nucleic acid digestion Units enter intestinal cells by active transport via membrane carriers. Units are absorbed into capillary blood in the villi and transported to the liver via the hepatic portal vein.Smallintestine Smallintestine FoodstuffNucleic acidsPancreatic ribo-nuclease and deoxyribonucleaseBrush borderenzymes(nucleosidasesand phosphatases)NucleotidesPath of absorptionEnzyme(s)and sourceSite ofactionSummary of Nucleic Acid Digestion

Digestive System II: Digestive ActivitiesDigestive Activities in the Mouth, SwallowingDigestive Activities in the StomachGastric juiceHormones that stimulate stomach acid and enzyme secretionContributions of the Liver in DigestionDigestion in the Small IntestinePancreatic juiceHormonal stimulation of pancreasMethods employed to digest and absorb carbohydrates, proteins, lipids, and nucleic acidsAbsorption in the Large Intestine and Feces Formation

Processes in the Large IntestineNo digestive enzymes are producedResident bacteria digest remaining nutrientsProduce some vitamin K and BRelease methane and hydrogen sulfide gasesWater and vitamins K and B are absorbedRemaining materials are eliminated via fecesCH4 H2SVitamin KVitamin B

Propulsion in the Large Intestine (Colon)Sluggish peristalsisMass movementsSlow, powerful movements(Three to four times per day)Lack of fiber and weakening of colon walls cause diverticulaPresence of feces in the rectum causes defecation reflexInternal anal sphincter is relaxedDefecation occurs with relaxation of the voluntary external anal sphincterNeural control of the rectum and anal sphicters

Digestive System II: Digestive ActivitiesDigestive Activities in the Mouth, SwallowingDigestive Activities in the StomachGastric juiceHormones that stimulate stomach acid and enzyme secretionContributions of the Liver in DigestionDigestion in the Small IntestinePancreatic juiceHormonal stimulation of pancreasMethods employed to digest and absorb carbohydrates, proteins, lipids, and nucleic acidsAbsorption in the Large Intestine and Feces Formation

**********The gallbladder is a small, think-walled green sac in a shallow fossa on the inferior surface of the liver. Bile backs up the cystic duct when not being used in digestion and enters the gall bladder for storage. Bile is concentrated in the gallbladder by the removal of water, sometimes resulting in the crystallization of the cholesterol to form gallstones. Contraction of the gallbladder or blockage of the common hepatic or bile ducts causes excruciating pain, backing up bile into the liver. Bile salts and bile pigments then enter the bloodstream causing jaundice. Normally jaundice is more common from hepatitis or cirrhosis (both inflammations) when the liver is damaged.

Central Role of the Liver in MetabolismThe liver plays a very important role in connection with diet and metabolism. Aside from the enzymes and bile it produces, the liver is the first stop for absorbed molecules that enter via the intestinal tract. The liver not only stores glucose, fatty acids, and amino acids, but also detoxifies drugs and alcohol, degrades hormones, and produces cholesterol, blood proteins (albumin and clotting proteins), and lipoproteins. The liver's phagocytic cells remove and destroy bacteria that get through the intestinal lining.The liver plays a key role in blood sugar regulation. The liver does a variety of metabolic processes. It oxidizes fatty acids into simpler substances like acetic acid (Acetyl CoA) and acetoacetic acid for insertion into the Krebs Cycle to make ATP. Blood proteins like albumin (critical for blood and tissue osmoregulation as a blood solute) and clotting proteins are made in the liver. Liver cells also synthesis nonessential amino acids and detoxify ammonia into urea. Nutrients not needed by the liver enter the blood circulation.

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