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Chapter 23The Digestive System
G.R. Pitts, Ph.D., J.R. Schiller, Ph.D. and James F. Thompson, Ph.D.Use the video clips:CH 23 Digestive System General Anatomy, CH 23 Anatomy of the Mouth and Esophagus, CH 23 Anatomy of the Stomach, CH 23 Anatomy of the Pancreas, CH 23 Anatomy of the Liver, CH 23 Anatomy of the Small Intestine and CH 23 Anatomy of the Large Intestine for a review of digestive system structure
Digestive ProcessesIngestionMovement of foodDigestionMechanical digestionChemical digestionAbsorptionDefecation
General StructureDigestive organs divided into 2 main groups GI (alimentary) tract Accessory structures cheeks, teeth, tongue, salivary glandsliver, gallbladder, pancreas
Salivary Glands3 pairs salivary glands Parotid glands Submandibular glands Sublingual glands
Salivary GlandsComposition of Saliva 99.5% water, 0.5% solutes Na+, K+, Cl-, HCO3-, and PO4-, proteins, waste productslysozyme salivary amylase digests carbohydrates
Saliva composition differs among the 3 glandsparotid - watery saliva, amylasesubmandibular - thicker mucous, amylasesublingual - mostly mucous, a little amylase
Salivary GlandsFunctions of SalivaWater dissolves food for taste and digestionMucous moistens and lubricates foodMucous lubricates oral surfaces for smooth actions in swallowing and speechCl- ions activate amylase HCO3- and PO4- ions buffer bacterial acidsIgA, lysozymes, cyanide, defensins: protect against microorganisms
Salivary GlandsSecretion of Saliva - 1-1.5 L l dayPrimarily under nervous controlParasympathetic (ANS) normal salivary secretionssaliva swallowedmost reabsorbedSympathetic (ANS) reduced flow (dry mouth)Food (mechanically, chemically) stimulates salivation behavioral memories from cortexstarts digestioncontinues after ingestion is completeirritating foods or nausea
Mechanical digestion Chewing = masticationFood mixed with saliva Shaped into a bolusChemical digestion salivary amylase breaks down and converts polysaccharides (starches) to disaccharides (maltose) and monosaccharides (glucose) [no enzymatic action with cellulose which is also a polymer of glucose]Physiology of Digestion in Mouth
Physiology of Deglutition (Swallowing)Moving bolus from mouth to stomachThree phasesFacilitated by saliva, mucous secretionsInvolves mouth, pharynx, esophagus
Buccal phaseVoluntaryMoves bolus to oropharynx
Physiology of DeglutitionPharyngeal phase Involuntary
Receptors in oropharynx stimulate medulla and pons to:Block mouth with tongueBlock nasopharynx with soft palateRaise larynx to seal epiglottis, blocking airwaysRelax upper esophageal sphincter
Bolus is moved through pharynx into esophagus
Physiology of DeglutitionEsophageal stageUpper esophageal sphincter closesGastroesopaheal sphincter opensEsophagus controls involuntary peristaltic movementEpiglottis reopensBolus moves from esophagus to stomach
EsophagusPeristalsisInvoluntary, rhythmic contraction of muscularisControlled by medullary centersA movement activity: inner circular layer of smooth muscle contracts behind bolus to push it forward; outer longitudinal muscle contracts to pull esophagus wall up
EsophagusPhysiologyUpper esophageal sphincterPeristalsisLower esophageal (cardiac) sphincterSharp transition from nonkeratinized stratified squamous epithelium to simple columnar epithiliumEsophageal epithelium resistant to abrasion but not to acid and proteolytic enzyme attack acid reflux disease
StomachPhysiology of digestion - Mechanical digestion peristaltic movement (mixing waves) back and forth between body and pylorus3 muscle layers: longitudinal, circular, and obliquechyme
StomachPhysiology of digestion - Chemical digestionparietal cells secrete intrinsic factor for B12 absorptionparietal cells secrete HCl by active transportkills microbes, denatures proteinscauses some acid hydrolysis of food moleculesstimulates secretion of hormones for bile & pancreatic juice flowchief cells secrete pepsinogen (inactive precursor)activated to pepsin by HCl acid and by other pepsinsonly an effective protease at acid pHcleaves proteins into smaller peptides
Stomach: MucosaGastric gland chief cellsSecrete pepsinogen (inactive precursor)activated to pepsin by HCl acid and by other activated pepsin enzymesonly an effective protease at acid pHcleaves proteins into smaller peptides
Secrete rennin in neonatescurdles milk to increase time for gastric processing
Secrete gastric lipase in neonatessplits short chain triglycerides common in milklimited role in digestion since it works best at pH 5-6
Stomach: MucosaSimple columnar epithelium with goblet cells and gastric pitsSecretes 2-3 L l day
Gastric gland parietal cellsSecrete intrinsic factor for B12 absorptionSecrete HCl by active transportkills microbes, denatures proteinscauses some acid hydrolysis of food moleculesstimulates secretion of hormones for bile & pancreatic juice flow
Goblet & gastric pit mucous cells secrete mucin1-3 mm mucus layer in the stomach prevents self-digestion
Stomach: MucosaGastric gland G cells (enteroendocrine)Secrete gastrin, histamine, serotonin, somatostatin
Absorption
Impermeable to diffusion of most molecules into the bloodstream
Absorbs a few lipid soluble compounds:certain drugs (e,g., aspirin) alcohol
Stomach: Regulation of Secretion and MotilityRegulated by combination of neuronal and hormonal factors
3 phasesCephalicGastricIntestinal
Stomach: Regulation of Secretion and MotilityCephalic phaseStimulisightsmelltastethoughts/memories
EffectParasympathetic impulses increase gastric secretion
Stomach: Regulation of Secretion and MotilityGastric phase Neural negative feedback mechanisms
Distension activates stretch receptors causing myenteric and vagovagal reflexes to release AchAch stimulates gastric juice secretion
Chemoreceptors respond to partially digested proteins, caffeine and rising pHStimulate gastrin secretion from G cells
Stomach: Regulation of Secretion and MotilityGastric phase (cont.)GastrinInhibited at pH < 2 Gastrin transported in the blood to the gastric glandsGreatly stimulates HCl secretionStimulates histamine secretionSlightly stimulates pepsinogen secretionContracts lower esophageal sphincterIncreases gastric motilityRelaxes pyloric sphincter
StomachGastric phase (continued)Control of HCl secreting parietal cells stimulation by three signal chemicalsgastrinacetylcholinehistamineAll three needed for strong H+ secretionH+ pumps work in conjunction with carbonic anhydraseblockage of the histamine H2 receptor decreases HCl secretionTagametZantacHClKHCO3
Stomach: Regulation of Secretion and MotilityIntestinal phase has excitatory and inhibitory components:
Excitatory Very short phase
Initiated by chyme entry into duodenum
Stretch receptors stimulate release of intestinal (enteric) gastrin
Chemoreceptors detect fatty acids, & glucose in the duodenumStimulate enteric gastrin release
Stomach: Regulation of Secretion and MotilityIntestinal phase (cont.)InhibitoryEnterogastric reflex: stretch receptors, chemoreceptors trigger 3 reflexes thatInhibit vagoval reflexInhibit myenteric reflexActivate sympathetic nervous system to close pyloric sphincterInhibit gastric secretion
Enterogastrone secretionEnteroendocrine cells in the small intestine release:Cholecystokinin (CCK) Gastric inhibitory peptide (GIP) SecretinVasoactive intestinal peptide (VIP)Hormones inhibit gastric secretion
Stomach: Regulation of Gastric EmptyingFood normally passes through stomach in 4 hours
Hormonal/neuronal reflexes regulate gastric emptying
Large meals and large amounts of liquid increase stomach distension increasing rate of emptying
Stomach emptying inhibited by the enterogastric reflex, enterogastrones, and fat in the duodenum
StomachSummary
PancreasPancreatic juice 1.2-1.5 L/day
Mostly water some salts, bicarbonate, enzymesalkaline, pH 7.1-8.2buffers acidic gastric juice, stops pepsin activity, creates proper alkaline pH for enzymes acting in the intestine
Enzymes include:pancreatic amylasetrypsinogen, chymotrypsinogen, procarboxypeptidase (inactive zymogens)pancreatic lipaseribonuclease and deoxyribonuclease
Regulation of Pancreatic SecretionNeural control from parasympathetic division of ANS via vagus nerveAutoregulation by sensing the presence of fatty acids and amino acids in the acidic chymeHormonal control by the secretion of enteroendocrines from duodenumSecretin stimulates secretion of water, HCO3- CCK stimulates secretion of enzymes
Liver: Blood SupplyTwo sourcesHepatic artery - oxygenated blood from aorta
Hepatic portal vein -deoxygenated blood: absorbed nutrients and toxins from the stomach and intestines hormones from the pancreasbreakdown products of RBCs from the spleenBlood mixes in the sinusoids
Hepatocytes (liver cells) modify and exchange molecules with the blood
LiverNote the portal triads and fenestrated capillary sinusoidshepatic portal venous blood and arterial blood mix in the sinusoidsCentral veins return blood to the systemic circulation via the hepatic vein and inferior vena cava
LiverSinusoids are patrolled by monocyte-derived stellate reticuloendothelial (Kupffer's) cells
Phagocytize damaged blood cells, bacteria, and other microbes
Phagocytize some foreign molecules such as toxins
Hepatocytes can also store or breakdown toxic molecules
LiverHepatocytes Receive, process and store nutrients from meals
Process heme and cholesterol breakdown products to make bile components
Liver: Bile SecretionBile from the hepatocytes enters bile capillaries (canaliculi)
Canaliculi empty into small bile ducts
Hepatic ducts join the cystic duct from the gallbladder to form the common bile duct
Gallbladder stores bile
Common bile duct meets pancreatic duct at the hepatopancreatic ampulla (of Vater)
LiverBile800-1000 ml/day
Yellow, brownish, or olive-green liquid
pH 7.6-8.6, mostly water, bile salts, bile acids, cholesterol, lecithin (phospholipid), bile pigments, ions
Part digestive secretion, part excretory productbile salts help in emulsification of ingested fatsbilirubin and other bile pigments are wastes from lipid catabolism
Liver: Bile SynthesisRegulation of bile production/secretionnervous control from parasympathetic division of ANS via vagus nerve
autoregulation by sensing the resence of fatty acids and amino acids in the acidic chyme
hormonal control by the secretion of the enteroendocrines, CCK and secretin, from the duodenumCholecystokinin = gall bladder moves
LiverPhysiology of the liver processes vital to lifeCarbohydrate metabolism regulates blood glucose levelsglycogenesis (insulin)glycogenolysis (glucagon)gluconeogenesis (glucagon)Lipid metabolism - stores, metabolizes some triglyceridessynthesizes new cholesterol degrades excess cholesterol for bile salt productionProtein metabolism -deaminates AAs by removing amino groups (-NH2) from AAsdeaminated AA's used for ATP production or changed to carbohydrates or fats as neededdetoxifies ammonia (NH3) by synthesizing urea (1 CO2 + 2 NH3 = urea)can convert AA's from one to another (transamination)synthesizes and secretes most plasma proteins
LiverPhysiology of the liver - processes vital to life Storage oil-soluble vitamins, iron, other nutrients and minerals PhagocytosisRemoval of dietary toxins, hormones, drugsdetoxify or store or secrete compounds into bile metabolize thyroid, steroid hormonesSynthesis of bile saltsExcretion of bile - bilirubinActivation of Vitamin D (?)
Pathologies of the liver hepatitis (viral, toxic), cirrhosis, cancer
Gall BladderPear-shaped sac, 7-10 cm longPhysiology stores and concentrates bile between mealsCCK stimulates bile release for fatty mealswhen the small intestine is empty, the hepatopancreatic sphincter closes, forcing bile into the gallbladder for storagePathology:gallstones
Summary: Digestive HormonesenteroendocrinesGastrinGastric Inhibitory Peptide
SecretinCholecystokinin
(There are others.)
Small Intestine: Segmentationprimary action of small intestine when food is present
a form of mechanical digestion
a mixing activity
alternate contraction, relaxation of antagonistic smooth (circular and longitudinal) muscle segments in the intestine
controlled by the autonomic nervous system
Small Intestine: Peristalsisas absorption continues, distension decreases and true peristalsis starts
a movement which propels chyme onward
these weak movements which occur only after most nutrients have been absorbed
Small Intestine: Motility and SecretionIntestinal secretions 1-2 L/day, pH 7.6 mostly water and mucusbicarbonate buffer neutralizes gastric acidprovide enzymes for final chemical digestion
Regulation of intestinal secretion and motility stimulated by distension and acidic chymelocal reflexes increase Ach releaseVIP stimulates production of intestinal secretionsbasal motility is controlled by autorhythmic pacemakerslocal hormones and parasympathetic ANS reflexes increase motility
Small Intestine: Chemical DigestionIntestinal secretions 1-2 L/day, pH 7.6 mostly water and mucusalong with pancreatic secretions provide acid neutralization, final chemical digestion, and more water for absorption Brush border enzymesbrush border enzymes complete digestion of protein and carbohydrate molecules
Small Intestine: Chemical DigestionBrush border enzymesEnteropeptidase (enterokinase) converts trypsinogen to trypsinTrypsin activates other zymogens
Various other brush border enzymes complete digestion of protein and carbohydrate molecules
Small Intestine: Chemical DigestionComplete digestion is a function of bile, pancreatic secretions and intestinal secretionsAlthough produced by different organs, they all function in the small intestine
Prior to small intestine, only limited activitymouth salivary amylase stomachpepsin lingual lipase
Small Intestine: Chemical DigestionChemical digestion in the small intestine:Carbohydrate digestion pancreatic amylase digests starchesdisaccharidases liberate monosaccharides
Protein digestion pancreatic proteases (trypsin, chymotrypsin, carboxypeptidase)finished by brush border proteases in the lining epithelium
Lipid digestion bile salts for emulsification pancreatic lipase
Nucleic acid digestion pancreatic ribonuclease and deoxyribonucleasebrush border enzymes digest nucleotides
Small Intestine: AbsorptionAbout 90% of all absorption takes place in small intestine
Nutrient absorption
Monosaccharides: facilitated diffusion and Na+-driven secondary active transport
Amino acids: primary and Na+-driven secondary active transport
Di- and tripeptides: H+-driven secondary active transportNutrients enter capillaries via diffusion, facilitated diffusion, or active transport
Nutrients are transported in the blood to the liver via the hepatic portal circulation
Small Intestine: Nutrient AbsorptionElectrolytes (minerals)Na+: Primary active transportK+: facilitated diffusionFe: Active transportCa2+: Active transport, vitamin D is a cofactor
VitaminsWater-soluble vitamins (B complex & C) absorbed by diffusion - B12 absorbed with intrinsic factor Fat-soluble vitamins (A, D, E, K) included with other lipids in micelles/chylomicrons
Small Intestine: Nutrient AbsorptionLipids (neutral fats, cholesterol, phospholipids, etc.) are emulsified by bile salts, forming micellesPancreatic Lipase breaks triglycerides into 2 fatty acids and 1 monoglyceride
monoglycerides, fatty acids and other lipids diffuse into cells
SER re-synthesizes triglycerides
all lipids packaged into chylomicrons by Golgi apparatuschylomicrons leave the cell by exocytosis and enter lacteals of the lymphatic system
Small Intestine: Water AbsorptionTotal volume added to the small intestine/day - 9.3 L ~2.3 L from ingestion~7.0 L from secretions
Small intestine absorbs ~8.3 L /daypassive absorption following nutrient moleculesosmosis
The rest of the water (~1.0L/day) passes to large intestine where most is reabsorbed (~0.9 L/day)
Large IntestineFunctionsCompletion of absorption, especially final absorption of H2ONormal flora manufacture certain vitamins (B complex, K)Formation and expulsion of fecesAnatomy 1.5 m L, 6.5 cm WDivided into 4 general areas:cecumcolonrectum anal canal
Large Intestine: DigestionMechanical digestion Chyme passage regulated by ileocecal sphinctervalve generally closed - slow passagefollowing a meal gastroileal reflex: ileal motility increases, sphincter relaxes, chyme moves to the cecumwhen the cecum is full, the sphincter contractsColon movements start when chyme passes sphincterhaustral churning haustra relaxed, distended until full then contract, squeeze contents into next haustrumperistalsis is slowmass peristalsis (gastrocolic reflex) during or immediately following a meal, 3-4 times daystrong peristaltic waves from middle of transverse colon push contents into the rectum
Large Intestine: DigestionChemical digestion
much mucus but no enzymes are secreted
some digestion of chyme by bacteria in colon
final breakdown of substances, mostly carbohydrates
bacteria produce some vitamins, B complex and K
some bacterial metabolites are toxic, but the liver normally deals successfully with them
Large IntestineAbsorption and feces formation Chymeafter 3-10 hours in the large intestine, chyme becomes solidified (due to water reabsorption) into feceslarge intestine absorbs water, electrolytes, some vitamins and any toxinsFeces water, inorganic salts, sloughed off intestinal epithelial cells, bacteria, products of bacterial decomposition, undigested parts of foodmost water is reabsorbed in small intestine, but the large intestine is also important in water reabsorption
Large IntestinePhysiology of defecation Mass peristalsispushes fecal matter into rectumdistension stimulates stretch receptors initiating reflex for defecation
Parasympathetic ANS stimulated by stretch receptorsstimulates contraction of rectumshortens and increases pressure in rectumparasympathetic stimulation relaxes internal sphincter
Conscious stimulation relaxes external sphincterfeces expelled