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