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Chapter 18. The Digestive System. 18-1. Chapter 18 Outline Functions of GI Tract Structure of Digestive System From Mouth to Stomach Stomach Small Intestine Large Intestine Liver Gall Bladder & Pancreas Control & Phases of Digestion Digestion & Absorption of Food Types. 18-2.

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Chapter 18

The Digestive System


Chapter 18 Outline

  • Functions of GI Tract

  • Structure of Digestive System

  • From Mouth to Stomach

  • Stomach

  • Small Intestine

  • Large Intestine

  • Liver

  • Gall Bladder & Pancreas

  • Control & Phases of Digestion

  • Digestion & Absorption of Food Types



  • Inside gastrointestinal (GI) tract, food is broken down by hydrolysis into molecular monomers

  • Absorption of monomers occurs in small intestine


Fig 18.1



  • Is movement of food through GI tract by means of:

    • Ingestion--taking food into mouth

    • Mastication--chewing food & mixing it with saliva

    • Deglutition--swallowing food

    • Peristalsis--rhythmic wave-like contractions that move food through GI tract



  • Includes release of exocrine & endocrine products into GI tract

  • Exocrine secretions include: HCl, H20, HC03-, bile, lipase, pepsin, amylase, trypsin, elastase, & histamine

  • Endocrine includes hormones secreted into stomach & small intestine to help regulate GI system

    • E.g. gastrin, secretin, CCK, GIP, GLP-1, guanylin, VIP, & somatostatin



  • Is passage of digested end products into blood or lymph


Storage and elimination
Storage and Elimination

  • Includes temporary storage & subsequent elimination of indigestible components of food


Digestive system
Digestive System

  • Is composed of GI tract (alimentary canal) & accessory digestive organs

  • GI tract is 30 ft long; extends from mouth to anus


Digestive system continued
Digestive System continued

Fig 18.2

  • Organs include oral cavity, pharynx, esophagus, stomach, & small & largeintestine

  • Accessory organs include teeth, tongue, salivary glands, liver, gallbladder, & pancreas


Layers of gi tract
Layers of GI Tract

  • Are called tunics

  • The 4 tunics are mucosa, submucosa, muscularis, & serosa

Fig 18.3



  • Is the absorptive & secretory layer lining lumen of GI tract

  • In places is highly folded with villi to increase absorptive area

  • Contains lymph nodules, mucus-secreting goblet cells, & thin layer of muscle

Fig 18.3



  • Is a thick, highly vascular layer of connective tissue where absorbed molecules enter blood & lymphatic vessels

  • Contains glands & nerve plexuses (submucosal plexus) that carry ANS activity to muscularis mucosae

Fig 18.3



  • Is responsible for segmental contractions & peristaltic movement through GI tract

  • Has an inner circular & outer longitudinal layer of smooth muscle

    • Activity of these layers moves food through tract while pulverizing & mixing it

    • Myenteric plexus between these layers is major nerve supply to GI tract

      • Includes fibers & ganglia from both Symp & Parasymp systems



  • Is outermost layer; serves to bind & protect

  • Consists of areolar connective tissue covered with layer of simple squamous epithelium

Fig 18.3


Regulation of gi tract
Regulation of GI Tract

  • Parasympathetic effects, arising from vagus & spinal nerves, stimulate motility & secretions of GI tract

  • Sympathetic activity reduces peristalsis & secretory activity

  • GI tract contains an intrinsic system that controls its movements--the enteric nervous system

  • GI motility is influenced by paracrine & hormonal signals


From mouth to stomach1
From Mouth to Stomach

  • Mastication (chewing) mixes food with saliva which contains salivary amylase

    • An enzyme that catalyzes partial digestion of starch


From mouth to stomach continued
From Mouth to Stomach continued

  • Deglutition (swallowing) begins as voluntary activity

    • Oral phase is voluntary & forms a food bolus

    • Pharyngeal & esophagealphases are involuntary & cannot be stopped

    • To swallow, larynx is raised so that epiglottis covers entrance to respiratory tract

    • A swallowing center in medulla orchestrates complex pattern of contractions required for swallowing


From mouth to stomach continued1
From Mouth to Stomach continued

  • Esophagus connects pharynx to stomach

    • Upper third contains skeletal muscle

    • Middle third contains mixture of skeletal & smooth

    • Terminal portion contains only smooth

    • Passes through diaphragm via esophageal hiatus


From mouth to stomach continued2
From Mouth to Stomach continued

  • Peristalsis propels food thru GI tract

    • = wave-like muscular contractions

    • After food passes into stomach, the gastroesophageal sphincter constricts, preventing reflux

Fig 18.4






  • Is most distensible part of GI tract

  • Empties into the duodenum

  • Functions in: storage of food; initial digestion of proteins; killing bacteria with high acidity; moving soupy food mixture (chyme) into intestine


Stomach continued
Stomach continued

  • Is enclosed by gastroesophageal sphincter on top & pyloric sphincter on bottom

  • Is divided into 3 regions:

    • Fundus

    • Body

    • Antrum

Fig 18.5


Stomach continued1
Stomach continued

  • Inner surface of stomach is highly folded into rugae

  • Contractions of stomach churn chyme, mixing it with gastric secretions

    • Eventually these will propel food into small intestine

Fig 18.5


Stomach continued2
Stomach continued

Fig 18.7

  • Gastric mucosa has gastric pits in its folds

  • Cells that line folds deeper in the mucosa, are exocrine gastric glands


Stomach continued3
Stomach continued

  • Gastric glands contain cells that secrete different products that form gastric juice

    • Goblet cells secrete mucus

    • Parietal cells secrete HCl & intrinsic factor (necessary for B12 absorption in intestine)

    • Chief cells secrete pepsinogen (precursor for pepsin)

Fig 18.7


Stomach continued4
Stomach continued

  • Enterochromaffin-like cells secrete histamine & serotonin

  • G cells secrete gastrin

  • D cells secrete somatostatin

Fig 18.7


Hcl in stomach
HCl in Stomach

Fig 18.8

  • Is produced by parietal cells which AT H+ into lumen via an H+/ K+ pump (pH ≈1)

  • Cl- is secreted by facilitated diffusion

  • H+ comes from dissociation of H2CO3

  • Cl- comes from blood side of cell in exchange for HC03-


Hcl in stomach continued
HCl in Stomach continued

  • Is secreted in response to the hormone gastrin; & ACh from vagus

    • These are indirect effects since both stimulate release of histamine which causes parietal cells to secrete HCl


Hcl in stomach continued1
HCl in Stomach continued

  • Makes gastric juice very acidic which denatures proteins to make them more digestible

  • Converts pepsinogen into pepsin

    • Pepsin is more active at low pHs

Fig 18.9


Digestion absorption in stomach
Digestion & Absorption in Stomach

  • Proteins partially digested by pepsin

  • Carbohydrate digestion by salivary amylase is soon inactivated by acidity

  • Alcohol & aspirin are only commonly ingested substances absorbed


Gastric and peptic ulcers
Gastric and Peptic Ulcers

  • Peptic ulcers are erosions of mucous membranes of stomach or duodenum caused by action of HCl

  • In Zollinger-Ellison syndrome, duodenal ulcers result from excessive gastric acid in response to high levels of gastrin

  • Helicobacter pylori infection is associated with ulcers

    • Antibiotics are useful in treating ulcers

  • Acute gastritis is an inflammation that results in acid damage due to histamine released by inflammation

    • Why histamine receptor blockers such as Tagamet & Zantac can treat gastritis


Protective mechanisms of stomach
Protective Mechanisms of Stomach

  • Include:

    • Impermeability of parietal & chief cells to HCl

    • A layer of alkaline mucus containing HC03-

    • Tight junctions between adjacent epithelial cells

    • Rapid rate of cell division (entire epithelium replaced in 3 days)

    • Prostaglandins (PGs) inhibit gastric secretions

      • Which is why PG blockers such as NSAIDs can cause ulcers


Small intestine si
Small Intestine (SI)

  • Is longest part of GI tract; approximately 3m long

  • Duodenum is 1st 25cm after pyloric sphincter

  • Jejunum is next 2/5s

  • Ileum is last 3/5s; empties into large intestine

Fig 18.10


Small intestine si continued
Small Intestine (SI) continued

  • Absorption of digested food occurs in SI

  • Facilitated by long length & tremendous surface area


Small intestine si continued1
Small Intestine (SI) continued

Fig 18.10

  • Surface area increased by foldings & projections

  • Large folds are plicae circulares

  • Microscopic finger-like projections are villi

  • Apical hair-like projections are microvilli


Small intestine si continued2
Small Intestine (SI) continued

  • Each villus is covered with columnar epithelial cells interspersed with goblet cells

  • Epithelial cells at tips of villi are exfoliated & replaced by mitosis in crypts of Lieberkuhn

  • Inside each villus are lymphocytes, capillaries, & central lacteal

Fig 18.12


Small intestine si continued3
Small Intestine (SI) continued

  • A carpet of hair-like microvilli project from apical surface of each epithelial cell

    • Create a brush border


Intestinal enzymes
Intestinal Enzymes

  • Attached to microvilli are brush border enzymes that are not secreted into lumen

    • Enzyme active sites are exposed to chyme


Intestinal contractions and motility
Intestinal Contractions and Motility

Fig 18.14

  • 2 major types of contractions occur in SI:

    • Peristalsis is weak & slow & occurs mostly because pressure at pyloric end is greater than at distal end

    • Segmentation is major contractile activity of SI

      • Is contraction of circular smooth muscle to mix chyme


Large intestine li or colon
Large Intestine (LI) or Colon

  • Has no digestive function but absorbs H20, electrolytes, B & K vitamins, & folic acid

  • Internal surface has no villi or crypts & is not very elaborate

  • Contains large population of microflora

    • LI bacteria produce folic acid & vitamin K & ferment indigestible food to produce fatty acids


Large intestine li or colon continued
Large Intestine (LI) or Colon continued

  • Extends from ileocecal valve at end of SI to anus

  • Outer surface bulges to form pouches (haustra)

  • Chyme from SI enters cecum, then passes to ascending colon, transverse colon, descending colon, sigmoid colon, rectum, & anal canal

Fig 18.17


Fluid electrolyte absorption in li
Fluid & Electrolyte Absorption in LI

  • SI absorbs most water but LI absorbs 90% of water it receives

    • Begins with osmotic gradient set up by Na+/K+ pumps

      • Water follows by osmosis

  • Salt & water reabsorption stimulated by aldosterone



  • After electrolytes & water have been absorbed, waste material passes to rectum, creating urge to defecate

  • Defecation reflex begins with relaxation of external anal sphincter allowing feces to enter anal canal

    • Longitudinal rectal muscles contract to increase rectal pressure; internal anal sphincter relaxes

    • Excretion is aided by contractions of abdominal & pelvic muscles which push feces from rectum





Structure of liver
Structure of Liver

  • Liver largest internal organ

  • Hepatocytes form hepatic plates that are 1–2 cells thick

    • Plates separated by sinusoids which are fenestrated & permeable even to proteins

      • Contain phagocytic Kupffer cells

Fig 18.20


Hepatic portal system
Hepatic Portal System

  • Food absorbed in SI is delivered 1st to liver

  • Capillaries in digestive tract drain into hepatic portal vein which carries blood to liver

  • Hepatic vein drains liver

  • Liver also receives blood from hepatic artery


Liver lobules
Liver Lobules

  • Are functional units formed by hepatic plates

  • In middle of each is central vein

  • At edge of each lobule are branches of hepatic portal vein & artery which open into sinusoids

Fig 18.20


Liver lobules1
Liver Lobules

  • Bile is secreted by hepatocytes in bile canaliculi

    • Empty into bile ducts which flow into hepatic ducts that carry bile away from liver

Fig 18.21


Enterohepatic circulation
Enterohepatic Circulation

  • Is recirculation of compounds between liver & intestine

  • Many compounds are released in bile, reabsorbed in SI, & returned to liver to be recycled

  • Liver excretes drug metabolites into bile to pass out in feces

Fig 18.22


Bile production and secretion
Bile Production and Secretion

  • Amounts to 250–1500 ml/day

  • Bile pigment (bilirubin) is produced in spleen, bone marrow, & liver

    • Is a derivative of heme groups (minus iron) from Hb

    • Carried in blood attached to albumin

  • Free bilirubin combines with glucuronic acid to form conjugated bilirubin that is secreted into bile

    • Converted by intestinal bacteria to urobilinogen

    • 30-50% of urobilogen is absorbed by intestine & enters hepatic vein

      • Thus enters enterohepatic circulation to be recycled or filtered by kidneys & excreted in urine


Bile acids
Bile Acids

  • Are formed in major breakdown pathway for cholesterol

  • Are mostly cholic & chenodeoxycholicacids

    • Form bile salts by combining with glycine or taurine

      • Bile salts aggregate as micelles

  • 95% of bile acids are absorbed by ileum

Fig 18.25


Detoxification of blood
Detoxification of Blood

  • Liver can remove hormones, drugs, & other biologically active molecules from blood by:

    • Excretion into bile

    • Phagocytosis by Kupffer cells

    • Chemical alteration of molecules

      • E.g. ammonia is produced by deamination of amino acids in liver

        • Liver converts it to urea which is excreted in urine


Detoxification of blood continued
Detoxification of Blood continued

  • Liver conjugates steroid hormones with groups that make them anionic

    • Which can be transported into bile or urine by multispecific organic anion transport carriers & excreted

    • Cytochrome P450 enzymes are involved in hepatic metabolism of steroids & drugs


Secretion of glucose triglycerides ketones
Secretion of Glucose, Triglycerides & Ketones

  • Liver helps regulate blood glucose by removing it from blood or releasing it to blood

    • Removes it via glycogenesis & lipogenesis

    • Or produces it via glycogenolysis & gluconeogenesis

  • Can convert free fatty acids into ketone bodies (ketogenesis) that can be used for energy during fasting


Production of plasma proteins
Production of Plasma Proteins

  • Albumin & most of plasma globulins are produced by liver

  • Albumin makes up 70% of total plasma protein & contributes most to colloid osmotic pressure of blood

  • Globulins transport cholesterol & hormones, inhibit trypsin, & are involved in blood clotting

    • Constitute many of the clotting factors



  • Is a sac-like organ attached to inferior surface of liver

  • Stores & concentrates bile continuously produced by liver

    • When SI is empty, sphincter of Oddi in common bile duct closes & bile is forced into gallbladder

      • Expands as it fills with bile

    • When food is in SI, sphincter of Oddi opens, gall bladder contracts, & bile is ejected thru cystic duct into common bile duct then to duodenum

Fig 18.26



  • Is located behind stomach

  • Has both endocrine & exocrine functions

    • Endocrine function performed by islets of Langerhans

      • Secretes insulin & glucagon

    • Exocrine secretions include bicarbonate solution & digestive enzymes

      • These pass in pancreatic duct to SI

      • Exocrine secretory units are acini


Fig 18.26


Pancreatic juice
Pancreatic Juice

  • Contains water, bicarbonate, & digestive enzymes

  • Digestive enzymes include amylase for starch, trypsin for proteins, and lipase for fats

    • Brush border enzymes are also required for complete digestion


Pancreatic juice1
Pancreatic Juice

  • Most pancreatic enzymes are produced in inactive form (zymogens)

    • Trypsin is activated by brush border enzyme, enterokinase

    • Trypsin activates other zymogens

Fig. 18.29

Fig 18.30


Neural and endocrine regulation
Neural and Endocrine Regulation

  • Neural & endocrine mechanisms modify activity of GI system

    • Vagus nerve is heavily involved in regulating & coordinating digestive activities

    • GI tract is both an endocrine gland & target for action of hormones

      • Hormones include secretin, gastrin, CCK, & GIP


Regulation of gastric function
Regulation of Gastric Function

  • Gastric motility & secretion occur automatically

  • Waves of contraction are initiated spontaneously by pacesetter cells & secretion occurs in absence of hormonal & neural input

    • ANS & hormonal effects are superimposed on automatic activity

  • Extrinsic control of gastric function is divided into cephalic, gastric, & intestinal phases


Cephalic phase
Cephalic Phase

  • Refers to control by brain of vagus activity

  • Stimulated by sight, smell, & taste of food

  • Activation of vagus:

    • Stimulates chief cells to secrete pepsinogen

    • Directly stimulates G cells to secrete gastrin

    • Directly stimulates ECL cells to secrete histamine

    • Indirectly stimulates parietal cells to secrete HCl

  • Continues into 1st 30 min of a meal


Gastric phase
Gastric Phase

  • Arrival of food in stomach stimulates gastric phase

  • Gastric secretion stimulated by distension of stomach & chemical nature of chyme


Gastric phase continued
Gastric Phase continued

Fig 18.31

  • Short polypeptides & amino acids stimulate G cells to secrete gastrin & chief cells to secrete pepsinogen

    • Gastrin stimulates ECL cells to secrete histamine which stimulates parietal cell secretin of HCl

    • This is a positive feedback mechanism: As more HCl & pepsinogen are secreted, more polypeptides & amino acids are released


Gastric phase continued1
Gastric Phase continued

Fig 18.31

  • Secretion of HCl is also regulated by a negative feedback mechanism:

    • HCl secretion decreases if pH < 2.5; at pH 1 gastrin secretion stops

    • D cells stimulate secretion of somatostatin which inhibits gastrin secretion


Intestinal phase
Intestinal Phase

  • Begins with inhibition of gastric activity when chyme enters SI

  • Arrival of chyme in SI is detected by sensory neurons of vagus

    • This causes a neural reflex that inhibits gastric motility & secretion

    • Fat in chyme stimulates SI to secrete enterogasterones--hormones that inhibit gastric motility & secretion

      • Enterogasterones include somatostatin, CCK, & GLP-1


Enteric nervous system
Enteric Nervous System

  • Submucosal & myenteric plexuses contain as many neurons as spinal cord

  • Includes preganglionic Parasymp axons, ganglion cell bodies, postganglionic Symp axons; & afferent intrinsic & extrinsic sensory neurons; interneurons, & glia

  • Peristalsis is controlled by enteric NS


Enteric nervous system continued
Enteric Nervous System continued

Fig 18.32

  • For peristalsis:

  • ACh & substance P stimulate smooth muscle contraction above bolus

  • NO, VIP, & ATP stimulate smooth muscle relaxation below bolus

Insert fig. 18.31


Paracrine regulators of intestine
Paracrine Regulators of Intestine

  • ECL cells release serotonin & motilin in response to pressure & chemical stimuli in SI

    • Serotonin stimulates intrinsic afferents which activate motor neurons in intrinsic NS

    • Motilin stimulates contraction in duodenum & stomach antrum

  • Guanylin, from ileum & colon, stimulates production of cGMP which inhibits absorption of Na+& causes secretion of Cl- & H20


Intestinal reflexes
Intestinal Reflexes

  • Can be mediated by enteric NS & paracrines; & regulated by ANS & hormones

  • Gastroileal reflex refers to increased motility of ileum & movement of chyme thru ileocecal sphincter in response to increased gastric activity

  • Ileogastric reflex decreases gastric motility in response to distension of ileum

  • Intestino-intestinal reflex causes relaxation of rest of intestine when any part is overdistended


Secretion of pancreatic juice
Secretion of Pancreatic Juice

  • Secretion of pancreatic juice & bile is stimulated by secretin & bile

  • Secretin is secreted in response to duodenal pH < 4.5

    • Stimulates release of HC03- into SI by pancreas & into bile by liver

  • CCK is secreted in response to fat & protein content of chyme in duodenum

    • Stimulates production of pancreatic enzymes

    • Enhances secretin

    • Stimulates contraction of sphincter of Oddi


Digestion absorption of carbohydrates
Digestion & Absorption of Carbohydrates

Fig 18.33

  • Most carbohydrates are ingested as starch--a polymer of glucose

  • Salivary amylase begins starch digestion

  • Pancreatic amylase converts starch to oligosaccharides

    • Oligosaccharides hydrolyzed by SI brush border enzymes


Digestion absorption of protein
Digestion & Absorption of Protein

  • Begins in stomach when pepsin digests proteins to form polypeptides

  • In SI, endopeptidases (trypsin, chymotrypsin, elastase) cleave peptide bonds in interior of polypeptides

  • SI exopeptidases (carboxypeptidase, aminopeptidase) cleave peptide bonds from ends of polypeptides


Digestion absorption of protein continued
Digestion & Absorption of Protein continued

  • Protein digestion in SI results in free amino acids, dipeptides, & tripeptides

    • Which are transported into SI cells where di- & tripeptides are broken down to amino acids

    • Which are secreted into blood

Fig 18.34


Digestion absorption of lipids
Digestion & Absorption of Lipids

  • Occurs in SI

  • Arrival of lipids in duodenum causes secretion of bile

  • Fat is emulsified by bile salt micelles

    • Forms tiny droplets of fat dissolved in bile salt micelles

      • Greatly increases surface area for fat digestion


Digestion absorption of lipids continued
Digestion & Absorption of Lipids continued

  • Pancreatic lipase hydrolyzes triglycerides to free fatty acids & monglycerides

  • Phospholipase A breaks down phospholipids into fatty acids & lysolecithin

Fig 18.35


Digestion absorption of lipids continued1
Digestion & Absorption of Lipids continued

  • Products of fat digestion dissolve in micelles forming mixed micelles which move to brush border

Fig 18.36


Digestion absorption of lipids continued2
Digestion & Absorption of Lipids continued

  • Free fatty acids, monoglycerides, & lysolecithin leave micelles & enter epithelial cells

    • Inside epithelial cells, they are resynthesized into triglycerides & phospholipids

Fig 18.37


Digestion absorption of lipids continued3
Digestion & Absorption of Lipids continued

  • Triglycerides & phospholipids combine with a protein to form small particles called chylomicrons

    • Which are secreted into central lacteals of SI villi

Fig 18.37


Transport of lipids
Transport of Lipids

  • In blood, endothelial lipoprotein lipase hydrolyzes triglycerides to free fatty acids & glycerol for use in cells

  • Cholesterol-containing remnants are taken up by liver


Transport of lipids continued
Transport of Lipids continued

  • Cholesterol & triglycerides from liver form VLDLs which are secreted & take triglycerides to cells

    • Once triglycerides are removed, VLDLs become LDLs

      • LDLs transport cholesterol to organs & blood vessels

    • HDLs transport excess cholesterol back to liver

      • High ratio of HDL-cholesterol to total cholesterol is believed to confer protection against atherosclerosis


  • Exit Quiz 11

  • 1. True/False: Bile is produced in the gall bladder.

  • Which structure increases surface area of the intestine? Plicae/villi/microvilli

  • True/False: The large intestine is the last part of the digestive system to digest food stuffs before elimination.