Ch. 16: The Digestive System
Breakdown
• Understand that digestion is a process and how that process works
• Know the general anatomy of the digestive tract and it organs and accessory organs
• Know the physiology of digestion and absorption for the macromolecules as well as vitamins and minerals
Digestion
• What is digestion and why is it necessary?
• How is digestion accomplish?
Stages of Digestion• Ingestion
• Digestion– Mechanical and chemical
• Absorption
• Compaction
• Defecationhttp://www.ootonline.com/OOTnews/?p=1507
Digestive System
General Anatomy
• Structure: common layers through out the system
• Mucosa
• Submucosa
• Muscularis externa
• Serosa
Tissue Layers of GI Tract
Figure 25.2
Mucosa:
Lamina propriaMuscularis mucosae
Submucosa: Esophageal gland
Lumen
Blood vessels
Diaphragm
Esophageal hiatus
Myenteric plexus
Submucosal plexus
Muscularis externa: Inner circular layer
Outer longitudinal layer
Serosa
Enteric nervous system:
Stratified squamousepithelium
Parasympathetic ganglion ofmyenteric plexus
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Enteric Nervous System
• Regulates motility, secretion, blood flow
• Independent of CNS– CNS greatly influences
• Two networks– Submucosal plexus – Myenteric plexus
Regulation of Digestive Tract• Motility and secretion of the digestive tract are controlled by
neural, hormonal, and paracrine mechanisms
• Neural control– short (myenteric) reflexes – stretch or chemical stimulation acts through
myenteric plexus• stimulates parastaltic contractions of swallowing
– long (vagovagal) reflexes - parasympathetic stimulation of digestive motility and secretion
• Hormones– chemical messengers secreted into bloodstream– gastrin and secretin
• Paracrine secretions– chemical messengers that stimulate nearby target cells
Mouth
• Oral or buccal cavity
• Site of ingestion
• Teeth, tongue, saliva
• Mechanical and chemical digestionFigure 25.4
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The Teeth• Dentition – the teeth
• Masticate food into smaller pieces– makes food easier to swallow– exposes more surface area
• 32 adult teeth – 20 deciduous (baby) teeth– from midline to the rear of each jaw
• 2 incisors – chisel-like cutting teeth used to bite off a piece of food• 1 canine – pointed and act to puncture and shred food• 2 premolars – broad surface for crushing and grinding• 3 molars – even broader surface for crushing and grinding
Saliva• Saliva
– moisten mouth– begin starch and fat digestion– cleanse teeth– inhibit bacterial growth– dissolves molecules so they can stimulate the taste buds– moistens food and bind it together into bolus to aid in swallowing
• Hypotonic solution of 97.0% to 99.5% water and the following solutes:– salivary amylase – begins starch digestion in the mouth– lingual lipase – activated by stomach acid and digests fats– mucus – binds and lubricates the mass of food and aids in swallowing– lysozyme – enzyme that kills bacteria– immunoglobulin A (IgA) – an antibody that inhibits bacterial growth– electrolytes - Na+, K+, Cl-, phosphate and bicarbonate
• pH of 6.8 to 7.0
Esophagus• Esophagus – a straight muscular tube 25-30
cm long– extends from pharynx to cardiac orifice of
stomach – lower esophageal sphincter
• prevents stomach content regurgitation• heartburn – burning sensation produced by acid
reflux into the esophagus
– nonkeratinized stratified squamous epithelium– esophageal glands in submucosa secrete mucus– skeletal muscle in upper one-third, mixture in
middle one-third, and only smooth muscle in the bottom one-third
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Stomach• Stomach – muscular sac in upper left abdominal cavity
immediately inferior to the diaphragm– primarily functions as a food storage organ
• internal volume of about 50 mL when empty• 1.0 – 1.5 L after a typical meal• up to 4 L when extremely full and extend nearly as far as the pelvis
• Mechanically breaks up food particles, liquefies the food, and begins chemical digestion of protein and fat– chyme – soupy or pasty mixture of semi-digested food in
the stomach
Gross Anatomy of Stomach
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Microscopic Anatomy
• Mucosa – simple columnar epithelium
• Gastric pits – depressions in mucosa– Cardiac, pyloric and gastric
glands– Produce gastric juice
Figure 25.13 b-c
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Cells of Gastric Glands• Mucous cells – secrete mucus
– predominate in cardiac and pyloric glands
• Regenerative (stem) cells – found in the base of the pit and in the neck of the gland– divide rapidly and produce a continual supply of new
cells
• Parietal cells – found mostly in the upper half of the gland– secrete hydrochloric acid (HCl), intrinsic factor,
hunger hormone ghrelin
• Chief cells – most numerous– secrete gastric lipase and pepsinogen– absent in pyloric and cardiac glands
• Enteroendocrine cells – concentrated in lower end of gland– secrete hormones and paracrine messengers that
regulate digestion Figure 25.13c
(c) Gastric gland
G cell
Mucous neck cell
Parietal cell
Chief cell
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Hydrochloric Acid
• gastric juice has a high concentration of hydrochloric acid– pH as low as 0.8
• parietal cells produce HCl and contain carbonic anhydrase (CAH) CAH– CO2 + H2O H2CO3 HCO3
- + H+
– H+ is pumped into gastric gland lumen by H+- K+ ATPase pump• antiporter uses ATP to pump H+ out and K+ in
– HCO3- exchanged for Cl- (chloride shift) from blood plasma
• Cl- (chloride ion) pumped into the lumen of gastric gland to join H+ forming HCl• elevated HCO3
- (bicarbonate ion) in blood causes alkaline tide increasing blood pH
Figure 25.14
Blood Parietal cell Lumen of gastric gland
HCO3–
Cl–
H2CO3
H+
K+
Cl–
CO2
HCO3–
H2OCO2 +
H+–K+ ATPase
Stomachacid
Alkalinetide
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Functions of Hydrochloric Acid• activates pepsin and lingual lipase
• breaks up connective tissues and plant cell walls– helps liquefy food to form chyme
• converts ingested ferric ions (Fe3+) to ferrous ions (Fe2+)– Fe2+ absorbed and used for hemoglobin synthesis
• contributes to nonspecific disease resistance by destroying most ingested pathogens
Production and Action of Pepsin
Figure 25.15
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HCl
Parietal cell
Chief cell
Gastric gland
Pepsin(active enzyme)
Pepsinogen(zymogen)
Removedpeptide
Dietaryproteins
Partially digestedprotein
Gastric Secretions
• Gastric lipase– Combines with lingual lipase– Begin to digest small amount of fats
• Intrinsic factor– Secreted by the parietal cells– Required for vitamin B12 absorption– Only essential function of stomach
Stomach Rumbles
• Digestion and Absorption– Partial digestion of proteins and lipids– Absorption of aspirin and lipid-soluble drugs
• Protection – Mucus– Tight junctions– Cell replacement
Regulation of Gastric Function• Cephalic phase
– Stomach responds to sensory and mental info
• Gastric phase– Occurs after ingestion and deglutition– Stimulate gastric activity (secretion)
• Ach, Histamine, Gastrin– Act on parietal, chief, enteroendocrine cells
– Positive and negative feedback loops
• Intestinal phase– Initially stimulates gastric secrestions– Enterogastric reflex- inhibitory signals – Secretion of secretin and cholecystokinin
• Suppress gastric secretion and motility
Accessory Organs• Liver
– Produces and secretes bile
• Gall bladder– Stores and concentrates bile– Bile
• Contains cholesterol• Bile acids and lecithin help digest fat• 80% of bile acids reabsorbed in ilieum
• Pancreas– Exocrine tissue secretes pancreatic juice– Mixture of zymogens, enzymes, NaCO3, and water– Zymogens include: trypsinogen, chymotrypsinogen, procarboxypeptidase– Enzyms include: lipase, amylase, ribonuclease, deoxyribonuclease– Regulation
• Ach, CCK, Secretin
Small Intestine• Digestion and absorption completed
• Duodenum– Pancreatic enzyme digestion
• Jejunum– Highly vascularized– Absorption
• Ileum– Less vascularized– Absorption– Ileocecal valve
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Intestinal Motility
• Segmentation
• Peristalsis– Migrating motor
complex
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Large Intestine
• Begins with cecum, 1.5m long
• Ascending, transverse, descending, sigmoid
• Rectum
• AnusCopyright © The McGraw-Hill Companies, Inc.
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Carbohydrate Digestion and Absorption
• Digestible dietary form is starch
• Primary enzyme – amylase– Digests starch into
oligosaccharides
• Secondary enzymes – dextrinase, glucoamylase, maltase, sucrase, lactase
Protein Digestion and Absorption• Three sources
– Dietary– Digested enzymes– Sloughed epithelial cells
• Digested by proteases– Pepsin (cleaves between tyrosine and phenylalanine)
• Stomach– Trypsin, chymotrypsin, carboxypeptidase
• Pancreatic secretions– aminopeptidase, dipeptidase
• Brush border enzymes
• Absorbed like carbohydrates
Figure 25.29
H N H
OC
HO
HN H
OC
HO
No chemical digestion occurs.
OC
HO
H N H
OC
HO
HN
H
OC
OH
H
HN
H
HN
OC
OH
OC
HO
HN H
OC
HO
HN H
OC
OH
H
HN
H
HN
OC
OH
OC
HO N
HN
H
CO OH
H
HN O
COH
H
HN
OC
OH
OC
OHH
HNH
N H
CO OH
OC
OH
H
HN
OC
OHH
HN
OC
OH
H
HN
OC
OH
H
HN O
COH
H
HN O
COH
H
HN
OC
OH
H
HN
OC
OH
H
HN O
COH
H
HN
Mouth
Stomach
Carboxypeptidase Aminopeptidase Dipeptidase
Protein
Protein
Polypeptides
Polypeptides Oligopeptides
Pepsin ( ) hydrolyzes certain peptide bonds, breaking protein down into smaller polypeptides.
Trypsin ( ) and chymotrypsin ( )hydrolyze other peptide bonds, breakingpolypeptides down into smalleroligopeptides.
Carboxypeptidase ( ) removes one amino acid at a time from the carboxyl (–COOH) end of an oligopeptide.
Carboxypeptidase ( ) of the brush border continues to remove amino acids from the carboxyl (–C OOH) end.
Aminopeptidase ( ) of the brush borderremoves one amino acid at a time from the amino (–N H ) end.
Dipeptidase ( ) splits dipeptides ( )into separate amino acids ( ).
Small intestineActions of brush border enzymes (contact digestion)
Blood capillaryof intestinal villus
Small intestineActions of pancreatic enzymes
Protein Digestionand Absorption
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Lipid Digestion and Absorption• Digested by lipases
• Emulsification
• Micelles– Transport to brush border
• FFAs, monoglycerides, fat-soluable vitamins, cholesterol• Resynthesized
– Chylomicrons and exocytosis
• Enter lacteals
Emulsification
Figure 25.30
Emulsification
Bile acid
Lecithin Hydrophilic regionHydrophobic region
Fat globule
Emulsificationdroplets
Fat globule is broken up and coated bylecithin and bile acids.
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Fat Hydrolysis and Micelles
Figure 25.30
Fat hydrolysis
Lipid uptake by micelles
Pancreatic lipasePancreatic lipase
Monoglyceride
Free fatty acid
Free fatty acid
Lecithin
Bile acid
Dietary lipid
Lipid core
MicellesFatty acids Fat-soluble vitamins
Bile acid
Monoglycerides Cholesterol
Emulsification droplets are acted upon bypancreatic lipase, which hydrolyzes thefirst and third fatty acids from triglycerides,usually leaving the middle fatty acid.
Micelles in the bile pass to the smallintestine and pick up several types ofdietary and semidigested lipids.
Triglyceride
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Nucleic Acids and Vitamins • Nucleic acid
– nucleases (deoxyribonuclease and ribonuclease) hydrolyze DNA and RNA to nucleotides
– nucleosidases and phosphatases of brush border split them into phosphate ions, ribose or deoxyribose sugar, and nitrogenous bases
• Vitamins – are absorbed unchanged– fat-soluble vitamins - A, D, E and K absorbed with other
lipids• if they are ingested without fat-containing food, they are not absorbed at
all, but are passed in the feces and wasted
– water-soluble vitamins, B complex and C, absorbed by simple diffusion and B12 if bound to intrinsic factor from the stomach
Minerals
• minerals (electrolytes)– are absorbed all along small intestine
– Na+ cotransported with sugars and amino acids
– Cl- exchanged for bicarbonate reversing chloride-bicarbonate exchange that occurs in the stomach
– iron and calcium absorbed as needed• iron absorption is stimulated by liver hormone hepcidin• absorptive cells bind ferrous ions (Fe2+) and internalize by active
transport• unable to absorb ferric ions (Fe3+) but stomach acid reduces ferric
ions to absorbable ferrous ions• transferrin (extracellular protein) transports iron in blood to bone
marrow, muscle, and liver
Water Balance• Digestive system is one of several systems involved in water
balance• Digestive tract receives about 9 L of water/day
– 0.7 L in food, 1.6 L in drink, 6.7 L in gastrointestinal secretions– 8 L is absorbed by small intestine and 0.8 L by large intestine– 0.2 L voided in daily fecal output
• Water is absorbed by osmosis following the absorption of salts and organic nutrients
• Diarrhea - occurs when large intestine absorbs too little water – feces pass through too quickly if intestine is irritated– feces contains high concentrations of a solute (lactose)
• Constipation – occurs when fecal movement is slow, too much water gets reabsorbed, and feces becomes hardened
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