Chapter 24: The Digestive System

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Chapter 24 The Digestive System
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Organisms

• Need to acquire nutrients from environment
• Metabolism
• Anabolism
• Uses raw materials to synthesize essential
  compounds (ex cell membrane lipids)
• Catabolism
• Decomposes organic molecules to provide energy
  (ATP) that cells need to function
• Usually requires oxygen

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Components of the Digestive System
Figure 241
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Digestive Tract

• Gastrointestinal (GI) tract or alimentary canal
• Is a muscular tube
• Extends from oral cavity to anus

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6 Functions of the Digestive System

• Ingestion
• occurs when materials enter digestive tract via
  the mouth
• Mechanical processing
• crushing and shearing
• makes materials easier to propel along digestive
  tract

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6 Functions of the Digestive System

• Digestion
• is the chemical breakdown of food into small
  organic fragments for absorption by digestive
  epithelium
• Secretion
• is the release of water, acids, enzymes, buffers,
  and salts
• by epithelium of digestive tract
• by glandular organs

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6 Functions of the Digestive System

• Absorption
• movement of organic substrates, electrolytes,
  vitamins, and water across digestive epithelium
  into interstitial fluid of digestive tract
• Excretion
• removal of waste products from body fluids

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Smooth Muscle

• Along digestive tract
• has rhythmic cycles of activity
• controlled by pacesetter cells
• Cells undergo spontaneous depolarization
• triggering wave of contraction through entire
  muscular sheet

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Peristalsis
Figure 244
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Peristalsis

• Consists of waves of muscular contractions
• Circular and longitudinal
• Moves a bolus along the length of the digestive
  tract

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Peristaltic Motion

• Circular muscles contract behind bolus
• while circular muscles ahead of bolus relax
• Longitudinal muscles ahead of bolus contract
• shortening adjacent segments
• Wave of contraction in circular muscles
• forces bolus forward

12
Segmentation

• Cycles of contraction
• Churn and fragment bolus
• mix contents with intestinal secretions
• Does not follow a set pattern
• does not push materials in any 1 direction

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The Regulation of Digestive Activities
Figure 245
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Neural Mechanisms

• Control
• movement of materials along digestive tract
• secretory functions
• Motor neurons
• control smooth muscle contraction and glandular
  secretion
• located in myenteric plexus

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Digestive Hormones

• At least 18 hormones that affect most aspects of
  digestive function
• Are produced by enteroendocrine cells in
  digestive tract
• Reach target organs after distribution in
  bloodstream

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The Oral Cavity
Figure 246
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4 Functions of the Oral Cavity

• Sensory analysis
• of material before swallowing
• Mechanical processing
• through actions of teeth, tongue, and palatal
  surfaces
• Lubrication
• mixing with mucus and salivary gland secretions
• Limited (chemical) digestion
• of carbohydrates and lipids

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The Epithelial Lining

• Of cheeks, lips, and inferior surface of tongue
  is nonkeratinized, and delicate
• Mucosa under tongue and inside cheeks is thin and
  vascular enough to rapidly absorb lipid-soluble
  drugs

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Lingual Papillae

• Fine projections on superior surface (dorsum) of
  tongue
• Covered in thick epithelium
• Assists in moving materials

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Lingual Glands

• Small glands extend into underlying lamina
  propria
• Secretions flush tongues epithelium
• Contain water, mucins, and enzyme lingual lipase

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Salivary Glands

• 3 pairs secrete products into oral cavity
• Each pair has distinctive cellular organization
  and produces saliva with different properties
• Produce 1.01.5 liters of saliva each day

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Saliva

• 99.4 water
• 0.6 includes
• electrolytes (Na, Cl, and HCO3)
• buffers
• glycoproteins (mucins)
• antibodies
• Enzymes (salivary amylase)
• waste products

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Control of Salivary Secretions

• By autonomic nervous system
• parasympathetic and sympathetic innervation
• _______________ stimulation accelerates secretion
  by all salivary glands

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The Swallowing Process
Figure 2411
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Swallowing

• Also called deglutition (2,400 X a day)
• Can be initiated voluntarily
• Proceeds automatically
• Is divided in 3 phases
• buccal phase
• pharyngeal phase
• esophageal phase

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Primary Peristaltic Waves

• Movements coordinated by afferent and efferent
  fibers in glossopharyngeal and vagus nerves
• Controlled by swallowing center of medulla
  oblongata

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The Stomach Lining
Figure 2413
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4 Functions of the Stomach

• Storage of ingested food
• Mechanical breakdown of ingested food
• (Preliminary) Disruption of chemical bonds in
  food material
• by acids and enzymes
• Production of intrinsic factor
• glycoprotein required for absorption of vitamin
  B12 in small intestine

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Gastric Pits

• Are shallow depressions that open onto the
  gastric surface
• Mucous cells
• at base, or neck, of each gastric pit
• actively divide, replacing superficial cells

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Gastric Glands

• In fundus and body of stomach
• extend deep into underlying lamina propria
• Each gastric pit leads to several gastric glands
• 2 main types of cells found in gastric glands
• parietal cells
• chief cells

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Parietal and Chief Cells

• Parietal cells
• Secrete hydrochloric acid (HCl)
• Chief cells
• Are most abundant near base of gastric gland
  (secrete pepsinogen)
• Pepsinogen is converted by HCl in the gastric
  lumen to pepsin

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Enteroendocrine Cells ofPyloric Glands

• Are scattered among mucus-secreting cells of
  pylorus
• G cells
• Produce gastrin (hormone that stimulates both the
  Chief and Parietal cells)
• D cells
• Release somatostatin (hormone that inhibits
  release of gastrin)

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The Phases of Gastric Secretion
Figure 2415
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The Cephalic Phase

• Begins when you see, smell, taste, or think of
  food
• directed by CNS
• prepares stomach to receive food

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The Gastric Phase

• Begins with arrival of food in stomach
• builds on stimulation from cephalic phase
• lasts 3-4 hours

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The Intestinal Phase

• Begins when chyme first enters small intestine
• After several hours of mixing contractions
• when waves of contraction sweep down length of
  stomach

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Digestion in the Stomach

• Stomach performs preliminary digestion of
  proteins by pepsin
• some digestion of carbohydrates (by salivary
  amylase)
• lipids (by lingual lipase)
• Chyme
• become more fluid
• pH approaches 2.0
• pepsin activity increases (proteins)

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Absorption in the Stomach

• Although some digestion occurs in the stomach,
  nutrients are not absorbed there
• Only small lipid-soluble particles can cross
  stomach lining
• Alcohol
• Drugs (aspirin)

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The Small Intestine

• Plays key role in digestion and absorption of
  nutrients
• 90 of nutrient absorption (lipids, proteins,
  carbohydrates) occurs in the small intestine

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The Intestinal Wall
Figure 2417
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The Duodenum

• The segment of small intestine closest to stomach
• 25 cm (10 in.) long
• Mixing bowl that receives
• chyme from stomach
• digestive secretions from pancreas and liver

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The Jejunum

• Is the middle segment of small intestine
• 2.5 meters (8.2 ft) long
• Is the location of most
• chemical digestion
• nutrient absorption

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The Ileum

• The final segment of small intestine
• 3.5 meters (11.48 ft) long

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Brush Border Enzymes

• Integral membrane proteins
• On surfaces of intestinal microvilli
• Break down materials in contact with brush border
• Ex Enterokinase
• A brush border enzyme
• Activates pancreatic proenzyme Trypsinogen

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Enteroendocrine Cells

• In intestinal glands
• Produce intestinal hormones
• Gastrin
• Cholecystokinin (CCK)
• Secretin

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Functions of the Duodenum

• Has few plicae and Small villi
• receives chyme from stomach
• neutralizes acids before they can damage the
  absorptive surfaces of the small intestine

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Intestinal Secretions

• Watery intestinal juice
• 1.8 liters per day enter intestinal lumen
• Moistens chyme
• Assists in buffering acids
• Keeps digestive enzymes and products of digestion
  in solution

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Intestinal Movements

• Chyme arrives in duodenum
• Weak peristaltic contractions move it slowly
  toward jejunum
• Segmentation will periodically mix everything up

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The Gastric Reflexes

• The Gastroenteric Reflex
• Stimulates motility and secretion along entire
  small intestines
• The Gastroileal Reflex
• Triggers relaxation of ileocecal valve
• Allows materials to pass from small intestine
  into large intestines

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Functions of the Pancreas

• Endocrine cells
• of pancreatic islets
• secrete insulin and glucagon into bloodstream
• Exocrine cells
• acinar cells
• Produce pancreatic juice (alkaline mixture of
  digestive enzymes, water, ions)

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Pancreatic Secretions

• 1000 ml (1 L) pancreatic juice per day
• Controlled by hormones from duodenum
• Contain pancreatic enzymes

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Pancreatic Enzymes

• Pancreatic alpha-amylase
• a carbohydrase
• breaks down starches
• similar to salivary amylase
• Pancreatic lipase
• breaks down complex lipids
• releases products (e.g., fatty acids) that are
  easily absorbed

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Pancreatic Enzymes

• Nucleases
• break down nucleic acids
• Proteolytic enzymes
• break certain proteins apart
• proteases break large protein complexes
• peptidases break small peptides into amino acids

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Trypsin

• An active protease
• Enterokinase in duodenum
• converts trypsinogen to trypsin

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Hepatic Blood Supply

• 1/3 of blood supply
• arterial blood from __________________
• 2/3 venous blood from _________________,
  originating at
• esophagus
• stomach
• small intestine
• most of large intestine

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Liver Histology
Figure 2420
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Liver Lobules

• The basic functional units of the liver
• Each lobe is divided
• by connective tissue
• into about 100,000 liver lobules
• about 1 mm diameter each

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Hepatocytes

• liver cells
• Adjust circulating levels of nutrients
• through selective absorption and secretion
• form a series of irregular plates arranged like
  wheel spokes
• Many Kupffer Cells (immune system macrophages)
  are located in sinusoidal lining

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A Portal Area

• Contains 3 structures
• branch of hepatic portal vein
• branch of hepatic artery proper
• small branch of bile duct

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Hepatocyte Function

• As blood flows through sinusoids
• hepatocytes absorb solutes from plasma
• and secrete materials such as plasma proteins

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Pressures in Hepatic Portal System

• Are usually low (average 10 mm Hg or less)
• Can increase markedly
• if blood flow is restricted by blood clot or
  damage
• Portal hypertension
• an abnormal rise in portal pressure
• can be a symptom of liver cirrhosis
• can cause esophageal varices

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Liver functions

1. Metabolic regulation
2. Hematological regulation
3. Bile production

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1. Metabolic Regulation

• Removal and storage of carbohydrates, lipids,
  amino acids
• Mobilizing or synthesizing energy reserves
• Vitamin and mineral storage
• detoxification

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2. Hematological Regulation

• Phagocytosis and antigen presentation
• Synthesis of plasma proteins
• Removal of circulating hormones
• Removal of antibodies
• Removal of RBC

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3. Bile production

• Synthesis and secretion of bile
• Dietary lipids are not water soluble
• Mechanical processing in stomach creates drops
  containing lipids
• Pancreatic lipase is not lipid soluble
• interacts only at surface of lipid droplet
• Bile needed for emulsification
• Helps pancreatic lipase do its job

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The Gallbladder and Bile Ducts
Figure 2421
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The Gallbladder

• Is a pear-shaped, muscular sac
• Stores and concentrates bile prior to excretion
  into small intestine
• Releases bile into duodenum
• only under stimulation of hormone cholecystokinin
  (CCK)

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Gallstones

• Are crystals of insoluble minerals and salts
• Form if bile is too concentrated
• Small stones may be flushed through bile duct and
  excreted

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Activities of Major Digestive Tract Hormones
Figure 2422
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Hormones of Enteroendocrine Cells

• Coordinate digestive functions
• Secretin
• cholecystokinin (CCK)
• gastric inhibitory peptide (GIP)
• vasoactive intestinal peptide (VIP)
• gastrin
• enterocrinin

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Secretin

• Is released when chyme arrives in duodenum
• Increases secretion of bile, buffers and enzymes
  by pancreas and liver

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Cholecystokinin (CCK)

• Is secreted by the duodenum
• when chyme contains lipids and partially digested
  proteins
• Relaxes hepatopancreatic sphincter and
  gallbladder
• ejects bile and pancreatic juice into duodenum

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Gastric Inhibitory Peptide (GIP)

• Is secreted when fats and carbohydrates enter
  small intestine
• Causes pancreas to release insulin
• Inhibits Gastrin

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Vasoactive Intestinal Peptide (VIP)

• Dilates capillaries of the villi

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Gastrin

• Is secreted by G cells in duodenum
• when exposed to incompletely digested proteins
• Promotes increased stomach motility
• Stimulates acids and enzyme production in the
  stomach

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Enterocrinin

• Is released when chyme enters small intestine
• Stimulates mucin production by submucosal glands
  of duodenum

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Intestinal Absorption

• It takes about 5 hours for materials to pass
• from duodenum to end of ileum
• Movements of the mucosa increases absorptive
  effectiveness
• stir and mix intestinal contents
• constantly change environment around epithelial
  cells

78
Splenic vein
Aorta
Hepatic portal vein
Superior mesenteric artery
Superior mesenteric vein
Inferior mesenteric vein
Right colic (hepatic) flexure
Inferior vena cava
Left colic (splenic) flexure
Greater omentum (cut)
TRANSVERSE COLON
DESCENDING COLON
Left colic vein
Middle colic artery and vein
Inferior mesenteric artery
Right colic artery and vein
Left colic artery
ASCENDING COLON
Haustra
Fatty appendices
Intestinal arteries and veins
Rectal artery
Ileocecal valve
Ileum
Sigmoid arteries and veins
Cecum
Taenia coli
Appendix
Sigmoid flexure
SIGMOID COLON
Rectum
Rectum
Ileocecal valve
Anal canal
Cecum (cut open)
Anal columns
Internal anal sphincter
External anal sphincter
Appendix
Anus
Cecum and appendix
Rectum, sectioned
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Functions of the Large Intestine

1. Reabsorption of water
2. Compaction of intestinal contents into feces
3. Absorption of important vitamins released by
   bacteria
4. Storage of fecal material prior to defecation

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The Large Intestine

• Also called large bowel
• Is about 1.5 meters (4.9 ft) long and 7.5 cm (3
  in) wide
• Three regions
• Cecum
• Colon
• Rectum

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The Rectum

• Forms last 15 cm of digestive tract
• Is an expandable organ for temporary storage of
  feces
• Movement of fecal material into rectum triggers
  urge to defecate

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Anal Sphincters

• Internal anal sphincter
• circular muscle layer of muscularis externa
• has smooth muscle cells, not under voluntary
  control
• External anal sphincter
• encircles distal portion of anal canal
• a ring of skeletal muscle fibers, is under
  voluntary control

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Characteristics of the Colon

• Lack villi
• Presence of distinctive intestinal glands
• Are deeper than glands of small intestine
• Are dominated by goblet cells
• Provides lubrication for fecal material
• Does not produce enzymes

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Physiology of the Large Intestine

• Less than 10 of nutrient absorption occurs in
  large intestine
• Prepares fecal material for ejection from the
  body

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Absorption in the Large Intestine

• Reabsorption of water
• Reabsorption of bile salts
• in the cecum
• transported in blood to liver
• Absorption of vitamins released by bacteria

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Vitamins

• Are organic molecules
• Important as cofactors or coenzymes in metabolism
• Normal bacteria in colon make 3 vitamins that
  supplement diet
• Vitamin K, Biotin, Pantothenic acid

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Organic Wastes

• Bacteria convert bilirubin to urobilinogens and
  stercobilinogens
• Bacteria break down peptides in feces and
  generate
• ammonia
• hydrogen sulfide
• Action on indigestible carbohydrates produce
  flatus, or intestinal gas

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Movements of the Large Intestine (1 of 3)

• Gastroileal and gastroenteric reflexes
• move materials into cecum while you eat
• Peristaltic waves move material from cecum to
  transverse colon
• very slow allowing hours for water absorption

89
Movements of the Large Intestine (2 of 3)

• Segmentation movements (haustral churning) mix
  contents of adjacent haustra
• Movement from transverse colon through rest of
  large intestine results from powerful peristaltic
  contractions (mass movements)

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Movements of the Large Intestine (3 of 3)

• Stimulus is distension of stomach and duodenum
  relayed over intestinal nerve plexuses
• Distension of the rectal wall triggers defecation
  reflex
• positive feedback loops triggered by stretch
  receptors in rectum

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Essential Nutrients

• A typical meal contains
• Carbohydrates (complex carbohydrates)
• Proteins (polypeptides)
• Lipids (triglycerides)
• water
• Electrolytes (Na, K)
• Vitamins (A,B,C, D,E,K)

92
Digestion and Absorption

• Digestive system handles each nutrient
  differently
• large organic molecules
• must be digested before absorption can occur
• water, electrolytes, and vitamins
• can be absorbed without processing
• may require special transport

93
Digestive Enzymes

• secreted by salivary glands, tongue, stomach,
  pancreas, (liver)
• Break molecular bonds in large organic molecules
• carbohydrates, proteins, lipids, and nucleic
  acids
• in a process called hydrolysis

94
Salivary Amylase and Pancratic Alpha-Amylase

• From parotid and submandibular salivary glands
  and pancreas
• Breaks down starches (complex carbohydrates)
• Produces
• disaccharides (2 simple sugars)
• trisaccharides (3 simple sugars)

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Carbohydrates

• Fragment disaccharides and trisaccharides into
  monosaccharides (simple sugars)
• lactase splits lactose into glucose and galactose

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Absorption of Monosaccharides

• Intestinal epithelium absorbs monosaccharides

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Lipid Digestion

• Involves
• lingual lipase from glands of tongue
• pancreatic lipase from pancreas
• Bile salts improve chemical digestion by
  emulsifying lipid drops into tiny droplets

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Triglycerides

• Are the most important and abundant dietary
  lipids
• Consist of 3 fatty acids attached to 1 molecule
  glycerol

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Lipid Absorption

• Triglycerides and other absorbed molecules are
  coated with proteins
• creating chylomicrons
• Lacteals pick up chylomicrons and they later
  enter left subclavian vein

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Protein Digestion (1 of 2)

• Is complex and time-consuming
• mechanical processing in oral cavity
  (mastication) and chemical processing in stomach
  acid (HCl) allows proteolytic enzymes to attack
  proteins

101
Protein Digestion (2 of 2)

• pepsin
• proteolytic enzyme
• works at pH 1.52.0
• breaks peptide bonds within polypeptide chain
• when chyme enters duodenum
• enterokinase from small intestine triggers
  conversion of trypsinogen to trypsin
• pH is adjusted to 78
• Other pancreatic proteases work through SI

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Digestive Secretion and Absorption
Figure 2427
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Water Absorption

• Cells cannot actively absorb or secrete water
• All movement of water across lining of digestive
  tract
• involves passive water flow down osmotic
  gradients

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Vitamins

• Are organic compounds required in very small
  quantities
• Are divided in 2 major groups
• fat-soluble vitamins
• water-soluble vitamins

105
Vitamin B12

• Cannot be absorbed by intestinal mucosa in normal
  amounts
• unless bound to intrinsic factor (glycoprotein
  secreted by parietal cells of stomach)

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5 Effects of Aging on the Digestive System

• Division of epithelial stem cells declines
• digestive epithelium becomes more susceptible to
  damage by abrasion, acids, or enzymes
• Smooth muscle tone and general motility
  decreases
• peristaltic contractions become weaker

107
5 Effects of Aging on the Digestive System

• Cumulative damage from toxins (alcohol, other
  chemicals) to liver and pancreas
• Rates of colon cancer and stomach cancer rise
• Decline in olfactory and gustatory sensitivities
• lead to dietary changes that affect entire body