Chapter 24: The Digestive System Biology 141 A

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Chapter 24 The Digestive SystemBiology 141 A
PBrashear-Kaulfers
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Anabolism

• Uses raw materials to synthesize essential
  compound
• Catabolism
• Decomposes substances to provide energy cells
  need to function
• Catabolic Reactions- Require two essential
  ingredients
• oxygen
• organic molecules broken down by intracellular
  enzymes
• e.g., carbohydrates, fats, and proteins

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Components of the Digestive System
3D Peel-Away of Digestive System
PLAY
3D Panorama of Digestive System
PLAY
Figure 241
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Digestive Tract

• Gastrointestinal (GI) tract or alimentary canal
• Is a muscular tube
• Extends from oral cavity to anus
• Passes through
• pharynx
• esophagus
• stomach
• small intestine
• large intestine
• 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
• Digestion
• is the chemical breakdown of food
• into small organic fragments
• for absorption by digestive epithelium

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6 Functions of the Digestive System

• Secretion
• is the release of water, acids, enzymes, buffers,
  and salts
• by epithelium of digestive tract
• by glandular organs
• 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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Lining of the Digestive Tract

• Protects surrounding tissues against
• corrosive effects of digestive acids and enzymes
• mechanical stresses, such as abrasion
• bacteria

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Bacteria

• Is ingested with food or resides in digestive
  tract
• Attacked by macrophages, and immune system cells
• In lamina propria (underlying layer of areolar
  tissue)

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Peritoneal Cavity

• Is located within the abdominopelvic cavity
• Lined with serous membrane consisting of
  superficial mesothelium covering a layer of
  areolar tissue
• Peritoneal Fluid-is produced by serous membrane
  lining
• Provides essential lubrication

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Mesenteries
Figure 242a, b
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Mesenteries
Figure 242c, d
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Mesenteries (1)

• Are double sheets of peritoneal membrane
• Suspend portions of digestive tract within
  peritoneal cavity by sheets of serous membrane
• that connect parietal peritoneum
• with visceral peritoneum

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Mesenteries (2)

• Areolar tissue between mesothelial surfaces
• provides an access route to and from the
  digestive tract
• for passage of blood vessels, nerves, and
  lymphatic vessels
• Stabilize positions of attached organs
• Prevent intestines from becoming entangled

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Histological Organization of the Digestive Tract

• Major layers of the digestive tract
• Mucosa- Is the inner lining of digestive tract
• Is a mucous membrane consisting of
• epithelium, moistened by glandular secretions
• submucosa
• muscularis externa
• serosa

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Structure of the Digestive Tract
Figure 243
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The Digestive Epithelium

• Mucosal epithelium is simple or stratified
• depending on location, function, and stresses
• Oral cavity, pharynx, and esophagus
• mechanical stresses
• lined by stratified squamous epithelium
• Stomach, small intestine, and most of large
  intestine
• absorption
• simple columnar epithelium with goblet cells

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Lining of Digestive Tract

• Folding increases surface area for absorption
• longitudinal folds, disappear as digestive tract
  fills
• permanent transverse folds (plicae)

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Muscularis Mucosae

• Narrow band of smooth muscle and elastic fibers
  in lamina propria
• Smooth muscle cells arranged in 2 concentric
  layersinner layer encircles lumen (circular
  muscle)
• outer layer contains muscle cells parallel to
  tract (longitudinal layer)

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The Submucosa

• Is a layer of dense irregular connective tissue
• Surrounds muscularis mucosae
• Has large blood vessels and lymphatic vessels
• May contain exocrine glands
• secrete buffers and enzymes into digestive tract

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Muscularis Externa Structure

• Is dominated by smooth muscle cells
• Are arranged in
• inner circular layer
• outer longitudinal layer
• Function
• Involved in
• mechanical processing
• movement of materials along digestive tract
• Movements coordinated by enteric nervous system
  (ENS)
• sensory neurons
• interneurons
• motor neurons

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The Movement of Digestive Materials

• By muscular layers of digestive tract
• consist of visceral smooth muscle tissue
• 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
Consists of waves of muscular contractions Moves
a bolus along the length of the digestive
tract-Bolus is a small, oval mass of digestive
contents
Figure 244
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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

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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
Neural mechanisms Hormonal mechanisms Local
mechanisms
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
• Short Reflexes control small segments of
  digestive tract

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Digestive Hormones

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

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Local Mechanisms

• Prostaglandins, histamine, and other chemicals
• Released into interstitial fluid
• Affect adjacent cells within small segment of
  digestive tract
• Coordinating response to changing conditions
• e.g., variations in local pH, chemical, or
  physical stimuli
• Affect only a portion of tract

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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 digestion
• of carbohydrates and lipids

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Oral Mucosa

• Lining of oral cavity
• Has stratified squamous epithelium
• The Mucosa Inferior to the Tongue
• Is thin and vascular enough to rapidly absorb
  lipid-soluble drugs
• Cheeks- Are supported by pads of fat and the
  buccinator muscles
• Lips- labia - the mucosa of each cheek is
  continuous with that of the lips

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Vestibule

• Space between the cheeks (or lips) and the teeth
• Gingivae (Gums)
• Ridges of oral mucosa
• Surround base of each tooth on alveolar processes
  of maxillary bones and mandible
• The Oral Cavity
• Roof formed by hard and soft palates
• Tongue dominates the floor

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Uvula

• A dangling process
• Helps prevent food from entering pharynx
  prematurely
• Is supported by posterior margin of soft palate
• Tonsils
• Lies between palatoglossal and palatopharyngeal
  arches, on each side

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The Tongue

• Manipulates materials inside mouth
• May bring foods into oral cavity
• 4 Functions of the Tongue
• Manipulation
• Mechanical processing
• compression, abrasion, and distortion
• assists in chewing
• prepares material for swallowing

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4 Functions of the Tongue

• Sensory analysis
• touch, temperature, and taste receptors
• Secretion
• mucins
• enzyme lingual lipase
• Lingual Papillae has fine projections on superior
  surface (dorsum) of tongue which are covered in
  thick epithelium

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Sublingual Glands

• Small glands extend into underlying lamina
  propria
• Secretions flush tongues epithelium
• Contain water, mucins, and enzyme lingual lipase
  - Enzyme, works over broad pH range (3.06.0)
• Starts lipid digestion immediately

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Muscles of the Tongue

• Extrinsic tongue muscles-Perform all gross
  movements of the tongue
• Are large muscles
• Intrinsic tongue muscles- Change shape of the
  tongue
• Assist extrinsic muscles during precise
  movements, as in speech

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The Salivary Glands
Figure 247
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Salivary Glands

• 3 pairs secrete into oral cavity
• Each pair has distinctive cellular organization
• and produces saliva with different properties
• Produce 1.01.5 liters of saliva each day
• 70 by submandibular glands
• 25 by parotids
• 5 by sublingual glands

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Saliva

• 99.4 water
• 0.6 includes
• electrolytes (Na, Cl, and HCO3)
• buffers
• glycoproteins (mucins)- Glycoproteins,
  responsible for lubricating action
• antibodies
• enzymes
• waste products

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4 Functions of Saliva

• Lubricating the mouth
• Moistening and lubricating materials in the mouth
• Dissolving chemicals that
• stimulate taste buds
• provide sensory information
• 4. Initiate digestion of
• complex carbohydrates by enzyme salivary amylase
  (ptyalin or alpha-amylase)
• lipids by enzyme lingual lipase

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Teeth
Figure 248
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The Teeth

• Tongue movements pass food across occlusal
  surfaces of teeth
• Chew (masticate) food
• Tooth Structure
• Dentin
• a mineralized matrix similar to that of bone
• does not contain cells
• Pulp cavity
• receives blood vessels and nerves through the
  root canal
• Root of each tooth sits in a bony socket
  (alveolus)
• A layer of cementum covers dentin of the root
• providing protection and anchoring periodontal
  ligament

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The Crown

• Exposed portion of tooth
• Projects beyond soft tissue of gingiva
• Dentin covered by layer of enamel
• Gingival Sulcus is a shallow groove surrounding
  the neck of each tooth

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Dental Arches

• Contain 4 types of teeth
• 1. Incisors- Blade-shaped teeth
• Located at front of mouth
• Used for clipping or cutting, Have a single
  root
• 2. Cuspids (canines)-Conical
• Sharp ridgeline, Pointed tip
• Used for tearing or slashing, Have a single
  root
• 3.Bicuspids (premolars)-Flattened crowns
• Prominent ridges, Used to crush, mash, and
  grind, Have 1 or 2 roots
• 4. Molars-Very large, flat crowns
• With prominent ridges, Used for crushing and
  grinding, Have 3 or more roots

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Primary and Secondary Dentitions
Figure 249
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Dental Succession

• During embryonic development, 2 sets of teeth
  form
• Primary dentition or deciduous teeth-
• Also called primary teeth, milk teeth, or baby
  teeth
• 20 temporary teeth of primary dentition
• 5 on each side of upper and lower jaws
• 2 incisors,1 cuspid, 2 deciduous molars
• Secondary dentition or permanent dentition
• Replaces deciduous teeth, 32 permanent teeth
• 8 on each side, upper and lower
• 2 incisors, 1 cuspid, 5 molars

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Mastication

• Also called chewing
• Food is forced from oral cavity to vestibule and
  back
• crossing and recrossing occlusal surfaces
• Mastication muscles close the jaws
• Slide or rock lower jaw from side to side
• Chewing involves mandibular
• elevation and depression
• protraction and retraction
• medial and lateral movement

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The Pharynx

• A common passageway for solid food, liquids, and
  air
• Nasopharynx
• Oropharynx
• Laryngopharynx
• Food passes through oropharynx and laryngopharynx
  to esophagus

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The Esophagus
Figure 2410
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The Esophagus

• A hollow muscular tube
• About 25 cm long and 2 cm wide
• Conveys solid food and liquids to the stomach
• Begins posterior to cricoid cartilage
• Is innervated by fibers from the esophageal plexus

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5 Characteristics of the Esophageal Wall

• Wall of esophagus has 3 layers
• mucosal
• submucosal
• muscularis
• Mucosa contains
• nonkeratinized and stratified squamous epithelium
  
• Mucosa and submucosa
• form large folds that extend the length of the
  esophagus
• Muscularis mucosae
• consists of irregular layer of smooth muscle

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5 Characteristics of the Esophageal Wall

• Submucosa contains esophageal glands
• which produce mucous secretion
• reduces friction between bolus and esophageal
  lining
• Muscularis externa
• has usual inner circular and outer longitudinal
  layers

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The Swallowing Process
Is also called deglutition Can be initiated
voluntarily, Proceeds automatically Is divided in
3 phases buccal phase-Compression of bolus
against hard palate, Retraction of tongue forces
bolus into oropharynx pharyngeal phase-Bolus
contacts arches and posterior pharyngeal wall
esophageal phase- Contraction of pharyngeal
muscles forces bolus through entrance to
esophagus Primary Peristaltic Waves
Figure 2411
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4 Functions of the Stomach

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

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The Stomach
Figure 2412a
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The Stomach
Figure 2412b
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Anatomy of the Stomach

• The stomach is shaped like an expanded J
• short lesser curvature forms medial surface
• long greater curvature forms lateral surface
• Anterior and posterior surfaces are smoothly
  rounded
• Shape and size vary
• from individual to individual
• from 1 meal to the next
• Stomach typically extends between levels of
  vertebrae T7 and L3

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4 Regions of the Stomach

• Cardia, Fundus, Body, Pylorus
• Muscularis mucosae and muscularis externa
• contain extra layers of smooth muscle cells
• in addition to circular and longitudinal layers

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The Stomach Lining
Simple columnar epithelium lines all portions of
stomach Epithelium is a secretory sheet produces
mucus that covers interior surface of stomach
Figure 2413
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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
• Chyme- Mixture of secretions and food in the
  stomach
• In fundus and body of stomach
• extend deep into underlying lamina propria
• Each gastric pit communicates with several
  gastric glands
• Secretory cells in gastric glands
• parietal cells
• chief cells

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The Secretion of Hydrochloric Acid
Parietal and Chief Cells- Secrete hydrochloric
acid (HCl) Chief Cells are most abundant near
base of gastric gland secrete pepsinogen
(inactive proenzyme)- Is converted by HCl in the
gastric lumen to pepsin (active proteolytic
enzyme)
Figure 2414
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Pyloric Glands

• Pyloric Glands in the pylorus
• produce mucous secretion
• G Cells produce gastrin
• D Cells in pyloric glands
• Release somatostatin
• hormone that inhibits release of gastrin

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The Phases of Gastric Secretion
Table 241
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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)
• Stomach contents
• become more fluid
• pH approaches 2.0
• pepsin activity increases
• protein disassembly begins
• Although digestion occurs in the stomach,
  nutrients are not absorbed there

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Associated Glandular Organs

• Stomach
• gastric juices
• stomach acids
• pepsin

• Pancreas
• digestive enzymes
• buffers
• Liver
• Bile

Bile is produced in liver Contains buffers and
bile salts Stored in gallbladder Discharge into
small intestine
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Segments of the Intestine
Figure 2416
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The Small Intestine

• The Small Intestine
• Plays key role in digestion and absorption of
  nutrients
• 90 of nutrient absorption occurs in the small
  intestine

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The Small Intestine plays key role in digestion
and absorption of nutrient, 90 of nutrient
absorption occurs in the small intestine

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

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The Intestinal Wall
plicae are transverse folds in intestinal
lining Are permanent features Do not disappear
when small intestine fills
Figure 2417
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Intestinal Villi

• A series of fingerlike projections
• in mucosa of small intestine
• Covered by simple columnar epitheliumcovered
  with microvilli
• Intestinal glands have goblet cells between
  columnar epithelial cells
• Eject mucins onto intestinal surfaces
• Brush Border Enzymes are integral membrane
  proteins ,on surfaces of intestinal microvilli
• Break down materials in contact with brush border
  by trypsinogen

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Enteroendocrine Cells

• In intestinal glands
• Produce intestinal hormones
• Gastrin, Cholecystokinin, Secretin
• Brunners glands-submucosal glands of duodenum
• Produce copious mucus
• when chyme arrives from stomach

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Functions of the Duodenum

• To receive chyme from stomach
• To neutralize acids before they can damage the
  absorptive surfaces of the small intestine
• Duodenum has few plicae
• Small villi

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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
• Intestinal Movements
• Chyme arrives in duodenum
• Weak peristaltic contractions move it slowly
  toward jejunum

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Peristaltic Contractions

• Myenteric reflexes,Not under CNS Parasympathetic
  stimulation
• accelerates local peristalsis and segmentation
• Gastroenteric Reflex Stimulates motility and
  secretion
• along entire small intestine
• Gastroileal Reflex triggers relaxation of
  ileocecal valve
• Allows materials to pass
• from small intestine into large intestine

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The Pancreas
Lies posterior to stomach from duodenum toward
spleen Is bound to posterior wall of abdominal
cavity Is wrapped in thin, connective-tissue
capsule

• A compound tubuloalveolar gland

Figure 2418
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Common Bile Duct from the liver and gallbladder
meets pancreatic duct near duodenum Lobules of
the Pancreas

• Are separated by connective tissue partitions
  (septa)
• Contain blood vessels and tributaries of
  pancreatic ducts
• In each lobule
• ducts branch repeatedly
• end in blind pockets (pancreatic acini)
• Pancreatic Islets -Endocrine tissues of pancreas
• Scattered (1 of pancreatic cells)

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Functions of the Pancreas

• Endocrine cells
• of pancreatic islets
• secrete insulin and glucagon into bloodstream
• Exocrine cells
• acinar cells
• epithelial cells of duct system
• Pancreatic Secretions - 1000 ml (1 qt) 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
• 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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Proteolytic Enzymes

• 70 of all pancreatic enzyme production
• Secreted as inactive proenzymes
• Activated after reaching small intestine
• Trypsin- An active protease
• Enterokinase in duodenum
• converts trypsinogen to trypsin

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The Liver

• Is the largest visceral organ (1.5 kg)
• Lies in right hypochondriac and epigastric
  regions
• Extends to left hypochondriac and umbilical
  regions
• Performs essential metabolic and synthetic
  functions

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The Anatomy of the Liver
Is wrapped in tough fibrous capsule Is covered by
visceral peritoneum Is divided into lobes
Figure 2419
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The Porta Hepatis

• Doorway to the liver
• Region where blood vessels and other structures
  converge on liver
• Vessels reach liver by traveling within
  connective tissue of lesser omentum

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Hepatic Blood Supply

• 1/3 of blood supply
• arterial blood from hepatic artery proper
• 2/3 venous blood from hepatic portal vein,
  originating at
• esophagus
• stomach
• small intestine
• most of large intestine
• Hepatocytes are liver cells
• Adjust circulating levels of nutrients
• through selective absorption and secretion
• Blood Leaving the Liver returns to systemic
  circuit
• Via hepatic veins
• which open into inferior vena cava

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Liver Histology
Liver lobule is 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
Figure 2420
87
Hepatocytes

• In a liver lobule form a series of irregular
  plates arranged like wheel spokes
• Blood enters liver sinusoids
• from small branches of hepatic portal vein
• from hepatic artery proper
• As blood flows through sinusoids
• hepatocytes absorb solutes from plasma
• and secrete materials such as plasma proteins

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The Bile Duct System

• Liver secretes bile fluid
• into a network of narrow channels (bile
  canaliculi)
• between opposing membranes of adjacent liver
  cells
• The Right and Left Hepatic Ducts Collect bile
  from all bile ducts of liver lobes
• Unite to form common hepatic duct which leaves
  the liver

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3 Functions of the Liver

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

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3 Functions of the Liver Metabolic Regulation

• The liver regulates
• composition of circulating blood
• nutrient metabolism
• waste product removal
• nutrient storage
• drug inactivation

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Composition of Circulating Blood

• All blood leaving absorptive surfaces of
  digestive tract
• enters hepatic portal system
• flows into the liver
• Liver cells extract nutrients or toxins from
  blood
• before it reaches systemic circulation through
  hepatic veins
• Liver removes and stores excess nutrients
• corrects nutrient deficiencies by mobilizing
  stored reserves or performing synthetic
  activities

92
Metabolic Activities of the Liver

• Carbohydrate metabolism
• Lipid metabolism
• Amino acid metabolism
• Waste product removal
• Vitamin storage
• Mineral storage
• Drug inactivation

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The Liver and Hematological Regulation (1 of 2)

• Largest blood reservoir in body
• Receives 25 of cardiac output
• Performs 6 hematological regulation functions

94
The Liver and Hematological Regulation (2 of 2)

1. Phagocytosis and antigen presentation
2. Synthesis of plasma proteins
3. Removal of circulating hormones
4. Removal of antibodies
5. Removal or storage of toxins
6. Synthesis and secretion of bile

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Lipid Digestion and Absorption

• Dietary lipids are not water soluble
• Mechanical processing in stomach creates large
  drops containing lipids
• Pancreatic lipase is not lipid soluble
• interacts only at surface of lipid droplet

96
Functions of Bile

• Bile salts break droplets apart (emulsification)
• increases surface area exposed to enzymatic
  attack
• creates tiny emulsion droplets coated with bile
  salts

97
The Gallbladder and Bile Ducts
Figure 2421
98
The Gallbladder

• Is a pear-shaped, muscular sac
• Stores and concentrates bile prior to excretion
  into small intestine
• Is located in the fossa on the posterior surface
  of the livers right lobe
• Cystic duct extends from gallbladder
• Union with common hepatic duct forms common bile
  duct
• Stores bile
• Releases bile into duodenum
• only under stimulation of hormone cholecystokinin
  (CCK)

99
Without CCK

• Hepatopancreatic sphincter remains closed
• Bile exiting liver in common hepatic duct cannot
  flow through common bile duct into duodenum
• Bile enters cystic duct
• is stored in gallbladder

100
CCK

• Is released whenever chyme enters duodenum
• Relaxes hepatopancreatic sphincter
• Stimulates contractions in gallbladder
• pushes bile into small intestine
• Amount secreted depends on lipid content of chyme

101
The Gallbladder and Bile Modification

• Full gallbladder contains 4070 ml bile
• Bile composition gradually changes in
  gallbladder
• water is absorbed
• bile salts and solutes become concentrated

102
Gallstones

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

103
Coordination of Secretion and Absorption

• Neural and hormonal mechanisms coordinate
  activities of digestive glands
• Regulatory mechanisms center around duodenum
• where acids are neutralized and enzymes added

104
Activities of Major Digestive Tract Hormones
Figure 2422
105
Intestinal Hormones

• Intestinal tract secretes peptide hormones with
  multiple effects
• in several regions of digestive tract
• in accessory glandular organs

106
Hormones of Duodenal Enteroendocrine Cells

• Coordinate digestive functions
• secretin
• cholecystokinin (CCK)
• gastric inhibitory peptide (GIP) -when fats and
  carbohydrates enter small intestine
• vasoactive intestinal peptide (VIP) - Inhibits
  acid production in stomach
• Gastrin- Stimulates acids and enzyme production
• Enterocrinin- when chyme enters small
  intestine,Stimulates mucin production

107
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

108
The Large Intestine
Is horseshoe-shaped Extends from end of ileum to
anus Lies inferior to stomach and liver Frames
the small intestine
Figure 2423
109
Functions of the Large Intestine

• Reabsorption of water
• Compaction of intestinal contents into feces
• Absorption of important vitamins produced by
  bacteria
• Storage of fecal material prior to defecation
• Also called large bowel
• Is about 1.5 meters long and 7.5 cm wide
• 3 Parts of the Large Intestine
• Cecum
• the pouchlike first portion -Stores materials and
  begins compaction
• Colon the largest portion, Has a larger diameter
  and thinner wall than small intestine
• The wall of the colon
• forms a series of pouches (haustra) which permit
  expansion and elongation of colon
• 3. Rectum the last 15 cm of digestive tract

110
The Appendix

• Also called vermiform appendix
• Is a slender, hollow appendage (about 9 cm long)
• Is dominated by lymphoid nodules (a lymphoid
  organ)
• Is attached to posteromedial surface of cecum
• mesoappendix connects appendix to ileum and cecum

111
4 regions the Ascending Colon

• Begins at superior border of cecum
• Ascends along right lateral and posterior wall of
  peritoneal cavity
• to inferior surface of the liver
• The Transverse Colon- Curves anteriorly from
  right colic flexure,
• Crosses abdomen from right to left
• Is supported by transverse mesocolon
• Is separated from anterior abdominal wall by
  greater omentum

112
The Descending Colon

• Proceeds inferiorly along left side
• to the iliac fossa (inner surface of left ilium)
• Is retroperitoneal, firmly attached to abdominal
  wall
• The Sigmoid Colon
• Is an S-shaped segment, about 15 cm long
• Starts at sigmoid flexure
• Lies posterior to urinary bladder
• Is suspended from sigmoid mesocolon
• Empties into rectum

113
Blood Supply of the Large Intestine

• Receives blood from tributaries of
• superior mesenteric and inferior mesenteric
  arteries
• Venous blood is collected from
• superior mesenteric and inferior mesenteric veins

114
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
• The Anal Canal-
• Is the last portion of the rectum
• Contains small longitudinal folds called anal
  columns
• Anus or anal orifice is exit of the anal canal
• Has keratinized epidermis like skin

115
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, under voluntary
  control

116
Mucosa and Glands of the Colon
Colon has a Lack of villi Abundance of goblet
cells Presence distinctive intestinal glands
Figure 2424
117
Glands of the Large Intestine

• Are deeper than glands of small intestine
• Are dominated by goblet cells
• Mucosa does not produce enzymes
• Provides lubrication for fecal material
• Physiology of the Large Intestine-
• Less than 10 of nutrient absorption occurs in
  large intestine
• Prepares fecal material for ejection from the
  body

118
Absorption in the Large Intestine

• Reabsorption of water
• Reabsorption of bile salts
• in the cecum
• transported in blood to liver
• Absorption of vitamins produced by bacteria-
  organic molecules
• Important as cofactors or coenzymes in metabolism
• Normal bacteria in colon make 3 vitamins that
  supplement diet
• Absorption of organic wastes

119
3 Vitamins Produced in the Large Intestine

• Vitamin K
• a fat-soluble vitamin
• required by liver for synthesizing 4 clotting
  factors, including prothrombin
• Biotin
• a water-soluble vitamin
• important in glucose metabolism
• Pantothenic acid
• a water-soluble vitamin
• required in manufacture of steroid hormones and
  some neurotransmitters

120
Organic Wastes

• Bacteria convert bilirubin to urobilinogens and
  stercobilinogens
• urobilinogens absorbed into bloodstream are
  excreted in urine
• urobilinogens and stercobilinogens in colon
  convert to urobilins and stercobilins by exposure
  to oxygen
• Bacteria break down peptides in feces and
  generate
• ammonia
• as soluble ammonium ions
• indole and skatole
• nitrogen compounds responsible for odor of feces
• hydrogen sulfide
• gas that produces rotten egg odor

121
The Defecation Reflex
Organic Wastes Bacteria feed on indigestible
carbohydrates (complex polysaccharides) produce
flatus, or intestinal gas, in large intestine
Figure 2425
122
Movements of the Large Intestine

• Gastroileal and gastroenteric reflexes
• move materials into cecum while you eat
• Movement from cecum to transverse colon is very
  slow allowing hours for water absorption
• Peristaltic waves move material along length of
  colon
• Segmentation movements (haustral churning) mix
  contents of adjacent haustra

123
Movements of the Large Intestine

• Movement from transverse colon through rest of
  large intestine results from powerful peristaltic
  contractions (mass movements)
• Stimulus is distension of stomach and duodenum
  relayed over intestinal nerve plexuses
• Distension of the rectal wall triggers defecation
  reflex
• 2 positive feedback loops
• both loops triggered by stretch receptors in
  rectum

124
Elimination of Feces

• Requires relaxation of internal and external anal
  sphincters
• Reflexes open internal sphincter, close external
  sphincter
• Opening external sphincter requires conscious
  effort

125
Essential Nutrients

• A typical meal contains
• Carbohydrates, proteins
• Lipids, water
• Electrolytes, vitamins
• 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

126
Summary Chemical Events in Digestion
Figure 2426
127
Processing Nutrients

• The digestive system
• breaks down physical structure of food
• disassembles component molecules
• Molecules released into bloodstream are
• absorbed by cells
• Broken down to provide energy for ATP synthesis
• used to synthesize carbohydrates, proteins, and
  lipids

128
Digestive Enzymes

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

129
Digestive Enzymes

• Are divided into classes by targets
• carbohydrases
• break bonds between simple sugars
• proteases
• break bonds between amino acids
• lipases
• separate fatty acids from glycerides
• Brush border enzymes break nucleotides into
• sugars
• phosphates
• nitrogenous bases

130
Complex Carbohydrate Digestion

• Proceeds in 2 steps
• carbohydrases (from salivary glands and
  pancreas)-Function at pH 6.77.5
• salivary amylase - From parotid and submandibular
  salivary glands, Breaks down starches (complex
  carbohydrates),Produces
• disaccharides (2 simple sugars)
• trisaccharides (3 simple sugars)
• pancreatic alpha-amylase-
• 2. brush border enzymes- Fragment disaccharides
  and trisaccharides into monosaccharides (simple
  sugars)maltase splits maltose into 2 glucose

131
Absorption of Monosaccharides

• Intestinal epithelium absorbs monosaccharides
• by facilitated diffusion and cotransport
• via a carrier protein

132
Facilitated Diffusionvs. Cotransport

• Facilitated diffusion
• moves only 1 molecule or ion through cell
  membrane, does not require ATP
• will not occur against opposing concentration
  gradient
• Cotransport
• moves more than 1 molecule or ion at the same
  time
• transported materials move in same direction
• may require ATP to preserve cell homeostasis
• can occur against opposing concentration gradient

133
Lipid Digestion

• Involves
• lingual lipase from glands of tongue-Begins
  triglycerides breakdown in mouth
• Continues for limited time within stomach
• Digests 20 of lipids before chyme enters
  duodenum
• Bile salts improve chemical digestion
• by emulsifying lipid drops into tiny droplets
• providing better access for pancreatic lipase
• pancreatic lipase from pancreas-Break off 2 fatty
  acids, leaving monoglycerides ,Are water-soluble
  enzymes, Attack only exposed surfaces of lipid
  drops
• Triglycerides
• Are the most important and abundant dietary
  lipids
• Consist of 3 fatty acids attached to 1 molecule
  glycerol

134
Lipid Absorption

• Intestinal cells synthesize new triglycerides
  from monoglycerides and fatty acids
• Triglycerides and other absorbed molecules are
  coated with proteins
• creating chylomicrons
• Intestinal cells secrete chylomicrons into
  interstitial fluid by exocytosis

135
Protein Digestion

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

136
Absorption of Amino Acids

• Dipeptidases
• enzymes on epithelial surfaces of small intestine
  
• break short peptide chains into individual amino
  acids
• After diffusing to basal surface of cell
• amino acids are released into interstitial fluid
• by facilitated diffusion and cotransport

137
Digestive Secretion and Absorption
Cells cannot actively absorb or secrete water All
movement of water across lining of digestive
tract involves passive water flow down osmotic
gradients
Figure 2427
138
Absorption of Ions and Vitamins
Table 244
139
Ion Absorption

• Osmosis does not distinguish among solutes
• determined only by total concentration of solutes
• To maintain homeostasis
• concentrations of specific ions must be regulated
• Sodium ion absorption
• rate increased by aldosterone (steroid hormone
  from adrenal cortex)
• Calcium ion absorption
• involves active transport at epithelial surface
• rate increased by parathyroid hormone (PTH) and
  calcitriol

140
Ion Absorption

• Potassium ion concentration increases
• as other solutes move out of lumen
• Other ions diffuse into epithelial cells along
  concentration gradient
• Cation absorption (magnesium, iron)
• involves specific carrier proteins
• cell must use ATP to transport ions to
  interstitial fluid
• Anions (chloride, iodide, bicarbonate, and
  nitrate)
• are absorbed by diffusion or carrier mediated
  transport
• Phosphate and sulfate ions
• enter epithelial cells by active transport

141
Vitamins

• Are organic compounds required in very small
  quantities
• Are divided in 2 major groups
• fat-soluble vitamins A, D, E, and K
• - Their structure allows them to dissolve in
  lipids
• 9 water-soluble vitamins
• all B vitamins (common in milk and meats)
• vitamin C (found in citrus)
• All but 1 of water-soluble vitamins easily
  diffuse across digestive epithelium
• Vitamin B12- Cannot be absorbed by intestinal
  mucosa in normal amounts
• unless bound to intrinsic factor (glycoprotein
  secreted by parietal cells of stomach

142
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
• 2. Smooth muscle tone and general motility
  decreasesperistaltic contractions become weaker
• Cumulative damage from toxins (alcohol, other
  chemicals) absorbed by digestive tract and
  transported to liver for processing

143
5 Effects of Aging on the Digestive System

• Rates of colon cancer and stomach cancer rise
  with age
• oral and pharyngeal cancers common among elderly
  smokers
• Decline in olfactory and gustatory sensitivities
• lead to dietary changes that affect entire body

144
The Digestive System and Other Systems
Figure 2428
145
SUMMARY (1 of 4)

• Digestive system
• digestive tract
• accessory organs
• Digestive system functions
• ingestion
• mechanical processing
• digestion
• secretion
• absorption

146
SUMMARY (2 of 4)

• Oral cavity
• Buccal cavity
• oral mucosa
• Tongue
• intrinsic tongue muscles
• extrinsic tongue muscles
• Salivary glands
• Teeth,
• Pharynx
• Esophagus
• Stomach
• cephalic phase
• gastric phase
• intestinal phase

147
SUMMARY (3 of 4)

• Small intestine
• gastroenteric reflex
• gastroileal reflex
• Pancreas
• Liver
• bile
• Gallbladder
• Intestinal hormones
• secretin
• cholecystokinin (CCK)
• gastric inhibitory peptide (GIP)
• vasoactive intestinal peptide (VIP)
• gastrin
• enterocrinin

148
SUMMARY (4 of 4)

• Large intestine
• cecum
• colon
• rectum
• Processing and absorption of nutrients
• Carbohydrate digestion and absorption
• Lipid digestion and absorption
• Protein digestion and absorption
• Water absorption
• Ion absorption
• Vitamin absorption