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Chapter 24:
The Digestive System
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
Components of
the Digestive System
Figure 24–1
Digestive Tract
 Gastrointestinal (GI) tract or alimentary
canal
 Is a muscular tube
 Extends from oral cavity to anus
6 Functions of the
Digestive System
1. Ingestion:

occurs when materials enter digestive tract
via the mouth
2. Mechanical processing:


crushing and shearing
makes materials easier to propel along
digestive tract
6 Functions of the
Digestive System
3. Digestion:

is the chemical breakdown of food into
small organic fragments for absorption by
digestive epithelium
4. Secretion:



is the release of water, acids, enzymes,
buffers, and salts
by epithelium of digestive tract
by glandular organs
6 Functions of the
Digestive System
5. Absorption:

movement of organic substrates,
electrolytes, vitamins, and water across
digestive epithelium into interstitial fluid of
digestive tract
6. Excretion:

removal of waste products from body fluids
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
Peristalsis
Figure 24–4
Peristalsis
 Consists of waves of muscular
contractions
 Circular and longitudinal
 Moves a bolus along the length of the
digestive tract
Peristaltic Motion
1. Circular muscles contract behind bolus:

while circular muscles ahead of bolus relax
2. Longitudinal muscles ahead of bolus contract:

shortening adjacent segments
3. Wave of contraction in circular muscles:

forces bolus forward
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
The Regulation of
Digestive Activities
Figure 24–5
Neural Mechanisms
 Control:
 movement of materials along digestive tract
 secretory functions
 Motor neurons:
 control smooth muscle contraction and
glandular secretion
 located in myenteric plexus
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
The Oral Cavity
Figure 24–6
4 Functions of the Oral Cavity
1. Sensory analysis:

of material before swallowing
2. Mechanical processing:

through actions of teeth, tongue, and palatal
surfaces
3. Lubrication:

mixing with mucus and salivary gland secretions
4. Limited (chemical) digestion:

of carbohydrates and lipids
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
Lingual Papillae
 Fine projections on superior surface
(dorsum) of tongue
 Covered in thick epithelium
 Assists in moving materials
Lingual Glands
 Small glands extend into underlying
lamina propria
 Secretions flush tongue’s epithelium
 Contain water, mucins, and enzyme
lingual lipase
Salivary Glands
 3 pairs secrete products into oral cavity
 Each pair has distinctive cellular
organization and produces saliva with
different properties
 Produce 1.0–1.5 liters of saliva each day
Saliva
 99.4% water
 0.6% includes:






electrolytes (Na+, Cl—, and HCO3—)
buffers
glycoproteins (mucins)
antibodies
Enzymes (salivary amylase)
waste products
Control of Salivary Secretions
 By autonomic nervous system:
 parasympathetic and sympathetic innervation
 _______________ stimulation accelerates
secretion by all salivary glands
The Swallowing Process
Figure 24–11
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
Primary Peristaltic Waves
 Movements coordinated by afferent and
efferent fibers in glossopharyngeal and
vagus nerves
 Controlled by swallowing center of medulla
oblongata
The Stomach Lining
Figure 24–13
4 Functions of the Stomach
1. Storage of ingested food
2. Mechanical breakdown of ingested food
3. (Preliminary) 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
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
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
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
Enteroendocrine Cells of
Pyloric 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)
The Phases of
Gastric
Secretion
Figure 24–15
The Cephalic Phase
 Begins when you see, smell, taste, or think
of food:
 directed by CNS
 prepares stomach to receive food
The Gastric Phase
 Begins with arrival of food in stomach:
 builds on stimulation from cephalic phase
 lasts 3-4 hours
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
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)
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)
The Small Intestine
 Plays key role in digestion and absorption
of nutrients
 90% of nutrient absorption (lipids, proteins,
carbohydrates) occurs in the small
intestine
The Intestinal Wall
Figure 24–17
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
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
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
Enteroendocrine Cells
 In intestinal glands
 Produce intestinal hormones:
 Gastrin
 Cholecystokinin (CCK)
 Secretin
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
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
 Segmentation will periodically “mix”
everything up
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
Functions of the Pancreas
1. Endocrine cells:


of pancreatic islets
secrete insulin and glucagon into
bloodstream
2. Exocrine cells:


acinar cells
Produce pancreatic juice (alkaline mixture
of digestive enzymes, water, ions)
Pancreatic Secretions
 1000 ml (1 L) pancreatic juice per day
 Controlled by hormones from duodenum
 Contain pancreatic enzymes
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
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
Trypsin
 An active protease
 Enterokinase in duodenum:
 converts trypsinogen to trypsin
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
Liver Histology
Figure 24–20
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
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
A Portal Area
 Contains 3 structures:
 branch of hepatic portal vein
 branch of hepatic artery proper
 small branch of bile duct
Hepatocyte Function
 As blood flows through sinusoids:
 hepatocytes absorb solutes from plasma
 and secrete materials such as plasma
proteins
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
Liver functions
1. Metabolic regulation
2. Hematological regulation
3. Bile production
1. Metabolic Regulation
•
•
•
•
Removal and storage of carbohydrates,
lipids, amino acids
Mobilizing or synthesizing energy reserves
Vitamin and mineral storage
detoxification
2. Hematological Regulation
•
•
•
•
•
Phagocytosis and antigen presentation
Synthesis of plasma proteins
Removal of circulating hormones
Removal of antibodies
Removal of RBC
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 it’s job
The Gallbladder and Bile Ducts
Figure 24–21
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)
Gallstones
 Are crystals of insoluble minerals and salts
 Form if bile is too concentrated
 Small stones may be flushed through bile
duct and excreted
Activities of
Major
Digestive Tract
Hormones
Figure 24–22
Hormones of Enteroendocrine Cells
 Coordinate digestive functions






Secretin
cholecystokinin (CCK)
gastric inhibitory peptide (GIP)
vasoactive intestinal peptide (VIP)
gastrin
enterocrinin
Secretin
 Is released when chyme arrives in
duodenum
 Increases secretion of bile, buffers and
enzymes by pancreas and liver
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
Gastric Inhibitory Peptide (GIP)
 Is secreted when fats and carbohydrates
enter small intestine
 Causes pancreas to release insulin
 Inhibits Gastrin
Vasoactive Intestinal
Peptide (VIP)
 Dilates capillaries of the villi
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
Enterocrinin
 Is released when chyme enters small
intestine
 Stimulates mucin production by
submucosal glands of duodenum
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
Splenic vein
Superior mesenteric artery
Inferior mesenteric vein
Aorta
Hepatic portal vein
Right colic
(hepatic) flexure
Superior
mesenteric vein
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
Left colic artery
Right colic
artery and vein
ASCENDING
COLON
Haustra
Fatty appendices
Intestinal arteries
and veins
Ileocecal valve
Ileum
Rectal
artery
Cecum
Sigmoid arteries
and veins
Taenia coli
Appendix
Sigmoid flexure
SIGMOID COLON
Rectum
Rectum
Ileocecal
valve
Cecum
(cut open)
Appendix
Anal canal
Anal
columns
Internal anal
sphincter
External anal
sphincter
Anus
Cecum and appendix
Rectum, sectioned
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
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
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
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
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
Physiology of the Large Intestine
 Less than 10% of nutrient absorption
occurs in large intestine
 Prepares fecal material for ejection from
the body
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
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
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
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
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)
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
Essential Nutrients
 A typical meal contains:






Carbohydrates (complex carbohydrates)
Proteins (polypeptides)
Lipids (triglycerides)
water
Electrolytes (Na+, K+)
Vitamins (A,B,C, D,E,K)
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
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
Salivary Amylase and Pancratic AlphaAmylase
 From parotid and submandibular salivary
glands and pancreas
 Breaks down starches (complex
carbohydrates)
 Produces:
 disaccharides (2 simple sugars)
 trisaccharides (3 simple sugars)
Carbohydrates
 Fragment disaccharides and
trisaccharides into monosaccharides
(simple sugars):
 lactase splits lactose into glucose and
galactose
Absorption of Monosaccharides
 Intestinal epithelium absorbs
monosaccharides
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
Triglycerides
 Are the most important and abundant
dietary lipids
 Consist of 3 fatty acids attached to 1
molecule glycerol
Lipid Absorption
 Triglycerides and other absorbed
molecules are coated with proteins:
 creating chylomicrons
 Lacteals pick up chylomicrons and they
later enter left subclavian vein
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
Protein Digestion (2 of 2)
 pepsin:
 proteolytic enzyme
 works at pH 1.5–2.0
 breaks peptide bonds within polypeptide chain
 when chyme enters duodenum:
 enterokinase from small intestine triggers
conversion of trypsinogen to trypsin
 pH is adjusted to 7–8
 Other pancreatic proteases work through SI
Digestive
Secretion
and
Absorption
Figure 24–27
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
Vitamins
 Are organic compounds required in very
small quantities
 Are divided in 2 major groups:
 fat-soluble vitamins
 water-soluble vitamins
Vitamin B12
 Cannot be absorbed by intestinal mucosa
in normal amounts:
 unless bound to intrinsic factor (glycoprotein
secreted by parietal cells of stomach)
5 Effects of Aging on
the Digestive System
1. 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
decreases:

peristaltic contractions become weaker
5 Effects of Aging on
the Digestive System
3. Cumulative damage from toxins (alcohol,
other chemicals) to liver and pancreas
4. Rates of colon cancer and stomach
cancer rise
5. Decline in olfactory and gustatory
sensitivities:

lead to dietary changes that affect entire
body