Download The Digestive System What are the organs of the digestive system

5
Unit
Environmental Exchange
Fundamentals of
Anatomy & Physiology
Frederic H. Martini
PowerPoint® Lecture Slides prepared by
Professor Albia Dugger, Miami-Dade College, Miami, FL
Professor Robert R. Speed, Ph.D., Wallace Community College, Dothan, AL
Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings
Chapter 24:
The Digestive System
What are the organs of
the digestive system and
their major functions?
Organisms

Acquire nutrients from environment
Anabolism

Uses raw materials to synthesize essential
compounds
Catabolism

Decomposes substances to provide energy
cells need to function
Catabolic Reactions

Require two essential ingredients:
1.
2.
oxygen
organic molecules broken down by intracellular
enzymes:

e.g., carbohydrates, fats, and proteins
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
Digestive Tract

Passes through:






pharynx
esophagus
stomach
small intestine
large intestine
anus
6 Functions of the
Digestive System
1.
Ingestion:

occurs when materials enter digestive tract via
the mouth
6 Functions of the
Digestive System
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
6 Functions of the
Digestive System
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 Functions of the
Digestive System
6.
Excretion:

removal of waste products from body fluids
Lining of the Digestive Tract

Protects surrounding tissues against:



corrosive effects of digestive acids and enzymes
mechanical stresses, such as abrasion
bacteria
Bacteria


Is ingested with food or resides in digestive
tract
Attacked by macrophages, and immune
system cells
Peritoneal Cavity


Is located within the abdominopelvic cavity
Lined with serous membrane consisting of:

superficial mesothelium covering a layer of
areolar tissue
Divisions of the
Serous Membrane

Serosa, or visceral peritoneum:


covers organs within peritoneal cavity
Parietal peritoneum:

lines inner surfaces of body wall
Peritoneal Fluid




Is produced by serous membrane lining
Provides essential lubrication
Separates parietal and visceral surfaces
Allows sliding without friction or irritation
Mesenteries
Figure 24–2a, b
Mesenteries
Figure 24–2c, d
Mesenteries (1 of 3)


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
Mesenteries (2 of 3)

Areolar tissue between mesothelial
surfaces:


provides an access route to and from the
digestive tract
for passage of blood vessels, nerves, and
lymphatic vessels
Mesenteries (3 of 3)


Stabilize positions of attached organs
Prevent intestines from becoming
entangled
The Lesser Omentum


Stabilizes position of stomach
Provides access route for blood vessels
and other structures entering or leaving
liver
The Dorsal Mesentery


Is on ventral surface of stomach
Enlarges to form an enormous pouch,
called the greater omentum
The Greater Omentum (1 of 2)

Extends inferiorly between:



the body wall and
the anterior surface of small intestine
Hangs like an apron:

from lateral and inferior borders of stomach
The Greater Omentum (2 of 2)

Adipose tissue in greater omentum:




conforms to shapes of surrounding organs
pads and protects surfaces of abdomen
provides insulation to reduce heat loss
stores lipid energy reserves
The Mesentery Proper (1 of 2)




Is a thick mesenterial sheet
Provides stability
Permits some independent movement
Suspends all but first 25 cm of small
intestine
The Mesentery Proper (2 of 2)


Is associated with initial portion of small
intestine (duodenum) and pancreas
Fuses with posterior abdominal wall,
locking structures in position
The Mesocolon

A mesentery associated with a portion of
the large intestine:


transverse mesocolon supports transverse colon
sigmoid mesocolon supports sigmoid colon
What is the
functional histology of
the digestive system?
Histological Organization
of the Digestive Tract

Major layers of the digestive tract:




mucosa
submucosa
muscularis externa
serosa
Structure of the Digestive Tract
Figure 24–3
The Mucosa


Is the inner lining of digestive tract
Is a mucous membrane consisting of:

epithelium, moistened by glandular secretions
The Digestive Epithelium (1 of 3)

Mucosal epithelium is simple or stratified:

depending on location, function, and stresses
The Digestive Epithelium (2 of 3)

Oral cavity, pharynx, and esophagus:


mechanical stresses
lined by stratified squamous epithelium
The Digestive Epithelium (3 of 3)

Stomach, small intestine, and most of large
intestine:


absorption
simple columnar epithelium with goblet cells
Enteroendocrine Cells


Are scattered among columnar cells of
digestive epithelium
Secrete hormones that:

coordinate activities of the digestive tract and
accessory glands
Lining of Digestive Tract

Folding increases surface area for
absorption:
1.
2.
longitudinal folds, disappear as digestive tract
fills
permanent transverse folds (plicae)
The Lamina Propria

Consists of a layer of areolar tissue that
contains:





blood vessels
sensory nerve endings
lymphatic vessels
smooth muscle cells
scattered areas of lymphoid tissue
Muscularis Mucosae


Narrow band of smooth muscle and elastic
fibers in lamina propria
Smooth muscle cells arranged in 2
concentric layers:


inner layer encircles lumen (circular muscle)
outer layer contains muscle cells parallel to tract
(longitudinal layer)
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
Muscularis Externa Structure


Is dominated by smooth muscle cells
Are arranged in:


inner circular layer
outer longitudinal layer
Muscularis Externa Function

Involved in:



mechanical processing
movement of materials along digestive tract
Movements coordinated by enteric nervous
system (ENS):



sensory neurons
interneurons
motor neurons
The Serosa

Serous membrane covering muscularis
externa:

except in oral cavity, pharynx, esophagus, and
rectum
Adventitia



Covers muscularis externa of oral cavity,
pharynx, esophagus, and rectum
Is a dense sheath of collagen fibers
Firmly attaches the digestive tract to
adjacent structures
How do materials move through
the digestive system?
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
Pacesetter Cells

Located in muscularis mucosae and
muscularis externa:

surrounding lumen of digestive tract
Peristalsis
Figure 24–4
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
Peristaltic Motion
Circular muscles contract behind bolus:
1.

while circular muscles ahead of bolus relax
Longitudinal muscles ahead of bolus contract:
2.

shortening adjacent segments
Wave of contraction in circular muscles:
3.

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
What mechanisms
regulate digestion?
Control of Digestive Function



Neural mechanisms
Hormonal mechanisms
Local mechanisms
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
activities of other systems
Digestive Hormones



Are peptides
Are produced by enteroendocrine cells in
digestive tract
Reach target organs after distribution in
bloodstream
Local Mechanisms (1 of 2)



Prostaglandins, histamine, and other
chemicals
Released into interstitial fluid
Affect adjacent cells within small segment
of digestive tract
Local Mechanisms (2 of 2)

Coordinating response to changing
conditions:


e.g., variations in local pH, chemical, or physical
stimuli
Affect only a portion of tract
What is the anatomy
of the oral cavity?
The Oral Cavity
Figure 24–6
4 Functions of the Oral Cavity

Sensory analysis:


of material before swallowing
Mechanical processing:

through actions of teeth, tongue, and palatal
surfaces
4 Functions of the Oral Cavity

Lubrication:


mixing with mucus and salivary gland secretions
Limited digestion:

of carbohydrates and lipids
Oral Mucosa


Lining of oral cavity
Has stratified squamous epithelium
Layer of Keratinized Cells

Covers only regions exposed to severe
abrasion
The Epithelial Lining

Of cheeks, lips, and inferior surface of
tongue:

is relatively thin, nonkeratinized, and delicate
The Mucosa Inferior
to the Tongue

Is thin and vascular enough to rapidly
absorb lipid-soluble drugs
The Mucosae of the Cheeks

Are supported by pads of fat and the
buccinator muscles
Labia


Also called lips
Anteriorly, the mucosa of each cheek is
continuous with that of the lips
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
Hard Palate

Is formed by:


palatine processes of maxillary bones
horizontal plates of palatine bones
Raphe


Prominent central ridge
Extends along midline of hard palate
Mucosae

Mucosa lateral and anterior to raphe:


is thick, with complex ridges
Thin, delicate mucosa:


covers posterior margin of hard palate
extends onto soft palate
Soft Palate

Lies posterior to hard palate
Uvula



A dangling process
Helps prevent food from entering pharynx
prematurely
Is supported by posterior margin of soft
palate
Tonsil

Lies between palatoglossal and
palatopharyngeal arches, on each side
What are the major
structures and functions of
the regions of the oral cavity?
The Oral Cavity
Figure 24–6
The Tongue


Manipulates materials inside mouth
May bring foods into oral cavity
4 Functions of the Tongue
1.
Mechanical processing:

2.
compression, abrasion, and distortion
Manipulation:


assists in chewing
prepares material for swallowing
4 Functions of the Tongue
3.
Sensory analysis:

4.
touch, temperature, and taste receptors
Secretion:


mucins
enzyme lingual lipase
Lingual Papillae



Fine projections on superior surface
(dorsum) of tongue
Covered in thick epithelium
Assists in moving materials
Lingual Frenulum



Is a thin fold of mucous membrane along
inferior midline
Prevents extreme movements of the
tongue
Connects:

body of tongue to floor of oral cavity
Sublingual Glands



Small glands extend into underlying lamina
propria
Secretions flush tongue’s epithelium
Contain water, mucins, and enzyme lingual
lipase
Lingual Lipase


Enzyme, works over broad pH range (3.0–
6.0)
Starts lipid digestion immediately
The Salivary Glands
Figure 24–7
Salivary Glands


3 pairs secrete into oral cavity
Each pair has distinctive cellular
organization:

and produces saliva with different properties
Parotid Salivary Glands


Inferior to zygomatic arch
Produce serous secretion:


enzyme salivary amylase (breaks down starches)
Drained by parotid duct (Stensen’s duct):

which empties into vestibule at second molar
Sublingual Salivary Glands


Covered by mucous membrane of floor of
mouth
Produce mucous secretion:

buffer and lubricant
Submandibular Salivary Glands



In floor of mouth
Secrete buffers, glycoproteins (mucins),
and salivary amylase
Submandibular ducts (Wharton’s ducts):


open immediately posterior to teeth
either side of lingual frenulum
Salivary Glands

Produce 1.0–1.5 liters of saliva each day:



70% by submandibular glands
25% by parotids
5% by sublingual glands
Saliva


99.4% water
0.6% includes:






electrolytes (Na+, Cl—, and HCO3—)
buffers
glycoproteins (mucins)
antibodies
enzymes
waste products
Mucins


Glycoproteins
Responsible for lubricating action
4 Functions of Saliva
1.
2.
3.
Lubricating the mouth
Moistening and lubricating materials in the
mouth
Dissolving chemicals that:


stimulate taste buds
provide sensory information
4 Functions of Saliva
4. Initiate digestion of:


complex carbohydrates by enzyme salivary
amylase (ptyalin or alpha-amylase)
lipids by enzyme lingual lipase
Teeth
Figure 24–8
The Teeth


Tongue movements pass food across
occlusal surfaces of teeth
Chew (masticate) food
Dental Arches

Contain 4 types of teeth:
1.
2.
3.
4.
incisors
cuspids (canines)
bicuspids (premolars)
molars
Incisors




Blade-shaped teeth
Located at front of mouth
Used for clipping or cutting
Have a single root
Cuspids (Canines)





Conical
Sharp ridgeline
Pointed tip
Used for tearing or slashing
Have a single root
Bicuspids (Premolars)




Flattened crowns
Prominent ridges
Used to crush, mash, and grind
Have 1 or 2 roots
Molars




Very large, flat crowns
With prominent ridges
Used for crushing and grinding
Have 3 or more roots
Primary and
Secondary Dentitions
Figure 24–9
Dental Succession

During embryonic development, 2 sets of
teeth form:


primary dentition, or deciduous teeth
secondary dentition, or permanent dentition
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
Permanent Dentition




Also called secondary dentition
Replaces deciduous teeth
32 permanent teeth
8 on each side, upper and lower:



2 incisors
1 cuspid
5 molars
Mastication


Also called chewing
Food is forced from oral cavity to vestibule
and back:

crossing and recrossing occlusal surfaces
Muscles of Mastication



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
PLAY
3D Movie of Digestive System
What are the anatomy and
functions of the pharynx?
The Pharynx

A common passageway for solid food,
liquids, and air
Regions of the Pharynx




Nasopharynx
Oropharynx
Laryngopharynx
Food passes through oropharynx and
laryngopharynx to esophagus
What are the anatomy and
functions of the esophagus?
The Esophagus
Figure 24–10
The Esophagus


A hollow muscular tube
About 25 cm long and 2 cm wide
The Esophagus



Conveys solid food and liquids to the
stomach
Begins posterior to cricoid cartilage
Is innervated by fibers from the esophageal
plexus
Resting Muscle Tone

In the circular muscle layer in the superior 3
cm of esophagus prevents air from entering
Histology of the Esophagus

Wall of esophagus has 3 layers:



mucosal
submucosal
muscularis
5 Characteristics of
the Esophageal Wall
1.
Mucosa contains:

nonkeratinized and stratified squamous
epithelium
5 Characteristics of
the Esophageal Wall
2.
Mucosa and submucosa:

form large folds that extend the length of the
esophagus
5 Characteristics of
the Esophageal Wall
3.
Muscularis mucosae:

consists of irregular layer of smooth muscle
5 Characteristics of
the Esophageal Wall
4.
Submucosa contains esophageal glands:


which produce mucous secretion
reduces friction between bolus and esophageal
lining
5 Characteristics of
the Esophageal Wall
5.
Muscularis externa:

has usual inner circular and outer longitudinal
layers
The Swallowing Process
Figure 24–11
Swallowing




Also called deglutition
Can be initiated voluntarily
Proceeds automatically
Is divided in 3 phases:



buccal phase
pharyngeal phase
esophageal phase
The Buccal Phase


Compression of bolus against hard palate
Retraction of tongue forces bolus into
oropharynx:


assists elevation of soft palate
seals off nasopharynx
The Pharyngeal Phase

Bolus contacts:



palatoglossal arches
palatopharyngeal arches
posterior pharyngeal wall
The Swallowing Reflex

Passage of the bolus stimulates tactile
receptors on palatal arches and uvula
The Esophageal Phase

Contraction of pharyngeal muscles forces
bolus through entrance to esophagus
Primary Peristaltic Waves

Movements coordinated by afferent and
efferent fibers in glossopharyngeal and
vagus nerves
What is the anatomy of the
stomach, its histological
features, and its roles in
digestion and absorption?
4 Functions of the Stomach
1.
2.
Storage of ingested food
Mechanical breakdown of ingested food
4 Functions of the Stomach
3.
Disruption of chemical bonds in food
material:

by acids and enzymes
4 Functions of the Stomach
4. Production of intrinsic factor:

glycoprotein required for absorption of vitamin
B12 in small intestine
The Stomach
Figure 24–12a
The Stomach
Figure 24–12b
Anatomy of the Stomach (1 of 3)

The stomach is shaped like an expanded J:


short lesser curvature forms medial surface
long greater curvature forms lateral surface
Anatomy of the Stomach (2 of 3)


Anterior and posterior surfaces are
smoothly rounded
Shape and size vary:


from individual to individual
from 1 meal to the next
Anatomy of the Stomach (3 of 3)

Stomach typically extends between levels
of vertebrae T7 and L3
4 Regions of the Stomach




Cardia
Fundus
Body
Pylorus
Smooth Muscle

Muscularis mucosae and muscularis
externa:


contain extra layers of smooth muscle cells
in addition to circular and longitudinal layers
The Stomach Lining
Figure 24–13
Histology of the Stomach


Simple columnar epithelium lines all
portions of stomach
Epithelium is a secretory sheet:

produces mucus that covers interior surface of
stomach
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
Gastric Glands

In fundus and body of stomach:


extend deep into underlying lamina propria
Each gastric pit communicates with several
gastric glands
2 Types of Secretory Cells

In gastric glands:


parietal cells
chief cells
The Secretion of
Hydrochloric Acid
Figure 24–14
Parietal and Chief Cells

Secrete hydrochloric acid (HCl)
Chief Cells

Are most abundant near base of gastric
gland:

secrete pepsinogen (inactive proenzyme)
Pepsinogen

Is converted by HCl in the gastric lumen:

to pepsin (active proteolytic enzyme)
Pyloric Glands

In the pylorus:

produce mucous secretion
Enteroendocrine Cells

Are scattered among mucus-secreting
cells:


G cells
D cells
G Cells


Abundant in gastric pits of pyloric antrum
Produce gastrin
D Cells


In pyloric glands
Release somatostatin:

hormone that inhibits release of gastrin
The Phases of Gastric Secretion
Figure 24–15
The Phases of Gastric Secretion
Table 24–1
3 Phases of Gastric Secretion



Cephalic phase
Gastric phase
Intestinal phase
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
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 (1 of 2)

Stomach performs preliminary digestion of
proteins by pepsin:


some digestion of carbohydrates (by salivary
amylase)
lipids (by lingual lipase)
Digestion in the Stomach (2 of 2)

Stomach contents:




become more fluid
pH approaches 2.0
pepsin activity increases
protein disassembly begins
Absorption in the Stomach

Although digestion occurs in the stomach,
nutrients are not absorbed there
What are the anatomical and
histological characteristics
of the small intestine?
Associated Glandular Organs

Stomach:



gastric juices
stomach acids
pepsin

Pancreas:



digestive enzymes
buffers
Liver:

bile
Bile




Produced in liver
Contains buffers and bile salts
Stored in gallbladder
Discharge into small intestine
Segments of the Intestine
Figure 24–16
The Small Intestine


Plays key role in digestion and absorption
of nutrients
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
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
The Intestinal Wall
Figure 24–17
What are the functions of
the intestinal secretions and how
are these secretory activities
regulated?
Plicae



Also called plicae circulares
Transverse folds in intestinal lining
Are permanent features:

Do not disappear when small intestine fills
Intestinal Villi

A series of fingerlike projections:


in mucosa of small intestine
Covered by simple columnar epithelium:

covered with microvilli
Intestinal Glands


Goblet cells between columnar epithelial
cells
Eject mucins onto intestinal surfaces
Brush Border Enzymes



Integral membrane proteins
On surfaces of intestinal microvilli
Break down materials in contact with brush
border
Enterokinase


A brush border enzyme
Activates pancreatic proenzyme
trypsinogen
Enteroendocrine Cells


In intestinal glands
Produce intestinal hormones:



gastrin
cholecystokinin
secretin
Brunner’s Glands


Submucosal glands of duodenum
Produce copious mucus:

when chyme arrives from stomach
The Duodenum


Has few plicae
Small villi
Functions of the Duodenum


To receive chyme from stomach
To neutralize 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
Peristaltic Contractions



Myenteric reflexes
Not under CNS control
Parasympathetic stimulation:

accelerates local peristalsis and segmentation
The Gastroenteric Reflex

Stimulates motility and secretion:

along entire small intestine
The Gastroileal Reflex


Triggers relaxation of ileocecal valve:
Allows materials to pass:

from small intestine into large intestine
What are the structure,
functions, and regulation
of the accessory
digestive organs?
The Pancreas

A compound tubuloalveolar gland
Figure 24–18
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
Regions of the Pancreas

Head:



Body:



broad
in loop of duodenum
slender
extends toward spleen
Tail:

short and rounded
Duct of Wirsung


Large pancreatic duct
Delivers digestive enzymes and buffers to
duodenum
Common Bile Duct


From the liver and gallbladder
Meets pancreatic duct near duodenum
Duodenal Ampulla

Chamber that receives secretions from:



common bile duct
pancreatic duct
Located halfway along length of duodenum
Pancreatic Islets


Endocrine tissues of pancreas
Scattered (1% of pancreatic cells)
Functions of the Pancreas
1.
Endocrine cells:


2.
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
Pancreatic Enzymes (1 of 4)

Pancreatic alpha-amylase:



a carbohydrase
breaks down starches
similar to salivary amylase
Pancreatic Enzymes (2 of 4)

Pancreatic lipase:


breaks down complex lipids
releases products (e.g., fatty acids) that are
easily absorbed
Pancreatic Enzymes (3 of 4)

Nucleases:

break down nucleic acids
Pancreatic Enzymes (4 of 4)

Proteolytic enzymes:



break certain proteins apart
proteases break large protein complexes
peptidases break small peptides into amino acids
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
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
The Anatomy of the Liver
Figure 24–19
Anatomy of the Liver



Is wrapped in tough fibrous capsule
Is covered by visceral peritoneum
Is divided into lobes
The Falciform Ligament


On anterior surface
Divides left lobe and right lobe
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
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
Hepatocyte Function

As blood flows through sinusoids:


hepatocytes absorb solutes from plasma
and secrete materials such as plasma proteins
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
The Common Bile Duct

Is formed by union of:





cystic duct
common hepatic duct
Passes within the lesser omentum toward
stomach
Penetrates wall of duodenum
Meets pancreatic duct at duodenal ampulla
3 Functions of the Liver
1.
2.
3.
Metabolic regulation
Hematological regulation
Bile production
Metabolic Regulation

The liver regulates:
1.
2.
3.
4.
5.
composition of circulating blood
nutrient metabolism
waste product removal
nutrient storage
drug inactivation
Composition of
Circulating Blood (1 of 3)

All blood leaving absorptive surfaces of
digestive tract:


enters hepatic portal system
flows into the liver
Composition of
Circulating Blood (2 of 3)

Liver cells extract nutrients or toxins from
blood:

before it reaches systemic circulation through
hepatic veins
Composition of
Circulating Blood (3 of 3)

Liver removes and stores excess nutrients:

corrects nutrient deficiencies by mobilizing stored
reserves or performing synthetic activities
Metabolic Activities of the Liver







Carbohydrate metabolism
Lipid metabolism
Amino acid metabolism
Waste product removal
Vitamin storage
Mineral storage
Drug inactivation
The Liver and Hematological
Regulation (1 of 2)



Largest blood reservoir in body
Receives 25% of cardiac output
Performs 6 hematological regulation
functions
The Liver and Hematological
Regulation (2 of 2)
1.
2.
3.
4.
5.
6.
Phagocytosis and antigen presentation
Synthesis of plasma proteins
Removal of circulating hormones
Removal of antibodies
Removal or storage of toxins
Synthesis and secretion of bile
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
Functions of Bile

Bile salts break droplets apart
(emulsification):


increases surface area exposed to enzymatic
attack
creates tiny emulsion droplets coated with bile
salts
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
Is located in the fossa on the posterior
surface of the liver’s right lobe
The Cystic Duct


Extends from gallbladder
Union with common hepatic duct forms
common bile duct
The Common Bile Duct

Meets pancreatic duct at duodenum:

before emptying into duodenal ampulla
The Duodenal Ampulla

Receives:



buffers and enzymes from pancreas
bile from the liver and gallbladder
Opens into duodenum at duodenal papilla
The Gallbladder


Stores bile
Releases bile into duodenum:

only under stimulation of hormone
cholecystokinin (CCK)
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
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
The Gallbladder
and Bile Modification


Full gallbladder contains 40–70 ml bile
Bile composition gradually changes in
gallbladder:


water is absorbed
bile salts and solutes become concentrated
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
Intestinal Hormones

Intestinal tract secretes peptide hormones
with multiple effects:


in several regions of digestive tract
in accessory glandular organs
Hormones of Duodenal
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 and buffers by
liver and pancreas
Cholecystokinin (CCK)

Is secreted in duodenum:


when chyme contains lipids and partially
digested proteins
Accelerates pancreatic production and
secretion of digestive enzymes
Cholecystokinin (CCK)

Relaxes hepatopancreatic sphincter and
gallbladder:

ejecting bile and pancreatic juice into duodenum
Gastric Inhibitory Peptide (GIP)

Is secreted when fats and carbohydrates
enter small intestine
Vasoactive Intestinal
Peptide (VIP)



Stimulates secretion of intestinal glands
Dilates regional capillaries
Inhibits acid production in stomach
Gastrin

Is secreted by G cells in duodenum:



when exposed to incompletely digested proteins
Promotes increased stomach motility
Stimulates acids and enzyme production
Enterocrinin


Is released when chyme enters small
intestine
Stimulates mucin production by
submucosal glands of duodenum
Intestinal Absorption (1 of 2)

It takes about 5 hours for materials
to pass:


from duodenum
to end of ileum
Intestinal Absorption (2 of 2)

Movements of the mucosa increases
absorptive effectiveness:


stir and mix intestinal contents
constantly change environment around epithelial
cells
Movements of the Mucosa

Microvilli:


are moved by supporting microfilaments
Individual villi:

are moved by smooth muscle cells
Movements of the Mucosa

Groups of villi:


are moved by muscularis mucosae
Plicae:

are moved by muscularis mucosae and
muscularis externa
What is the gross and
histological structure
of the large intestine?
The Large Intestine
Figure 24–23
The Large Intestine




Is horseshoe-shaped
Extends from end of ileum to anus
Lies inferior to stomach and liver
Frames the small intestine
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
The Large Intestine


Also called large bowel
Is about 1.5 meters long and 7.5 cm wide
3 Parts of the Large Intestine
1.
Cecum:

2.
Colon:

3.
the pouchlike first portion
the largest portion
Rectum:

the last 15 cm of digestive tract
What are the
regional specializations
of the large intestine?
The Ileum


Attaches to the medial surface of cecum
Opens into the cecum at the ileocecal valve
The Cecum



Is an expanded pouch
Receives material arriving from the ileum
Stores materials and begins compaction
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
The Colon


Has a larger diameter and thinner wall than
small intestine
The wall of the colon:


forms a series of pouches (haustra)
Haustra permit expansion and elongation of
colon
Colon Muscles

3 longitudinal bands of smooth muscle
(taeniae coli):




run along outer surfaces of colon
deep to the serosa
similar to outer layer of muscularis externa
Muscle tone in taeniae coli creates the
haustra
4 Regions of the Colon
1.
2.
3.
4.
Ascending colon
Transverse colon
Descending colon
Sigmoid colon
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
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
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
The Anus



Also called anal orifice
Is exit of the anal canal
Has keratinized epidermis like skin
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
Mucosa and Glands of the Colon
Figure 24–24
Characteristics of the Colon



Lack of villi
Abundance of goblet cells
Presence distinctive intestinal glands
Glands of the Large Intestine


Are deeper than glands of small intestine
Are dominated by goblet cells
Mucosa of the Large Intestine


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
What is the significance
of the large intestine in
the absorption of nutrients?
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
Absorption of organic wastes
Vitamins



Are organic molecules
Important as cofactors or coenzymes in
metabolism
Normal bacteria in colon make 3 vitamins
that supplement diet
3 Vitamins Produced
in the Large Intestine
1.
Vitamin K:


a fat-soluble vitamin
required by liver for synthesizing 4 clotting
factors, including prothrombin
3 Vitamins Produced
in the Large Intestine
2.
Biotin:


a water-soluble vitamin
important in glucose metabolism
3 Vitamins Produced
in the Large Intestine
3.
Pantothenic acid:


a water-soluble vitamin
required in manufacture of steroid hormones
and some neurotransmitters
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
Organic Wastes

Bacteria break down peptides in feces and
generate:

ammonia:


indole and skatole:


as soluble ammonium ions
nitrogen compounds responsible for odor of feces
hydrogen sulfide:

gas that produces “rotten egg” odor
Organic Wastes

Bacteria feed on indigestible carbohydrates
(complex polysaccharides):

produce flatus, or intestinal gas, in large intestine
The Defecation Reflex
Figure 24–25
Movements of the
Large Intestine (1 of 4)

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
Movements of the
Large Intestine (2 of 4)


Peristaltic waves move material along
length of colon
Segmentation movements (haustral
churning) mix contents of adjacent haustra
Movements of the
Large Intestine (3 of 4)


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
Movements of the
Large Intestine (4 of 4)

Distension of the rectal wall triggers
defecation reflex:


2 positive feedback loops
both loops triggered by stretch receptors in
rectum
2 Positive Feedback Loops
1.
Short reflex:

2.
triggers peristaltic contractions in rectum
Long reflex:


coordinated by sacral parasympathetic system
stimulates mass movements
Rectal Stretch Receptors

Trigger 2 reflexes important to voluntary control of
defecation:

a long reflex:



mediated by parasympathetic innervation in pelvic nerves
causes relaxation of internal anal sphincter
a somatic reflex:


motor commands carried by pudendal nerves
stimulates contraction of external anal sphincter (skeletal
muscle)
Elimination of Feces



Requires relaxation of internal and external
anal sphincters
Reflexes open internal sphincter, close
external sphincter
Opening external sphincter requires
conscious effort
What nutrients are
required by the body?
Essential Nutrients

A typical meal contains:






carbohydrates
proteins
lipids
water
electrolytes
vitamins
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
What chemical events are
responsible for digestion
of organic nutrients?
Summary: Chemical
Events in Digestion
Figure 24–26
Processing Nutrients (1 of 2)

The digestive system:


breaks down physical structure of food
disassembles component molecules
Processing Nutrients (2 of 2)

Molecules released into bloodstream are:


absorbed by cells
Broken down to provide energy for ATP
synthesis:

used to synthesize carbohydrates, proteins, and
lipids
Digestive Enzymes (1 of 4)

Are secreted by:




salivary glands
tongue
stomach
pancreas
Digestive Enzymes (2 of 4)

Break molecular bonds in large organic
molecules:


carbohydrates, proteins, lipids, and nucleic acids
in a process called hydrolysis
Digestive Enzymes (3 of 4)

Are divided into classes by targets:

carbohydrases:


proteases:


break bonds between simple sugars
break bonds between amino acids
lipases:

separate fatty acids from glycerides
Digestive Enzymes (4 of 4)

Brush border enzymes break nucleotides
into:



sugars
phosphates
nitrogenous bases
What mechanisms are involved
in the absorption of organic and
inorganic nutrients?
Complex Carbohydrate Digestion

Proceeds in 2 steps:
1.
2.
carbohydrases (from salivary glands and
pancreas)
brush border enzymes
Salivary and Pancreatic Enzymes

Function at pH 6.7–7.5:


salivary amylase
pancreatic alpha-amylase
Salivary Amylase



From parotid and submandibular salivary
glands
Breaks down starches (complex
carbohydrates)
Produces:


disaccharides (2 simple sugars)
trisaccharides (3 simple sugars)
Brush Border Enzymes

Fragment disaccharides and
trisaccharides into monosaccharides
(simple sugars):



maltase splits maltose into 2 glucose
sucrase splits sucrose into glucose and
fructose
lactase splits lactose into glucose and
galactose
Absorption of Monosaccharides

Intestinal epithelium absorbs
monosaccharides:


by facilitated diffusion and cotransport
via a carrier protein
Facilitated Diffusion
vs. Cotransport (1 of 3)

Facilitated diffusion:


moves only 1 molecule or ion through cell
membrane
Cotransport:


moves more than 1 molecule or ion at the same
time
transported materials move in same direction
Facilitated Diffusion
vs. Cotransport (2 of 3)

Facilitated diffusion:


does not require ATP
Cotransport:

may require ATP to preserve cell homeostasis
Facilitated Diffusion
vs Cotransport (3 of 3)

Facilitated diffusion:


will not occur against opposing concentration
gradient
Cotransport:

can occur against opposing concentration
gradient
The Cotransport System

For transporting glucose and sodium ions
into cell:

both sodium ion and glucose molecule must bind
to carrier protein before they can move into cell
Cotransport Systems

For simple sugars and amino acids:


deliver nutrients to cytoplasm
but bring in sodium ions that must be ejected by
the sodium–potassium exchange pump
Facilitated Diffusion

Simple sugars transported into cell at apical
surface:



diffuse through cytoplasm
reach interstitial fluid by facilitated diffusion
across basolateral surfaces
diffuse into capillaries of villus for transport to
liver
Lipid Digestion

Involves:


lingual lipase from glands of tongue
pancreatic lipase from pancreas
Triglycerides


Are the most important and abundant
dietary lipids
Consist of 3 fatty acids attached to 1
molecule glycerol
Lingual and Pancreatic Lipases



Break off 2 fatty acids, leaving
monoglycerides
Are water-soluble enzymes
Attack only exposed surfaces of lipid drops
Lingual Lipase



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

Breaks apart triglycerides:

to form fatty acids and monoglycerides
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
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
pancreatic proteases begin working
Absorption of
Amino Acids (1 of 2)

Dipeptidases:


enzymes on epithelial surfaces of small intestine
break short peptide chains into individual amino
acids
Absorption of
Amino Acids (2 of 2)

After diffusing to basal surface of cell:


amino acids are released into interstitial fluid
by facilitated diffusion and cotransport
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
Absorption of Ions and Vitamins
Table 24–4
Ion Absorption (1 of 5)

Osmosis does not distinguish among
solutes:


determined only by total concentration of solutes
To maintain homeostasis:

concentrations of specific ions must be regulated
Ion Absorption (2 of 5)

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
Ion Absorption (3 of 5)

Potassium ion concentration increases:


as other solutes move out of lumen
Other ions diffuse into epithelial cells along
concentration gradient
Ion Absorption (4 of 5)

Cation absorption (magnesium, iron):


involves specific carrier proteins
cell must use ATP to transport ions to interstitial
fluid
Ion Absorption (5 of 5)

Anions (chloride, iodide, bicarbonate, and
nitrate):


are absorbed by diffusion or carrier mediated
transport
Phosphate and sulfate ions:

enter epithelial cells by active transport
Vitamins


Are organic compounds required in very
small quantities
Are divided in 2 major groups:


fat-soluble vitamins
water-soluble vitamins
Fat-Soluble Vitamins


Vitamins A, D, E, and K
Their structure allows them to dissolve in
lipids
Water-Soluble Vitamins

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)
What are the effects of aging on
the digestive system?
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
5 Effects of Aging on
the Digestive System
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) absorbed by digestive
tract and transported to liver for
processing
5 Effects of Aging on
the Digestive System
4.
Rates of colon cancer and stomach
cancer rise with age:

oral and pharyngeal cancers common among
elderly smokers
5 Effects of Aging on
the Digestive System
5.
Decline in olfactory and gustatory
sensitivities:

lead to dietary changes that affect entire body
The Digestive System
and Other Systems
Figure 24–28
SUMMARY (1 of 9)

Digestive system:


digestive tract
accessory organs
SUMMARY (2 of 9)

Digestive system functions:






ingestion
mechanical processing
digestion
secretion
absorption
excretion
SUMMARY (3 of 9)


Oral cavity
Buccal cavity:


Tongue:




oral mucosa
intrinsic tongue muscles
extrinsic tongue muscles
Salivary glands
Teeth
SUMMARY (4 of 9)



Pharynx
Esophagus
Stomach:



cephalic phase
gastric phase
intestinal phase
SUMMARY (5 of 9)

Small intestine:


gastroenteric reflex
gastroileal reflex
SUMMARY (6 of 9)


Pancreas
Liver:


bile
Gallbladder
SUMMARY (7 of 9)

Intestinal hormones:






secretin
cholecystokinin (CCK)
gastric inhibitory peptide (GIP)
vasoactive intestinal peptide (VIP)
gastrin
enterocrinin
SUMMARY (8 of 9)

Large intestine:



cecum
colon
rectum
SUMMARY (9 of 9)







Processing and absorption of nutrients
Carbohydrate digestion and absorption
Lipid digestion and absorption
Protein digestion and absorption
Water absorption
Ion absorption
Vitamin absorption