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PowerLecture:
Chapter 7
Digestion and Nutrition
Learning Objectives



Understand the structure and function of the
component parts of the human digestive
system.
Describe how digestion functions in
homeostasis.
Explain how the human body manages to
meet the energy and nutritional needs of the
various body parts even though the person
may be feasting sometimes and fasting at
other times.
Learning Objectives (cont’d)

Summarize the daily nutritional
requirements of a 25-year-old man who
works at a desk job and exercises very little.
State what he needs in terms of energy,
carbohydrates, proteins, and lipids, and
name at least six vitamins and six minerals
that he needs to include in his diet every
day.
Impacts/Issues
Hormones and Hunger
Hormones and Hunger
Sixty percent of Americans are
considered overweight or obese.




Adipose cells store fat to help us
through times when food might be
scarce (which is not often for most
of us in today’s world!)
The hormones leptin and ghrelin
may have effects on hunger and
eating.
The digestive system works with
other organ systems to maintain
homeostasis.
How Would You Vote?
To conduct an instant in-class survey using a classroom response
system, access “JoinIn Clicker Content” from the PowerLecture main
menu.
 Should
fast-food items be required to carry
health warnings?


a. Yes, obesity is becoming an epidemic and
warning labels will help alleviate the crisis.
b. No, it is up to individuals to use common
sense and avoid too much fat or too many
calories.
Section 1
The Digestive System:
An Overview
The Digestive System: An Overview
The digestive system (gastrointestinal
tract or GI tract) is a 6-9 meter long tube
where food processing takes place to bring
nutrients into the body.



Food is transported through the lumen of the
tube from the mouth to the anus.
Specialized regions include the mouth,
pharynx, esophagus, stomach, small intestine,
large intestine, rectum, and anus.
The Digestive System: An Overview
The digestive tube has four layers.




The mucosa is the epithelial lining that faces
the lumen of the gut.
Underlying the mucosa is the submucosa, a
layer of connective tissue containing blood and
lymph vessels plus nerve networks.
A smooth muscle layer (longitudinal and
circular) lies outside the submucosa.
The Digestive System: An Overview


The serosa forms the outer covering; it is a thin
serous membrane.
Circular arrays of smooth muscle make up the
sphincters, which close off a passageway to
control forward movement of the tract’s
contents.
lumen
mucosa
submucosa
circular
muscle
cells
smooth
muscle
layer
longitudinal
muscle
cells
serosa
Fig. 7.2, p. 121
The Digestive System: An Overview
Digestive system operations contribute to
homeostasis in key ways.




Mechanical processing and motility include
movements that break up, mix, and transport
ingested food material.
Secretion is the release of needed enzymes
and other fluids.
Digestion is the chemical breakdown of food
matter to molecules small enough to cross the
gut lining.
The Digestive System: An Overview


Absorption is the passage of digested
nutrients into the blood and lymph.
Elimination is the expulsion of undigested and
unabsorbed residues from the end of the GI
tract.
food, water intake
oxygen intake
DIGESTIVE
SYSTEM
nutrients,
water,
salts
RESPIRATORY
SYSTEM
oxygen
carbon
dioxide
CIRCULATORY
SYSTEM
elimination
of food
residues
elimination
of carbon
dioxide
URINARY
SYSTEM
water,
solutes
rapid transport
to and from all
living cells
elimination of
excess water,
salts, wastes
Fig. 7.3, p. 121
Major Components:
Mouth (Oral cavity)
Accessory Organs:
Salivary Glands
Pharynx
Esophagus
Stomach
Liver
Gallbladder
Pancreas
Small intestine
Large intestine (colon)
Rectum
Anus
Fig. 7.1, p.120
Section 2
Chewing and
Swallowing: Food
Processing Begins
Chewing and Swallowing: Food
Processing Begins
Processing starts with the teeth and salivary
gland enzymes.


Mechanical breakdown of food begins in the
mouth (oral cavity).
•
•
•
Teeth (32 in adults, 20 in children) chew the food.
Each tooth has an enamel coat (crown), a dentin
core, and an inner pulp.
Incisors bite off chunks, cuspids tear, and molars and
premolars grind food.
molars (12)
premolars (8)
canines (4)
incisors (8)
lower jaw
upper jaw
Fig. 7.4a, p. 122
enamel
crown
dentin
pulp cavity
(contains
nerves and
blood vessels)
gingiva
(gum)
root canal
periodontal
membrane
root
bone
Fig. 7.4b, p. 122
Chewing and Swallowing: Food
Processing Begins
Salivary glands secrete a fluid (saliva)
into the mouth.



Parotid glands lie just in front of the ear;
submandibular glands are located in the lower
jaw; sublinguals are under the tongue.
Saliva contains salivary amylase to begin
carbohydrate digestion, bicarbonate to
neutralize acids, and mucins to help form the
food into a bolus.
parotid gland
submandibular gland
sublingual gland
Fig. 7.4c, p. 122
Chewing and Swallowing: Food
Processing Begins

The tongue presses food against the hard
palate of the roof of the mouth to aid mixing,
then muscle contractions force the bolus into
the pharynx (throat) and then into the
esophagus.
Chewing and Swallowing: Food
Processing Begins
Swallowing has voluntary and involuntary
phases.




The tongue (with the help of the palate) pushes
the bolus into the pharynx where receptors
initiate the swallowing reflex into the
esophagus.
The epiglottis closes off the trachea to prevent
choking.
Muscle contractions called peristalsis move
the food along the tract to the stomach.
VOLUNTARY
PHASE
INVOLUNTARY
PHASE
hard
palate
food
bolus
epiglottis
Contracted
muscles close
off esophagus.
trachea
(airway) open
Larynx rises; trachea
closes, and muscle
contractions squeeze
food into esophagus.
Fig. 7.5a, p. 123
muscles
relaxed
Circular muscles
contract, squeezing
bolus toward
the stomach.
stomach
muscles
relaxed
bolus
Lower
esophageal
sphincter
opens and food
enters stomach.
Fig. 7.5bc, p. 123
Section 3
The Stomach: Food
Storage, Digestion, and
More
The Stomach: Food Storage, Digestion,
and More
The stomach is a muscular sac that stores
and mixes food, secretes substances to
dissolve and degrade food, and controls the
rate at which food enters the small intestine.


Cells of the stomach
lining secrete intrinsic
factor, required for
absorption of vitamin B12.
Figure 7.6a
esophagus
serosa
1
pyloric sphincter
longitudinal
muscle
circular
muscle
2
oblique
muscle
submucosa
3
duodenum
mucosa
Fig. 7.6, p. 124
The Stomach: Food Storage, Digestion,
and More

Gastric juice includes hydrochloric acid,
pepsinogens, and mucus; the hormone gastrin
stimulates secretion of these materials.
•
•

HCl dissolves bits of food to form a thick mixture
called chyme; it also converts pepsinogen (inactive)
to pepsin (active), and kills microbes.
Pepsin begins the digestion of proteins.
Normally, mucus and bicarbonate ions protect
the stomach lining; however, a bacterium
(Helicobacter pylori) can interfere with mucus
production, resulting in a peptic ulcer.
The Stomach: Food Storage, Digestion,
and More
Peristaltic contractions churn the chyme
and keep the pyloric sphincter at the
stomach’s exit closed, but small amounts of
chyme are released at intervals into the
small intestine.



Depending on the acidity and fat content of
chyme, it can take the stomach 2-6 hours to
empty; an empty stomach forms folds called
rugae.
Water and alcohol are two of the few
substances absorbed across the stomach wall.
Section 4
The Small Intestine: A
Huge Surface Area for
Digestion and
Absorption
The Small Intestine: A Huge Surface Area
for Digestion and Absorption
The vast majority of nutrients are absorbed
in the small intestine.
The key to the small intestine’s ability to
absorb is the structure of its wall.




The mucosa is highly folded.
Absorptive surface area is increased by
fingerlike projections of the intestine lining
called villi, which bear even smaller microvilli,
giving each cell a “brush border.”
submucosa
serosa (thin
connective
tissue layer)
thin layer
thick layer
of longitudinal of circular
muscle
muscle
mucosa
Extensive circular folds
of mucosa
Fig. 7.7a, p. 125
Villi (many
fingerlike,
epitheliumcovered
projections
from the
mucosa)
epithelium
blood
capillaries
lymph
vessel
connective
tissue
vesicles
artery
vein
lymph
vessel
Villi on one of the folds,
longitudinal section
One villus
Fig. 7.7cd, p. 125
microvilli at
free surface
of absorptive
cells
ctyoplasm
absorption
mucus secretion
(goblet cell)
hormone secretion
phagocytosis
lysozyme secretion
Fig. 7.7e, p. 125
Section 5
Accessory Organs: The
Pancreas, Gallbladder,
and Liver
Accessory Organs: The Pancreas,
Gallbladder, and Liver
The pancreas produces a variety of
digestive enzymes.




The pancreas lies near the stomach; it
contains both exocrine and endocrine glands.
Four kinds of digestive enzymes are
produced—one kind for each of the four major
categories of food (carbohydrates, proteins,
lipids, and nucleic acids).
Hormones that regulate blood sugar and
bicarbonate to neutralize the pH of chyme are
also produced.
duodenum
main
pancreatic
duct
gland cells
stomach
pancreatic
hormones
secreted
into blood
endocrine portion
of pancreas
(Islets of Langerhans)
exocrine portion of pancreas
enzymes secreted into duodenum
Fig. 7.8, p. 126
Accessory Organs: The Pancreas,
Gallbladder, and Liver
The gallbladder stores bile.



The gallbladder is a small sac that stores bile
produced by the liver.
Bile is emptied into the small intestine when
food is present to aid in the digestion and
absorption of fats.
The liver is a multipurpose organ.


The liver is one of the largest organs in the
body.
Accessory Organs: The Pancreas,
Gallbladder, and Liver

The bile secreted by the liver contains bile
salts, which are derived from cholesterol.
•
•

Bile salts help emulsify fats by breaking large fat
globules into smaller bits.
Excess cholesterol is also emulsified; if the level of
cholesterol is much higher than the amount of bile
salts present, however, gallstones can form.
The liver processes nutrient-laden blood from
the small intestine; blood enters the liver via the
hepatic portal vein.
inferior vena cava
hepatic vein
liver capillary beds
liver
gallbladder
stomach
(spleen)
hepatic portal vein
pancreas
ascending colon
of large intestine
small intestine
descending
colon of large
intestine
appendix
rectum
Fig. 7.9, p. 127
Accessory Organs: The Pancreas,
Gallbladder, and Liver

The liver has many important functions:
•
•
•
•
•
•
•
•
It converts glucose to glycogen, a storage form.
It produces plasma proteins.
It forms urea.
It makes bile, which is useful in fat emulsification.
It detoxifies harmful substances.
It degrades worn-out red blood cells.
It aids in the immune response by removing foreign
particles.
It absorbs and stores factors needed for red blood
cell production.
Table 7.1, p. 127
Section 6
Digestion and
Absorption in the Small
Intestine
Digestion and Absorption
in the Small Intestine
Roughly 9 liters of fluid enters the
duodenum (the first section of the small
intestine) each day.
Nutrients are released by chemical and
mechanical means.



Secretions from the pancreas (pancreatic
juice) are added to chyme in the duodenum.
•
•
Trypsin and chymotrypsin digest proteins to
peptide fragments; peptidases break the fragments
down into amino acids.
Bicarbonate from the pancreas buffers the acid from
the stomach.
Table 7.2, p. 128
Digestion and Absorption
in the Small Intestine


Bile salts speed up fat digestion by
emulsification: triglycerides tend to form large
globules, but when smaller fat droplets become
coated with bile salts, the negative charges on
the droplets repel and cause them to stay
separated.
Repeated segmentation contractions create
an oscillating movement that mixes the food.
p. 128
Digestion and Absorption
in the Small Intestine
Different nutrients are absorbed by different
mechanisms.



Monosaccharides (glucose), amino acids, and
some nucleotides cross the gut lining by active
transport and enter the bloodstream.
Absorption of fatty acids and monoglycerides is
aided by bile salts.
•
•
Free fatty acids diffuse into the gut epithelium, aided
by micelle formation.
Fats combine with proteins to form particles that
enter lymph vessels called lacteals, which drain into
the blood.
Lumen of
intestine
carbohydrates
proteins
monosaccharides
amino acids
Epithelial
Cell
Internal
Environment
© 2007 Thomson Higher Education
Fig. 7.10, p. 129
bile salts
bile salts
+
+
Micelles
Fat Globules
(triglycerides)
Emulsification
fatty acids,
monoglycerides
triglycerides + proteins
© 2007 Thomson Higher Education
Fig. 7.10, p. 129
Section 7
The Large Intestine
The Large Intestine
The large intestine begins as a blind, cupshaped pouch (cecum) where it joins the
small intestine.



It continues as the colon (ascending,
transverse, and descending).
It ends in the sigmoid colon, which connects
with the rectum.
The Large Intestine
The colon concentrates and stores feces—
a mixture of undigested and unabsorbed
material, water, and bacteria.



Sodium is actively transported out of the colon
and water follows by osmosis.
About 30 percent of the feces is bacteria that
produce vitamins and intestinal gases.
The Large Intestine
The appendix projects from the cecum; it
has no known function.




Feces can become trapped in the appendix,
causing inflammation called appendicitis.
If the appendix bursts, released bacteria can
cause peritonitis in the abdominal cavity.
When feces distend the rectal wall,
defecation occurs; feces move into the
anal canal and eventually out of the body
through the anus.
transverse
colon
ascending
colon
descending
colon
ileum of small
intestine
fat deposit
cecum
appendix
rectum
anal sphincter
anal canal
anus
Fig. 7.11, p. 130
Table 7.5, p. 140
Section 8
Managing Digestion and
the Processing of
Nutrients
Managing Digestion and the Processing
of Nutrients
Nerves and hormones regulate digestion.



Distention of the gut wall after a meal
stimulates the release of signals from sensory
receptors, which respond with muscle action
and enzyme secretion.
Four major gastrointestinal hormones regulate
digestion:
•
•
Gastrin, produced by the stomach lining, stimulates
secretion of acids into the stomach to aid digestion of
protein; somatostatin inhibits acid production.
Secretin, from the intestinal lining, stimulates the
pancreas to release bicarbonate.
sight, smell,
taste of food
CENTRAL NERVOUS SYSTEM
emotional
states
LONG
REFLEX
PATHWAYS
gut
wall
sensory
receptors
gut
lumen
Stimulus
change in food
volume,
composition in lumen
© 2007 Thomson Higher Education
nerve
network
smooth
muscle
or gland
Response
gut wall moves or
substances
secreted into lumen
Fig. 7.12, p. 130
Managing Digestion and the Processing
of Nutrients
•
•

Cholecystokinin (CCK) enhances the actions of
secretin and stimulates gallbladder contractions;
both CCK and secretin slow the rate of gastric
emptying.
GIP (glucose insulinotropic peptide) is released in
the presence of glucose and fat, and stimulates
insulin secretion for glucose uptake.
Blood carries the absorbed nutrients to the
liver for delivery throughout the body.
Other
body
tissues
Brain
Liver
Muscle
Small
intestine
Fig. 7.13, p. 131
Section 9
Digestive System
Disorders
Digestive System Disorders
The GI tract is open to many kinds of
disease-causing organisms.


Bacteria, viruses, and other pathogens can
enter the GI tract through contaminated food
and water.
•
One common result of GI tract infection is diarrhea
(watery feces) where too little water is absorbed by
the intestines; loss of water can lead to dangerous
dehydration.
Digestive System Disorders
• “Food poisoning” can result when bacteria such as
Salmonella or E. coli O157:H7 colonize the gut
following consumption of contaminated food.
Figure 7.14a-b
Digestive System Disorders


Tooth decay (dental caries) results
when lack of brushing leaves food
residues on the teeth that support
bacterial growth; untreated,
gingivitis (inflammation of the
gums) or periodontal disease
(inflammation of the periodontal
membrane) can also occur.
Helicobacter pylori, introduced as a
cause of peptic ulcers, has also
been linked to gastritis
(inflammation of the intestines) and
stomach cancer.
Figure 7.14c-d
Digestive System Disorders
Colon disorders range from inconvenient to
life-threatening.


Constipation occurs when food residues
remain in the colon for too long, resulting in
hard, dry feces that are difficult to eliminate.
•
•
Bulk (soluble and insoluble fiber) in the diet is
important for moving material through the large
intestine at the proper speed.
Too little bulk can result in the formation of
diverticula, knoblike sacs protruding from the
intestinal wall (this condition is called
diverticulosis); if the knobs become inflamed this is
called diverticulitis and can lead to ruptures of the
diverticula.
Fig. 7.15, p. 133
Digestive System Disorders


The most common bowel disorder is irritable
bowel syndrome (IBS); as a result of
disturbances to peristalsis, individuals alternate
between diarrhea and constipation.
Colon cancer is the second most common
cancer diagnosed in the United States; polyps
forming in the colon wall can turn malignant and
spread.
Digestive System Disorders
Malabsorption disorders prevent some
nutrients from being absorbed.


A malabsorption disorder is anything that
interferes with the uptake of nutrients across
the lining of the small intestine.
•
•
Lactose intolerance results
from a deficiency of lactase.
Persons suffering from
cystic fibrosis do not produce
the pancreatic enzymes
necessary for normal digestion
and absorption of nutrients.
Figure 7.17
Digestive System Disorders

Crohn’s disease is an inflammatory disease that
can so severely damage the intestinal lining that
portions of the tract must be removed.
Video: The Problem with Pork
CLICK
TO PLAY

From ABC News, Biology in the Headlines, 2005 DVD.
Section 10
The Body’s Nutritional
Requirements
The Body’s Nutritional Requirements
Complex carbohydrates are best.


Complex carbohydrates are the main source of
energy taken into the body; they are also the
best types of carbohydrates to intake.
•

They are degraded to glucose, the main source of
energy available to individual cells.
Fruits, vegetables, and grains are also high in
fiber needed for bulk in the feces.
Figure 7.17
The Body’s Nutritional Requirements

Refined sugars represent “empty calories”
because they do not contribute to our nutritional
needs.
•
•
Eating such sugars leads to surges in blood sugar
levels and changes to how fat is utilized for energy
by the body.
Too much sugar eventually increases fat storage and
increases the risk of heart disease and type 2
diabetes.
The Body’s Nutritional Requirements
There are good fats and bad fats.




Phospholipids and cholesterol are important
components of membranes; fats are energy
reserves, provide insulation and cushioning,
and store vitamins.
Essential fatty acids are those not made by
the body itself and which must be acquired
through food; whole foods and vegetable oils
provide plenty of them in the diet.
Animal fats and trans fats are also common in
our food, but eating too much of them can
damage the cardiovascular system.
The Body’s Nutritional Requirements
Proteins are vital to life.



Of the 20 different amino acids in proteins, eight
are essential amino acids and are not made
by the body; they must be supplied in the diet.
Most animal proteins are complete, possessing
ratios of amino acids that match our needs;
nearly all plant proteins are incomplete,
meaning they lack one or more of the
essentials.
total protein
intake
isoleucine
leucine
lysine
methionine
phenylalanine
threonine
tryptophan
valine
essential
amino
acids
Fig. 7.16, p. 134
The Body’s Nutritional Requirements
Guidelines for healthy eating.


The classic “food pyramid” has been replaced
with a set of nutritional guidelines published by
the Food and Drug Administration.
•
•
These guidelines call for 55% of daily calories to
come from complex carbohydrates and further
recommend limiting fat intake to 20-30% of daily
calories.
More whole grains, legumes, dark green and orange
vegetables, fruits, and milk products should be
added to our diets.
p. 135
The Body’s Nutritional Requirements


The Mediterranean diet is an example of an
alternative diet that meets the goal of lowering
heart disease and other ills; it emphasizes
whole grains, fruits and vegetables, and
reduced meat intake.
Other alternative diets, such as those that
promote high protein, have little scientific data
to support their effectiveness.
Figure 7.17
Section 11
Vitamins and Minerals
Vitamins and Minerals



Humans need small amounts of at least 13
organic molecules called vitamins to assist
in cellular metabolism; these vitamins must
come from a balanced diet.
Inorganic substances called minerals (Ca,
Mg, K, Fe, for example) are also needed.
A balanced diet will normally meet all
requirements for these substances;
excessive intake is at least wasteful and at
worst harmful.
Table 7.3, p. 136
Table 7.3, p. 136
Table 7.4, p. 137
Section 12
Calories Count: Food
Energy and Body Weight
Calories Count:
Food Energy and Body Weight
Obesity is an excess of fat in the body’s
adipose tissue; the World Health
Organization has declared obesity a major
global health concern.



A male is obese if his body
is 20% body fat.
A female is obese if her
body is 24% body fat.
Calories Count:
Food Energy and Body Weight


One indicator of obesity-related health risk
is the body mass index (BMI); the BMI is a
number that relates body weight with height.
The basal metabolic rate (BMR) is the
amount of energy needed to sustain the
body at rest; the number of kilocalories
needed to maintain “acceptable” body
weight can be calculated relative to your
amount of physical activity and age.
Weight Guidelines for Women:
Starting with an ideal weight
of 100 pounds for a woman
who is 5 feet tall, add five
additional pounds for each
additional inch of height.
Examples:
Weight Guidelines for Men:
Starting with an ideal
weight of 106 pounds for
a man who is 5 feet tall,
add six additional
pounds for each
additional inch of height.
Examples:
Height
(feet)
Weight
(pounds)
Height
(feet)
5’2”
5’3”
5’4”
5’5”
5’6”
5’7”
5’8”
5’9”
5’10”
5’11”
6’
110
115
120
125
130
135
140
145
150
155
160
5’2”
5’3”
5’4”
5’5”
5’6”
5’7”
5’8”
5’9”
5’10”
5’11”
6’
Weight
(pounds)
118
124
130
136
142
148
154
160
166
172
178
Fig. 7.18, p. 138
Calories Count:
Food Energy and Body Weight
Genes, weight control, and exercise.




The hormones leptin and ghrelin influence
appetite, hunger, storage of fat in the body, and
other weight-related factors; hormone
production, as well as other genetic factors,
differ between different individuals.
To lose weight, caloric intake must be balanced
with calories burned; this usually requires
dietary controls paired with exercise.
Emotions also affect weight; anorexia nervosa
and bulimia are eating disorders related to
emotional problems.
Table 7.5, p. 139
Fig. 7.20, p. 142