Chapter 12: Digestive System
and Nutrition
12-1
The Digestive Tract
The human digestive tract is a tube with
specialized regions and organs
between the mouth and the anus.
Food is ingested, mechanically
processed, and chemically digested to
small molecules that are absorbed;
indigestible remains are eliminated.
Parts of the digestive tract produce
digestive enzymes.
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Digestive system
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The Mouth
Lips and cheeks enclose the mouth.
Taste buds on the tongue provide the
sense of taste; skeletal muscle in the
tongue allows it to move.
The roof of the mouth is formed by the
hard and soft palates that separate it
from the nasal cavities.
The soft palate ends in a finger-shaped
projection called the uvula.
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Tonsils at the back sides of the mouth
protect against infections.
Tonsillitis results when the tonsils
become inflamed; the infection can
spread to the middle ears.
Three pairs of salivary glands send saliva
(containing salivary amylase for
digestion of starch to maltose) into the
mouth.
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The Teeth
Twenty deciduous (baby) teeth are
replaced by 32 adult teeth.
Each tooth has a crown and a root.
The crown has a layer of enamel, dentin,
and an inner pulp with nerves and
blood vessels that extend into the root.
The tongue mixes the chewed food with
saliva and then forms the mixture into a
mass called a bolus in preparation for
swallowing.
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Adult mouth
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Longitudinal section of a tooth
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The Pharynx
The air passage and food passage cross
in the pharynx because the trachea is
ventral to the esophagus.
Swallowing occurs in the pharynx and is
a reflex action.
During swallowing, the air passage is
usually blocked off by the soft palate
and uvula, and the trachea moves
under the epiglottis to cover the glottis
opening to the windpipe.
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Swallowing
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The Esophagus
The esophagus is a muscular tube that
conducts food through the thoracic
cavity and diaphragm into the stomach.
Peristalsis begins in the esophagus; this
collapsed tube moves the bolus of food
downward after swallowing occurs.
Heartburn is a burning pain when acidic
stomach contents enter the esophagus.
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No chemical digestion occurs in the
esophagus.
The entrance of the esophagus to the
stomach is marked by a constriction,
called a sphincter; the sphincter must
relax in order for food to enter the
stomach.
The sphincter prevents food from
backing up into the esophagus.
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The Wall of the Digestive Tract
The digestive tract wall has four layers:
Mucosa (mucous membrane – secretes
digestive enzymes and mucus),
Submucosa (loose connective tissue –
houses blood and lymph vessels),
Muscularis (two layers of smooth muscle
- for peristalsis), and
Serosa (serous membrane – secretes
serous fluid to prevent sticking).
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Wall of the digestive tract
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The Stomach
The stomach expands to store food.
Food in the stomach is churned, mixing
the food with gastric juices containing
hydrochloric acid and pepsin for the
digestion of protein to peptides.
Alcohol, but not food, is absorbed here.
In 2–6 hours, the soupy chyme leaves the
stomach.
Ulcers are usually caused by a bacterial
infection.
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Anatomy and histology of the
stomach
12-16
Ulcer
12-17
The Small Intestine
The small intestine, averaging about 6
meters in length, is small in diameter.
The first 25 cm is the duodenum that
receives bile from the gallbladder and
pancreatic juice containing pancreatic
lipase and trypsin for digestion of
protein to peptides, as well as lipase for
digestion of fat to glycerol and fatty
acids.
Pancreatic juice contains NaHCO3 that is
basic and neutralizes the acidic chyme.
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Enzymes that finish the process of
digestion are produced by the intestinal
wall.
Walls of the small intestine have fingerlike projections called villi where
nutrient molecules are absorbed into
the cardiovascular and lymphatic
systems.
Villi have microvilli that increase the
surface area available for absorption.
The small lymphatic capillary in a villus
is called a lacteal.
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Anatomy of the small intestine
12-20
Regulation of Gastric Secretions
Both the nervous system and chemicals
called hormones regulate digestive
juice secretion.
In response to eating protein foods, the
hormone gastrin is produced by the
lower part of the stomach and flows
through the bloodstream to stimulate
the stomach to produce digestive juice.
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The duodenal wall produces gastric
inhibitory peptide (GIP) to inhibit
gastric gland secretion.
The hormones secretin and
cholecystokinin (CCK) are produced by
the duodenal wall and stimulate the
pancreas to secrete digestive juice and
the gallbladder to release bile.
Acidic chyme stimulates the secretion of
secretin, while fatty chyme with protein
triggers CCK release.
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Hormonal control of digestive
gland secretions
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The Large Intestine
The large intestine consists of the
cecum, colon, rectum and anal canal.
The large intestine does not produce
digestive enzymes but does absorb
water, salts, and some vitamins.
The colon includes the ascending colon,
the transverse colon, the descending
colon, and the sigmoid colon.
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The appendix is an extension of the
cecum.
Indigestible material is stored in the
rectum until the anus allows
defecation.
Anaerobic bacteria in the feces break
down indigestible material and produce
some vitamins.
Water tests that show the presence of the
bacterium Escherichia coli indicate
water is contaminated.
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Junction of the small intestine
and the large intestine
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Polyps are small growths arising from
the epithelial lining that may be benign
or cancerous.
Diarrhea and constipation are two
common complaints of the large
intestine.
Causes of diarrhea include infection of
the lower tract and nervous stimulation,
both moving feces more rapidly than
normal, but also causing dehydration if
prolonged.
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Water and fiber in the diet can prevent
constipation where the feces become
too dry and hard.
Hemorrhoids are enlarged and inflamed
blood vessels at the anus; this
condition is associated with chronic
constipation.
Regular elimination reduces the time the
colon wall is exposed to cancerpromoting agents in the feces and may
help prevent cancer.
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Defecation reflex
12-29
Three Accessory Organs
The pancreas, liver, and gallbladder are
accessory organs of digestion; their
secretions assist digestion.
Accessory organs are not part of the
digestive tube but produce enzymes
and other substances that assist
digestion.
These three accessory organs send
secretions to the duodenum via ducts.
12-30
The Pancreas
The pancreas produces pancreatic juice,
which contains digestive enzymes for
carbohydrate (pancreatic amylase),
protein (trypsin), and fat (lipase), along
with sodium bicarbonate (NaHCO3) to
neutralize acid in chyme.
The pancreas is also an endocrine gland
that secretes insulin and glucagon,
hormones that keep blood glucose
within normal limits.
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The Liver
The liver produces bile, which is stored
in the gallbladder.
Bile emulsifies fats; it is a yellowishgreen substance containing bilirubin
from hemoglobin breakdown and bile
salts derived from cholesterol.
The liver acts as gatekeeper to the blood
and receives blood from the small
intestine by way of the hepatic portal
vein.
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Hepatic lobules
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The functions of the liver are many:
detoxifies blood,
stores iron and vitamins,
makes plasma proteins,
stores glucose as glycogen,
produces urea from amino acids,
removes bilirubin after dismantling blood
cells, and
regulates blood cholesterol level when
producing bile salts.
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Hepatic portal system
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Liver Disorders
When a person has a liver disorder,
jaundice may occur.
Jaundice is a yellowish tint to eyes and
skin, indicating abnormal levels of
blood bilirubin.
Hepatitis is inflammation of the liver;
different strains of virus cause hepatitis
A, B, etc.
Cirrhosis is scar tissue that can form
when the liver is diseased or killed by
exposure to alcohol.
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The Gallbladder
The gallbladder is a pear-shaped
muscular organ that stores bile until it
is sent to the duodenum.
Water is reabsorbed in the gallbladder
making the bile thick and mucus-like.
Bile enters the duodenum via the
common bile duct.
Gallstones are crystals of cholesterol.
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Digestive Enzymes
Digestive enzymes are present in
digestive juices and introduce water at
specific bonds to break down food into
sugars, amino acids, fatty acids, and
glycerol.
Starches are broken down into simpler
sugars by salivary amylase and
pancreatic amylase.
Pepsin in the stomach, and trypsin from
the pancreas break proteins into
peptides.
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Peptidases and maltase, produced by the
small intestine, complete the digestion
of proteins and starches, respectively.
Glucose and amino acids are absorbed
into the blood capillaries of the villi.
Fatty acids and glycerol rejoin in the villi
to produce lipoprotein droplets which
enter the lacteals.
Digestive enzymes speed specific
reactions and function best at a warm
body temperature and optimum pH.
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Conditions for Digestion
For digestion to occur the correct
enzyme, optimum pH, optimum
temperature, and the correct substrate
must be present.
Exact conditions can be determined
during laboratory experiments.
Most digestive enzymes, aside from
pepsin, require a basic pH.
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Digestion experiment
12-41
Nutrition
The U.S. Department of Agriculture
recommends a diet high in
carbohydrates (whole grains), at least
five servings of fruits and vegetables,
and limited but adequate protein as
illustrated in a “food pyramid.”
Fats and sweets should be used
sparingly.
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Food guide pyramid: A guide to
daily food choice
12-43
Carbohydrates
Complex carbohydrates from foods like
breads and pasta can be converted to
glucose and used rapidly.
Body cells can utilize fatty acids as an
energy source, but brain cells require
glucose, thus carbohydrates are an
essential part of the diet.
Complex, rather than simple,
carbohydrates should make up the bulk
of the diet.
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Simple carbohydrates like table sugar
(sucrose) contribute to energy needs
and weight gain without supplying
other nutrients.
Insoluble fiber helps regularity and may
help prevent cancer by limiting the time
substances are in contact with the
intestinal wall.
Soluble fiber combines with bile acids
and cholesterol in the intestine and
prevents them from being absorbed.
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Complex carbohydrates
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Proteins
Meat, milk or eggs are complete proteins;
they provide all 20 essential amino
acids.
Because individual vegetables do not
provide all essential amino acids,
vegetarians must be careful to
consume a combination of legumes,
grains, vegetables, seeds and nuts to
secure complementary proteins.
The amino acid pool relies on continual
uptake; amino acids are not stored.
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Ancient versus modern diet of
native Hawaiians
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Lipids
Fat and cholesterol are lipids.
Lipids, found in fats and oils, should be
used sparingly.
Current guidelines suggest that fat
should account for 30% or less of daily
calories.
High-density lipoproteins (HDL) carry
cholesterol to the liver and is
considered to be “good”.
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Low density lipoprotein (LDL) takes
cholesterol to the cells and may
contribute to the development of
plaque on blood vessels walls; it is
considered to be “bad”.
Saturated fatty acids lack double bonds
and raise LDL cholesterol levels.
Linoleic acid and linolenic acid are two
essential fatty acids the body cannot
make; polyunsaturated fats supply
these.
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Fake Fat
Olestra looks, tastes, and acts like real
fat but the digestive system cannot
digest it; therefore, it is called “fake
fat”.
However, fat-soluble vitamins are taken
up by olestra and pass through the
digestive system unabsorbed.
Those who consume olestra have
reduced carotenoids in their blood.
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Vitamins
Vitamins are organic compounds that the
body cannot produce but needs for
metabolic purposes; some are portions
of coenzymes.
Vitamins A, E, and C are antioxidants that
protect cell contents from damage due to
free radicals.
Free radicals donate an electron to DNA,
proteins, enzymes, membranes, etc. and
can damage cell structures or cause
cancer.
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Vitamin D
A precursor molecule in skin is
converted to vitamin D after exposure
to ultraviolet (UV) light.
Vitamin D is modified first in the kidneys
and then the liver until it becomes
calcitriol, which is needed for calcium
absorption in intestines.
In the U.S., milk is often fortified by
vitamin D.
Vitamin D is a fat-soluble vitamin.
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Illnesses due to vitamin
deficiency
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Minerals
The body contains more than 5 grams of
each major minerals and less than 5
grams of each trace minerals.
Calcium and phosphorus are in bones
and teeth.
Potassium and sodium are involved in
nerve conduction.
Trace minerals are critical in various
enzymes and hormones.
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Minerals in the body
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Calcium
Calcium is needed to have strong bones.
Older women in particular are at risk for
osteoporosis, a degenerative bone
disease due to insufficient intake of
calcium because bone cells are
constantly building and eroding bone
tissue.
Calcium supplement with vitamin D (and
also estrogen for women) can help
prevent this bone loss.
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Sodium
Most Americans have too much salt in
their diet.
High sodium intake is linked to
hypertension in some persons.
About one-third of the sodium we
consume occurs naturally in foods;
another one-third is added during
commercial processing; and the final
one-third is added during cooking or at
the table in the form of table salt.
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Eating Disorders
Obesity is defined as a body weight of
more than 20% above the ideal weight
for that person.
Obesity can have hormonal, metabolic,
and social causes.
For many, a commitment to a sensible
diet and exercise program can prevent
obesity or a harmful cycle of weight
gain-and-loss.
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Recognizing obesity
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Bulimia nervosa is characterized by a
restrictive diet, binging, and purging.
Psychotherapy and antidepressants may
help.
Anorexia nervosa is characterized by a
distorted body image and feeling fat
even when emaciated.
It can be life-threatening and carries the
same risks as starvation.
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Recognizing bulimia nervosa
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Recognizing anorexia nervosa
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Chapter Summary
The human digestive tract is a tube with
specialized regions and organs
between the mouth and the anus.
Food is ingested, mechanically
processed, and chemically digested to
small molecules that are absorbed;
indigestible remains are eliminated.
12-64
The mouth, pharynx, esophagus,
stomach, small and large intestines
have distinct functions and hormones
control digestive gland secretions.
The pancreas, liver, and gallbladder are
accessory organs of digestion; their
secretions assist digestion.
The products of digestion are small
molecules, such as amino acids and
glucose, that can cross plasma
membranes.
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Digestive enzymes are specific and have
an optimum temperature and pH.
Proper nutrition supplies the body with
energy and nutrients, including
essential amino acids and fatty acids,
and all vitamins and minerals.
Anorexia nervosa, bulimia, and obesity
are primary eating disorders in the
United States.
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