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BIOLOGY
A GUIDE TO THE NATURAL WORLD
FOURTH EDITION
DAVID KROGH
Transport and Exchange 2:
Digestion, Nutrition, and Elimination
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30.1 The Digestive System
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• There are four central functions of the digestive
system.
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Four Functions of the Digestive
System
1. To get the foods the body ingests into a form
the body can use.
2. To move food in this form out of the digestive
tract and into blood circulation.
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Four Functions of the Digestive
System
3. To retain the waste that remains after nutrients
from the food have been transferred.
4. To eliminate this waste from the body.
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The Digestive System
• The central structure of the digestive system is
a muscular tube called the digestive tract that
passes through the body from the mouth to the
anus.
• It receives input along its length from accessory
digestive organs such as the gallbladder, liver,
and pancreas.
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The Digestive System
salivary
gland
mouth
(oral cavity)
salivary
glands
tongue
pharynx
esophagus
liver
stomach
gallbladder
pancreas
duodenum
jejunum
colon
small
intestine
ileum
cecum
large
intestine
appendix
rectum
anus
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Figure 30.1
The Digestive System
• Digestion is a process of breaking down
foods—first into small pieces and then into
small molecules.
• These small molecules leave the digestive
system for transport through the rest of the
body.
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30.2 Structure of the Digestive System
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Structure of the Digestive System
• The digestive tract begins with the mouth (or
oral cavity).
• It continues through the pharynx, esophagus,
stomach, small intestine, and large intestine.
• It ends at the rectum and anus.
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Structure of the Digestive System
• Most of the digestive tract is a tube composed
of four layers:
– The serosa, or outermost layer.
– The muscularis externa, whose two sets of muscles
produce the waves of contraction, called peristalsis,
that move food along.
– The submucosa, which contains blood vessels and
nerve tissue that control digestion.
– The mucosa, which absorbs digested food.
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Structure of the Digestive System
absorption of
nutrients
microvilli
epithelium
lymphatic
vessel
villus
capillary
network
small
intestine
mucosa: highly folded lining of
intestine where absorption occurs
submucosa: connective tissue
containing blood vessels and nerves
muscularis externa: circular and
longitudinal muscle layers used
for peristalsis
serosa: connective tissue
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Figure 30.2
Structure of the Digestive System
• Absorbed food is taken up by adjacent
capillaries (in the case of carbohydrates and
proteins) or lymphatic vessels (in the case of
fats).
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30.3 Steps in Digestion
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Steps in Digestion
Digestion begins in the mouth through both
mechanical means (chewing) and chemical means
(enzymes breaking down carbohydrates).
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The Pharynx and Esophagus
• Food is pushed by the tongue into the pharynx
(or throat) and moves by muscle contraction
from there to the esophagus.
• The esophagus’ own muscle contractions work
with gravity to push food into the stomach.
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The Stomach
• The stomach digests food partly by the
mechanical means of churning it and partly by
chemical means.
• Proteins are a primary target of the stomach’s
digestive juices.
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The Stomach
• The stomach’s contents are very acidic—a
quality that is valuable both for breaking food
down and for killing microorganisms that come
in with the food.
• The material that leaves the stomach is a
mixture of food and digestive juices called
chyme.
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The Stomach
esophagus
muscular
wall
stomach
pyloric
sphincter
duodenum
of small
intestine
rugae
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Figure 30.3
The Small Intestine
• Eighty percent of the digestive tract’s
absorption of nutrients takes place within the
small intestine.
• It begins at the stomach and ends at the large
intestine.
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The Small Intestine
small intestine
duodenum:
receives chyme
from stomach
and secretions
from pancreas
and liver
jejunum:
region of
most digestion
and nutrient
absorption
ileum:
absorption
continues
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large
intestine
Figure 30.4
The Pancreas
• The pancreas is an organ that secretes, into the
small intestine, three classes of digestive
enzymes that help break down fats, proteins,
and carbohydrates.
• It also secretes “buffers” that raise the pH of the
acidic chyme coming from the stomach.
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The Liver, Pancreas, and Gallbladder
liver
right
lobe
left
lobe
common
hepatic
duct
hepatic
ducts
common
bile duct
pancreas
cystic
duct
pancreatic
duct
gallbladder
duodenum of
small intestine
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Figure 30.5
The Gallbladder
• The gallbladder stores and concentrates bile, a
substance produced by the liver that aids in the
digestion of fats.
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The Liver
• The liver is a large organ that plays a central
role in digestion.
• All blood carrying nutrients from the digestive
tract is channeled through the hepatic portal
vein to the liver, making the liver a first stop for
much digested material.
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The Liver
• The liver controls which nutrients it will store
and which nutrients it will send to the rest of
the body.
• The liver is also the first stop in the body for
ingested toxins such as alcohol, making it
vulnerable to damage.
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The Large Intestine
• The large intestine holds and compacts material
left over from digestion, turning it into feces.
• It also returns water to general circulation and
absorbs vitamins produced by resident bacteria.
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The Large Intestine
• One of the large intestine’s regions, the rectum,
is usually empty except when peristaltic
contractions force fecal materials into it.
• The resulting stretching triggers the urge to
defecate.
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30.4 Human Nutrition
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Human Nutrition
• Nutrition is the study of the relationship
between food and health.
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Human Nutrition
• A nutrient is a substance, contained in food, that
does at least one of three things:
– Provides energy.
– Provides a structural building block for the body.
– Helps regulate a process in the body.
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Human Nutrition
• There are six classes of nutrients:
–
–
–
–
–
–
water
minerals
vitamins
proteins
carbohydrates
lipids
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30.5 Water, Minerals, and Vitamins
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• Water makes up 66 percent of the human body
and is vital to health.
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Water
• People generally do not need to be concerned
about whether they are getting too much or too
little water in their diet.
• Recent research has shown that thirst is the
best guide as to whether the body is
sufficiently hydrated.
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Minerals
• Minerals are chemical elements needed by the
body either to help form bodily structures or to
facilitate chemical reactions.
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Minerals
• Dietary minerals are divided into two
categories:
– Major minerals, which are needed in amounts of
100 milligrams or more per day.
– Trace minerals, which are needed in amounts of less
than 100 milligrams per day.
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Minerals Required by the Human Body
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Table 30.1
Vitamins
• Vitamins differ from minerals in that vitamins
are compounds, rather than elements.
• In addition, vitamins do not help form bodily
structures and thus are needed strictly in small
quantities—sometimes milligram quantities per
day, but often microgram quantities.
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Vitamins
• There are 13 vitamins that the body must have
to function properly.
• Of these, eight are B vitamins that facilitate
chemical reactions by acting as coenzymes or
parts of coenzymes.
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Vitamins as Coenzymes
coenzyme
substrate
enzyme
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Figure 30.8
Vitamins in the Human Diet
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Table 30.2
Do We Need Supplements?
• Most nutritionists have concluded that, while a
well-balanced diet can supply all the vitamins
and minerals the average American needs, there
is little harm to be had, and possibly some gain
to be realized, from taking a one-a-day vitaminand-mineral supplement.
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Do We Need Supplements?
• Nutritionists do not, however, recommend
taking large “megadoses” of any vitamin.
• The average American diet may sometimes be
deficient in vitamins A and C and often seems
to be deficient in the minerals calcium and iron.
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30.6 Calories and the
Energy-Yielding Nutrients
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Calories and Energy-Yielding Nutrients
• All the calories in the human diet are provided
by three classes of nutrients: proteins,
carbohydrates, and lipids.
• Calories are units of energy, not units of weight.
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Calories and Energy-Yielding Nutrients
• In nutrition, a calorie (sometimes written as
Calorie) is the amount of energy it takes to raise
the temperature of 1,000 grams of water by 1
degree Celsius.
• Proteins, carbohydrates, and lipids sometimes
are referred to as the energy-yielding nutrients
in recognition of the fact that they alone
provide the body with energy.
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Calories and Energy-Yielding Nutrients
• Quantities of energy-yielding nutrients that we
consume but do not burn up end up being
stored within us, mostly as fat tissue.
• Foods are caloric in accordance with how much
energy they contain per unit of weight.
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Calories and Energy-Yielding Nutrients
• Lipids are much more caloric than either
proteins or carbohydrates.
• Each gram of lipids yields 9 calories of energy.
• Each gram of proteins or carbohydrates yields 4
calories of energy.
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Daily Calorie Requirements
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Table 30.3
Calories in Portions of Food
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Table 30.4
30.7 Proteins
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Proteins
• Proteins are composed of chemical building
blocks called amino acids.
• Tens of thousands of human proteins are put
together from a starting set of only 20 amino
acids.
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Proteins
• Proteins have many structural and regulatory
functions in the body, but they provide very
little energy compared to carbohydrates or
lipids.
• This is so in part because the body has no
stored reservoir of proteins.
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Proteins
• As such, the body can only derive energy from
proteins by breaking apart proteins already in
use.
• This is an inefficient process that the body turns
to only when its supplies of carbohydrates and
lipids are running low.
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Proteins
• Proteins consumed in food are broken down in
the digestive tract into their amino acid building
blocks.
• It is these amino acids, rather than fully formed
proteins, that move into circulation.
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Proteins
• Eleven of the 20 amino acids needed for protein
synthesis can be produced by the body and are
called nonessential amino acids.
• There are, however, nine amino acids that
cannot be produced by the body and that must
be obtained from food: the essential amino
acids.
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Proteins
• Nearly every source of animal protein supplies
a complete, balanced set of essential amino
acids.
• Among plant proteins, however, only soy
protein contains all nine essential amino acids
in their proper proportions.
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Protein Complementation
methionine
histidine
Rice contains only
8 of the 9 essential
amino acids
isoleucine
leucine
phenylalanine
threonine
tryptophan
valine
lysine
Beans contain only
8 of the 9 essential
amino acids
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Figure 30.10
30.8 Carbohydrates
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Carbohydrates
• The primary function of dietary carbohydrates
is to provide energy.
• The building blocks of carbohydrates are
molecules called monosaccharides.
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Carbohydrates
• A single monosaccharide can be linked to a
second monosaccharide to form a molecule
called a disaccharide.
• Monosaccharides and disaccharides form one
of the three principal classes of dietary
carbohydrates, the simple sugars.
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Carbohydrates
• The other two classes of dietary carbohydrates,
collectively known as complex carbohydrates,
are:
– Starches, defined as complex carbohydrates that
are digestible.
– Fibers, defined as complex carbohydrates that are
indigestible.
• Both are composed of thousands of linked units
of the monosaccharide glucose.
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Carbohydrates and Nutrition
• Nutritionists recommend a diet that is:
– Low in simple sugars that have been added to
processed foods.
– High in complex carbohydrates that come from
fresh fruits, vegetables, and whole-grained cereals.
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Processed and Unprocessed
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Figure 30.11
Carbohydrates and Nutrition
• One of the reasons for this advice is that foods
high in added simple sugars often are sources of
“empty” calories, meaning calories that are not
accompanied by vitamins or minerals.
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Carbohydrates and Nutrition
• The consumption of large quantities of simple
sugars can result in a surge of glucose into the
bloodstream.
• The consumption of fresh and whole-grained
complex carbohydrates results in a slow, steady
entry of glucose into the bloodstream.
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Carbohydrates and Nutrition
• Glucose surges result in surges in blood levels
of insulin, a phenomenon that has been linked
to:
– increased hunger
– higher levels of lipids in the bloodstream
– a lowering of levels of “good” cholesterol, among
other effects
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Glycemic Load
• One means of measuring the effect of different
carbohydrates on blood levels of glucose is the
glycemic load: a measure of how blood glucose
levels are affected by defined portions of given
carbohydrates.
• Carbohydrates with low glycemic loads are in
general more healthy than carbohydrates with
high glycemic loads.
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Fiber
• The indigestible complex carbohydrates known
as fibers have several good health effects,
among them a lowering of blood levels of
cholesterol and a reduction in glycemic load.
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Fiber
• Whole, unrefined grains tend to be high in
fiber, while processed, “white” grains have less
fiber, and added simple sugars have no fiber at
all.
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Phytochemicals
• Most carbohydrates come from plant sources.
• Plants in general—and fresh fruits and
vegetables in particular—are rich sources of
phytochemicals: nonnutritive substances found
in plants that promote health.
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Phytochemicals
• Phytochemicals seem to protect against heart
disease, cancer, and age-related macular
degeneration.
• Nutritionists recommend choosing from among
a “rainbow” spectrum of brightly colored fruits
and vegetables to ensure an adequate intake of
phytochemicals.
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Phytochemicals
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Figure 30.13
30.9 Lipids
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Lipids
• Lipids are a great source of energy, but they
also have significant structural functions in the
body as they are major components of all cell
membranes.
• In addition, lipids are used to make hormones,
and they provide insulation and serve a shockabsorbing function.
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Lipids
• The building blocks of most dietary lipids are
molecules called triglycerides that are
composed of a “head” composed of glycerol
and three attached fatty acid chains.
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Triglycerides
glycerol
fatty acids
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Figure 30.14
Triglycerides
• Triglycerides can be broken down into these
component parts to yield energy or can be
stored in fat cells.
• When the body has sufficient energy,
carbohydrates and proteins are transformed into
triglycerides that are then stored away.
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Oils and Fats
• In everyday speech, all lipids are referred to as
fats.
• Nutritionists, however, make a distinction
between:
– Oils, which are dietary lipids that are liquid at room
temperature.
– Fats, which are dietary lipids that are solid at room
temperature.
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Saturated and Unsaturated Fatty Acids
• Fats are saturated or unsaturated in accordance
with their fatty acid makeup.
• A “fat”—meaning a fat or oil—that is saturated
is predominantly composed of saturated fatty
acids.
• A fat that is unsaturated is predominantly
composed of unsaturated fatty acids.
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Saturated and Unsaturated Fatty Acids
• Fatty acids are defined in a saturated-tounsaturated spectrum in accordance with the
number of carbon double bonds they have in
their hydrocarbon chains.
• A saturated fatty acid is a fatty acid with no
double bonds between the carbon atoms of its
hydrocarbon chain.
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Saturated and Unsaturated Fatty Acids
• A monounsaturated fatty acid is a fatty acid
with one double bond between carbon atoms.
• A polyunsaturated fatty acid is a fatty acid
with two or more double bonds between carbon
atoms.
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Saturated and Unsaturated Fatty Acids
Palmitic acid
saturated (no double bonds)
Oleic acid
monounsaturated (one double bond)
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Figure 30.15
Saturated and Unsaturated Fatty Acids
• Lipids that contain a high proportion of
unsaturated fatty acids tend to be oils, while
those that contain a high proportion of saturated
fatty acids tend to be fats.
• In general, lipids are healthy to the extent that
they are unsaturated rather than saturated.
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Saturated and Unsaturated Fatty Acids
• Trans fats and saturated fats increase the longterm risk of heart disease.
• Polyunsaturated and monounsaturated fats are
at least neutral with respect to it.
• Polyunsaturated fats containing omega-3 fatty
acids seem to protect against it.
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Saturated and Unsaturated Fatty Acids
• Omega-3 fats are found in greatest abundance
in certain kinds of fatty fish.
• The usual source for other polyunsaturated and
monounsaturated fats is plant-based oils.
• Saturated fats normally are found in animalbased fats.
• Trans fats are found in a number of packaged
and fast foods.
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Saturated and Unsaturated Fatty Acids
• Trans fats are the product of an industrial
process in which naturally occurring oils are
turned into fats by partially hydrogenating
them—by infusing them with hydrogen atoms.
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30.10 Elements of a Healthy Diet
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Elements of a Healthy Diet
• Diets can be judged as healthy to the extent that
they contain a high proportion of healthy foods.
• Food pyramids are one means of getting a sense
of proportionality in food intake.
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Foods in the Proper Proportion
White rice,
white bread,
white pasta,
potatoes,
soda, and
sweets
Red meat,
butter
Use sparingly
Dairy or calcium
supplement,
1–2 times/day
Multiple vitamins
for most
Fish, poultry, eggs, 0–2 times/day
Nuts, legumes,
1–3 times/day
Alcohol in
moderation
(if appropriate)
Vegetables (in abundance)
Whole grain foods (at most meals)
Fruits, 2–3 times/day
Plant oils (olive, canola, soy,
corn, sunflower, peanut,
and other vegetable oils)
Daily exercise and weight control
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Figure 30.16
30.11 The Urinary System in Overview
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The Urinary System
• The urinary system:
– Filters waste materials from the blood.
– Regulates blood volume.
– Conserves useful materials, such as water, nutrients,
and ions.
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30.12 Structure of the Urinary System
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Structure of the Urinary System
• It consists of:
– Two kidneys that produce urine.
– The left and right ureters that the urine travels
through on leaving the kidneys.
– The muscular urinary bladder, which receives the
urine from the ureters and temporarily stores it.
– The tube called the urethra through which urine
passes from the bladder out of the body.
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Structure of the Urinary System
vena cava
aorta
renal artery
renal vein
kidney:
produces urine
ureter:
transports urine toward
urinary bladder
urinary bladder:
temporarily stores urine
urethra:
transports urine out of body
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Figure 30.17
Structure of the Urinary System
• The working unit of the kidneys is the nephron,
composed of a nephron tubule, its associated
blood vessels, and the interstitial fluid in which
both are immersed.
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Structure of the Urinary System
• Several nephron tubules empty into a common
collecting duct, which will merge with other
such ducts to feed into the renal pelvis, which
feeds into the ureter.
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Structure of the Urinary System
Proximal tubule:
Water and nutrients
move back into
capillaries.
The Nephron
Glomerulus:
Blood flows into glomerulus
under pressure, driving some
blood components out into
the Bowman’s capsule.
nephron
tubule
H2O
nutrients
Bowman’s
capsule
renal
arteriole
Distal tubule:
Water continues to
move back into
capillaries; toxins
ions, and acids
move from
capillaries into
tubule.
blood from
renal artery
blood to
renal vein
Kidney
Collecting duct:
As water continues
to move back into
blood circulation,
waste concentrates,
becoming urine
that drains to the
renal pelvis.
renal
pelvis
ureter
urine drained
to renal pelvis
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Figure 30.18
30.13 How the Kidneys Function
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How the Kidneys Function
• A knotted network of capillaries within a
nephron, the glomerulus, receives arterial blood
and is porous enough to allow much of the fluid
portion of the blood to flow out of it along with
smaller molecules such as vitamins, nutrients,
and waste products.
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How the Kidneys Function
• These materials enter the surrounding
Bowman’s capsule, thus moving into the
nephron’s tubule as a fluid called filtrate.
• At the nephron’s next structure, called the
proximal tubule, much of the original water and
almost all the original nutrients are moved back
into blood circulation.
• Waste products remain in the nephron tubule,
however, because of their chemical
composition.
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How the Kidneys Function
• This general process continues over the length
of the nephron tubule: water and nutrients move
back into circulation, while waste products
become ever more concentrated within the
tubule.
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How the Kidneys Function
• By the time the filtrate has reached the
collecting duct, it has become urine.
• The body is able to control how much water the
kidneys send to the bladder (in urine) or retain
in circulation.
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How the Kidneys Function
urine from kidneys
ureter
ligament
bladder
ureteral
openings
internal
sphincter
prostate
gland
urethra
external
sphincter
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Figure 30.19
How the Kidneys Function
• Retention of water by the kidneys is regulated
by a part of the brain, called the hypothalamus,
that controls the secretion of antidiuretic
hormone (ADH).
• An increased secretion of ADH means that
more water will move out of the kidney’s
tubules and collecting ducts and back into
circulation.
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30.14 Urine Storage and Excretion
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Urine Storage and Excretion
• Waves of muscle contraction squeeze urine out
of the renal pelvis, moving it through the
ureters and into temporary storage in the
urinary bladder.
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Urine Storage and Excretion
• The tube called the urethra then carries the
urine from the urinary bladder to the exterior of
the body.
• An internal sphincter muscle provides
involuntary control over the discharge of urine.
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Urine Storage and Excretion
• The urethra also contains an external sphincter
that is under voluntary control.
• We become aware of the need to urinate when
the bladder is stretched beyond a certain
threshold.
• We then relax the voluntary, external sphincter,
which relaxes the involuntary internal
sphincter, and the urine moves out of the
bladder and the body.
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The Urinary System
PLAY
Animation 30.1: The Urinary System
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