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Chapter 21
Nutrition, Digeston,
and Excretion
Lectures by
Gregory Ahearn
University of North Florida
Copyright © 2009 Pearson Education, Inc..
21.1 How Do Animals Regulate The
Composition Of Their Bodies?
 A nutrient is any substance that an animal
needs but cannot synthesize or produce in
its own body, and hence must acquire it
from its environment as it eats or drinks.
 Digestion is the process whereby an animal
physically grinds up and chemically breaks
down its food, producing small, simple
molecules that can be absorbed into the
circulatory system.
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21.1 How Do Animals Regulate The
Composition Of Their Bodies?
 Nutrition includes taking food into the body,
converting it into usable forms, absorbing
the resulting molecules from the digestive
tract into the circulatory system, and using
the nutrients in the animal’s own
metabolism.
 Excretion is the disposal of indigestible,
toxic, or surplus materials.
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21.2 What Nutrients Do Animals Need?
 Animal nutrients fall into six major
categories:
•
•
•
•
•
•
Lipids
Carbohydrates
Proteins
Minerals
Vitamins
Water
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21.2 What Nutrients Do Animals Need?
 The primary sources of energy are lipids
and carbohydrates.
• Energy is provided mostly from lipids,
carbohydrates, and to lesser extent, proteins.
• Energy in food is measured in Calories.
• A Calorie is the amount of energy needed to
raise the temperature of 1 gram of water by 1
degree Celsius.
• The average person at rest burns 1,550
Calories per day at rest.
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21.2 What Nutrients Do Animals Need?
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21.2 What Nutrients Do Animals Need?
 Lipids include fats, phospholipids, and
cholesterol.
• Fats and oils are used primarily as a source of
energy.
• Cholesterol is used to make cell membranes
and several hormones, including testosterone
and estrogen.
• Essential fatty acids, such as linoleic acid,
cannot be synthesized and must be obtained
in the diet.
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21.2 What Nutrients Do Animals Need?
 Fats store energy in concentrated form.
• In humans, energy is stored primarily as fat.
• When more Calories are eaten than are used,
the excess fats, carbohydrates, and proteins
are all converted to fat for storage.
• Fats has twice as much energy per unit weight
as the other nutrients.
• Lipids are hydrophobic and do not cause
water to be accumulated within the fats in the
body.
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21.2 What Nutrients Do Animals Need?
 For example, the
ruby-throated
hummingbird
migrates across the
Gulf of Mexico in the
fall, getting its
energy from stored
lipids.
Fig. 21-1
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21.2 What Nutrients Do Animals Need?
 Carbohydrates are a source of quick energy.
• Carbohydrates include simple sugars and
longer chains of sugars called
polysaccharides.
• During digestion, simple sugars, like glucose,
are derived from the breakdown of more
complex carbohydrates, such as sucrose and
starch.
• Animals and humans store sugars as
glycogen, a large branched chain of glucose
molecules.
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21.2 What Nutrients Do Animals Need?
 Proteins provide amino acids for building
new proteins.
• Protein provides these amino acids after they
are digested.
• Dietary protein comes from meat, milk, eggs,
corn, and beans.
• Our bodies can synthesize certain amino
acids, but eight cannot be made by our
biochemistry and must be supplied in our
diet—they are called “essential amino acids”.
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21.2 What Nutrients Do Animals Need?
 Protein deficiency
can cause a variety
of debilitating
conditions, including
kwashiorkor.
Fig. 21-2
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21.2 What Nutrients Do Animals Need?
 Minerals are elements required by the body.
• A mineral is a chemical element that is
required for proper bodily function.
• Minerals are needed for strong bones and
teeth, for muscles contraction, for nerve
functions, and for proper blood cell functions.
• Metals are also important since they act as
parts of enzymes in certain body reactions
(e.g., zinc, copper, selenium).
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21.2 What Nutrients Do Animals Need?
 Vitamins play many roles in metabolism.
• Vitamins are a diverse group of organic
compounds that animals require in very small
amounts.
• The body cannot synthesize them, so they
must be obtained in the diet.
• Vitamins are grouped into two categories:
water soluble and fat soluble.
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Copyright © 2009 Pearson Education Inc.
21.2 What Nutrients Do Animals Need?
 Water-soluble vitamins
• These substances dissolve in water or blood
plasma and are excreted by the kidney; they
therefore do not build up in the body.
• They include vitamin C and the B-vitamin
complex.
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21.2 What Nutrients Do Animals Need?
 A deficiency in
niacin, a B-vitamin,
causes cracked,
scaly skin and
digestive and
nervous system
disorders.
Fig. 21-3
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21.2 What Nutrients Do Animals Need?
 Fat-soluble vitamins
• Fat soluble vitamins can accumulate in the
body and be toxic if present in too high a
concentration.
• This group includes:
• Vitamin K: regulates blood clotting
• Vitamin A: produces visual pigments in the
eyes for vision
• Vitamin D: promotes strong bones
• Vitamin E: prevents cellular damage; is an
antioxidant
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21.2 What Nutrients Do Animals Need?
 Vitamin D deficiency
can lead to a
condition called
Rickets, which is a
deterioration of
bone.
Fig. 21-4
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21.2 What Nutrients Do Animals Need?
 The human body is about two-thirds water.
• Water is the principal component of saliva,
blood, lymph, extracellular fluid, and
cytoplasm within each cell.
• The average human requires about 2,500
milliliters (10 cups) of water per day, but this
can change with exercise, temperature, and
humidity.
• We obtain about half of our water from the
food we eat and the rest is obtained from the
fluids we drink.
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21.2 What Nutrients Do Animals Need?
 Nutritional guidelines help people obtain a
balanced diet.
• Nutritional guidelines, called “My Pyramid,”
are posted to a U.S. government interactive
website.
• Other sources of nutritional information are
found on the labels of commercially packaged
foods; they contain information about calorie,
fat, sugar, and vitamin content.
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21.2 What Nutrients Do Animals Need?

Are you too heavy?
•
A simple way to calculate whether your
weight is likely to pose a health risk is to
calculate your body mass index (BMI).
• The BMI takes into account your weight and
height to arrive as an estimate of body fat.
• Two ways to calculate your BMI are:
1. Weight (in kilograms)/height2 (in meters)
2. Weight (in pounds) x 703/height2 (in
inches)
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21.2 What Nutrients Do Animals Need?
 Are you too heavy? (continued)
• A BMI between 18.5 and 25 is considered
healthy.
• People with anorexia have a BMI of 17.5 or
lower.
• A BMI between 25 and 30 indicates you are
probably overweight.
• A BMI over 30 indicates your are obese.
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21.3 What Are The Major Processes Of
Digestion?
 All digestive systems must accomplish
certain tasks.
• Ingestion: food is brought into the digestive
tract through the mouth
• Mechanical breakdown: the physical
breakdown of food into small pieces
• Chemical breakdown: digestive enzymes
convert the large molecules in food into small
molecules
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21.3 What Are The Major Processes Of
Digestion?
 All digestive systems must accomplish
certain tasks (continued).
• Absorption: the transfer of small molecules
across the gut to the blood and then to cells of
the body
• Elimination: indigestible materials are expelled
from the body
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21.4 What Is The Diversity Of Digestive
Systems In Non-Human Animals?
 In sponges, digestion occurs within single
cells.
• Sponges rely exclusively on individual cells to
digest their food.
• Sponges circulate seawater through pores in
their bodies, and collar cells filter microscopic
organisms from the water and ingest them by
phagocytosis.
• Phagocytized food is digested inside these
cells in sacs called lysosomes, which contain
digestive enzymes.
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21.4 What Is The Diversity Of Digestive
Systems In Non-Human Animals?
 Intracellular digestion in a sponge
Water, uneaten food,
and wastes are expelled
through the large opening
at one end of the sponge
Waste products are
expelled by exocytosis
H2O
H2O
(a) Tube sponges
The food
vacuole merges
with a lysosome
collar cell
H2O carrying
food particles
enters the pores
H2O
H2O
Food particles
are filtered from the
water by the collar
(b) A simple sponge
Food enters the
collar cell by phagocytosis,
forming a food vacuole
food vacuole
(c) Collar cell
lysosome
with
digestive
enzymes
Fig. 21-6
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21.4 What Is The Diversity Of Digestive
Systems In Non-Human Animals?
 Jellyfish and their relatives have digestive
systems consisting of a sac with a single
opening.
• The most simple digestive tract occurs in sea
anemones, coral, and jellyfish, which possess
a sac with one opening.
• Both food and waste pass through the single
opening.
• Food is chemically broken down in the sac by
digestive enzymes, and the nutrients are
absorbed by cells lining the sac.
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21.4 What Is The Diversity Of Digestive
Systems In Non-Human Animals?
 Digestion in a sac
prey
(a) Hydra with prey
Tentacles with
stinging cells capture
the prey and carry
it into the mouth
mouth
Gland cells secrete
digestive enzymes into
the digestive sac and
begin extracellular
digestion
prey
Nutritive cells engulf
food particles and
complete digestion
within food vacuoles
digestive
sac
(b) Food processing in Hydra
Fig. 21-7
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21.4 What Is The Diversity Of Digestive
Systems In Non-Human Animals?
 Most animals have digestive systems
consisting of a tube with several specialized
compartments.
• The tube performs different functions along its
length; food is first mechanically broken down,
then chemically altered, then the nutrients are
absorbed, and finally, wastes are eliminated.
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21.4 What Is The Diversity Of Digestive
Systems In Non-Human Animals?
 Worms, mollusks, arthropods, and
vertebrates are examples of animals with
this type of gut.
Soil with
food particles
is ingested
Indigestible remnants
are expelled
intestine
anus
mouth
pharynx
esophagus
crop
Food is ground
up in the gizzard
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gizzard
Food is digested
and absorbed in the
intestine
Fig. 21-8
21.5 How Do Humans Digest Food?
 Humans have a tubular digestive tract with
several compartments in which food is
broken down, physically and chemically,
before being absorbed into the circulatory
system.
 Digesting and absorbing food requires
coordinated action from the various
structures of the digestive system.
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Salivary glands: Secrete
lubricating fluid and
starch-digesting enzymes
Pharynx: Shared digestive
and respiratory passage
Oral cavity. tongue,
teeth: Grind food,
mix with saliva
Epiglottis: Directs food
down the esophagus
Esophagus: Transports
food to the stomach
Stomach: Breaks
down food and
begins protein
digestion
Liver: Secretes bile (also
has many non-digestive
functions)
Gallbladder: Stores bile
from the liver
Pancreas: Secretes buffers
and several digestive
enzymes
Large intestine: Absorbs
vitamins, minerals, and
water; houses bacteria;
produces feces
Small intestine:
Food is digested
and absorbed
Rectum: Stores feces
Fig. 21-9
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21.5 How Do Humans Digest Food?
 Breakdown of food begins in the mouth.
• Mechanical food breakdown is due to the
action of 32 teeth of different shapes and
sizes including incisors, canines, premolars,
and molars.
• Three pairs of salivary glands secrete saliva,
which lubricates the food, as well as amylase,
which starts the chemical breakdown of
sugars in the mouth.
Fig. 21-10
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21.5 How Do Humans Digest Food?
 Teeth begin the
mechanical
breakdown of food.
incisors
canine
premolars
molars
Fig. 21-10
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21.5 How Do Humans Digest Food?
PLAY
Animation—Digestion in the Mouth
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21.5 How Do Humans Digest Food?
 The pharynx connects the mouth to the rest
of the digestive system.
• With the help of the muscular tongue, the food
is manipulated into a mass and pressed
backward into the pharynx, which connects
the mouth with the esophagus.
• The swallowing reflex elevates the larynx, so
that the epiglottis blocks off the opening to the
trachea and guides food to the esophagus.
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21.5 How Do Humans Digest Food?
 The challenge of swallowing
tongue
roof of mouth
The
tongue forces
food into the
esophagus
food
The tongue
manipulates
food while
chewing
pharynx
The larynx moves
up and the epiglottis
folds over the larynx
epiglottis
epiglottis
The epiglottis is
elevated to allow air
to flow through the
pharynx into the larynx
(a) Before swallowing
food
esophagus
esophagus
larynx
Food enters
the esophagus
larynx
(b) During swallowing
Fig. 21-11
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21.5 How Do Humans Digest Food?
 The esophagus conducts food to the
stomach.
• Swallowing forces food into the esophagus, a
muscular tube that propels the food from the
mouth to the stomach.
• Muscles surrounding the esophagus produce
a wave of contraction, called peristalsis, that
begins above the swallowed food and
progresses down the esophagus, forcing the
food to the stomach.
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21.5 How Do Humans Digest Food?
 The stomach stores and breaks down food.
• The human stomach is an expandable
muscular sac capable of holding as much as a
gallon of food and liquids.
• The stomach has three functions:
• It stores food and releases it gradually into
the small intestine for digestion and
absorption.
• It assists in the mechanical food
breakdown.
• It has a role in chemical food breakdown.
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21.5 How Do Humans Digest Food?
PLAY
Animation—Digestion in the Stomach
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21.5 How Do Humans Digest Food?
 Most digestion occurs in the small intestine.
• The small intestine is 1 inch in diameter and
10 feet long.
• It digests food into small molecules and
absorbs them into the bloodstream.
• This process of digestion is accomplished with
the aid of secretions from the liver, the
pancreas, and the cells of the small intestine
itself.
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21.5 How Do Humans Digest Food?
 The liver and gallbladder provide bile.
• The liver stores glycogen and detoxifies many
poisonous substances.
• It also produces bile for digestion; bile is a
complex mixture of bile salts, other salts,
water, and cholesterol.
• Bile is stored in the gallbladder and is
released into the small intestine where it aids
in fat digestion.
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21.5 How Do Humans Digest Food?
 The pancreas secretes digestive
substances.
• The pancreas consists of two major types of
cells:
• One type produces hormones that regulate
blood sugar.
• The other type produces a digestive
secretion called pancreatic juice; this
contains water, sodium bicarbonate, and
several digestive enzymes that break down
sugars, lipids, and proteins.
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21.5 How Do Humans Digest Food?
 The intestinal wall completes the digestive
process.
• Digestive enzymes are embedded in the
plasma membrane of the cells that line the
small intestine, so that the final phase of
digestion occurs as the nutrient is being
absorbed into the cell.
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21.5 How Do Humans Digest Food?
 Most absorption occurs in the small
intestine.
• The small intestine is the major site of nutrient
absorption into the blood.
• It has numerous folds and projections that
give it an internal surface area 600 times
greater than a smooth tube of the same
length.
• Fingerlike projections called villi (singular,
villus) cover the entire surface of the intestinal
wall.
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21.5 How Do Humans Digest Food?
 Most absorption occurs in the small intestine
(continued).
• Each cell on a villus has microscopic
projections called microvilli, which increase
the area for absorption even more.
• Within each villus is a network of blood
capillaries and a single lymph capillary called
a lacteal.
• Most nutrients pass through the cells of the
small intestine and enter the capillaries, but
breakdown products of fats pass across the
cells and enter the lacteals.
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21.5 How Do Humans Digest Food?
 The small intestine
lacteal
fold of
intestinal
lining
microvilli
villi
capillaries
arteriole
intestinal
gland
(a) Small intestine
(b) A fold of the
intestinal lining
(c) A villus
lymph
vessel
venule
(d) Cells of a villus
Fig. 21-12
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21.5 How Do Humans Digest Food?
 The large intestine absorbs water, minerals,
and vitamins, and forms feces.
• The large intestine in an adult human is about
5 feet long and 3 inches in diameter; the first
part is called the colon and the last 6 inches is
the rectum.
• Bacteria in the colon synthesize vitamin B12,
thiamin, riboflavin, and vitamin K.
• Large intestine cells absorb water, minerals,
and vitamins.
• Feces is formed in the large intestine.
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21.5 How Do Humans Digest Food?
PLAY
Animation—Digestion in the Intestines
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21.5 How Do Humans Digest Food?
PLAY
Animation—Absorption of Nutrients
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21.5 How Do Humans Digest Food?
 Digestion is controlled by the nervous
system and hormones.
• The secretions and muscular activity of the
digestive tract are regulated by both nerves
and hormones.
• Sensory signals initiate digestion.
• The sight, smell, taste, and just the thought
of food generate signals from the brain that
act on the digestive tract.
• For example, nerve impulses stimulate the
salivary glands and cause the stomach to
secrete acid and mucus.
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21.5 How Do Humans Digest Food?
 Hormones help regulate digestive activity
through negative feedback.
• Gastrin is secreted from stomach cells in
response to the presence of protein
breakdown products, and stimulates acid
secretion by the stomach.
• Secretin and cholecytokinin are secreted by
the small intestine in response to chyme
coming from the stomach; they stimulate the
secretion of digestive enzymes and sodium
bicarbonate by the pancreas and bile from the
liver.
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21.5 How Do Humans Digest Food?
 Gastric inhibitory peptide, secreted by the
small intestine in response to fatty acids and
sugars in chyme, stimulates the pancreas to
release insulin.
 This in turn stimulates body’s cells to absorb
sugar from the blood.
 It also inhibits stomach peristalsis, which
slows its emptying rate.
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21.6 What Are The Functions Of Urinary
Systems?
 All urinary systems of animals function
similarly.
• First, the blood is filtered, with water and small
dissolved molecules moving from the blood
into the urinary system.
• Next, nutrients are selectively reabsorbed
back into the blood.
• Some highly toxic substances are actively
secreted from the blood into the urinary
system.
• Finally, wastes and excess nutrients are
excreted from the body.
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21.7 What Is The Diversity Of Urinary
Systems In Non-Human Animals?
 In a few animals, like sponges, individual
cells dump wastes into the surrounding
water.
 Most animals have complex urinary
systems, under nervous and hormonal
control, that regulate which substances are
excreted and which are retained in the
body’s fluids.
 Flame cells are urinary structures in
flatworms, while nephridia have the same
role in earthworms.
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21.7 What Is The Diversity Of Urinary
Systems In Non-Human Animals?
 Flame cells filter fluids in flatworms.
• Because flatworms largely live in freshwater, a
major function of their excretory system is to
regulate water balance.
• The flatworm’s excretory system consists of a
network of tubes that branch throughout the
body.
• At intervals, the branches end blindly in singlecelled bulbs called flame cells.
• Water and dissolved substances are filtered
from the body by these bulbs, and are
expelled through pores on the body surface.
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21.7 What Is The Diversity Of Urinary
Systems In Non-Human Animals?
 The simple
eyespot
flame
tubule
excretory system of
cell
a flatworm
cilia
fluid
excretory
pores
Fig. 21-13
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21.7 What Is The Diversity Of Urinary
Systems In Non-Human Animals?
 Nephridia filter fluids in earthworms.
• Earthworms, mollusks, and other invertebrates
have simple filtering structures called
nephridia, which resemble the filtering
structures found in vertebrate kidneys.
• Each segment of the worm contains a pair of
nephridia that filter each segment of wastes
and nutrients.
• The resulting urine is stored in a bladder-like
portion of the nephridium and is excreted
through pores in the body wall.
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21.7 What Is The Diversity Of Urinary
Systems In Non-Human Animals?
 The excretory
system of the
earthworm
nephridia
intestine
excretory pore
nerve cord
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Fig. 21-14
21.8 How Does The Human Urinary System
Work?
 The human urinary system produces,
transports, and excretes urine.
• The kidneys are organs in which the fluid
portion of the blood is collected and filtered.
• From this fluid, water and important nutrients
are then reabsorbed into the blood.
• The remaining fluid, called urine—consisting
of toxic substances, cellular waste products,
excess vitamins, salts, some hormones, and
water—stays behind and is excreted from the
body.
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21.8 How Does The Human Urinary System
Work?
 The urinary system is crucial for
homeostasis.
• It regulates blood levels of ions such as
sodium, potassium, chloride, and calcium.
• It regulates the water content of the blood.
• It maintains proper pH of the blood.
• It retains important nutrients such as glucose
and amino acids in the blood.
• It eliminates cellular waste products such as
urea.
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21.8 How Does The Human Urinary System
Work?
 A flow diagram
showing the
formation and
excretion of urea
amino acid
In cells, amino acids
are broken into simpler
molecules, releasing
ammonia
ammonia
carried
in blood
In the liver, ammonia
is converted to urea
urea
carried
in blood
In the kidneys, urea and
other water-soluble wastes
are filtered from the blood
excreted
in
urine
Fig. 21-15
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21.8 How Does The Human Urinary System
Work?
 The urinary system consists of the kidneys,
ureter, urinary bladder, and urethra.
• Human kidneys are paired organs located on
either side of the spinal cord, slightly above
the waist.
• The kidneys produce urine, which leaves each
kidney through a narrow, muscular tube called
a ureter.
• The ureters transport the urine to the urinary
bladder.
• The urethra is a short tube from the bladder to
the outside world.
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21.8 How Does The Human Urinary System
Work?
 The human urinary
system
left renal
artery
left kidney
left renal
vein
aorta
left ureter
vena cava
urinary
bladder
urethra
(in penis)
Fig. 21-16
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21.8 How Does The Human Urinary System
Work?
PLAY
Animation—Human Urinary System
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21.8 How Does The Human Urinary System
Work?
 Urine is formed in the nephrons of the
kidneys.
• Each kidney contains a solid outer layer where
urine forms and an inner chamber that collects
urine and funnels it into the ureter.
• The outer layer of each kidney contains about
a million tiny tubes called nephrons, which
filter the blood, process the filtered fluid, and
form urine.
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21.8 How Does The Human Urinary System
Work?
 Cross section of a
kidney
renal pelvis
(cut away
to show the
path of urine)
renal
artery
renal
vein
ureter
(cut away
to show
the path
of urine)
collecting
duct
urine
nephron
enlargement of a single
nephron and collecting duct
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to
bladder
Fig. 21-17
21.8 How Does The Human Urinary System
Work?
 Each nephron has three parts:
• The glomerulus: capillaries from which fluid is
filtered from the blood and collected
• Bowman’s capsule: captures filtered fluid from
the glomerulus
• The tubule: receives filtered fluid from
Bowman’s capsule
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21.8 How Does The Human Urinary System
Work?
 An individual
nephron and its
blood supply
collecting
duct
distal tubule
proximal tubule
Bowman’s
capsule
glomerulus
arterioles
branch of
renal artery
branch of
renal vein
loop of Henle
capillaries
Fig. 21-18
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21.8 How Does The Human Urinary System
Work?
 Different portions of the tubule selectively
modify the fluid as it travels through them;
nutrients are selectively reabsorbed, while
wastes remain behind to form urine.
• The Bowman’s capsule channels fluid into the
proximal tubule.
• The fluid then moves through the loop of
Henle and the distal tubule.
• The distal tubules of multiple nephrons drain
into a collecting duct that conducts urine to the
ureter.
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21.8 How Does The Human Urinary System
Work?
 Blood is filtered by the glomerulus.
• Urine formation starts with the process of
filtration.
• Blood enters each nephron by an arteriole that
branches off the renal artery.
• The arteriole branches into capillaries that
form the glomerulus.
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21.8 How Does The Human Urinary System
Work?
 Blood is filtered by the glomerulus
(continued).
• Blood pressure within the capillaries forces
water and dissolved substances through the
wall of the glomerulus.
• The resulting watery fluid is called the filtrate.
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21.8 How Does The Human Urinary System
Work?
 The filtrate is converted to urine in the
tubules of the nephron.
• This filtrate contains a mixture of wastes,
essential nutrients, and water.
• The nephron must restore the nutrients and
most of the water to the blood while retaining
the wastes for elimination.
• This process is accomplished by the two
processes of tubular reabsorption and tubular
secretion.
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21.8 How Does The Human Urinary System
Work?
 Tubular reabsorption moves water and
nutrients from the nephron to the blood.
• From Bowman’s capsule, the filtrate passes
through the proximal tubule where most of the
water and nutrients in the filtrate move from
the proximal tubule into the capillaries; this
process is called tubular reabsorption.
• Salts and nutrients are actively transported out
of the proximal tubule into the extracellular
fluid, and then diffuse into the surrounding
capillaries to return to the blood.
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21.8 How Does The Human Urinary System
Work?
 Tubular secretion moves wastes from the
blood into the nephron.
• In tubular secretion, wastes such as hydrogen
ions, potassium, ammonia, and many drugs
are moved from the capillaries into the
nephron.
• Cells of the distal tubule actively transport
wastes from the surrounding extracellular
space into the tubule, creating a concentration
gradient from blood in the capillaries to the
extracellular fluid; the wastes thus diffuse out
of the capillaries.
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21.8 How Does The Human Urinary System
Work?
 Urine becomes concentrated in the
collecting ducts.
• Concentration of urine occurs in the collecting
ducts through the removal of water.
• As filtrate travels through the collecting ducts
to the renal pelvis, it passes through areas of
increasingly concentrated extracellular fluid.
• Water leaves the filtrate by osmosis and is
carried off by the surrounding capillaries.
Copyright © 2009 Pearson Education Inc.
21.8 How Does The Human Urinary System
Work?
 Urine formation in the nephron and
collecting duct
Filtration: Water, nutrients,
and wastes are filtered from the
glomerular capillaries into the
Bowman’s capsule of the nephron
Tubular reabsorption: In the
proximal tubule, most water and nutrients
are reabsorbed into the blood
Tubular secretion:
In the distal tubule,
additional wastes are
actively secreted into the
tubule from the blood
Concentration: In
the collecting duct,
additional water may
leave, creating urine
that is more concentrated
than the blood
Fig. 21-19
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21.8 How Does The Human Urinary System
Work?
 Negative feedback regulates the water content of
the blood.
• The amount of water reabsorbed into the blood is
controlled by negative feedback.
• Antidiuretic hormone (ADH) regulates the amount of
water reabsorbed by the collecting ducts.
• It does this by increasing the permeability of the distal
tubule and the collecting ducts to water.
• The release of ADH from the pituitary is regulated by
receptor cells in the brain that monitor blood
concentration.
Copyright © 2009 Pearson Education Inc.
21.8 How Does The Human Urinary System
Work?
 Dehydration
stimulates ADH
release and water
retention.
Heat causes water
loss and dehydration
Receptors in the brain detect
the low water content of the
blood and signal the pituitary
gland
The pituitary gland
releases ADH into the
bloodstream
ADH increases the
permeability of the distal tubule
and the collecting duct,
allowing more water to be
reabsorbed into the blood
Water is retained in the body
and concentrated urine is
produced
Fig. 21-20
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21.8 How Does The Human Urinary System
Work?
PLAY
Animation—Urine Formation
Copyright © 2009 Pearson Education Inc.