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What Happens to the Food You Eat?
teeth
salivary glands
tongue
salivary glands
esophagus
liver
stomach
bile duct
large intestine
small intestine
appendix
rectum
anus
a.
Carbohydrates
Protein
Lipids
amylase
pepsin
Stomach
pancreatic pancreatic pancreatic
proteases lipases
amylase
Small
Intestine
Simple
Sugars
b.
Mouth
Amino
Acids
Simple Lipids
and Fats
Nutrients Absorbed into Blood Stream
Figure E7.8 The digestive system of the human body
a. Location of organs and tissues involved in digestion, b. Examples
of enzyme action in breaking down food. Notice the compartments
in which the enzymes for specific substrates act.
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Unit 3
ESSAY: What Happens to the Food You Eat?
Have you ever watched a pizza commercial
on television and heard your stomach growl
as the actor pulls up a warm slice with
stringy cheese trailing behind? When you
feel hungry—whether in response to your
body’s actual need for nutrients or just over
thoughts of a tasty slice of pizza—hormonal
signals begin to prepare the digestive system
for action. How does this work when your
body truly needs nutrients? A decrease in
nutrient levels in your blood sends a signal
to the hunger center in your brain’s
hypothalamus. The hypothalamus responds
by triggering the release of digestive juices
into the stomach. A feeling of hunger then
motivates you to find food.
Why do humans often eat when their
bodies are not really in need of nutrients?
Areas of the brain in addition to the
hypothalamus are involved in controlling
eating. Signals from sensory organs about the
smell, taste, and sight of food may trigger the
perception of hunger. Memories and attitudes
that you have developed about food also
affect your eating behavior.
Regardless of how your feelings of
hunger originated, the initial responses to
hunger stimulate the secretion of hormones
such as gastrin. Those hormones, in turn,
stimulate further secretion of digestive juices
in the stomach. Thus, a feedback system
alerts your body that it needs (or wants) food.
You respond by changing your behavior to
locate the desired food.
Humans and most other animals
(sponges and parasitic worms are some of
the exceptions) must bring food inside their
bodies to provide proper conditions for
digestion. Figure E7.8 shows the components
of the human digestive system. The first steps
are familiar: obtain food, chew it, and then
swallow it.
Similarly, chewed bread still has its
carbohydrates intact, mainly in the form of
long starch molecules. Your body must break
starch down into sugars to use it. Starch is a
molecule manufactured in plants. As shown
in Figure E7.9, a starch molecule consists of
many sugar molecules bonded together in a
branching pattern.
How are large molecules broken down into
small ones? The chemical action of special
proteins known as enzymes helps make this
happen. Enzymes catalyze, or speed up, specific
molecular reactions that otherwise would take
place very slowly. Enzymes act
on molecules called
Proteins
O
substrates. Substrates bind to
O
specific places on enzymes.
OH
N
O
Carbohydrate and protein
N
OH
N
molecules are substrates of
OH
many enzyme-catalyzed
amino acids: carbon,
proteins: many amino acids reactions, particularly those
nitrogen, hydrogen,
bonded together in a chain
that occur during digestion.
and oxygen
that folds into a precise
The reaction catalyzed by an
shape
enzyme changes the enzyme’s
Carbohydrates
substrate into a different
molecule. Enzymes that help
accomplish the breakdown of
food are present in your
mouth, your stomach, and
your small intestines. Do you
remember the discussion
simple sugar: carbon,
starch: many simple
hydrogen, and lots of
sugars bonded together
earlier about saliva? Saliva
oxygen
in branching structures
contains the enzyme amylase.
Amylase breaks down starch
Lipids
into maltose molecules.
0
(Maltose consists of two
0
0
glucose molecules joined
together.) In your stomach,
0
the enzyme pepsin binds to
fatty acid: carbon,
protein molecules. Combined
simple fat: consists of
hydrogen, and a small
three fatty acid
withthe action of other
amount of oxygen
molecules joined to a
molecule of glycerol
digestive enzymes in the
Chewing performs an important
digestive function. As you chew, the surface
area of your food increases greatly. Increased
surface area means that the chemical
reactions involved in digestion can occur
more quickly. In addition, chewing moistens
the food with saliva. Salivary glands located
under your tongue secrete saliva.
The mechanical breakdown of food is not
enough. No matter how finely you chew bits
of steak, the proteins remain intact. If your
body is to use the proteins, it must break
them down into subunit parts: amino acids.
Figure E7.9 Examples of macromolecules and their components Macromolecules, such as proteins,
carbohydrates, and lipids, are chains of smaller molecules. For example, proteins are made up of long chains of many
different amino acids, and complex carbohydrates such as starch and glycogen are made up of long chains of simple
sugars. (Starch and glycogen are storage molecules in plants and animals, respectively.) The lipids known as simple
fats are made up of three long chains of fatty acids combined with glycerol, a small alcohol.
ESSAY: What Happens to the Food You Eat?
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ROLE OF SOME DIGESTIVE ENZYMES
Type of Enzyme
amylase
proteases
lipases
General Reaction
Optimal pH
starch → double sugars (maltose)
protein → amino acids
fats → fatty acids + glycerol
pH 6.9 – 7.0
pH 2.0 (pepsin) pH 7.0 – 8.0 (most others)
pH 8.0
Figure E7.10
small intestines, pepsin breaks down protein
into amino acids, as illustrated in Figure
E7.8b.
In general, digestive enzymes break
down complex molecules into their simple
components. Figure E7.10 lists some
examples of digestive enzymes, the reactions
they carry out, and the pH conditions under
which they function.
After you swallow, it takes less than
10 seconds for the chewed and moistened
food to pass through the esophagus into
the stomach. Smooth muscles encircle the
digestive tract. These muscles contract in a
coordinated fashion, called peristalsis, to
move food through the digestive tract. In the
stomach, food churns and mixes with
digestive fluids for three or four hours. The
stomach stretches during this process and
makes you feel full. This sensation reduces
your motivation to continue eating (unless
the food tastes so good that you ignore these
signals and continue eating anyway). This is
another example of feedback, which you
studied in Chapter 5.
From the stomach, partially digested
food passes into the small intestine. Final
digestion of the complex food molecules,
breaking down into their simple
components, occurs here. The cells that line
the small intestine contribute fluids that
contain digestive enzymes. Other organs,
including the pancreas and liver, also deliver
digestive enzymes to the small intestine. The
pancreas is an organ in the abdomen that
produces 1.4 liters of fluid per day. The fluid
contains enzymes that contribute to the final
digestion of the remaining macromolecules
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Unit 3
ESSAY: What Happens to the Food You Eat?
in the small intestine. The liver produces
0.8 to 1.0 liter of bile per day. Bile is stored
in the gall bladder and released into the
small intestine after a meal. Bile contains
bile salts, which act in a manner similar to
detergents. Bile breaks fat into tiny droplets,
increasing the surface area of the fats so that
enzymes can work on them easily.
In addition to being the site of digestion,
the small intestine is the organ where nearly
all absorption takes place. The digested
nutrients consist of simple sugar, amino acid,
and fatty acid molecules. These building
block molecules, obtained through digestion,
are small enough to pass through the small
intestine cell membranes and into the
bloodstream.
Liver cells monitor nutrient levels in
the blood and adjust them as necessary.
Substances present in excess amounts are
removed and stored. Substances that are
lacking are increased. The liver accomplishes
that by releasing stored forms or by
stimulating synthesis processes. For example,
when you exercise heavily and glucose levels
drop in your bloodstream, sensors trigger the
liver to release glucose that it had stored as
glycogen. The glycogen is quickly broken
down into glucose to replenish the exhausted
supply. Liver cells also remove and correct
potentially toxic substances (such as alcohol
and other drugs) from the blood.
Undigested remains of food cannot pass
through the wall of the small intestine.
Instead, this part of the food enters the large
intestine. The large intestine absorbs water
and returns it to the blood. The body then
eliminates the compacted solid wastes.
Her self-discipline is the envy of
all her friends. She denies her
hunger and exercises relentlessly
until she is convinced that she has
burned off any “excess” calories she might
have consumed.
Left untreated, Christine likely will
continue starving herself—possibly to death.
As her nutritional base deteriorates, she will
experience profound physical changes. Her
hormone levels will continue to drop. Her
heart muscle will become weak and thin.
Her digestive system will begin to function
less and less efficiently. Electrical activity in
her brain may become abnormal. Electrolyte
imbalances in her body will put her at risk
for sudden heart failure.
Because the underlying causes of
anorexia nervosa are complex and involve
self-image and mental attitudes, treatment
of the disorder also is complex. Successful
treatment must take into account the whole
individual: the physical self, the cultural self,
and the psychological self. Not surprisingly,
treatment is most successful when the entire
family is involved and participates honestly
in the process.
Anorexia Nervosa:
Dying to Be Thin
She’s been getting increasingly moody, she
hasn’t menstruated in three months, and the
circles under her eyes suggest to others that
she hasn’t been sleeping well. The cold she
caught two weeks ago has lingered, despite
her efforts to shake it. Though she denies
feeling tired, she seems to have less energy
each day. Yesterday, her father noticed her
swaying a bit—dizzy, perhaps?—when she
jumped up to answer the phone. Yet, after
dinner (an unhappy meal in which her
parents pushed her to eat and Christine
insisted she was not hungry), she went out to
run her customary 3 kilometers (1.9 miles).
Although Christine doesn’t know it
and probably wouldn’t admit it, she has
anorexia nervosa. Anorexia nervosa is an
eating disorder that affects an estimated
1 million individuals, mostly teenage girls,
in the United States. Christine doesn’t see
that she is undernourished. In fact, she will
insist against all evidence to the contrary
that she is fat and needs to lose weight.
The Structural Basis
of Physical Mobility
Mary, James, Lolita, Madonna, Rodriguez . . .
Writing your name seems simple enough,
doesn’t it? To do even this simple task,
however, requires a highly coordinated series
of muscle movements in your arm, hand, and
fingers. Energy is necessary for all of these
movements. Indeed, energy is required even
to transmit nerve impulses.
All physical activities require some type
of structure that can translate the energy of
food into useful biological work. The
muscles and skeleton of your arm, hand, and
fingers as well as the neurons that transmit
nerve impulses are biological structures. The
functions of these structures are quite
specific. A neuron alone cannot move your
fingers, nor can a muscle carry nerve
impulses. These examples illustrate that
there is a close relationship between a
physical structure and its function.
Consider, for example, the organization
and function of skeletal muscle. Skeletal
ESSAY: The Structural Basis of Physical Mobility
Unit 3
341
tendon
Figure E7.11 The
human arm functions like a
lever through the action of
biceps and triceps muscles
on a fulcrum point, the
elbow. Although these
muscles are attached to
the bones in the upper and
lower arms, muscle
contraction causes only the
lower bone to move.
biceps
contract
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Unit 3
biceps
relax
tendon
triceps
relax
triceps
contract
tendon
flexed
extended
muscle produces the movements of your limbs.
To generate most types of movement, muscles
must work in opposing groups against the
skeleton. Figure E7.11 illustrates the
organization of muscles in your arm. Notice
that the biceps and triceps attach to the bones
of the upper and lower arm by tendons.
Tendons are flexible cords of connective tissue.
The biceps’ tendon attaches to the bone of the
lower arm on the inside of the elbow joint.
When you contract the biceps, your arm
bends. By contrast, the triceps’ tendon attaches
to the bone of the lower arm on the outer side
of the elbow. When you contract the triceps,
your arm straightens.
Figure E7.12
Structure and function
in moles and cheetahs
The type of movement
possible in an organism
depends on the precise
arrangements of skeleton
and muscle. a. Short,
heavy bones like those of
the mole are typical of
animal skeletons that
require power. b. Thin,
light bones like those of the
cheetah favor speed. Think
about the effect of
applying forces to the two
muscle attachment sites.
tendon
Muscles in a vertebrate limb work on
the bone just as two sets of pulleys work
on a lever. A relatively small amount of
shortening of either produces a large
movement. Even though this is the case in
a wide variety of organisms, the details can
vary greatly. These differences mean that
different organisms are capable of different
types of movements.
This variation is especially evident in
organisms that have the same overall body
plan but have adapted to different situations.
Compare, for example, the forelimb of the
cheetah to that of the mole in Figure E7.12.
Both are vertebrates, and their limbs work
a.
b.
extensor muscle
mole
ESSAY: The Structural Basis of Physical Mobility
muscle
attachment
site
cheetah
according to the same principles (and even
use the same muscles) as the human arm.
The primary functions of these limbs differ
greatly, however. The cheetah’s limbs are
adapted for running after fleet-footed prey.
The mole’s limbs are adapted for burrowing
in the ground. What structural details
underlie these adaptations?
The mole’s digging forelimbs must
generate power rather than speed. As the
diagram in Figure E7.12a shows, the bones
of such limbs are short and thick. In
addition, the projection at the elbow to
which the extensor muscles attach is quite
long in proportion to the lower limb bone.
Because of this structural arrangement, the
extensor muscles generate great power in the
lower limb when they contract. In the limb
of a running animal such as the cheetah,
however, the extension at the elbow to which
the extensor muscles attach is very short in
proportion to the long lower limb bone
(Figure E7.12b). As a result, the same
amount of contraction by the extensor
muscles moves a cheetah’s foot much farther
than a mole’s foot. Therefore, a cheetah can
run at great speeds.
The importance of structure to the
function of muscles is certainly apparent in
the size and shape of the limbs. However, it
also is apparent at the microscopic level.
Examine the internal organization of skeletal
muscle in Figure E7.13. As you can see, a
muscle consists of many bundles of muscle
fibers (Figure E7.13a). The thin and thick
lines visible in Figure E7.13c are filaments,
specialized structures within muscle fibers.
Filaments consist of two types of long, thin
protein molecules. When you contract a
muscle, energy enables the filaments within
each fiber to slide past each other, much as
your interlocked fingers can slide past each
other when you move your hands together.
The sliding of the individual filaments
shortens the larger muscle fiber. Together,
the shortening of many muscle fibers
shortens the whole muscle. When you relax
your muscle, these filaments return to their
original positions, and the muscle regains its
initial appearance and shape.
Studying muscle fibers at a subcellular
level explains why it is important that
muscles work together. While the movement
of the molecular filaments past each other
can shorten the muscle, it cannot lengthen
the muscle again. That is, when a muscle
relaxes, it cannot return to its normal length
by itself. Because a muscle cannot lengthen, a
muscle cannot push on anything. It can only
pull. For every set of muscles that pulls a limb
bone in one direction, another set pulls it
back the other way. Were that not the case,
many movements would not be possible.
The advantages of different structures
also are evident in organisms that have body
plans different from ours. In vertebrates,
groups of muscles work in opposing pairs
against an internal support system, the bony
skeleton. Invertebrates have different types
muscle fibers bundled
in sheath
a.
group of fibers
muscle fiber
myofibril
b.
contracted filaments
myosin
actin
c.
relaxed filaments
Figure E7.13 a. A muscle is composed of many muscle
fibers bundled in a sheath. b. Each muscle fiber is made up of
many parallel myofibrils. c. Each myofibril is made up of protein
molecules organized into thick and thin filaments.
ESSAY: The Structural Basis of Physical Mobility
Unit 3
343
a.
b.
bristles
Figure E7.14 The
earthworm has a hydrostatic
skeleton, each segment of
which contains a fluid-filled
cavity. a. When the circular
muscles around the segments
of the worm’s body contract,
the fluid in those segments is
squeezed, and the segments
become longer and thinner.
b. When the longitudinal
muscles contract, the segments
become shorter and thicker.
The earthworm moves by
anchoring one part of its body
with its bristles while it extends
another part.
segment
intestine
fluid-filled
cavity
circular
muscles
longitudinal
muscles
of support systems. For example, many softbodied invertebrates have a support system
composed of a surprising substance: water. A
water-based support system, or hydrostatic
skeleton, is not as odd as it might sound.
Water, like other liquids, is not compressible.
This characteristic means that although a
flexible container filled with water may
change shape in response to pressure, its
volume remains constant. As Figure E7.14
shows, the contraction of the circular
muscles around the segments of a worm’s
body squeezes on the watery fluid in those
segments, causing them to become longer
and thinner. Contraction of the opposing
longitudinal muscles, on the other hand,
causes the segments to become shorter and
thicker. When the worm crawls along, it
alternately extends and contracts different
parts of its body in this way. The worm uses
stiff bristles on each segment to anchor some
sections while extending others. Many softbodied animals such as slugs and jellyfish
have variations on this system: an internal
hydrostatic skeleton surrounded by opposing
groups of muscles.
Another common type of invertebrate
support system is the exoskeleton. An
exoskeleton is a hard skeleton on the outside
of the body. (An endoskeleton is a hard
internal skeleton such as in vertebrates.) The
grasshopper shown in Figure E7.15a is a
b.
a.
lower
leg
tendons
extensor muscle
relaxed
upper
flexor
leg
muscle
contracted
Figure E7.15
opposing pairs.
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Unit 3
chitin
extensor muscle
contracted
flexor muscle
relaxed
Exoskeletons have muscles attached to the inside of the skeleton, but these muscles still work in
ESSAY: The Structural Basis of Physical Mobility
good example of an animal that has an
exoskeleton. Figure E7.15b shows diagrams
of a grasshopper’s leg. Note the opposing set
of muscles. When a grasshopper draws up its
leg, the lower muscle contracts while the
upper muscle remains relaxed. What happens
when a grasshopper needs to hop? The upper
muscle contracts while the lower muscle
remains relaxed. Although these muscles
attach to the inside of an external skeleton,
the mechanical aspects of the system are
similar to those of the human arm. In
addition to being quite strong for its mass, an
exoskeleton also provides a layer of armor
that protects the soft parts of the animal’s
body. The essay The Ant That Terrorized
Milwaukee considers what happens if an
animal with an exoskeleton gets too big.
The Ant That
Terrorized Milwaukee
The huge, black thorax towered over
Damien, blocking the light. The enormous
insect was rearing up on its back two pairs of
legs. Its front legs pawed at the air like a
huge stallion, or more accurately, like a
menacing and hideous monster. Its antennae
quivered and twisted in the air, searching for
anything that might challenge it. The air was
heavy with the animal’s stench.
Now that he was this close, Damien
could understand why his pitiful attempts to
bring down the beast had failed so miserably.
The animal’s body was encased in a shiny,
hard, black substance. The armor seemed
impenetrable to any weapon Damien could
get his hands on. Through his fear he vaguely
remembered something important he knew
about insects. . . yes, that was it! A hard outer
skeleton—what did Mrs. Baxter call it?—but
no, too late. . . . As the moving mouth parts
drew nearer, Damien’s last thought was, But
she insisted they couldn’t get this big. . . .
You may have heard the following
statements. “An ant can carry 10 times its
own weight, so an ant the size of a person
could lift a car.” “A grasshopper the size of a
horse could jump the length of a football
field.” An overused plot in horror films has
insects or other tiny creatures become
gigantic. You have seen that multicellular
organisms exhibit a wide
range of body plans that
work by the same basic
principles. Is there any limit
to how big, how fast, or how strong an
organism might be?
For physical reasons, giant creatures
usually are not possible because basic structural
and mechanical considerations limit the sizes
for which particular body plans are suitable.
For example, growth is one limitation to
animals with exoskeletons. Because the
exoskeleton encases the whole body in armor,
ESSAY: The Ant That Terrorized Milwaukee
Unit 3
345
it must be shed completely every time there is
significant growth. The animal is relatively
helpless and vulnerable while the new
exoskeleton hardens. Another major limitation
of such a body is due to the material that
makes up exoskeletons. The material is a
complex carbohydrate called chitin. Hollow
tubes of chitin are very strong for their mass.
In larger sizes, however, the mass of the chitin
needed would increase to impossible levels for
sufficient body support and bracing against
muscle contractions. An ant the size of a
person probably couldn’t even pick itself up, let
alone wreak havoc on Milwaukee.
Energy’s Role in Making
Structures Functional
The structure of muscle fibers explains how a
muscle contracts. But where do muscles get
the energy needed for contraction? Scattered
among muscle fibers are many mitochondria.
Mitochondria are oblong-shaped
compartments, or organelles, located within
cells, as shown in Figure E7.16. Chemical
reactions that involve oxygen, water, and
food occur within the mitochondria and
result in the release of energy. This process is
called cellular respiration. You can think of
it as aerobic production of molecules such as
ATP that the body uses for energy. Aerobic
means “occurring in the presence of oxygen.”
Aerobic energy production fuels most of our
physical activity most of the time.
generalized
plant cell
generalized
animal cell
mitochondria: small
organelles that are the
site of energy-releasing
reactions in all cells;
enclosed by double
membrane with inner
membrane much folded
Figure E7.16 Multicellular organisms such as plants, animals, and other eukaryotes have mitochondria in their
cells. These organelles are the site of cellular respiration, an aerobic breakdown process that converts energy
stored in matter to a more useable form in the chemical bonds of ATP.
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Unit 3
ESSAY: Energy’s Role in Making Structures Functional
When you need a sudden burst of
energy—for example, to catch a bus pulling
away from its stop—the supply of oxygen to
your muscles may not be enough for aerobic
energy production. When that happens, your
muscles can shift to another energy-producing
process. The alternate process, fermentation,
or anaerobic energy production, does not
require oxygen. In comparison to cellular
respiration, fermentation provides much less
energy per glucose molecule. Still, it can
allow your muscles to continue working
for a minute or two. A disadvantage of
fermentation is that it creates a by-product
called lactic acid. The build-up of lactic acid
rapidly causes muscle fatigue. At one time,
physiologists believed that the build-up
of lactic acid was the reason for the sore
muscles that people often have after
participating in strenuous exercise. These
scientists now recognize, however, that lactic
acid is rapidly transported to the liver, which
converts it back into glucose and energy
storage molecules. Physiologists who studied
muscle tissue samples from marathon
runners before and after races found
microscopic evidence of tears and other
damage to muscle fibers. They believe this
damage is a primary reason for delayed
muscle soreness. Full recovery from extreme
anaerobic exercise may require several days
of rest, with adequate oxygen delivery and
nutrient intake.
Because contracting muscles demand
more oxygen to produce energy, vigorous
exercise requires a large increase in
circulation. The blood flow to exercising
muscles may reach 15 times the normal
levels. The increased blood flow delivers
enough oxygen for aerobic exercise and also
removes waste products. Regardless, vigorous
exercise eventually results in muscle fatigue.
Muscle fatigue is a condition in which the
muscles’ glycogen supplies are so depleted
that energy can no longer be released.
(Glycogen is a form of stored sugar.) The
only solution for extreme fatigue is rest.
With sufficient time and proper food,
glycogen is replenished and normal
functioning can resume.
Factors Influencing
Performance
Genetic and Gender Differences. Are
great athletes or great dancers born or made?
They are probably a combination of both
inheritance and training. On the one hand,
as humans, we are born with a certain basic
set of physical capabilities. As individuals, we
may have special abilities in a particular area.
On the other hand, many things that we do
and don’t do affect how well we are able to
use our inborn capabilities.
First, we are humans, not cheetahs,
ants, or any other type of creature. As
humans, we are capable of performing
certain functions because our bodies are
able to acquire and use energy in
particular ways. As individuals, we
inherit traits that may enhance or
limit our capacity to perform
particular activities. For example,
inheritance largely determines our height. A
person’s height may affect whether or not
he or she is likely to become a professional
basketball player. Inheritance also appears
to be important for skills required for
gymnastics. Most successful gymnasts have
small, compact bodies. Our gender and
genetic makeup also influence other
physical factors. These include skeletal and
muscle mass, lung capacity, and the rate at
which our bodies use energy. All of those
factors may influence the types of physical
activity that we can perform best.
ESSAY: Factors Influencing Performance
Unit 3
347
Gender clearly has an effect on the
body’s physical development. Testosterone
levels typically increase in young men during
puberty. Rising testosterone levels cause an
increase in muscle mass. Increased muscle
mass, in turn, results in increased muscle
strength. Therefore, males at puberty and
older tend to be stronger than females of the
same age and height. Anabolic steroids are
popular among some athletes because they
mimic some of the effects of testosterone
(testosterone is a type of steroid). Although
steroids may improve athletic performance,
such substances have a number of serious
and sometimes irreversible effects. These
effects include high blood pressure,
alterations in heart muscle, and reduced
fertility. For those reasons and others, the
National Collegiate Athletic Association
(NCAA) and the International Olympic
Committee (IOC) have banned steroid use
as a performance-enhancing technique.
Metabolism is the approximate rate at
which your body uses food for energy. This
rate differs among individuals, too. In
general, females tend to have a lower
metabolic rate than males. The combination
of your metabolic rate, diet, and level of
exercise determines your body mass. If your
food intake is balanced with your body’s
nutritional demands, metabolic rate, and
activity level, then your body mass will
remain about the same. If you take in excess
food, your body will store it as fat. A slight
excess in food intake is necessary for proper
development during adolescence and the
teen years. This is because the bodies of
these individuals are growing, that is,
producing more body tissues such as
muscles, fat, and blood. Remember the
increase in muscle mass during puberty in
boys? Girls also experience an increase in
body fat during adolescence in response to
the release of estrogen. Estrogen and other
hormones, along with the increase in body
fat, are necessary for ovulation to occur. All
of this growth, as boys and girls become men
and women, requires additional energy.
In moderate, controlled dieting, food
intake is slightly less than your body’s needs.
Your body will then use stored fat for the
matter and energy it needs. Long-term
fasting or starving depletes the body’s stored
fat supplies. In such a case, the body breaks
down its own structural
components, such as muscle,
to keep itself alive. That is
why conditions like anorexia
nervosa may cause muscles,
including the heart, to
become thin and weak.
Figure E7.17 Factors
Influencing Performance
Colored-pencil drawing by
Nina Behrend, senior,
Hartford Union High School.
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Unit 3
ESSAY: Factors Influencing Performance
Conditioning. Despite gender and
genetic differences, human performance also
is based in large part on general physical
fitness. Consider what happens, for example,
with a group of hikers. Those who normally
engage in an exercise program soon take the
lead, while others lag behind. The slower
ones may breathe heavily and later suffer
from aching muscles. One basis for these
differences is the way the body changes
during a regular exercise program. A regular
exercise program is called conditioning.
Conditioning can improve both general
and athletic fitness in several ways. The
major effect of regular exercise is to bring
about changes in the structure and function
of the body. Muscles enlarge and become
stronger. The number and size of
mitochondria in the muscle cells increase.
The muscles’ capacity for glycogen storage
and blood supply increases. The net effect is
a greater ability to convert fuel into useful
energy.
What is a reasonable amount of exercise
for staying fit? As little as 20 to 30 minutes
of moderate exercise two to three times
a week can help your circulatory and
respiratory systems work more effectively.
Conditioning lowers your resting heart rate
and increases heart output. Conditioning
builds muscle mass and tone (firmness). It
strengthens the skeleton by maintaining, or
increasing, bone mass. It improves the
DIET COMPARISON
FOOD GROUPS (DAILY SERVINGS)
Milk
Maintenance Diet
Weight-Loss Diet
CarbohydrateLoading Diet
communication between nerves and muscle.
Better strength, coordination, and endurance
are the rewards. Aerobic activities such as
jogging, brisk walking, bicycling, or
swimming can accomplish these goals.
Behavior. The lifestyle that an
individual adopts also influences fitness. In
addition to exercise, you decide what and
how much you eat and drink. For example,
individuals who wish to stay at a constant
weight must keep the number of kcals they
consume equal to the number they expend
over the long term. On the other hand, people
who are interested in losing weight must take
in fewer kcals than they expend. Athletes who
are training for a marathon may want to
increase the amount of carbohydrates that
they consume. Carbohydrate loading increases
the amount of glycogen available to muscles.
Figure E7.18 describes diets in each of these
categories so that you may compare the
relative amounts of servings in each food
group.
A good mental attitude can lead to
behaviors that promote good general fitness.
In contrast, mental and emotional disorders
can lead to behaviors that endanger fitness.
For example, an estimated 2 million
individuals in the United States suffer from
eating disorders. An eating disorder is a
condition in which chronic abuses in an
individual’s eating patterns can endanger
life itself.
3*
3*
3*
Meat
2–3
2
3 or more
Fruit
Vegetable
Grain
Fats, Oils, and Sweets
2–4
2
7 or more
3–5
3–5
5 or more
6–11
6
11 or more
**
**
**
* teenagers and young adults; 2 for older adults
** You can select foods from the Fats, Oils, and Sweets category only if you can afford the kcals after eating the recommended servings from the
essential food groups.
Note: The diets listed in this table are approximations based on information from the U.S. Department of Agriculture’s Daily Food Guide.
These do not constitute dietary recommendations. Individuals should check with their physician before going on any diet.
Figure E7.18
ESSAY: Factors Influencing Performance
Unit 3
349
Toxins. The consumption of toxins
also influences performance. Toxins are
substances that ultimately cause diminished
performance or impairment of health. Even
medications such as anti-inflammatory drugs
(ibuprofen, for example) may be toxins under
certain circumstances, especially if they are
taken at higher doses than directed.
Illegal, or so-called “street drugs,”
diminish performance as do legal toxins such
as alcohol and tobacco. While illegal drugs
may produce temporary feelings of pleasure,
they also cause negative, long-term
consequences. Our bodies respond to some
drugs by building up a tolerance to them.
Increasingly larger amounts are required to
produce the same effect. Our bodies may
become dependent on addictive drugs. An
addicted person cannot function normally
without the drug. Withdrawal from the drug
can be an extremely painful and difficult
experience.
Although alcohol is a legal drug,
consuming it can have negative
consequences. The consumption of alcohol
may initially produce a temporary sense of
well-being. Thus, under the influence of
alcohol, we may think we are feeling and
performing better. Actually, alcohol depresses
the central nervous system, which causes
a loss of coordination and impaired
performance. Alcohol causes cells to use
oxygen less efficiently and to produce less
energy. Consuming large amounts of alcohol
over long periods of time damages brain and
liver tissue. As noted previously, even small
quantities of alcohol impairs judgment.
Like alcohol, the purchase of tobacco is
legal, but restricted. Tobacco contains many
substances that can adversely influence
performance. Burning tobacco produces
carbon monoxide. Carbon monoxide binds
to hemoglobin faster than oxygen can.
(Hemoglobin is the molecule in red blood
cells that is designed to carry oxygen to the
cells in the body.) Hemoglobin that is bound
to carbon monoxide cannot carry oxygen. A
350
Unit 3
ESSAY: Factors Influencing Performance
smoker who smokes two cigarette packs per
day generates enough carbon monoxide to
reduce his or her blood oxygen level to that
of a nonsmoker who is experiencing the thin
air of high mountain altitudes (3,048 meters
or 10,000 feet) for the first time. Both
tobacco smoke and unburned tobacco such
as chew or dip release high levels of nicotine
into the bloodstream. Nicotine affects
performance directly because it constricts
blood vessels, thus impairing oxygen delivery.
In addition, nicotine is one of the most
addictive drugs known.
Other important aspects of lifestyle that
affect fitness include the amount of sleep a
person gets and how one handles stress. By
the choices that we make, humans can
control many, though not all, of the factors
influencing fitness.
Technology. Technology can help us
both measure and improve our individual
fitness. Athletes use weight machines and
computerized aerobic exercise machines to
build and measure fitness. Specialized
clothing and equipment enhance
performance by increasing comfort level and
efficiency. Computers can model the stresses
that various activities produce and help
researchers design athletic shoes for specific
sports. Sports equipment companies
continually use technological advances to
produce better equipment. For example,
lighter-weight tennis rackets and more
flexible vaulting poles can give athletes a
competitive edge.
Athletes also use devices
that simulate competitive
conditions to improve their
skills. At the Olympic
Training Center in Colorado
Springs, Colorado, swimmers
test their fitness in a device
called a flume, shown in
Figure E7.19. A flume is a
simulator containing water
that runs at gauged speeds.
The swimmer can swim in
place against moving water
to increase speed and
endurance. Ski team
members can practice all
year long on roller skis that
duplicate the feel of crosscountry skis. Therefore, snow
is not a training requirement!
The use of such
technologies may raise
ethical questions. For
example, do these
technologies give an unfair
advantage to competitors
who can afford them? Do
wealthy nations produce
Figure E7.19 Swimmers training for the Olympic Games
can test their speed and endurance by swimming in a flume. This
equipment controls the speed and direction of water flow through
the swimming tank.
superior athletes? Are these uses of
technology fair? To address those and other
issues, regulatory agencies exist for each
major sport and for large events such as the
Olympics. Many of these agencies are
international in scope.
We have seen that several factors affect
human physical performance. These include
genetic, behavioral, and technological
factors, and they all are related to how
effectively we use our body’s energy
supplies.
ESSAY: Factors Influencing Performance
Unit 3
351