The pancreas is located just behind the
stomach. This gland produces two
important hormones called insulin and
glucagon. They work to keep the right
amount of sugar in the blood.
Ovaries and Testes
Ovaries sit in the pelvis, or lower belly,
of females. They make hormones that
cause breasts, wide hips, and other
female features to develop. Testes sit in a
pouch behind the penis in males. They
make hormones that cause a deep voice,
facial hair, and other male features to
develop. In addition to hormones, ovaries
produce female sex cells, or eggs,
and testes make male sex cells, or sperm.
For this reason, ovaries and testes also
belong to the reproductive system.
Problems with the Endocrine
System
Diseases of the endocrine system cause
the glands to make too much or too
little of a hormone. One type of the
disease called diabetes happens when the
pancreas does not produce enough insulin.
If the pituitary gland does not make
the right amount of growth hormone,
children will grow too much or too
little.
Problems with the thyroid gland can
cause many symptoms. Too much thyroid
hormone in the blood causes
weight loss, a high heart rate, nervousness,
sweating, bulging eyes, and a swelling
in the neck. Too little thyroid
hormone causes weight gain, a slow
heart rate, dry skin, and a feeling of
tiredness.
#More to explore
Diabetes • Hormone • Pancreas
One of the hormones made by the pituitary
gland controls the growth of the body.
136 Endocrine System BRITANNICA STUDENT ENCYCLOPEDIA
Energy
Energy is another word for power.
Energy makes things move. It makes
machines work. Energy also makes living
things grow.
Types of Energy
Energy exists in many different forms.
Animals get energy from eating food.
Electrical energy is associated with the
tiny units called atoms that make up
everything in the universe. The energy is
created when particles called electrons
move from one atom to another. Heat
and light are also forms of energy. One
form of energy can also be transferred
into another. Through a battery, stored
chemical energy changes into electric
energy. In a lightbulb, electric energy
changes to light and heat.
Each of these forms of energy can be
described as either potential energy or
kinetic energy. Potential energy is
stored energy. An object with potential
energy has the ability, or potential, to
move. Potential energy comes from the
position or the shape of an object. For
example, potential energy is stored in a
rock perched on a cliff and in an arrow
stretched back on a bowstring. If the
cliff crumbles under the rock, the rock
falls. If the string is let go, it moves
forward and pushes the arrow through
the air.
As the rock and the arrow move, they
gain kinetic energy. Kinetic energy is the
energy of moving things. All moving
objects have kinetic energy—even
atoms.
Sources of Energy
The sun is the source of almost all
energy on Earth. The sun makes plants
grow. When animals eat the plants they
gain energy from the chemicals that
make up the plants. When plants die
they decay, or break down. Over millions
of years this process creates what
are called fossil fuels. Fossil fuels include
petroleum, natural gas, and coal. These
are used to run factories, heat homes,
and power automobiles.
Fossil fuels pollute the atmosphere
when they are used. They are also being
used up very quickly. Scientists
therefore have tried to find other
sources of energy to replace the fossil
fuels. Some of these sources have been
used directly for many years. These
include wind, water, and heat from
Earth. Sailboats harness the power of
the wind to move them along. Water
mills use water flowing through rivers
to turn wheels that grind grain. In
ancient times some people used hot
water and gases from Earth to heat
Wind turbines in California produce electricity.
BRITANNICA STUDENT ENCYCLOPEDIA Energy 137
People
The people of England are a mixture of
many different ethnic groups. The early
people of the area were called Celts.
Invaders from Germany, Denmark,
France, and other parts of Europe came
later. Most of the English people are
descendants of these peoples. People
from Asia, Africa, and the Caribbean
islands make up smaller groups. Most of
the population lives in cities.
Most of England’s people are Christian.
The Church of England, which is Protestant,
is the country’s official church.
There are also large groups of Muslims,
Sikhs, Hindus, and Jews.
Economy
Services such as tourism, banking, and
insurance are the most important part of
the English economy. London is a major
world center for finance. Manufacturing
businesses make foods, chemicals, textiles,
computers, automobiles, and aircraft.
England’s major crops include
wheat, barley, and potatoes and other
vegetables. Cattle and sheep are raised
for meat and milk.
History
People have lived on the land that is
now England for thousands of years.
The Celts lived there as early as 500 BC.
The Romans came later. In the AD 400s
and 500s three peoples from northern
Europe—Angles, Saxons, and Jutes—
conquered England. Their language,
Anglo-Saxon, later developed into
English. The region became known as
