The trachea separates into two main
branches, or bronchi—one for each
lung. Within the lungs, the bronchi
divide into smaller and smaller
branches. The branches end with tiny
air sacs called alveoli. There are more
than 600 million alveoli in the lungs.
The alveoli contain tiny blood vessels
called capillaries.
The Breathing Cycle
The brain controls breathing. It controls
the diaphragm, a muscle at the base of
the lungs. It also controls the muscles
between the ribs.
When the diaphragm moves down and
the ribs move out, the area inside the
chest grows larger. Air then flows into the
lungs and makes them expand. Blood in
the capillaries picks up oxygen from the
air in the alveoli. The blood then brings
the oxygen through the cardiovascular
system to all parts of the body.
As the blood picks up oxygen, it also
brings carbon dioxide from the body to
the alveoli. The rib muscles and the diaphragm
then relax. As the area inside the
chest shrinks, the lungs decrease in size.
The smaller size forces out air, including
the carbon dioxide.
Newborn babies breathe up to 44 times
a minute. Adults breathe only about 16
Tiny structures in the lungs called alveoli
and capillaries exchange oxygen for carbon
dioxide.
BRITANNICA STUDENT ENCYCLOPEDIA Respiratory System 55
times per minute. When people are
asleep, they breathe more slowly, about
six to eight times per minute. If people
are afraid or worried, they can breathe
up to 100 times per minute.
Problems with the Respiratory
System
Hiccups are a harmless problem with the
respiratory system. They happen when
the diaphragm moves in a fast, jerky
way. Colds and influenza (the flu) are
infections that start in the nose and the
throat. Bronchitis is a swelling of the
bronchi. Pneumonia is an infection of
the lungs. Tuberculosis, emphysema,
and lung cancer are serious diseases of
the lungs.
#More to explore
Lung • Pneumonia • Tuberculosis
Reunion
The island of Reunion is a department
(a type of province) of France. It lies
near Africa in the Indian Ocean.
Reunion’s capital is Saint-Denis.
Geography
Reunion is about 420 miles (680 kilometers)
east of Madagascar, an island
country to the southeast of Africa.
Reunion has many mountains. Some of
them are active volcanoes. The island
has a warm climate with dry and rainy
seasons. Tropical storms called cyclones
are common.
Plants and Animals
Reunion has many flowering plants and
tropical fruit trees. Tropical forests grow
in the higher areas. Animals on the
island include chameleons, frogs, birds,
and butterflies.
People
Many of the people of Reunion have a
mixture of African, European, and South
Asian roots. Whites and South Asians
form other large groups. There are small
groups of Chinese and Africans.
French is the language of government,
but Creole is the language of everyday
Diaphragm, ribs, and lungs all move when
a person breathes in and out.
A green valley lies between mountains in
Reunion.
56 Reunion BRITANNICA STUDENT ENCYCLOPEDIA
life. Roman Catholicism is the main
religion. Most people live in cities or
towns.
Economy
Services, especially tourism, are the most
important part of Reunion’s economy.
The island also produces sugar, rum,
and lobsters. Farmers grow sugarcane,
fruits, vegetables, vanilla beans, tobacco,
and geraniums, which are used to make
perfume. Reunion trades mainly with
France.
History
Portuguese explorers discovered the
island in the 1500s. No one lived there
until the 1600s, when the French built a
settlement. The French brought Africans
to work as slaves on coffee and sugar
plantations. After slavery ended in 1848,
workers from Asia arrived.
Reunion was a French colony until
1946, when it became a department of
France. The people of Reunion then
became citizens of France.
#More to explore
France • Indian Ocean
Revere, Paul
Paul Revere was a hero of the American
Revolution. He is most famous for
riding his horse through the countryside
near Boston to warn the colonists that
the British were coming. Revere was also
a skilled silversmith (maker of silverware).
Paul Revere was born on January 1,
1735, in Boston, Massachusetts. His
father’s family was French and the family
name was changed from De Revoire
to Revere. Paul learned silversmithing
from his father.
As a young man Revere joined the Sons
of Liberty, a club for colonists who disliked
British rule. Such people were
known as patriots. In 1773 he and other
patriots protested a tax on tea by throwing
British tea into Boston Harbor. This
action became known as the Boston Tea
Party.
On April 16, 1775, Revere rode to warn
the patriots of Concord, Massachusetts,
to hide their weapons from the British.
Two days later he rode again, from Boston
to Lexington, Massachusetts. This
time he warned the patriots John Han-
A painting shows Paul Revere holding a
silver teapot that was made in his own
workshop.
BRITANNICA STUDENT ENCYCLOPEDIA Revere, Paul 57
cock and Samuel Adams that the British
were coming to arrest them.
He also warned the Lexington minutemen
that the British were coming. The
minutemen were fighters who boasted
that they needed only a minute to get
ready. On April 19, the Lexington minutemen
