Cardiovascular
System
Humans and many other animals
depend on blood flowing through their
bodies to keep them alive. The blood
travels through a system that includes
the heart and a network of blood vessels.
This is called a cardiovascular system.
Heart
The human heart is a pear-shaped organ
about the size of a fist. It is made up of a
special type of muscle called cardiac
muscle, which is not found anywhere
else in the body. The heart acts as a
pump to push the blood throughout the
body.
The heart is separated into four chambers,
or parts. The upper chambers are
called atria, and the lower chambers are
called ventricles. A valve, or a flap that
can open and shut, connects each
atrium to the ventricle below it. The
valves control the movement of blood
through the heart.
Blood Vessels
The blood vessels are a system of tubes
that carry the blood throughout the
body. The main vessels are arteries,
veins, and capillaries. Arteries carry
blood out from the heart. Veins return
blood to the heart. Arteries are the
thickest of all blood vessels. They
The male northern cardinal is a colorful
visitor to birdfeeders in many parts of North
America.
Blood flows from the heart through arteries
and into capillaries. It then returns to the
heart through veins.
38 Cardiovascular System BRITANNICA STUDENT ENCYCLOPEDIA
expand and contract to keep blood moving
away from the heart. Veins contain
valves that prevent blood from flowing
backward. Capillaries are tiny passages
that connect the arteries and the veins to
the body’s tissues.
Blood flows through the vessels in two
very specific paths. In the first path,
called pulmonary circulation, blood
travels from the heart to the lungs. In
the lungs carbon dioxide and other
waste gases leave the blood. The blood
takes in oxygen and then returns to the
heart.
In the second path, called systemic circulation,
the blood travels away from
the heart throughout the body. The
arteries carrying the blood branch out
into smaller vessels called arterioles and
finally into capillaries. In the capillaries
the blood transfers oxygen and nutrients
to cells in the body’s tissues. The blood
in the capillaries also collects waste
products from the cells. From the capillaries
the blood flows into small vessels
called venules. These unite to form
veins. The blood flows through the veins
back to the heart.
Other Organs
In addition to the lungs, many other
organs in the body interact with the
cardiovascular system. Blood carries
away nutrients from the intestines,
which are part of the digestive system.
The spleen and the liver filter, or clean,
the blood by removing old blood cells
and bacteria. The kidneys take water,
minerals, and other waste products from
the blood.
Blood continuously flows from the heart,
throughout the human body, and back to
the heart.
BRITANNICA STUDENT ENCYCLOPEDIA Cardiovascular System 39
Diseases of the Cardiovascular
System
When the force of the blood against the
walls of the blood vessels is too high, a
person has hypertension, or high blood
pressure. Hypertension can weaken the
heart and damage other organs.
Sometimes fatty deposits thicken and
harden the walls of the arteries. When
this happens a person has arteriosclerosis.
Arteriosclerosis slows the flow of
blood, which can lead to a heart attack.
#More to explore
Blood • Heart • Lung
Carib
The Carib were American Indians who
lived in South America and on islands in
the Caribbean Sea. The sea was named
after the Carib.
The Carib first lived in small villages in
northern South America. They built
houses of pole frames covered with palm
leaves. They got their food by hunting
and farming.
After AD 1000 many Carib moved to the
Caribbean islands called the Lesser Antilles.
The island Carib were more warlike
than the South American Carib. They
drove the Arawak people off the islands.
The island Carib were fierce fighters.
After torturing and killing enemy men,
they took the women into their tribe as
wives or slaves. The Carib may have
eaten the bodies of their enemies. This
practice, called cannibalism, may have
been part of the tribe’s religion.
Spanish explorers began arriving in
Carib lands in the late 1400s. The South
American Carib were wiped out by
fighting and by diseases brought by the
Spanish. But the Spanish avoided the
islands where the Carib lived. There was
no gold on them, and the island Carib
were too hard to defeat in battle. Other
As blood travels through capillaries, it delivers
oxygen and nutrients to the body’s cells.
It also picks up carbon dioxide and other
wastes.
A Carib man weaves baskets on
the island of Dominica.
40 Carib BRITANNICA STUDENT ENCYCLOPEDIA
Europeans conquered the islands in the
1600s. Only a few Carib survived.
Today their descendants live on the
islands of Saint Vincent and Dominica.
The Garifuna are another group of
people with Carib roots. The Garifuna
originated when slaves brought from
Africa mixed with island Carib. Garifuna
today live in the Central American
countries of Belize, Honduras, and
Nicaragua.
#More to explore
Arawak • Caribbean Sea • Native
Americans
Caribbean Sea
The Caribbean Sea sits between the
