The Circulatory System
The human circulatory system is made up of the blood, the heart, and
the blood vessels. The function of the circulatory system is to
transport substances around the body. It moves nutrients absorbed
from the intestine to all of the body’s cells. Blood flows through the
lungs to pick up oxygen and then flows through the body to deliver it
to active cells. Blood also carries wastes from the body tissues for
disposal. It carries carbon dioxide to the lungs, where it is released
into the air. Other waste substances are carried to the kidneys
where the substances are filtered out and excreted.
Among the circulatory system’s other vital functions are the
regulation of body temperature and the transport of disease-fighting
white blood cells to areas of the body where there are viruses or
bacteria.
BLOOD
Blood is a type of connective tissue that circulates throughout all
parts of your body. The blood consists of four components:
 Red blood cells are the most plentiful of the body’s blood cells.
Red blood cells contain a protein called hemoglobin, which
allows them to transport oxygen throughout the body.
Hemoglobin makes the cells appear red.
 White blood cells are infection fighting cells. They recognize
and destroy invading bacteria and viruses. They are the only
blood cells to have a nucleus.
 Platelets are tiny cells that help in blood clotting.
 Plasma is a protein rich liquid that carries the blood cells along.
THE HEART
The heart is made up of three different types of tissue: cardiac
muscle tissue, nerve tissue, and connective tissue. All of the cardiac
muscle tissue in each part of the heart contracts at the same time.
This makes the heart contract and moves the blood around the body.
The heart pumps with a regular beat. The frequency of the beat (the
heart rate) changes depending on physical activity and other factors
such as stress, temperature and your general health.
BLOOD VESSELS
Three types of blood vessels form a network of tubes throughout the
body to transport the blood.
Arteries carry blood away from the heart. Because the blood in the
arteries is being pumped away from the heart, it is under greater
pressure than the blood in the other blood vessels. The walls of
arteries are thicker than the walls of other blood vessels to
withstand this pressure.
Veins carry blood toward the heart. This blood is at lower pressure,
so the walls of the veins are not as thick.
Arteries and veins are linked by the capillaries
Capillaries are tiny blood vessels with very thin walls that allow
substances to diffuse between the blood and other body fluids and
tissues. Oxygen and nutrients diffuse from the blood into the
surrounding tissues. Carbon dioxide and other wastes pass from the
body tissues into the blood to be carried away for disposal.