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Unit 19: Biology Part 2 Page 49 of 137

sure you understand that atria get smaller amount of pressure. Veins
blood in and ventricles pump blood contain valves, which stop the
out. blood they are carrying from
moving backwards.
Along with the heart, another
important part of the
cardiovascular system are blood
vessels. There are three main
types of blood vessels; arteries,
veins, and capillaries.

Arteries are blood vessels that
carry blood away from the heart.
Arteries have thick walls. They
need these thick walls because
there is a lot of pressure on the
blood in arteries. Figure 26 – As blood flows

through the veins back to the
Every time the heart contracts, it heart, valves stop the blood
creates a force on the walls of the from flowing backwards.
arteries. This force creates

pressure. You’ve probably heard of The last types of blood vessels are
blood pressure, and had your blood capillaries. Capillaries connect
pressure taken during a check-up. veins and arteries, and they are
Blood pressure is a measure of the tiny. Their walls are only one cell
pressure on the walls of the thick, and they are so narrow,
arteries caused by your beating blood cells have to go through
heart. them single file.

The next types of blood vessels are
veins. In many ways, veins are
the opposite of arteries. While
arteries move blood away from the

heart, veins bring blood back to
the heart. While arteries have
thick walls to handle blood under
high pressure, the walls of veins
are much thinner, because the
blood they carry is under a much

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