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 The Circulatory SystemThe collection of blood vessels, also known as the circulatory system, is differentiated into several types of vessels.
After passing through the capillaries, blood is no longer oxygen-rich and is called venous blood. It is collected into larger vessels called venules, which join together into large ones called veins. Veins lead back to the heart, where blood gets pumped to the lungs.
In the lungs the blood exchanges carbon dioxide for oxygen and
becomes oxygen-rich, or arterial blood again. From there it comes back to the heart,
where its journey around the body starts once again.
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Blood leaves the heart through large vesels called arteries, which
then split up into smaller ones called arterioles. Those carry blood
to near the tissue that requires oxygen, where they split up into many
small vessels called capillaries. It is in the capillaries that
gas exchange takes place.