Posted by TOM on Saturday, February 6, 2010 at 2:24pm.
Please try some of these sites:
http://www.google.com/search?q=why+veins+contain+valves&ie=utf-8&oe=utf-8&aq=t&rls=org.mozilla:en-US:official&client=firefox-a
Sra
To prevent the backflow of blood.
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