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Why do they always wrap your arm with a rubber band (tourniquet) before taking blood test?

  During venipuncture (taking blood test) a nurse will wrap a rubber band around your arm to prevent blood from squirting thru the opening created by a needle. 

  Remember, your "blood pressure" (120/80 or so) is a real pressure inside your cardiovascular system.  It will make your blood squirt out whenever there is an opening.  Sometimes, when an unexperienced medical student takes a blood test and forgets to apply a tourniquet, the blood may shoot across the room, a very frightening experience for the patient and the medical student.