text: I am guessing that you're wanting to know how blood samples are collected from a vein.  I will stick with that, no pun intended.  The techniques for arterial blood are somewhat similar, however.  Then there are fingersticks and cord-blood collection, which I won't discuss here.\n\nFirst, the phlebotomist or nurse (or veterinary technician)--the "collector" for this discussion-- places a rubber tourniquet around the arm or forelimb (standard, though other sites may be used as needed). The collector then palpates the site to locate a suitable vein.  The collector then cleans the site with alcohol or some other antiseptic.  The collector inserts a needle into the vein and gently withdraws the desired amount of blood, taking care to be GENTLE so as not to "blow" the vein (cause it to rupture or tear) or cause the red blood cells to break (hemolysis).  As soon as blood appears in the syringe or special vacuum-type collection tube, the tourniquet is loosened.  In veterinary medicine, we often use a syringe that has been treated with an anticoagulant such as heparin to keep it from clotting until the sample is put into the appropriate tubes (depending on the tests to be done).  Some vets and most human doctors use special tubes that provide a vacuum and allow the blood to be collected more quickly.  If we use vacuum tubes that have anticoagulants (substances that keep blood from clotting) or other substances, we keep the arm down while drawing to prevent backflow and accidental injection of these substances into the patient.  After the blood is drawn, the collector applies pressure to the venipuncture site with a bit of gauze or cotton to prevent bruising and stop bleeding.  For humans, a Band-aid with cotton or gauze is applied for a short time as a pressure bandage to ensure that the site doesn't begin bleeding again later.\n\nHope that helps.