Question

explain how syringe works during blood test ??

Answer 1

A syringe is just a hollow tube with a plunger at one end. After the needle is inserted into the vein (usually), the plunger is moved distally from the needle end, (pull the plunger back), and a negative air space is created that has less pressure that the vein. 
The blood will, like water, find it’s way to the easiest escape route, i.e., the hollow tube, and the blood will flow into the syringe. 

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