A syringe consists of a plunger fitted to a tube, called the barrel, which has a small opening on one end. Syringes are used to transfer small amounts of liquids or gasses to or from otherwise inaccesible areas. It operates on the principle of suction by filling the barrel with the substance at the opening when the plunger is drawn out, and expelling the substance when the plunger is depressed. The process of administering a substance with a syringe and needle is called an injection . The word syringe comes from the Greek syrinx , which means "tube".

Syringes are used in conjunction with hypodermic needles for injections of liquid or gasses into body tissues, or for their removal from the body. They may also be used for injections across the rubber septum of a medical device, container, or scientific apparatus such as in certain types of chromatography . The injection of air into a blood vessel is undesirable, as it may cause a gas embolism . Prevention of embolisms by removing air from the syringe is the source of the familiar image of holding a syringe upside down, tapping it, and expelling a small amount of liquid before an injection into the bloodstream .

Source: wikipedia.org