A syringe plunger is one of the key parts of a syringe. It is the part that fits inside the tube or barrel of the syringe. When depressed, the plunger forces fluid or gas out the opening of the syringe. When drawn back, it can pull fluids and gases into the syringe. The head of the syringe can be fitted with needles, nozzles, or tubing, depending on how it is being used. Syringe plungers are intended to be used once with an individual syringe and then discarded.
Each syringe plunger creates a tight seal inside the syringe, usually with the assistance of a small rubber or plastic gasket. This prevents the contents of the syringe from escaping out the rear of the syringe, keeping dosages consistent. When the plunger is pulled back, a vacuum is created inside the barrel and the vacuum will suck up fluids or gases the syringe is in contact with. If the plunger is not the right size or is defective, it can be difficult to pull it back or push it forward, and this can interfere with medication administration.
To fill a syringe with medication for administration, people insert the tip of the syringe into a container of medication and pull back on the plunger. Markings along the side of the syringe indicate how much material has been drawn up, and people can stop pulling back when they have reached the correct dosage. The syringe may be held upright and gently tapped to encourage air bubbles to escape if it is being prepared for an injection, to avoid introducing air bubbles to a vein.
Syringes used in medical care are typically designed to be disposable to prevent contamination. Specially designed safety syringes may have springs that pull the needle up into the barrel after an injection, usually when the care provider pulls the syringe plunger back hard or snaps it. These syringes improve safety by reducing the risk of a needlestick accident, where someone comes into contact with a needle that has been used on someone else.
It is also possible to use a syringe for other activities where sterility is not required. Cooks sometimes use syringes for basting, filling pastries, and other activities, and they can also be used to apply lubricants and greases to mechanical components. In these cases, the syringe can be used multiple times and the syringe plunger is designed to hold its seal through numerous uses.