A syringe assembly includes a syringe barrel having an inside surface defining a chamber and an outlet. A plunger assembly is disposed within the syringe barrel and includes an elongate plunger rod having a distal end surface, a plunger head having a distal sealing surface and a proximal end surface having a continuous perimeter, and a breakable neck portion integrally connecting the plunger rod and the plunger head. A retaining ring lock is disposed on the inside surface of the barrel and defines an engagement surface about a perimeter of the chamber. The retaining ring lock is adapted to engage and retain the plunger head in a locked position within the chamber. The engagement surface of the retaining ring lock engages the continuous perimeter of the proximal end surface of the plunger head in a perimetrical engagement when the plunger head is in the locked position.