A syringe assembly comprising: a barrel including a cylindrical sidewall having an interior surface defining a chamber for retaining fluid, an open proximal end and a distal end including a distal wall having an opening therethrough in fluid communication with said chamber, said sidewall including a rib adjacent said proximal end and a ramp having an axial length extending from the rib towards said proximal end such that the diameter of the barrel increases along the ramp from the rib towards the proximal end; an elongate plunger rod including a proximal end, a distal end, and a main body extending between the proximal and distal end, the plunger rod being distally and proximally movable within said chamber, the proximal end including a thumb press, the distal end including a stopper-engaging portion, and the plunger rod further including at least one frangible portion comprising a narrowed frangible connection disposed in a proximal end of the plunger rod; a stopper having a proximal end and a distal end, the stopper attached to the stopper-engaging portion of the plunger rod, characterized in that, the plunger rod further including a flexible protrusion between the thumb press and the main body, the protrusion having a diameter greater than the diameter of the barrel at the rib and the stopper being distally and proximally movable relative to the stopper-engaging portion for a pre-selected axial distance such that when the distal end of the stopper is in contact with the distal wall of the barrel, the protrusion is permitted to advance distally past the rib in the barrel and lock the plunger rod in the barrel, the stopper-engaging portion comprising a visual display being fully visible before an initial distally directed force is applied to the plunger rod and the visual display being blocked after the distally directed force is applied to the plunger rod to move the stopper-engaging portion of the plunger rod distally for the pre-selected axial distance. a barrel including a cylindrical sidewall having an interior surface defining a chamber for retaining fluid, an open proximal end and a distal end including a distal wall having an opening therethrough in fluid communication with said chamber, said sidewall including a rib adjacent said proximal end and a ramp having an axial length extending from the rib towards said proximal end such that the diameter of the barrel increases along the ramp from the rib towards the proximal end; an elongate plunger rod including a proximal end, a distal end, and a main body extending between the proximal and distal end, the plunger rod being distally and proximally movable within said chamber, the proximal end including a thumb press, the distal end including a stopper-engaging portion, and the plunger rod further including at least one frangible portion comprising a narrowed frangible connection disposed in a proximal end of the plunger rod; a stopper having a proximal end and a distal end, the stopper attached to the stopper-engaging portion of the plunger rod, characterized in that, the plunger rod further including a flexible protrusion between the thumb press and the main body, the protrusion having a diameter greater than the diameter of the barrel at the rib and the stopper being distally and proximally movable relative to the stopper-engaging portion for a pre-selected axial distance such that when the distal end of the stopper is in contact with the distal wall of the barrel, the protrusion is permitted to advance distally past the rib in the barrel and lock the plunger rod in the barrel, the stopper-engaging portion comprising a visual display being fully visible before an initial distally directed force is applied to the plunger rod and the visual display being blocked after the distally directed force is applied to the plunger rod to move the stopper-engaging portion of the plunger rod distally for the pre-selected axial distance.