A fluid pressurizing unit for an injection system, the unit comprising: a syringe including a proximal opening and an inner surface; a plunger shaft including a terminal portion, the terminal portion including a distal terminal end; and a plunger mounted within the syringe, the plunger including a wiper sidewall, a deformable distal end wall and a cavity; the wiper sidewall of the plunger including at least one external sealing ridge and an external sealing lip for reversible sealing against the inner surface of the syringe, the wiper sidewall further including a radially expandable-contractible portion that includes an internal surface with a plurality of longitudinally extending ribs to facilitate the expandable and contractible nature of the radially expandable-contractible portion, without sacrificing a structural integrity of the wiper sidewall for sealing; and the cavity of the plunger being defined by the wiper sidewall and the deformable distal end wall and having an opening located opposite the distal end wall; wherein the terminal portion of the plunger shaft has a size and shape such that, when the terminal portion is fully inserted into the cavity, a gap exists between the distal terminal end of the terminal portion and the deformable distal end wall of the plunger; and the external sealing lip of the wiper sidewall of the plunger is located in proximity to the deformable distal end wall of the plunger, such that, when the terminal portion of the plunger shaft is fully inserted into the cavity of the plunger, a pressure within the syringe, which is above a given threshold pressure, deforms the end wall into the gap, thereby forcing the external sealing lip radially outward and into tighter sealing engagement with the inner surface of the syringe. a syringe including a proximal opening and an inner surface; a plunger shaft including a terminal portion, the terminal portion including a distal terminal end; and a plunger mounted within the syringe, the plunger including a wiper sidewall, a deformable distal end wall and a cavity; the wiper sidewall of the plunger including at least one external sealing ridge and an external sealing lip for reversible sealing against the inner surface of the syringe, the wiper sidewall further including a radially expandable-contractible portion that includes an internal surface with a plurality of longitudinally extending ribs to facilitate the expandable and contractible nature of the radially expandable-contractible portion, without sacrificing a structural integrity of the wiper sidewall for sealing; and the cavity of the plunger being defined by the wiper sidewall and the deformable distal end wall and having an opening located opposite the distal end wall; wherein the terminal portion of the plunger shaft has a size and shape such that, when the terminal portion is fully inserted into the cavity, a gap exists between the distal terminal end of the terminal portion and the deformable distal end wall of the plunger; and the external sealing lip of the wiper sidewall of the plunger is located in proximity to the deformable distal end wall of the plunger, such that, when the terminal portion of the plunger shaft is fully inserted into the cavity of the plunger, a pressure within the syringe, which is above a given threshold pressure, deforms the end wall into the gap, thereby forcing the external sealing lip radially outward and into tighter sealing engagement with the inner surface of the syringe.