A disposable subassembly for a fluid pressurizing unit of an injection system (100; 200), the subassembly comprising: a syringe including a proximal opening, an inner surface, a first length, over which the inner surface defines a first inner diameter of the syringe, and a second length, over which the inner surface defines a second inner diameter of the syringe, the first length extending from the proximal opening toward the second length; and a plunger including a proximal end, a distal end, a wiper sidewall and a mating feature, the wiper sidewall extending between the proximal and distal ends and defining a cavity of the plunger, characterized in that: the wiper sidewall including a radially expandable-contractible portion surrounding an opening to the cavity at the proximal end of the plunger, the radially expandable-contractible portion including 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 mating feature being formed in the expandable-contractible portion of the wiper sidewall and projecting into the cavity for engaging with a plunger shaft of the fluid pressurizing unit, when the plunger shaft is inserted into the cavity and the expandable-contractible portion of the sidewall is contracted; wherein the plunger is mounted at a first position, within the opening of the syringe, so that the expandable-contractible portion of the wiper sidewall of the plunger is located within the first length of the syringe and the expandable-contractible portion of the wiper sidewall is expanded; and the second inner diameter is smaller than the first inner diameter so that, when the plunger is moved by the inserted plunger shaft from the first position to a second position, at which the expandable-contractible portion of the wiper sidewall is located within the second length of the syringe, the expandable-contractible portion is contracted and the mating feature operably engages with the inserted plunger shaft. a syringe including a proximal opening, an inner surface, a first length, over which the inner surface defines a first inner diameter of the syringe, and a second length, over which the inner surface defines a second inner diameter of the syringe, the first length extending from the proximal opening toward the second length; and a plunger including a proximal end, a distal end, a wiper sidewall and a mating feature, the wiper sidewall extending between the proximal and distal ends and defining a cavity of the plunger, characterized in that: the wiper sidewall including a radially expandable-contractible portion surrounding an opening to the cavity at the proximal end of the plunger, the radially expandable-contractible portion including 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 mating feature being formed in the expandable-contractible portion of the wiper sidewall and projecting into the cavity for engaging with a plunger shaft of the fluid pressurizing unit, when the plunger shaft is inserted into the cavity and the expandable-contractible portion of the sidewall is contracted; wherein the plunger is mounted at a first position, within the opening of the syringe, so that the expandable-contractible portion of the wiper sidewall of the plunger is located within the first length of the syringe and the expandable-contractible portion of the wiper sidewall is expanded; and the second inner diameter is smaller than the first inner diameter so that, when the plunger is moved by the inserted plunger shaft from the first position to a second position, at which the expandable-contractible portion of the wiper sidewall is located within the second length of the syringe, the expandable-contractible portion is contracted and the mating feature operably engages with the inserted plunger shaft. the wiper sidewall including a radially expandable-contractible portion surrounding an opening to the cavity at the proximal end of the plunger, the radially expandable-contractible portion including 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 mating feature being formed in the expandable-contractible portion of the wiper sidewall and projecting into the cavity for engaging with a plunger shaft of the fluid pressurizing unit, when the plunger shaft is inserted into the cavity and the expandable-contractible portion of the sidewall is contracted; wherein the plunger is mounted at a first position, within the opening of the syringe, so that the expandable-contractible portion of the wiper sidewall of the plunger is located within the first length of the syringe and the expandable-contractible portion of the wiper sidewall is expanded; and the second inner diameter is smaller than the first inner diameter so that, when the plunger is moved by the inserted plunger shaft from the first position to a second position, at which the expandable-contractible portion of the wiper sidewall is located within the second length of the syringe, the expandable-contractible portion is contracted and the mating feature operably engages with the inserted plunger shaft.