A pressure-regulating medical assembly comprising: a syringe assembly comprising: a housing having a longitudinal axis, a proximal portion, and a distal portion; a plunger being at least partially received in the housing and movable relative to the housing; a first reservoir located in the proximal portion of the housing and configured to contain regulating fluid, at least one side of the first reservoir being bounded by a sidewall of the housing; a regulating port in fluid communication with the first reservoir via a regulating channel; a second reservoir located in the distal portion of the housing and configured to contain medical liquid; an access port in fluid communication with the second reservoir via an access channel; a distal seal fixed to a distal end of the plunger and slidably coupled with an inner surface of the housing; and wherein, when the plunger moves relative to the housing, the distal seal slides longitudinally relative to the housing and the volume of the second reservoir changes as a function of the amount of longitudinal movement of the distal seal relative to the housing; and an adapter assembly configured to couple with the syringe assembly and with a flexible, variable volume container having an interior configured to contain medical liquid; wherein, when the adapter assembly is coupled with the syringe assembly and the container, the first and second reservoirs are in fluid communication with the interior of the variable volume container, and wherein the pressure-regulating medical assembly is configured to enable medical fluid to be introduced from the second reservoir into the interior of the container with none or substantially no regulating fluid being transferred from the container into the first reservoir. a syringe assembly comprising: a housing having a longitudinal axis, a proximal portion, and a distal portion; a plunger being at least partially received in the housing and movable relative to the housing; a first reservoir located in the proximal portion of the housing and configured to contain regulating fluid, at least one side of the first reservoir being bounded by a sidewall of the housing; a regulating port in fluid communication with the first reservoir via a regulating channel; a second reservoir located in the distal portion of the housing and configured to contain medical liquid; an access port in fluid communication with the second reservoir via an access channel; a distal seal fixed to a distal end of the plunger and slidably coupled with an inner surface of the housing; and wherein, when the plunger moves relative to the housing, the distal seal slides longitudinally relative to the housing and the volume of the second reservoir changes as a function of the amount of longitudinal movement of the distal seal relative to the housing; and a housing having a longitudinal axis, a proximal portion, and a distal portion; a plunger being at least partially received in the housing and movable relative to the housing; a first reservoir located in the proximal portion of the housing and configured to contain regulating fluid, at least one side of the first reservoir being bounded by a sidewall of the housing; a regulating port in fluid communication with the first reservoir via a regulating channel; a second reservoir located in the distal portion of the housing and configured to contain medical liquid; an access port in fluid communication with the second reservoir via an access channel; a distal seal fixed to a distal end of the plunger and slidably coupled with an inner surface of the housing; and wherein, when the plunger moves relative to the housing, the distal seal slides longitudinally relative to the housing and the volume of the second reservoir changes as a function of the amount of longitudinal movement of the distal seal relative to the housing; and an adapter assembly configured to couple with the syringe assembly and with a flexible, variable volume container having an interior configured to contain medical liquid; wherein, when the adapter assembly is coupled with the syringe assembly and the container, the first and second reservoirs are in fluid communication with the interior of the variable volume container, and wherein the pressure-regulating medical assembly is configured to enable medical fluid to be introduced from the second reservoir into the interior of the container with none or substantially no regulating fluid being transferred from the container into the first reservoir.