A regulator for in-line modulation of the flow rate of fluid during blood collection includes a fluid transfer device for transferring fluid from a patient to a collection device. The fluid transfer device comprises a fluid passageway defined by a tubular sidewall and a flexible member associated with a portion of the tubular sidewall. The flexible member is configured for movement with respect to the fluid passageway upon exposure of the fluid transfer device to a differential pressure during a blood collection procedure to prevent collapse of a patients blood vessel. The flexible member of the regulator can be a frame surrounding a flexible diaphragm. Alternatively, the flexible member can include a flexible material integrally formed within the tubular sidewall of the pressure regulator.
