A regulator for modulation of the flow rate of blood out of a patient's blood vessel during blood collection is disclosed. The regulator includes a housing having a housing inlet, a housing outlet, and a wall defining a housing interior. At least a portion of the wall includes a flexible member. A valve is associated with the flexible member and is in communication with the housing interior. The regulator is designed so that upon an application of a differential pressure within the housing interior, the flexible member and/or valve automatically move with respect to either the housing inlet or housing outlet to modulate a flow of blood moving through the housing. A manual over-ride device can be provided to enable a user to over-ride the automatic regulation and manually regulate the flow of blood through the housing.
