A receptacle device for separating a mixture, in particular blood, into a lighter phase with a lower density and a heavier phase with a relatively higher density, comprising - a receptacle having a first end and a second end spaced apart from one another in the direction of a longitudinal axis, the first end being open and the second end being closed by a base, and a side wall having an internal face and an external face extends between the first end and the second end, and the internal face of the side wall develops into a base surface formed by the base in a transition region, wherein the side wall and the base delineate a receptacle chamber, - a separating element, which is disposed in the receptacle chamber and is made entirely and continuously of an elastically deformable material, the density of the separating element being between the densities of the phases (3, 4) to be separated, - a removable closure unit, which closes the first, open end of the receptacle, - wherein the separating element in its undeformed initial state, has a predominantly spherical three-dimensional shape and is disposed in the region of the second end, which is closed by the base, before the receptacle chamber is filled with the mixture to be separated and is in contact with the internal face of the side wall at least in a contact plane oriented perpendicular to the longitudinal axis, - an external diameter of the separating element being selected to be larger in its undeformed initial state than a cross-sectional dimension of the receptacle chamber in the contact plane, and the separating element is thus held in position against the internal face of the side wall in a prestressed standby position, - and a first part section of the internal face, which is directly adjacent to the first open end is designed to be almost cylindrical with respect to the longitudinal axis, characterized in that - a second part section of the internal face is designed to be almost cylindrical with respect to the longitudinal axis, and this second part section extends over an axial length in the direction towards the first open end, starting from the transition region of the internal face to the base, said axial length corresponding to at least 50%, preferably 60% of the filling volume of the receptacle chamber, - at least one passage is disposed or formed between the base and the second part section of the internal face, said passage connecting partial chambers of the receptacle chamber disposed on both sides of the contact plane when the separating element is disposed in the receptacle chamber, and - the at least one passage is formed either by recesses in the internal face of the side wall and/or by webs or protrusions formed to protrude in the radial direction over the internal face in the direction towards the longitudinal axis. - a receptacle having a first end and a second end spaced apart from one another in the direction of a longitudinal axis, the first end being open and the second end being closed by a base, and a side wall having an internal face and an external face extends between the first end and the second end, and the internal face of the side wall develops into a base surface formed by the base in a transition region, wherein the side wall and the base delineate a receptacle chamber, - a separating element, which is disposed in the receptacle chamber and is made entirely and continuously of an elastically deformable material, the density of the separating element being between the densities of the phases (3, 4) to be separated, - a removable closure unit, which closes the first, open end of the receptacle, - wherein the separating element in its undeformed initial state, has a predominantly spherical three-dimensional shape and is disposed in the region of the second end, which is closed by the base, before the receptacle chamber is filled with the mixture to be separated and is in contact with the internal face of the side wall at least in a contact plane oriented perpendicular to the longitudinal axis, - an external diameter of the separating element being selected to be larger in its undeformed initial state than a cross-sectional dimension of the receptacle chamber in the contact plane, and the separating element is thus held in position against the internal face of the side wall in a prestressed standby position, - and a first part section of the internal face, which is directly adjacent to the first open end is designed to be almost cylindrical with respect to the longitudinal axis, - a second part section of the internal face is designed to be almost cylindrical with respect to the longitudinal axis, and this second part section extends over an axial length in the direction towards the first open end, starting from the transition region of the internal face to the base, said axial length corresponding to at least 50%, preferably 60% of the filling volume of the receptacle chamber, - at least one passage is disposed or formed between the base and the second part section of the internal face, said passage connecting partial chambers of the receptacle chamber disposed on both sides of the contact plane when the separating element is disposed in the receptacle chamber, and - the at least one passage is formed either by recesses in the internal face of the side wall and/or by webs or protrusions formed to protrude in the radial direction over the internal face in the direction towards the longitudinal axis.