The present invention relates to a valve (1) for controlling the flow of a medium from a first system (24) to a second system (26) comprising a deformable dome-shaped structure (3) forming a reservoir (25) and including a flow path opening (9), a valve cover (5) including a first connector opening (15) to connect the valve (1) to the first system (24) and a contact element (23) for interacting with the flow path opening (9) to open and close it and a valve base (7) being airtightly connected to the valve cover (5) and to the dome-shaped structure (3) and including a second connector opening (17) to connect the valve (1) to the second system (26) and being coupled to the reservoir (25) The deformable dome-shaped structure (3) is configured such that in an open state of the valve (1) the deformable dome-shaped structure (3) is deformed and the flow path opening (9) is not connected to the contact element (23) to allow a medium to flow from the first system (24) to the second system (26) through the first (15) and second (17) connector openings and the flow path opening (9) and in a closed state of the valve (1) the deformable dome-shaped structure (3) is in its original dome shape and the flow path opening (9) is connected to the contact element (23) and thus closed. The present invention further relates to a pump device (47) and a breast pump device (41) including a valve (1) as described above.
