A functional apparatus for a vehicle seat having at least one expansion member having at least one cavity, the volume of which can be changed, at least in part, by a fluid which can flow into and out of said cavity, a fluid reservoir comprising at least one cavity being provided, which cavity is connected in a fluid-conducting manner to the cavity of the expansion member. The fluid reservoir cooperates with an actuator assembly by means of which the cavity of the fluid reservoir can be actively compressed, with at least partial displacement of the fluid contained therein into the cavity of the expansion member.