Valve

A valve is a mechanical device that regulates the flow of fluids (either gases and liquids) by opening, closing, or partially obstructing various passageways.

A valve doesn't necessarily mean a man-made device; there are several valves inside the human body, for instance in the heart or in veins.

Because their effect on a flow of electrons is analogous to the fluid devices, the electronic devices known in the US as "vacuum tubes" are called "thermionic valves" in Britain.