Spring: Located inside the chamber, it returns the piston to its starting position after each press, creating the suction necessary to draw liquid for the next dose.

Inlet Valve (Liquid & Air): A one-way valve (often part of the piston) that opens to allow liquid and air into the chamber on the upstroke (when the spring returns) and closes on the downstroke to force the mixture out.

Foaming Chamber / Mixing Screen: This is the most critical part that differentiates a foam pump from a liquid pump. It is a chamber filled with one or more fine-mesh screens (like a sieve). As the liquid-air mixture is forced through these tiny holes under pressure, it is sheared and mixed, transforming into a foam.