The pump includes a housing containing a hollow elongated part, a piston arranged to move back and forth inside an elongated part, an inlet port and an outlet port arranged so that a fluid can be sucked through the inlet port into a chamber during an instroke of the piston and expelled through the outlet port during an outstroke. A linearly and/or angularly actuable valve system has a valve holder mounted on the pump housing which includes at least one through-hole extending from the piston chamber to the housing outer surface. The valve holder contains an inlet and/or outlet to alternately connect the inlet and outlet ports of the volumetric pump.