The invention relates to a positive displacement pump (10) comprising a pump body (11, 101, 201) comprising an inlet end (16, 116, 216) and an outlet end (18, 118, 218), a pumping chamber (30, 130, 230) arranged between said inlet end (16, 116, 216) and said outlet end (18, 118, 218), at least one membrane (20, 120, 220) active in the pumping chamber (30, 130, 230) and mobile between an expanded configuration in which the volume of the pumping chamber (30, 130, 230) is maximum and a retracted configuration in which the volume of the pumping chamber (30, 130, 230) is minimum, a delivery valve (46, 146, 246) arranged close to the outlet end (18, 118, 218) of the pump body (11, 101, 201), an intake valve (26, 126, 226) comprising an intake mouth (27, 127, 227), an outlet mouth (28, 128, 228) and a valve wall (29, 129, 229) that joins the intake mouth (27, 127, 227) to the outlet mouth (28, 128, 228), the intake mouth (27, 127, 227) being coupled to the inlet end (16, 116, 216) of the pump body (11, 101, 201) and the outlet mouth (28, 128, 228) being inserted in the pumping chamber (30, 130, 230). Said at least one membrane (20, 120, 220), when in the retracted configuration, adheres to the valve wall (29, 129, 229) of the intake valve (26, 126, 226) and the intake valve has the outlet mouth closed.