In a motor-driven air pump, a bottom wall and a surrounding barrier wall cooperatively confine a receiving space in which an impeller is mounted. The barrier wall is provided with angularly displaced first and second internal ports that communicate fluidly with first and second chambers, respectively. A first external port is disposed upstream of and communicates with one of the first and second chambers. A second external port is disposed downstream of and communicates with the other one of the first and second chambers.