We present and discuss velocity profiles of microflows obtained by micro particle-image velocimetry in a transmission setup. We have measured suspensions of beads in water and on human blood, using the red blood cells as a natural particle seeding. The limitations imposed by our optical system on the spatial resolution normal to the focal plane, the so-called focal depth, have also been analyzed. The first direct observations of the influence of the focal depth on the observed velocity profiles are presented. Good agreement is obtained between observations and calculated profiles modified by the finite focal depth through a weight function.