A blood separation chamber is provided for separating blood into its constituent parts by rotation about an axis. The chamber has a central hub coinciding with the axis and spaced apart high-G and low-G walls which define therebetween a separation channel. The chamber further includes a plurality of parallel radial walls extending outwardly from the central hub and defining a radial fluid flow passage between each adjacent pair of walls. Each fluid flow passage communicates between the central hub and the separation channel. One of the radial walls includes a radially outer end portion defining a barrier in the separation channel intermediate the low-G and high-G walls and having generally parallel upstream and downstream sides. The separation channel has a recessed portion located radially outward of the high-G wall and extending between the upstream and downstream sides of the barrier.