<qa>  What is the liquid part of the blood?3. leukapheresis to collect white blood cells and plasmapheresis to collect plasma the liquid part of the blood. For both of these procedures, blood is collected through a needle placed in an arm vein. The blood flows into a special machine that separates it into its components by spinning. The desired components white cells or plasma are removed and the rest of the blood is returned to the body through the same needle or a second needle in the other arm. 