Blood is separated by centrifugation,thats like when you spin a bucket round ur head and it stays put which is centrifugal force. Well, blood is spun round and the heavier particles, red blood cells, go to the outside, middle platelets stay in the middle and the lighter plasma is balanced on the top.
Centrifugal force comes from the Latin meaning Centrum "center" and fugere "to flee". It is made when the substance is spun and pushed to the outside; the particles that are heavier have more gravitational pull so they are pulled to the bottom of the container they are in.
The machine that blood is separated in is called the centrifuge they are used in laboratories and on an industrial scale for separating solids in suspensions of liquids.