Centrifuges are useful to, among other things, remove red blood cells from whole blood and retain platelets and other factors in a reduced volume of plasma. Platelet rich plasma (PRP) can be obtained rapidly and is ready for immediate injection into the host. Embodiments may include valves, operated manually or automatically, to open ports that discharge the excess red blood cells and the excess plasma while retaining the platelets and other facts. Highs speeds used allow simple and small embodiments to be used at the patient's side during surgical procedures. The embodiments can also be used for the separation of liquids or slurries in other fields such as, for example, the separation of pigments or lubricants.