Centrifugal acceleration forces have been used very effectively to separate solid particles from fluids or to separate different density fluids. In the medical industry, urine and blood components can be separated using centrifugal acceleration forces. For example, a normal blood solution, which has a density of approximately 1060 kg m−3, can be separated into its components, because of their density gradients, by accelerating it centrifugally in a test tube. The red blood cells, which have a density of approximately 1100 kg m−3, end up at the bottom of the test tube, the white blood cells, which have a density of approximately 1050 kg m−3, end up on top of the red blood cells in the test tube, and the blood plasma, which has a density of approximately 1025 kg m−3, ends up on top of the white blood cells in the test tube. Centrifugal acceleration forces have been used for centuries to separate milk into cream and skimmed milk, to separate yeast cells from beer, to separate solids and water in sewage sludge, to remove dust particles from the air, and so on, in different types of centrifuges.