Why spinning sorts blood into layers
Blood is a mixture of liquid and cells, not a single uniform fluid. In a centrifuge, a rapidly rotating rotor makes material move outward from the axis of rotation. The denser blood cells migrate toward the tube’s outer end more readily than the less-dense liquid, creating layers that can be separated after the spin.
For a sample to stay divided into components, it is commonly collected with an anticoagulant—a substance that prevents it from clotting. Without one, clotting would trap cells in a solid mass instead of leaving them available for separation.
The thin layer between cells and plasma
The layers are not limited to red cells and plasma. Between them is usually a narrow, pale band called the buffy coat, made mostly of white blood cells and platelets. It can be hard to see because it occupies so little of the tube, but it matters: white cells help defend against infection, while platelets help blood form clots.
The boundaries are not perfectly sharp. Cells and platelets can remain suspended near the interfaces, and the exact appearance depends on the sample and how it is spun. A centrifuge separates components by their physical properties; it does not identify them or determine whether they are healthy.
Why the separation is useful
Once the layers are separated, a lab can examine a particular component or use it in a test without the others obscuring the result. The same basic sorting principle also helps prepare blood components for clinical use. A small amount of blood can therefore become several distinct materials—each with different contents and a different role.
