Sedimentation
Sedimentation is the movement of suspended cells through plasma under centrifugal force. Denser cells travel outward faster, which is what separates blood into layers. Erythrocytes settle first, then leukocytes and platelets, with plasma remaining above.
Why blood separates
Figure. Denser fractions sediment faster under force. The density gap between plasma, platelets, leukocytes and erythrocytes is the entire basis of separation, and it is also why no machine can concentrate platelets faster than physics allows.
In practice
Sedimentation speed depends on the density difference between the cell and the surrounding plasma, the cell's volume, and the force applied. Because plasma density and cell counts vary between patients, the same protocol produces slightly different layering in different people.
- Stokes' lawStokes' law describes how fast a particle sediments through a fluid under force.
- Buffy coatThe buffy coat is the thin layer that forms between the red cells and the plasma after centrifugation.
- Relative centrifugal force (RCF)RCF is the force a centrifuge applies to a sample, expressed as a multiple of Earth's gravity and written as g.