Hematocrit Centrifuge: Blood Separation by Density

Centrifuge Controls

Rotor speed 12,000 RPM

Patient RBC volume (true Hct) 45%

Simulation speed 4×

Live Readouts

Elapsed spin time

0.0 s

Relative centrifugal force

0 × g

Measured hematocrit

— %

Status

Mixed

Tube Layers

Plasma (ρ ≈ 1.025 g/mL) Buffy coat — WBC + platelets (ρ ≈ 1.06–1.08) Packed red cells (ρ ≈ 1.10 g/mL)

How it works

Whole blood is a suspension of cells of different density in plasma. A centrifuge replaces weak gravity with a strong, controllable centrifugal field so the suspension separates in minutes instead of hours. This is exactly how a lab measures hematocrit (packed cell volume, PCV) — the fraction of blood volume occupied by red cells.

Relative centrifugal force:
  RCF = 1.118 × 10⁻⁵ · r · N²   (× g)
  r = rotor radius (cm), N = rotor speed (RPM)

Stokes settling velocity of a particle in the spinning tube:
  v = 2·a²·(ρ_cell − ρ_plasma)·g_eff / (9·η)
  a = cell radius, η = plasma viscosity, g_eff = RCF·g

Denser, larger red blood cells (ρ ≈ 1.10 g/mL) settle fastest and pack at the outer end of the tube first. Lighter white cells and platelets (ρ ≈ 1.06–1.08 g/mL, the buffy coat) settle more slowly and form a thin band on top of the red-cell column. Plasma (ρ ≈ 1.025 g/mL) stays on top. Because v ∝ g_eff, doubling the rotor speed roughly quadruples RCF (v ∝ N²) and cuts spin time accordingly.

Rotor speed — sets the RCF (×g) applied to the sample; higher speed settles cells faster and more completely.
Patient RBC volume — the true hematocrit the sample was drawn with (anemia ≈ 20–35%, normal ≈ 38–50%, polycythemia ≈ 52–60%); this is the value the readout should converge to once separation finishes.
Simulation speed — a time-lapse multiplier only; the relative order and final layer thicknesses follow the physics above.
Measured hematocrit — computed live from the height of the packed red-cell column relative to the total column, exactly as a technician reads a spun capillary tube against a hematocrit chart.

Real-world relevance: this same density separation underlies the microhematocrit test used to screen for anemia and polycythemia, and the buffy-coat layer it produces is where white cells are harvested for a manual differential count or flow cytometry.

0 RPM

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