Components of Blood

Test tube showing 55% plasma, 1% white blood cells & platelets, 44% red blood cells The majority of blood is plasma which is a water based solution containing plasma proteins, electrolytes and other dissolved constituents.
The remainder of blood is cellular consisting of

  • Red blood cells (erythrocytes)
  • White blood cells (lymphocytes, leucocytes)
  • Platelets (Thrombocytes)

If a sample of blood is spun in a centrifuge then three layers form, plasma at the top a middle layer of white blood cells and platelets and a bottom layer of red blood cells. The percentage of each can then be worked out (see diagram - right).


This information is used in clinical practice to work out the cellular fraction called the packed cell volume (PCV) or haematocrit of blood. This is the percentage of red blood cells, white blood cells and platelets.

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