HEMATOCRIT
A common laboratory test can tell a physician a great deal about the volume of
red cells in a blood sample. The volume of RBCs refers to the amount of space that the
RBCs occupy within the blood. If whole blood (the cellular portion together with the plasma) is placed in a special hematocrit tube (a small test tube) and then
spun very rapidly in a centrifuge, the heavier components will quickly settle to the
bottom of the tube (Figure 2). When the centrifuge spins, the RBCs are forced to the
bottom of the tube because they are the heaviest element in the blood. The WBCs and
platelets are lighter so, as the hematocrit tube spins, they come to rest on top of
the heavier RBCs in a layer called the buffy coat. Above the buffy coat rests
the plasma. From the hematocrit tube, one can approximate the percentage of space that
the RBCs occupy in the total sample. At sea level, the hematocrit of a normal adult
male averages about 47, which means that 47% of the blood volume is RBCs, while that
of a normal adult female is 42.
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Figure 2. Blood can be separated into its components by putting it
into a centrifuge and "spinning it down." The parts separate according to their relative "weights." This test tube shows the components of blood in their relative ratios. It shows a he
matocrit of 45 because the RBC layer together with the "buffy coat" layer make up 45% of the total volume of centrifuged blood (4.5 m. out of 10 ml).
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