White blood cells (leukocytes)
White cells, unlike red cells, are nucleated and independently
motile. Highly differentiated fortheir specialized functions,
they do not undergo mitosis (ordinary cell division)
in the bloodstream, but some retain the capability of
cell division. As a group they are involved in the body's
defense mechanisms and reparative activity. The number
of leukocytes in normal blood ranges between 4,500 and
11,000 per cubic millimetre. Fluctuations occur during
the day; lower values are obtained during rest and higher
values during exercise. Violent physical exertion may
cause the count to exceed 20,000 per cubic millimetre.
Most of the leukocytes are outside the circulation,
and the few in the bloodstream are in transit from one
site to another. As living cells, their survival depends
on their continuous production of energy. The chemical
pathways utilized are more complex than those of the
red cells and are similar to those of other tissue cells.
Leukocytes, containing a nucleus and able to produce
RNA, can synthesize protein. They comprise three classes
of cells, each unique as to structureand function, that
are designated granulocytes, monocytes, and lymphocytes.
