Why does the number of white blood cells increases in a patient after heart surgery?Please explain one possible reason of this.

The number of white cells in the blood (WBC) is measured
in the laboratory by a machine.  The WBC count is part of a complete blood count (CBC),
in which WBC and red cells (RBC) are counted. Measurements of the RBC size shape are
noted, and the five types of WBC are tallied.  The five WBC types are Neutrophils,
Lymphocytes, Monocytes, Basophils and Eosinophils.

An
elevated WBC count is called leukocytosis, reduced WBC,
leukopenia.

Common causes of leukocytosis are infection,
tissue injury or damage, leukemia and severe stress.

Some
causes of leukopenia are bone marrow failure, enlarged spleen and
radiation.

Open-heart surgery causes leukocytosis for
several reasons.  First of all the surgery represents a severe stress.  The stress
hormones such as adrenaline and cortisol cause mobilization of WBC from the bone marrow
and tissue storage sites into the blood stream, raising the WBC count.   Hemorrhage
(bleeding) is a cause of leukocytosis, and open-heart surgery may involve a significant
degree of blood loss.  Finally, tissue damage causes elevation of WBC, and some degree
of tissue damage occurs in any form of major surgery.