Polycythemia

Secondary Polycythemia. Whenever the tissues become
hypoxic because of too little oxygen in the breathed
air, such as at high altitudes, or because of failure of
oxygen delivery to the tissues, such as in cardiac
failure, the blood-forming organs automatically
produce large quantities of extra red blood cells. This
condition is called secondary polycythemia, and the
red cell count commonly rises to 6 to 7 million/mm3,
about 30 per cent above normal.
A common type of secondary polycythemia, called
physiologic polycythemia, occurs in natives who live at
altitudes of 14,000 to 17,000 feet, where the atmospheric
oxygen is very low.The blood count is generally
6 to 7 million/mm3; this allows these people to perform
428 Unit VI Blood Cells, Immunity, and Blood Clotting
reasonably high levels of continuous work even in a
rarefied atmosphere.
Polycythemia Vera (Erythremia). In addition to those
people who have physiologic polycythemia, others
have a pathological condition known as polycythemia
vera, in which the red blood cell count may be 7 to 8
million/mm3 and the hematocrit may be 60 to 70 per
cent instead of the normal 40 to 45 per cent. Polycythemia
vera is caused by a genetic aberration in the
hemocytoblastic cells that produce the blood cells.The
blast cells no longer stop producing red cells when too
many cells are already present.This causes excess production
of red blood cells in the same manner that a
breast tumor causes excess production of a specific
type of breast cell. It usually causes excess production
of white blood cells and platelets as well.
In polycythemia vera, not only does the hematocrit
increase, but the total blood volume also increases, on
some occasions to almost twice normal.As a result, the
entire vascular system becomes intensely engorged. In
addition, many blood capillaries become plugged by
the viscous blood; the viscosity of the blood in polycythemia
vera sometimes increases from the normal of
3 times the viscosity of water to 10 times that of water.