Effect of Polycythemia on Function of the Circulatory System

Because of the greatly increased viscosity of the blood
in polycythemia, blood flow through the peripheral
blood vessels is often very sluggish. In accordance with
the factors that regulate return of blood to the heart,
as discussed in Chapter 20, increasing blood viscosity
decreases the rate of venous return to the heart. Conversely,
the blood volume is greatly increased in polycythemia,
which tends to increase venous return.
Actually, the cardiac output in polycythemia is not far
from normal, because these two factors more or less
neutralize each other.

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The arterial pressure is also normal in most people
with polycythemia, although in about one third of
them, the arterial pressure is elevated.This means that
the blood pressure–regulating mechanisms can usually
offset the tendency for increased blood viscosity to
increase peripheral resistance and, thereby, increase
arterial pressure. Beyond certain limits, however, these
regulations fail, and hypertension develops.
The color of the skin depends to a great extent on
the quantity of blood in the skin subpapillary venous
plexus. In polycythemia vera, the quantity of blood in
this plexus is greatly increased. Further, because the
blood passes sluggishly through the skin capillaries
before entering the venous plexus, a larger than
normal quantity of hemoglobin is deoxygenated. The
blue color of all this deoxygenated hemoglobin masks
the red color of the oxygenated hemoglobin. Therefore,
a person with polycythemia vera ordinarily has a
ruddy complexion with a bluish (cyanotic) tint to the
skin.