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, increasing blood viscosity decreases
the rate of venous return to the heart. Con-versely, the blood volume is
greatly increased in poly-cythemia, 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.
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. There-fore, a person with polycythemia vera ordinarily has a ruddy
complexion with a bluish (cyanotic) tint to the skin.