Secondary Polycythemia. Whenever the tissues becomehypoxic 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 ataltitudes of 14,000 to 17,000 feet, where the atmos-pheric oxygen is very low. The blood count is generally 6 to 7 million/mm3; this allows these people to perform reasonably high levels of continuous work even in a rarefied atmosphere.
Polycythemia Vera (Erythremia). In addition to thosepeople who have physiologic polycythemia, others have a pathological condition known as polycythemiavera, in which the red blood cell count may be 7 to 8million/mm3 and the hematocrit may be 60 to 70 per cent instead of the normal 40 to 45 per cent. Poly-cythemia 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 pro-duction 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 poly-cythemia vera sometimes increases from the normal of 3 times the viscosity of water to 10 times that of water.
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.