Final answer:
Secondary erythrocytosis causes an elevated red blood cell count due to increased demand for or decreased supply of oxygen. It differs from polycythemia vera, which is a bone marrow disease causing overproduction of RBCs and is not caused by anemia or leukemia.
Step-by-step explanation:
Secondary erythrocytosis is a major cause of an increase in the red blood cell (RBC) count. It is often a compensatory response to hypoxia and may be normal in certain situations, such as living at high altitudes, or it could be pathological in response to conditions like chronic obstructive pulmonary disease. When the body experiences a decreased oxygen level, it may produce more RBCs to enhance the oxygen-carrying capacity of the blood. In polycythemia vera, a bone marrow disease, there is an overproduction of RBCs, which increases the viscosity of blood and raises blood pressure, putting more strain on the heart. This condition is different from secondary erythrocytosis, which is not primarily caused by a bone marrow disease but by an increased demand for oxygen or a decreased supply of oxygen. Secondary erythrocytosis does not typically refer to iron-deficiency anemia, leukemia, or polycythemia vera itself.