What causes erythrocytosis seen in relative polycythemia?

Relative polycythemia is characterized by an increase in the concentration of red blood cells (RBCs) without a corresponding increase in total red blood cell mass. This condition often arises due to a decrease in plasma volume, which can occur in situations such as dehydration or conditions causing fluid loss. When plasma volume decreases, the proportion of red blood cells in the blood relative to plasma increases, leading to an appearance of erythrocytosis.

In contrast, decreased arterial oxygen saturation, increased erythropoietin levels, and increased erythropoiesis in the bone marrow are associated with primary or secondary causes of absolute polycythemia, where there is an actual increase in red blood cell mass due to stimulation of erythropoiesis. In relative polycythemia, the key factor is the reduction in plasma volume rather than an increase in red blood cell production or oxygen levels. Thus, the mechanism behind the observed erythrocytosis in relative polycythemia specifically highlights the importance of plasma volume changes.