What condition is characterized by a reduction in the number of circulating red blood cells?

Anemia is characterized by a reduction in the number of circulating red blood cells (RBCs) or a decrease in the hemoglobin concentration in the blood. This condition can lead to fatigue, weakness, and pallor, as the body's tissues are not getting enough oxygen due to insufficient RBCs. Anemia can result from various factors, including nutritional deficiencies (such as iron, vitamin B12, or folate), chronic diseases, bone marrow disorders, or excessive blood loss.

In contrast, thrombocytopenia refers to a low platelet count, which affects blood clotting rather than the levels of red blood cells. Leukemia is a type of cancer involving the proliferation of abnormal white blood cells, which can lead to an overcrowding of these cells in the bone marrow and a decrease in healthy blood cell production, but it is not specifically defined by the reduction of red blood cells alone. Hemolysis is the destruction of red blood cells, which can lead to anemia as a secondary effect but is a different process that entails the breakdown rather than a straightforward reduction in RBC levels.