The term  anemia  is a general term that refers to a decrease in red blood cells. Anemia can occur from either a decrease in the number of red blood cells, a decrease in the hemoglobin content, or both. Red blood cells live for approximately four months in the bloodstream.he RBC is a count of the number of red blood cells per cubic millimeter of blood. In response to hypoxia, the hormone erthyropoietin, secreted by the kidneys, stimulates the bone marrow to produce red blood cells. The formation of red blood cells is known as erthyropoiesis.