Decreased red blood cells means that the number of red blood cells in the peripheral blood is lower than normal and anemia is usually present. There are three clinical causes of red blood cell decline, the first of which is insufficient red blood cell production, such as aplastic anemia, pure red blood cell aplastic anemia, as well as iron deficiency anemia and macrocytic anemia caused by a lack of hematopoietic material. Insufficient red blood cell production due to various reasons leads to a decrease in the number of red blood cells. The second type of cause is excessive destruction of red blood cells, which is commonly seen in various hemolytic diseases. For example, after a long-distance runner’s strenuous exercise, the red blood cells may lyse and rupture due to the continuous impact, and the number of red blood cells may drop. There is also autoimmune hemolytic anemia, in which the red blood cells are destroyed by autoantibodies, and the red blood cells may drop. The third type of cause is a significant decrease in red blood cell count if there is blood loss. For example, if there is bleeding from a chronic peptic ulcer, there will be a decrease in the number of red blood cells.