Red blood cell decline means that the number of red blood cells in the peripheral blood is decreasing; a decrease in the number of red blood cells is a decrease in red blood cell volume, and often there is a condition of anemia. There are three causes of a decrease in the number of red blood cells. i. Decrease in red blood cell production: It is common in various blood system diseases, such as aplastic anemia, acute leukemia, and myelodysplastic syndrome. In addition, the lack of hematopoietic raw materials caused by the reduction in the number of red blood cells, including iron deficiency anemia, megaloblastic anemia and so on. Second, excessive destruction of erythrocytes: the decrease in the number of erythrocytes caused by the destruction of erythrocytes is most commonly associated with various hemolytic diseases, autoimmune hemolytic anemia, thalassemia, sericosis, etc. Third, red blood cell loss: red blood cell loss means that the human body has blood loss, such as peptic ulcer bleeding, bleeding from malignant tumors of the digestive tract, bleeding from excessive menstruation in women, long-term bleeding from hemorrhoids, etc., will have a decrease in red blood cells.