There are two types of low red blood cells in routine blood tests: first, the relative decrease in the number of red blood cells caused by an increase in plasma volume, such as after a large number of transfusions or a large number of drinks, due to a decrease in the density of red blood cells caused by the expansion of blood volume, there will be a low number of red blood cells. Second, due to the decrease in the number of red blood cells produced, it is common in anemia caused by various reasons, such as aplastic anemia, hemolytic anemia, iron deficiency anemia, megaloblastic anemia and long-term chronic blood loss, such as blood loss from hemorrhoids, blood loss from excessive menstruation in women and bleeding caused by peptic ulcers or tumors, etc. Regardless of the type of anemia, there will be a low number of red blood cells, as well as a decrease in hemoglobin concentration and a decrease in red blood cell pressure. If low red blood cells are present, first rule out the presence of dilated blood volume, and if there is no dilated blood volume, consider low red blood cells caused by anemia.