Low hemoglobin represents anemia. Below 120g/L is mild anemia, below 90g/L is moderate anemia and below 60g/L is severe anemia. There are three main causes of anemia: insufficient or reduced erythropoiesis, excessive destruction of erythrocytes and blood loss, as follows: 1. Insufficient or reduced erythropoiesis: the body’s hematopoietic cells, hematopoietic microenvironment and regulatory factors and hematopoietic raw materials determine the production of erythropoiesis, when these three substances are abnormal, it will often lead to the body’s erythropoiesis is not enough to produce or reduce the phenomenon, and thus anemia occurs. 2. Excessive destruction of red blood cells: when the red blood cells themselves are defective or affected by external factors, the life span of the red blood cells will be affected and hemolytic anemia will occur. 3. Blood loss: there are two kinds of blood loss: acute blood loss and chronic blood loss. Chronic blood loss is the most common cause of anemia, while acute blood loss mainly causes hemodynamic changes.