The word hemoglobin is formed from heme meaning iron and globin meaning protein. The iron-containing substance in the cytoplasm of red blood cells is called hemoglobin. Due to this the blood is red in color. 33% of the weight of a red blood cell is hemoglobin. Each human red blood cell contains about 270 million hemoglobin molecules. 100 ml of blood contains 15/16 grams of hemoglobin. Each gram of hemoglobin can carry 1.36-1.40 ml of oxygen. If hemoglobin is low in the body, it is called oligocythemia. Again, if the amount of hemoglobin is increased, it is called polycythemia.