The main function of hemoglobin is to carry oxygen from the lungs to several organs or tissues in our body. It contains iron and is found in the red blood cells of all vertebrates. Some invertebrates also contain hemoglobin. Oxygen supports aerobic respiration to generate energy for organs so that they could function properly. Every 100 ml of blood contains 12 to 16 grams of hemoglobin in a healthy person.
Source: https://en.wikipedia.org/wiki/Hemoglobin