Hemoglobin is a protein that can be found in erythrocytes or red blood cells. This protein is able to bind oxygen in the lungs and carry it to the rest of the body. There it releases the oxygen to enable aerobic respiration which creates energy. There are approximately 12-20 grams of hemoglobinIn in every 100 ml of blood. Anemia occurs when the amount of hemoglobin is lower than the normal range. It is defined as a decrease in the ability of the blood to carry oxygen, a decrease of the total amount of red blood cells, or a decrease in the amount of hemoglobin in the blood. When a person has anemia, there is not enough oxygen being transported to different parts of the body, which affects energy production. That is why symptoms of anemia include lightheadedness and tiredness.