Hemoglobin

Medical Author: Siamak Nabili, MD, MPH
Medical Editor: William C. Shiel, Jr., MD, FACP, FACR

• What is hemoglobin?
• How is hemoglobin measured?
• What are normal hemoglobin values?
• What does a low hemoglobin level mean?
• What does a high hemoglobin level mean?
• What is sickle cell disease?
• What is thalassemia?
• Patient Discussions: Hemoglobin - Helped With Your Diagnosis

What is hemoglobin?

Hemoglobin is the protein molecule in red blood cells that carries oxygen from the lungs to the body's tissues and returns carbon dioxide from the tissues to the lungs.

Hemoglobin is made up of four protein molecules (globulin chains) that are connected together. The normal adult hemoglobin (Hbg) molecule contains 2 alpha-globulin chains and 2 beta-globulin chains. In fetuses and infants, there are only a few beta chains and the hemoglobin molecule is made up of 2 alpha chains and 2 gamma chains. As the infant grows, the gamma chains are gradually replaced by beta chains.

Each globulin chain contains an important central structure called the heme molecule. Embedded within the heme molecule is iron that transports the oxygen and carbon dioxide in our blood. The iron contained in hemoglobin is also responsible for the red color of blood.

Hemoglobin also plays an important role in maintaining the shape of the red blood cells. Abnormal hemoglobin structure can, therefore, disrupt the shape of red blood cells and impede its function and its flow through blood vessels.

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