What Is the Function of Hemoglobin?

The Protein That Carries Oxygen in Blood

Hemoglobin is an iron-containing protein in red blood cells (RBCs) that gives blood its red color. It has two primary functions: It transfers oxygen from your lungs to tissues throughout your body, and it carries carbon dioxide from cells back to the lungs so it can be expelled.

When hemoglobin is too low, it can indicate certain types of anemia. There are also abnormal types of hemoglobin that cause anemia as well as illnesses such as sickle cell disease.

This article explains the functions of hemoglobin in your body. It also looks at some of the conditions associated with high and low hemoglobin.

Risks of High and Low Hemoglobin Levels - Illustration by JR Bee

Verywell / JR Bee

The Function of Hemoglobin

Hemoglobin binds and transports oxygen from the lungs to the tissues in the body. It also transports carbon dioxide from tissues back to the lungs.

Nitric oxide and carbon monoxide can also bind with hemoglobin. Carbon monoxide binds to hemoglobin much more strongly than oxygen. Its presence keeps oxygen from binding to hemoglobin. This is why carbon monoxide poisoning is so serious.

Hemoglobin contains around 70% of the iron in your body and gives red blood cells their distinctive red color.

Structure and Abnormalities

Amino acids are the building blocks of proteins. Hemoglobin, a protein, is made up of four amino acid chains. Each of these chains contains heme. Heme is a compound that contains iron. One of the key functions of heme is to transport oxygen in the bloodstream.

Hemoglobin is what gives RBCs their shape. RBCs usually look like donuts, but with a thin center instead of a hole.

There are several different types of hemoglobin, the two most common of which are:

  • Hemoglobin A (HgbA): This is the most common type found in healthy adults.
  • Hemoglobin F (HgbF): Also known as fetal hemoglobin, this type is found in fetuses and newborn. It is replaced by HgbA shortly after birth.

There are abnormal types of hemoglobin that affect both the shape of RBCs but also their ability to transport oxygen and carbon dioxide, including:

  • Hemoglobin S (HgbS): This type of hemoglobin is found in sickle cell disease that causes RBCs to become stiff and crescent-shaped.
  • Hemoglobin C (HgbC): This type of hemoglobin does not carry oxygen well and is associated with mild anemia.
  • Hemoglobin E (HgbE): This type of hemoglobin is mostly found in people of Southeast Asian descent that may cause mild anemia or no symptoms at all.

Conditions Involving Low Hemoglobin

Low hemoglobin is called anemia. Anemia can be caused by anything that interferes either with the amount or function of hemoglobin or RBCs.

Causes of anemia include:

Blood Loss

Any type of blood loss can cause anemia. This includes:

Premenopausal females are more likely to have a low hemoglobin level than males.

Lack of Production

In some conditions, the bone marrow may not produce enough RBCs. These conditions include:

  • Aplastic anemia and other conditions that cause bone marrow failure
  • Cancers like leukemia, lymphoma, or tumors that have spread from other parts of the body into the bone marrow


Hemolysis is the breakdown of RBCs. This may happen with conditions like:

  • Severe infections
  • Damage from toxins
  • Malaria

Nutritional Deficiencies

Certain nutritional deficiencies can cause anemia. These include:

Kidney Disease

Healthy kidneys release erythropoietin. This is a hormone that promotes RBC production. People with kidney disease may not produce enough of this hormone.

Conditions Involving Elevated Hemoglobin

Several conditions are associated with high hemoglobin levels. These include:

In these cases, the body responds to low oxygen by increasing hemoglobin.

If you are dehydrated, your hemoglobin test may show an artificially high hemoglobin level. This is because hemoglobin is more concentrated when the fluid in the body is low.

Hemoglobin may also be high in people living at high altitudes. This is because of the low levels of oxygen in the air.

Conditions with Abnormal Hemoglobin

Conditions where hemoglobin has an abnormal structure include:

Sickle Cell Anemia

Sickle cell anemia is an inherited condition. In people with this condition, abnormal hemoglobin causes sickle-shaped RBCs. These cells can get "stuck" in blood vessels. This can cause:

  • Pain
  • Blood clots
  • Increased risk of stroke


Thalassemia is another inherited disease. Different types are characterized by different hemoglobin abnormalities.

Alpha thalassemia and beta thalassemia are the two main types. There are also many different subtypes. Symptoms vary from none to severe.

People with thalassemia often have lifelong anemia. They may require frequent blood transfusions.

Thalassemia intermedia is also called "non-transfusion-dependent thalassemia." This kind of thalassemia may not be discovered until adulthood.


Conditions like sickle cell anemia and thalassemia can cause hemoglobin to have an abnormal structure. 

Hemoglobin Tests

Hemoglobin levels are usually measured as part of a complete blood count (CBC). The results of other lab tests may help identify the cause of hemoglobin problems. These include:

Normal Hemoglobin Levels

Normal hemoglobin ranges vary by age and sex. They're measured in grams per deciliter (g/dL). Normal ranges are:

  • Adult male: 13.8-17.2 g/dL
  • Adult female: 12.1-15.1 g/dL


Hemoglobin is a protein in red blood cells that carries oxygen from your lungs to your tissues. Problems with hemoglobin can cause symptoms like fatigue and rapid heart rate.

Hemoglobin levels that are too high or too low can lead to health problems. In conditions like sickle cell anemia, hemoglobin can have an abnormal shape. This can lead to serious problems like pain and blood clots. 

Hemoglobin levels are usually checked as part of a complete blood count. Other lab tests can help confirm the cause of hemoglobin issues. 

A Word From Verywell

The word hemoglobin is often used when discussing heavy bleeding. There are a wide range of other disorders that can cause high or low hemoglobin, though. Abnormal types of hemoglobin can also cause disease.

If you have high or low hemoglobin, your doctor will use a range of tools to find the cause. This may include asking questions, doing a physical exam, and ordering blood tests.

Frequently Asked Questions

  • What are the signs that my red blood cell count is low?

    Fatigue is the number one sign. This is caused by anemia. Anemia is a blood disorder resulting from a lack of hemoglobin. This is the essential protein found in red blood cells. Other symptoms may include headache, dizziness, weakness, pale skin, feeling cold, and trouble breathing.

  • Does exercising a lot cause anemia?

    Intense training, especially among endurance athletes, can cause a condition called “sports anemia.” Sports anemia refers to well-conditioned athletes’ relatively low hemoglobin levels. This is caused by an increased plasma volume. It actually has positive effects. It lets the blood better circulate oxygen during exercise.

  • How do healthcare providers tell if you have a problem with your hemoglobin?

    A blood test called electrophoresis measures the types of hemoglobin in your blood. This test identifies abnormal hemoglobin including:

    • HgbS: Related to sickle cell disease
    • HgbC: Causes mild anemia
    • HgbE: Causes mild anemia 
9 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. MedlinePlus. Hemoglobin electrophoresis.

  2. University of Rochester Medical Center. Health Encyclopedia. What are red blood cells?

  3. Karakochuk CD, Hess SY, Moorthy D, et al. Measurement and interpretation of hemoglobin concentration in clinical and field settings: a narrative review. Ann N Y Acad Sci. 2019;1450(1):126-146. doi:10.1111/nyas.14003

  4. Auerbach M. Patient education: anemia caused by low iron in adults (beyond the basics). UpToDate.

  5. MedlinePlus. Hemoglobin test.

  6. MedlinePlus. Thalassemia.

  7. MedlinePlus. RBC indices.

  8. Cleveland Clinic. Low hemoglobin.

  9. Mairbäurl H. Red blood cells in sports: effects of exercise and training on oxygen supply by red blood cells. Front Physiol. 2013;0. doi:10.3389%2Ffphys.2013.00332

By Lynne Eldridge, MD
 Lynne Eldrige, MD, is a lung cancer physician, patient advocate, and award-winning author of "Avoiding Cancer One Day at a Time."