What Is the Function of Hemoglobin?

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.1

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

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.2

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.

Structure and Abnormalities

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:1

• 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.

RBCs usually look like donuts, but with a thin center instead of a hole.

Conditions Involving Low Hemoglobin

Low hemoglobin is called anemia.3 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:

• Blood loss from surgery
• Heavy menstrual periods
• Bleeding in the gastrointestinal tract4

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

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:

• Iron deficiency
• Vitamin B12 deficiency
• Folate deficiency

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:

• Lung diseases like COPD and pulmonary fibrosis5
• Congenital heart disease, or heart disease that is present at birth
• Cor pulmonale, or right-sided heart failure

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

Thalassemia is another inherited disease. Different types are characterized by different hemoglobin abnormalities.6

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.

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:

• Total RBC count
• Red blood cell indices, including mean corpuscular hemoglobin concentration (MCHC), mean corpuscular hemoglobin (MCH), and mean corpuscular volume (MCV)7
• Serum ferritin level, which measures iron stores in the body

Summary

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 number of red blood cells in the body is a balance of production, loss, and destruction. The function of red blood cells is impacted by the amount of red blood cells present, the lifespan of the red blood cells, and the type of hemoglobin present. Your healthcare provider may use a combination of your genetic background, symptoms, and lab test results to evaluate your hemoglobin status.

— STEFFINI STALOS, DO, MEDICAL EXPERT BOARD