Written by Ada’s Medical Knowledge Team
Hemoglobin is a protein in red blood cells that carries oxygen to cells and gives blood its distinctive red color. After transporting oxygen to the tissues that need it for their cell metabolism, hemoglobin takes carbon dioxide, a waste substance the cells produce, and transports it to the lungs, where it leaves the body.
There are a few things that can go wrong with hemoglobin that can have a serious impact on our health. It might be that there is too little hemoglobin present in the blood, a condition called anemia. Too much hemoglobin, on the other hand, might be caused by diseases of the bone marrow, smoking and dehydration. Problems with hemoglobin can also cause red blood shells to become malformed, as seen in sickle cell disease. 1 2
What is hemoglobin?
Hemoglobin is a protein found in red blood cells. It contains iron, which allows it to transport oxygen. Each hemoglobin molecule can bind up to four oxygen molecules. 3 This way, it can transport the oxygen from the lungs to the tissues that need it.
Oxygen is important for normal cellular metabolism. It allows cells to extract energy for their processes. 4 Hemoglobin is red when combined with oxygen, giving blood its color. When hemoglobin isn’t bound to oxygen, it is a deep purple color.
Red blood cells containing normal hemoglobin are disk-shaped and smooth. Some inherited conditions, such as sickle cell disease, can lead to malformed red blood cells due to problems with hemoglobin. In sickle cell disease, red blood cells appear in the shape of the letter C. This is problematic because the C-shaped cells are more likely to stick together and block blood vessels. 5
Hemoglobin is created in the bone marrow. At the end of its lifecycle, hemoglobin is broken down into iron for reuse and bilirubin for excretion from the body. 6
Why is hemoglobin important?
Several of hemoglobin’s functions are essential for normal bodily function.
- It carries oxygen from the lungs to the tissues
- It carries carbon dioxide from the tissues to the lungs.
- It provides red blood cells with their color and shape.
What causes low hemoglobin?
Hemoglobin levels usually depend on age, sex, and lifestyle habits. In general, the levels in newborns are the highest (14-24 g/dL.) Afterward, they decrease until age 6, from which point they start increasing again slightly.
The average hemoglobin levels for adult men and women are different:
- Adult men: 14-18 g/dL
- Adult women: 12-16 g/dL
More information on what are considered low, high and normal hemoglobin levels can be found on our specified hemoglobin levels page.
If a blood test concludes that your values are below the normal range, you have low hemoglobin. This can have various causes. In some people, low hemoglobin may be normal for a while. For example, during menstruation, after donating blood, or because of significant changes in altitude.
However, if hemoglobin continues to be low, one of the following may be the cause:
- Blood loss in any form. This can be due to conditions such as hemorrhoids, ulcers, or heavy menstrual bleeding.
- The use of medications that lower your hemoglobin count, such as certain blood pressure and chemotherapy medication 7 8
- Fluid retention, which is excess fluid buildup in the body 9
- Inherited conditions such as thalassemia and malformations of hemoglobin 10
You might see a combination of low hemoglobin and hematocrit in test results. Low hemoglobin refers to the amount of oxygen-carrying molecules in the blood, while hematocrit refers to the total volume of red blood cells. A decrease in hematocrit will cause a decrease in hemoglobin, as hemoglobin forms a part of the red blood cells. 11
What are low hemoglobin symptoms?
Low levels of hemoglobin cause a condition called anemia. People with anemia may not have enough iron, vitamin B12, or vitamin B9. They may experience symptoms such as: 9 12
- pale skin
- trouble breathing
Who should test their hemoglobin level?
People who are experiencing symptoms of anemia should test their hemoglobin levels. You should also have your hemoglobin levels tested if you have family members who suffer from conditions such as thalassemia, sickle cell disease, or other inheritable blood diseases. 12
Frequently asked questions
Q: How is hemoglobin defined?
Hemoglobin is an oxygen-carrying protein found in red blood cells. It contains iron, and it gives the blood its red color.
Q: What does hemoglobin do?
Hemoglobin is vital to the body because it can bind to oxygen, carbon dioxide, and other groups such as sulfide and nitrogen oxide. When bound to hemoglobin, these substances are transported through the blood.
Q: When do I need to seek medical help?
If you are experiencing symptoms such as fatigue, dizziness, weakness, headaches, and trouble breathing, you should get your blood tested for low hemoglobin.
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National Cancer Institute (2022). PDQ Chronic Myeloproliferative Neoplasms Treatment. Accessed on 30 March 2022.
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Mairbäurl H, Weber RE (2012). Oxygen transport by hemoglobin. Accessed on 8 April 2022.
Center for Disease Control and Prevention (2020). What is Sickle Cell Disease?. Accessed on 8 April 2022.
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