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Why Is Blood Red?


Updated May 23, 2011

Written or reviewed by a board-certified physician. See About.com's Medical Review Board.

Question: Why Is Blood Red?
Answer: Blood cells are red because they contain hemoglobin. Blood cells get their red color when hemoglobin passes through the lungs where it is exposed to oxygen.

So, you wonder, what is hemoglobin?

Hemoglobin is a protein found in red blood cells. The protein in hemoglobin is full of iron: Iron is the power that enables hemoglobin to move oxygen from the lungs to all of the tissues and cells of the body.

Iron is also, more precisely, the reason blood is red. When oxygen attaches to iron, it becomes iron oxide, which, just like rust, causes the red color of blood.

However, the job of hemoglobin does not stop with the release of oxygen. Empty hemoglobin cells then begin their second job: Removing carbon dioxide and gas waste products from the body.

What Happens When You Don't Have Enough Red Blood Cells?

When you have a low red blood cell count, it usually means that you have anemia. Having anemia, or being anemic, means that your heart and lungs must work harder to move oxygen, carbon dioxide, and other waste from your body. This can result in a number of symptoms of which fatigue is most common.

See: What Is Anemia?

Can I Have Too Many Red Blood Cells?

Yes, in fact, for over 1 million Americans, hemochromatosis or iron overload is devastating. A gene mutation causes hemochromatosis when can lead to severe consequences including organ damage. Never use iron supplements without first checking with your physician.

See: What Is Iron Overload?


What Is Anemia; NHLBI; http://www.nhlbi.nih.gov/health/dci/Diseases/anemia/anemia_whatis.html. Accessed 05/06/2011.

What is Iron-Deficiency Anemia; NHLBI; http://www.nhlbi.nih.gov/health/dci/Diseases/ida/ida_whatis.html. Accessed 05/06/2011.

Why Is Blood Red. NLB Singapore. http://blogs.nlb.gov.sg/ask/children/81. Accessed 05/20/2011.

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