Why can red blood cells carry oxygen?
The short answer
Red blood cells carry oxygen because they contain hemoglobin, a protein that can hold oxygen for a short time. Blood carries the cells from the lungs to the body's cells.
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1. Picked up in the lungs
Oxygen enters the blood and attaches to hemoglobin.
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2. Carried in the blood
Red blood cells carry hemoglobin with oxygen through the blood.
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3. Released near cells
Near body cells, hemoglobin releases oxygen for them to use.
- Oxygen
- Red blood cell
- Hemoglobin
Hemoglobin holds oxygen
Hemoglobin is a protein found in large amounts inside red blood cells. It contains iron in parts called heme groups. Oxygen can attach to this iron for a short time. Some oxygen dissolves directly in blood, but most travels attached to hemoglobin.
Oxygen is picked up in the lungs and released near cells
In the lungs, oxygen moves from the air into the blood, where hemoglobin picks it up. Red blood cells carry it through the body.
Body cells use oxygen, so there is less oxygen near them. This difference helps oxygen leave hemoglobin and move into the cells. Near active cells, carbon dioxide levels and temperature are often higher. These changes also help hemoglobin release oxygen. When it returns to the lungs, hemoglobin can pick up oxygen again.
Red blood cells are built for the job
As a red blood cell matures, it loses its nucleus, leaving more room for hemoglobin. Its thin, indented-disk shape gives oxygen and carbon dioxide a wide area to pass in and out. Its flexibility also helps it move through narrow blood vessels.
Carbon dioxide also returns to the lungs
Carbon dioxide made by cells travels back to the lungs in blood. Most carbon dioxide is carried as bicarbonate, a form that dissolves in blood. Red blood cells help make it. Some carbon dioxide dissolves directly in blood, and some attaches to hemoglobin. In the lungs, bicarbonate changes back into carbon dioxide, which leaves the body when we breathe out.