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Red blood cell

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Red blood cells^[1] (also called RBCs or erythrocytes) are the most common type of cell in the blood. Their primary function is to deliver oxygen from the lungs to the tissues; they also carry a waste product, carbon dioxide, from the tissues back to the lungs.

A red blood cell is a concave disk, microscopic in size (about 7 micrometers in diameter), and contains hemoglobin. As the main element of the circulating blood, its main function is to transport oxygen, which is carried by the hemoglobin. The number of cells per cu mm of blood is usually between 4.5 million and 5.5 million in men and between 4.2 million and 4.8 million in women. The number varies with age, activity, and environment. For example, an increase to a level of 8 million per cu mm can normally occur at more than 10,000 feet above sea level. An erythrocyte usually lives for 110 to 120 days. It is then removed from the bloodstream and broken down in the body's cells. New erythrocytes are produced in the marrow of long bones at a steady rate so that a constant level is usually maintained. With acute blood loss, hemolytic anemia, or chronic lack of oxygen, erythrocyte production may increase greatly.

^[2]Many people may not know this, but red blood cells are different from other cells in the body. Other cells have a nucleus, mitochondria, and other cellular "machinery" for energy production and replication, but red blood cells lack these. They are essentially "bags of hemoglobin" held in by a very sophisticated cell membrane structure, with the primary function of carrying oxygen to tissues and carbon dioxide waste away from tissues.