red blood cell

red blood cell, component of blood, millions of which in the circulation of vertebrates give the blood its characteristic color and carry oxygen from the lungs to tissues throughout the body. Red blood cells also participate in transporting carbon dioxide from tissues back toward the lungs. Their distinctive red color comes from hemoglobin, an iron-containing protein that binds oxygen. Red blood cells are the most abundant type of cell in human blood.

Form and structure

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The mature human red blood cell is small, round, and biconcave; it appears dumbbell-shaped in profile. The cell is flexible and assumes a bell shape as it passes through extremely small blood vessels. It is covered with a membrane composed of lipids and proteins. Each red blook cell lacks a nucleus and contains hemoglobin—a red iron-rich protein that binds oxygen.

Oxygen transport

The function of the red cell and its hemoglobin is to carry oxygen from the lungs (or gills, in fishes) to all the body tissues and to carry carbon dioxide, a waste product of metabolism, to the lungs, where it is excreted. In invertebrates, oxygen-carrying pigment is carried free in the plasma; its concentration in red cells in vertebrates, so that oxygen and carbon dioxide are exchanged as gases, is more efficient and represents an important evolutionary development. The mammalian red cell is further adapted by lacking a nucleus—the amount of oxygen required by the cell for its own metabolism is thus very low, and most oxygen carried can be freed into the tissues. The biconcave shape of the cell allows oxygen exchange at a constant rate over the largest possible area.

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The absence of a nucleus and other organelles leaves more internal space for hemoglobin. Mature mammalian red blood cells also lack mitochondria (ATP-generating organelles) and produce their own ATP primarily through glycolysis. Consequently, they do not consume the oxygen they are carrying. In addition, their flexible membranes allow them to deform as they squeeze through capillaries that may be narrower than the cells themselves.

Production and life cycle

The red cell develops in bone marrow in several stages. From a hemocytoblast, a multipotential cell in the mesenchyme, it becomes an erythroblast (normoblast); during two to five days of development, the erythroblast gradually fills with hemoglobin, and its nucleus and mitochondria disappear. In a late stage the cell is called a reticulocyte, which ultimately becomes a fully mature red cell.

The average red cell in humans lives 100–120 days; there are some 5.2 million red cells per cubic millimeter of blood in the adult human. Aging or damaged cells are removed primarily by macrophages (white blood cells that help eliminate degrading or foreign materials) in the spleen and liver. Components that may still be useful are recycled; these can include iron from hemoglobin, which can be sent back to the bone marrow for the generation of new red blood cells. Remaining components of heme can be metabolized into substances that facilitate their elimination.

Role in health and disease

Alhough red blood cells are usually round, a small proportion are oval in healthy individuals; in certain hereditary states a higher proportion may be oval. Some diseases also display red cells of unusual shape—examples include an oval form in pernicious anemia, a crescent shape in sickle cell anemia, and a form with projections giving a thorny appearance in the hereditary disorder acanthocytosis.

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The number of red cells and the amount of hemoglobin vary among different individuals and under different conditions; the number is higher, for example, in individuals who live at high altitudes and in the disease polycythemia. At birth the red cell count is high; it falls shortly after birth and gradually rises to adult levels at puberty.