RED BLOOD CELLS (ERYTHROCYTES)
A major function of RBCs, also known as erythrocytes, is to transport hemoglobin, which, in turn, carries oxygen from the lungs to the tissues.
The RBCs have other functions besides transport of hemoglobin. For instance, they contain a large quantity of carbonic anhydrase, an enzyme that catalyzes the reversible reaction between carbon dioxide (CO2) and water to form carbonic acid (H2CO3), increasing the rate of this reaction several thousand fold. The hemoglobin in the cells is an excellent acid-base buffer (as is true of most proteins), so the RBCs are responsible for most of the acid-base buffering power of whole blood.
Shape and Size of Red Blood Cells
Size: Normal RBCs are biconcave disks having a mean diameter of about 7.8 micrometers and a thickness of 2.5 micrometers at the thickest point and 1 micrometer or less in the center. The average volume of the RBC is 90 to 95 cubic micrometers.
Shape: The shapes of RBCs can change remarkably as the cells squeeze through capillaries. Actually, the RBC is a “bag” that can be deformed into almost any shape.
Concentration of Red Blood Cells in the Blood
In healthy men, the average number of RBCs per cubic millimeter is 5,200,000 (±300,000); in women, it is 4,700,000 (±300,000).
Quantity of Hemoglobin in the Cells
RBCs have the ability to concentrate hemoglobin in the cell fluid up to about 34 grams in each 100 milliliters of cells. The concentration does not rise above this value because this is the metabolic limit of the cell’s hemoglobin-forming mechanism.
When the hematocrit (the percentage of blood that is in cells—normally, 40 to 45 percent) and the quantity of hemoglobin in each respective cell are normal, the whole blood of men contains an average of 15 grams of hemoglobin per 100 milliliters; for women, it contains an average of 14 grams per 100 milliliters.
PRODUCTION OF RED BLOOD CELLS
Areas of the Body That Produce Red Blood Cells. In the early weeks of embryonic life, primitive, nucleated RBCs are produced in the yolk sac. During the middle trimester of gestation, the liver is the main organ for production of RBCs, but reasonable numbers are also produced in the spleen and lymph nodes. Then, during the last month or so of gestation and after birth, RBCs are produced exclusively in the bone marrow.
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| Relative rates of red blood cell production in the bone marrow of different bones at different ages. |
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Hematology