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The size of red blood cells usually falls within a range of 6 to 8 micrometers in random blood samples that have been analyzed by laboratory testing. Mature forms of the red blood cells (RBCs) are called erythrocytes, which constitute around 40 to 45% of human red blood cells. Immature forms of red blood cells are termed reticulocytes, and these usually account for only about 1 to 2% of the red blood cells.
Erythrocytes are larger red blood cells that have a shape similar to a round hard candy with a hole in the center, having a structure that is called “biconcave.” The structure is naturally flexible and bendable, however, in order to provide easier passage through the blood vessels going around the body. Their shape also allows these cells to absorb greater amounts of oxygen during their circulation throughout the blood, since it provides a greater surface area for absorption. This is important since one of the functions of erythrocytes is to provide a sufficient oxygen exchange for body tissues and organs.
Several lab tests can be used to measure the characteristics of an average red blood cell. A mean corpuscular volume (MCV) test measures the average size of the cell. A mean corpuscular hemoglobin (MCH) test measures the cell’s content of hemoglobin — a red protein substance that carries oxygen within the cell. A mean corpuscular hemoglobin concentration (MCHC) test measures the ratio of hemoglobin to cell size.
These tests are important in that they can show the presence of anemia — a condition that arises when the hemoglobin content in regular red blood cells is too low to provide the body with sufficient oxygen. This condition is defined according the size of red blood cells and the amount of hemoglobin the cells carry. Red blood cell indices on the lab tests help to determine if anemia is present, by establishing normal hemoglobin ranges for males and females, generally within a range of 12 to 17 grams per deciliter.
The size of red blood cells depends largely upon their maturity. RBCs are produced on a continual basis in the body within the bone marrow, such as the large bone of the femur. Once an erythrocyte is created, it grows to maturity in approximately 7 days, and each cell lives approximately 120 days. Moreover, a mammalian erythrocyte is distinguished from other vertebrate species — those having a vertebra, or spine — by the fact that mammalian erythrocytes do not have a cell nucleus, a center containing genetic material. All other vertebrate erythrocytes retain the capacity that human cells lose once they develop to maturity.