Red Blood Cells (Erythrocytes)
Red blood cells serve many important functions in the body, including transporting hemoglobin (and oxygen), carrying catalysts for gas exchange and containing hemoglobin, which can act as an acid-base buffer (Guyton 1991). Forty to forty-five percent of the blood consists of cells; there are about 5 million red blood cells in every cubic centimeter of blood in the average healthy person (Guyton 1991). Normal red blood cells have an average life span of circulating through the blood of 120 days.
Red blood cells are produced by cells in the bone marrow. Once the soon-to-be red blood cells reach 34 percent hemoglobin concentration, the nucleus condenses and is transported out of the cell (Guyton 1991). At this stage the cell is called a reticulocyte and moves through the pores of the capillary membrane into the capillary (Guyton 1991). Once in the capillaries the cell circulates one or two days until it loses more of its organelles (i.e. mitochondria, Golgi apparatus), when it can be considered a mature erythrocyte (Guyton 1991).
Red blood cells as they move through capillaries and become deoxygenated. Image used and modified with permission from Dr. Kenneth Bridges. http://sickle.bwh.harvard.edu |
Normal, mature red blood cells are shaped as biconcave discs, with a diameter of 7.5 micrometers and a thickness of 1 to 1.9 micrometers (Guyton 1991). They are not full so that they can change shape while squeezing through tiny capillaries without bursting (Guyton 1991). |
After about 120 days in the blood stream, a red blood cell is considered old and its membrane becomes fragile. At this point the red blood cell usually ruptures when trying to fit through a small space; this rupturing often occurs in the spleen, where trabeculae can be as small as three micrometers wide (Guyton 1991).
Hemoglobin A
A hemoglobin chain is made up of a heme prosthetic group (which contains an atom of iron) and a globin polypeptide. Each hemoglobin molecule is made up of four hemoglobin chains and therefore carries four iron atoms. Each iron atom allows the binding of one oxygen molecule, giving each hemoglobin molecule an oxygen capacity of four molecules.
Hemoglobin A is the most common type of hemoglobin found in adults. It is made up of two subunits of alpha globin chains and two subunits of beta gobin chains. Other forms of hemoglobin can be found in healthy adults, including combinations with gamma and delta chains, but hemoglobin A is the most common of all of them. |
A single hemoglobin A molecule, made up of two alpha and two beta globin chains, each containing a heme prosthetic group with an iron atom. |
Hemoglobin is a great oxygen carrier because it “combines loosely and reversibly” with oxygen (Guyton 1991). The oxygen is kept in its molecular form when bound to the hemoglobin molecule through a very loose bond with the iron atom (Guyton 1991). This loose and reversible bond allows oxygen to be easily picked up when oxygen concentrations are high in the lungs and to be easily released when the oxygen concentration is low in the tissues.