Red Blood Cells (Erythrocytes)
A major function of red blood cells, also known as erythrocytes, is to transport hemoglobin, which in turn carries oxygen from the lungs to the tissues. In some lower animals, hemoglobin circulates as free protein in the plasma, not enclosed in red blood cells. When it is free in the plasma of the human being, about 3 percent of it leaks through the capillary membrane into the tissue spaces or through the glomerular membrane of the kidney into the glomerular filtrate each time the blood passes through the capillaries. Therefore, hemoglobin must remain inside red blood cells to effectively perform its functions in humans. The red blood cells 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 rapidity of this reaction makes it possible for the water of the blood to transport enormous quantities of CO₂ in the form of bicarbonate ion (HCO₃) from the tissues to the lungs, where it is reconverted to CO₂ and expelled into the atmosphere as a body waste product. The hemoglobin in the cells is an excellent acid-base buffer (as is true of most proteins), so the red blood cells are responsible for most of the acid-base buffering power of whole blood.
Concentration of Red Blood Cells in the Blood
In healthy men, the average number of red blood cells per cubic millimeter is 5,200,000 (±300,000); in women, it is 4,700,000 (±300,000). Persons living at high altitudes have greater numbers of red blood cells, as discussed later. Normal red blood cells, shown in Figure 32-3, are biconcave discs 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 red blood cell is 90 to 95 cubic micrometers.