Red Blood Cells (Erythrocytes)
The major function of red blood cells, also known as erythrocytes, is to trans-port 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 per cent 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, for hemo-globin to remain in the human blood stream, it must exist inside red blood cells.
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 thou-sandfold. 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 that the red blood cells are responsible for most of the acid-base buffering power of whole blood.
Shape and Size of Red Blood Cells. 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.
The shapes of red blood cells can change remarkably as the cells squeeze through capillaries. Actually, the red blood cell is a “bag” that can be deformed into almost any shape. Furthermore, because the normal cell has a great excess of cell membrane for the quantity of material inside, deformation does not stretch the membrane greatly and, consequently, does not rupture the cell, as would be the case with many other cells.
Concentration of Red Blood Cells in the Blood. In normal men, the average numberof red blood cells per cubic millimeter is 5,200,000 (±300,000); in normal women, it is 4,700,000 (±300,000). Persons living at high altitudes have greater numbers of red blood cells. This is discussed later.
Quantity of Hemoglobin in the Cells. Red blood cells have the ability to concentratehemoglobin 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 hemo-globin-forming mechanism. Furthermore, in normal people, the percentage of hemoglobin is almost always near the maximum in each cell. However, when hemo-globin formation is deficient, the percentage of hemo-globin in the cells may fall considerably below this value, and the volume of the red cell may also decrease because of diminished hemoglobin to fill the cell.
When the hematocrit (the percentage of blood that is cells—normally, 40 to 45 per cent) 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 of cells; for women, it contains an average of 14 grams per 100 milliliters.
As discussed in connection with blood transport of oxygen, each gram of pure hemoglobin is capable of combining with 1.34 milliliters of oxygen. Therefore, in a normal man, a maximum of about 20 milliliters of oxygen can be carried in combination with hemoglobin in each 100 milliliters of blood, and in a normal woman, 19 milliliters of oxygen can be carried.