Red blood cells (erythrocytes)
The red blood cells are highly specialized, well adapted for their primary function of transporting oxygen from the lungs to all of the body tissues. Red cells are approximately 7.8 μm (1 μm = 0.000039 inch) in diameter and have the form of biconcave disks, a shape that provides a large surface-to-volume ratio. When fresh blood is examined with the microscope, red cells appear to be yellow-green disks with pale centres containing no visible internal structures. When blood is centrifuged to cause the cells to settle, the volume of packed red cells (hematocrit value) ranges between 42 and 54 percent of total volume in men and between 37 and 47 percent in women; values are somewhat lower in children. Normal red blood cells are fairly uniform in volume, so that the hematocrit value is determined largely by the number of red cells per unit of blood. The normal red cell count ranges between four million and six million per cubic millimetre.
The red blood cell is enclosed in a thin membrane that is composed of chemically complex lipids, proteins, and carbohydrates in a highly organized structure. Extraordinary distortion of the red cell occurs in its passage through minute blood vessels, many of which have a diameter less than that of the red cell. When the deforming stress is removed, the cell springs back to its original shape. The red cell readily tolerates bending and folding, but, if appreciable stretching of the membrane occurs, the cell is damaged or destroyed. The membrane is freely permeable to water, oxygen, carbon dioxide, glucose, urea, and certain other substances, but it is impermeable to hemoglobin. Within the cell the major cation is potassium; in contrast, in plasma and extracellular fluids the major cation is sodium. A pumping mechanism, driven by enzymes within the red cell, maintains its sodium and potassium concentrations. Red cells are subject to osmotic effects. When they are suspended in very dilute (hypotonic) solutions of sodium chloride, red cells take in water, which causes them to increase in volume and to become more spheroid; in concentrated salt solutions they lose water and shrink.
When red cell membranes are damaged, hemoglobin and other dissolved contents may escape from the cells, leaving the membranous structures as “ghosts.” This process, called hemolysis, is produced not only by the osmotic effects of water but also by numerous other mechanisms. These include physical damage to red cells, as when blood is heated, is forced under great pressure through a small needle, or is subjected to freezing and thawing; chemical damage to red cells by agents such as bile salts, detergents, and certain snake venoms; and damage caused by immunologic reactions that may occur when antibodies attach to red cells in the presence of complement. When such destruction proceeds at a greater than normal rate, hemolytic anemia results.
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