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 micrometres in diameter and have the
form of biconcave disks, a shape that provides a large
surface-to-volume ratio. When blood is centrifuged to
cause the cells to settle, the volume ofpacked red cells
(hematocrit value) ranges between 42 and 54 percent
of total volume in menand 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 4,000,000 and 6,000,000 per cubic
millimetre. Hemoglobin constitutes about one-third of
the weight of each red cell. The amount of hemoglobin
in blood is related to the hematocrit valueand to the
red cell count, and in normal adults ranges between
14 and 18 grams per 100 millilitres. When fresh blood
is examined with the microscope, red cells appear to
be yellow-green disks with pale centres containing no
visible internal structures.
The red 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 freelypermeable
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 predominantly 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. In distilled
water red cells continue to swell until they become
spherical, whereupon they disrupt, releasing the dissolved
hemoglobin into the surrounding fluid (hemolysis).
