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468 Part VI: The Erythrocyte Chapter 31: Structure and Composition of the Erythrocyte 469

in the differential diagnosis of anemias. Normal human red cells have TABLE 31–1. Human Erythrocyte Protein and Water
a diameter of 7 to 8 μm, and the diameter decreases slightly with cell
age. The size decrease likely results from loss of membrane surface area Content
during erythrocyte life span by spleen-facilitated vesiculation. The cells Component mg/mL RBC Reference(s)
have an average volume of approximately 90 fL and a surface area of Water 721 ± 17.3 71
approximately 140 μm. The membrane is present in sufficient excess to
2
allow the cell to swell to a sphere of approximately 150 fL or to deform so Total protein 371 71
as to enter a capillary with a diameter of 2.8 μm. The normal erythrocyte Nonhemoglobin 9.2 71, 72
stains reddish-brown with Wright-stained blood films and pink with protein
Giemsa stain. The central third of the cell appears relatively pale com- Insoluble stroma 6.3 72
pared with the periphery, reflecting its biconcave shape. Many artifacts protein
can be produced in the preparation of the blood film. They may result Enzyme proteins 2.9 72
from contamination of the glass slide or coverslip with traces of fat, deter-
gent, or other impurities. Friction and surface tension involved in the Extensive study by 73, 74
preparation of the blood film produce fragmentation, “doughnut cells” proteomic methods
or anulocytes, and crescent-shaped cells. Observed under the phase-
contrast or interference microscope, the red cell shows a characteristic
48
internal scintillation known as red cell flicker. The scintillation results process required recalculation of published data. These recalculations
from thermally excited undulations of the red cell membrane. Frequency assume a hematocrit value of 45 percent and 33 g of hemoglobin per
analysis of the surface undulations has provided an estimate of the mem- deciliter of red cells. To obtain concentration per gram of hemoglobin,
brane curvature elastic constant and of changes in this constant resulting the concentration per milliliter red blood cell can be multiplied by 3.03.
from alcohol, cholesterol loading, and exposure to cross-linking agents. The tables list only some of the most commonly referred to constituents
of the erythrocyte. The reference on which each value is based is the first
RED CELL SHAPE AND SURVIVAL IN number presented in the last column of each table. Where applicable,
additional confirmatory references are given. In some instances, only
CIRCULATION the percentage of the total of the type of constituent present is given.
The red cell spends most of its circulatory life within the capillary chan- Chapter 46 discusses the detailed protein composition of the red cell
nels of the microcirculation. During its 100- to 120-day life span, the membrane and its various protein constituents.
red cell travels approximately 250 km and loses approximately 15 to 20
percent of its cell surface area. The long survival of the red cell is at ERYTHROCYTE DEFORMABILITY
least partially a result of the unique capacity of its membrane to “tank
tread”—that is, to rotate around the red cell contents and thereby facil- During its 120-day life span, the erythrocyte must undergo extensive
itate more efficient oxygen delivery. The physical arrangement of mem- passive deformation and must be mechanically stable to resist fragmen-
brane skeletal proteins in a uniform shell of highly folded hexagonal tation and cellular deformability is an important determinant of red cell
spectrin lattice permits this unusual behavior. 49–51 The arrangement also survival in the circulation. Red cell deformability is influenced by three
is responsible for the characteristic biconcave shape of the resting cell.
Red cells must also be able to withstand large shear forces and must be
able to undergo extensive reversible deformation during transit through TABLE 31–2. Human Erythrocyte Phospholipids
the microvasculature and in transiting from the splenic red cell pulp Lipid Amount Reference
back into circulation. The resiliency and fluidity of the membrane to
deformation is regulated by the spectrin-based membrane skeleton. Total phospholipids 2.98 ± 0.20 mg/ 75
49
A deficiency in the amount of spectrin or the presence of mutant spec- mL RBC
trin in the submembrane skeleton results in abnormally shaped cells in Cephalin 1.17 (0.38–1.91) 75
hereditary spherocytosis, elliptocytosis, and pyropoikilocytosis (Chap. mg/mL RBC
46). In regions of circulatory standstill or very slow flow, red cells Ethanolamine 29% of total 75
49
travel in aggregates of two to 12 cells, forming rouleaux. Within large phosphoglyceride phospholipid
vessels, increased shear forces disrupt this aggregation.
Mean plasmalogen 67% of eth- 75
content anolamine
RED CELL COMPOSITION phosphoglyceride
The erythrocyte is a complex cell. The membrane is composed of lipids Serine 10% of total 75
and proteins, and the interior of the cell contains metabolic machinery phosphoglyceride phospholipid
designed to sustain the cell through its 120-day life span and maintain Mean plasmalogen 8% of serine 75
the integrity of hemoglobin function. Each component of red blood cells content phosphoglyceride
may be expressed as a function of red cell volume, grams of hemoglo-
bin, or square centimeters of cell surface. These expressions are usually Lecithin 0.32 (0.03–0.95) 76
interchangeable, but under certain circumstances each may have spe- mg/mL
cific advantages. However, because disease may produce changes in the Sphingomyelin 0.12–1.13 mg/mL 76
average red cell size, hemoglobin content, or surface area, the use of any Lysolecithin 1.82% of total 77
of these measurements individually may, at times, be misleading. For phospholipids
convenience and uniformity, data in the accompanying tables (Tables
31–1 through 31–6) are expressed in terms of cell constituent per mil- note: Some results are given as mean ± standard deviation.
liliter of red cell and per gram of hemoglobin. In many instances, this RBC, red blood cell.

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