Biomedical Engineering Reference
In-Depth Information
10
µ
m
FIGURE 17.2
Scanning electron microscope photograph of erythrocytes.
17.2.1 E
RYTHROCYTES
Erythrocytes are best known as red blood cells. These cells are doughnut-shaped and approximately
5-8 μm in diameter (Figure 17.2). Blood of a healthy individual contains approximately 4
10
9
cells/mL. The red color of the cells is caused by the presence of oxygenated hemoglobin. The main
function of erythrocytes is the transport of oxygen and carbon dioxide to and from tissues and organs.
They occupy approximately 40-50% of the total blood volume. The ratio of blood cells
(of which over 99% are red blood cells) to plasma is called the hematocrit. Too high or too low
hematocrit values demonstrate a defect in the formation or removal of erythrocytes. For profes-
sional athletes, for instance cyclists, a hematocrit value of 0.5 or higher is a strong indication (but no
proof) for the use of illegal drugs like erythropoietin (EPO). This hormone increases the formation
of erythrocytes, enhancing the possible uptake of oxygen in the blood and thus the performance of
the athlete.
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5
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17.2.2 L
EUKOCYTES
Leukocytes, also known as white blood cells, consist of phagocytic cells, granulocytes, and
lymphocytes.
3,4
Leukocytes are approximately 10-15 μm that comprise only a small part of the
blood (4
10
6
/mL). White blood cells can be divided into nonlymphatic white cells, formed
predominantly in the bone marrow, and lymphocytes, formed in the lymphatic system. There
are several different nonlymphatic white blood cells, namely, monocytes (approximately 5% of
circulating white blood cells), polymorphonuclear neutrophils (PMN, 62%), basophils (0.5%), and
eosinophils (2.5%).
Monocytes and PMNs can easily leave the blood stream and infi ltrate into peripheral tissues.
Upon translocation into tissues, monocytes are called tissue macrophages. They play an essential
role by performing phagocytosis, clearing pathogens and foreign bodies from tissues upon injury
or infection.
The neutrophils, basophils, and eosinophils contain a large number of granules and are there-
fore often referred to as granulocytes. Granulocytes release the contents of their granules upon
activation, for instance by pathogens or a synthetic surface. High doses of active oxygen species,
peroxides, histamines, and enzymes are released in order to attack the foreign body. In general
one can state that the nonlymphatic white blood cells play an important role in the innate immune
response.
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6
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